The Story Behind the Dynojet Chassis Dyno - The Truth Meter

1/4

It's a story as old as hot rodding itself. It starts with the sales pitch-"Buy my widget and your engine will gain 50 hp"-and ends with a disappointed customer with a car that sure doesn't feel like it picked up 50 hp. A dragstrip doesn't offer much proof one way or the other on incremental changes because there are too many variables involved, so the seat of the pants was for a long time the only way to tell if a modification or part really helped. That all changed in the '90s with the introduction of the Dynojet, a portable chassis dyno that was in the financial grasp of most every mom-and-pop performance shop. Finally, power claims were proved or disproved as soon as the stuff was installed. And with the emergence of custom computer tuning, the Dynojet has proven invaluable to these shops; they can now tune the car without ever having to blast up and down a city street. They can thank Mark Dobeck, the machine's creator.
Dobeck got his start tuning English sports cars in a Portland, Oregon, garage in the late '70s. He had hot-rodded the shop's Sun infrared exhaust analyzer to improve response time and became a wiz at using exhaust-gas carbon monoxide to optimize power on the go. The trouble came later when he moved on in 1980 to open a motorcycle shop in Wisconsin. Cars were one thing, but there was no way to haul a gas analyzer as big as a TV set on a motorcycle. So Dobeck talked his inventor/fabricator father into building a stationary rolling-road that could support the rear wheel of a motorcycle on a moving drum so he could continue tuning while "driving" with the big infrared analyzer.
The rolling road was designed with a hydraulic system that could be adjusted to work a bike engine harder at a given speed, something like the resistance controls on a Stairmaster machine. But because Dobeck and his dad were mechanics rather than mathematicians, they made the rolling drum heavy, and the homebuilt dyno had a surprising amount of inertia. It was accidentally pretty good at simulating a motorcycle's ability to accelerate.

2/4 On the road, Dobeck tested a bunch of cars in secret so he could watch the Dynojet 248 in action without too many people knowing what he was doing. The Dynojet often disappointed optimistic enthusiasts and tuners, but it just as often helped find free power.

Dobeck's new bike shop opened just in time for the arrival of Japanese superbikes equipped with constant velocity (CV) carbs, which were new to motorcycling. CV carbs provided good performance, economy, and emissions, but they could not be tuned and jetted using traditional methods. Many people recommended replacing them, a $600 solution. But Dobeck understood CV carbs from the days of wrenching on English cars and modified them to allow the new motorcycles to run with performance pipes and air cleaners. Before long, bikers were traveling from all over the upper Midwest for Dobeck's dyno-jetting service. Meanwhile, in the evenings, Dobeck read magazine stories of hot rod bikes running exhaust-system shootouts on the torque-cell dynos of famous California super tuners.
Performance magazines loved dynamometers because they brought science to hot rodding. But torque-cell dynos, which load an engine by forcing it to pump water or generate electricity, are expensive, and using them has often required removing the engine from the vehicle.
"I started to realize I was doing something that no one else was doing," says Dobeck, who was using his homebuilt inertial dyno to tune bikes with the goal of improving acceleration and responsiveness. "Eventually I built a few jet kits to see what we could do with them."

3/4 The front end of a Chrysler K-car-the cheapest vehicle Dobeck could find-was used to spin up this research chassis dyno at Dynojet. A robotic controller could use the K-car to the spin the dyno drums to high speed to observe balance issues and then leap off the drums to remove all drag.

Dobeck named his company Dynojet. His first big customer was K&N Filters, and it wasn't long before he was selling lots of jet kits. His company grew at a rapid pace, and sure enough, a competitor sprang up with a similar product. "Their advertising was working," Dobeck says. "They were taking away sales. But the product didn't work. Not at all." To prove it, he called several of the top engine-dyno suppliers to see if they would help him develop an affordable version of his homebuilt inertial chassis dyno that could live in the shops of Dynojet dealers to show the world what worked and exactly how well. "Every one of them laughed at me," Dobeck remembers.
One of the biggest headaches of Dynojet's go-it-alone chassis-dyno project was figuring out how to assign meaningful power numbers in the face of unknown inertia from the moving parts of the hundreds or thousands of engine, drivetrain, and tire combinations. Wrestling to fully understand inertia and powertrain losses, Dobeck and his team quickly realized that the standard physics formula of weight, time, and distance for converting acceleration into horsepower simply didn't work-the derived number was always lower than accepted numbers. They poured on resources and burned up time and money investigating it, but no matter what they did, the math never added up.
Dynojet's final number-fudge was arbitrarily based on a number from the most powerful road-going motorcycle of the time, the '85 1,200cc Yamaha VMax. The VMax had 145 advertised factory horsepower, which was far above the raw 90hp number spit out by the formula. Meanwhile, existing aftermarket torque-cell engine dynamometers delivered numbers that clustered around 120. Always a pragmatist, Dobeck finally ordered his Chief Engineer to doctor the math so that the Dynojet 100 measured 120 hp for a stock VMax. And that was that: For once and forever, the power of everything else in the world would be relative to the '85 Yamaha VMax and a fudged imaginary number. Dobeck's engineering staff was dismayed by the decision, but the Dynojet 100 exclusively measured surplus power available to accelerate the vehicle's mass-no more, no less-and that was true even if the modification was a low-inertia flywheel or lightweight wheels. As long as the inertial dyno's numbers were repeatable, the critical question (did a particular modification make the engine accelerate faster or slower?) would be answered correctly.

4/4 Demonstrating the Dynojet 100 bike dyno in communist China, Dobeck entertained these bikers in the hills above Macao.

Dobeck then turned his attention to providing the dyno to bike shops across the country. The first 20 early adopters of Dynojet kits were customers who had defeated the replace-your-CV-carbs drumbeat seven years earlier. "These guys believed in what we were doing," says Dobeck. "I called, said I've got this dyno, and it costs $6,500. And they said, 'Send it.'"
When a small network of the most important dealers had dynos, Dobeck took to the road with a mobile bike dyno mounted in a trailer. He would ask performance-shop owners, "Aren't you sick of being the scapegoat for stuff that doesn't work as advertised?" They were, and they started buying dynos. In subsequent years, Dobeck demonstrated his bike dyno everywhere from Montana to communist China. Then he took on the world of cars.
The pre-Dynojet world of hot rods circa 1993 had a lot of information, misinformation, and disinformation. You can't feel a 5-10hp boost on a car, so many engine modifications were faith-based efforts made with a screwdriver and a prayer. Hot rodding had left more than a few hapless victims with fading dreams of glory and empty pockets. The onset of computerized engine controls in the '80s made increasing horsepower even more complicated-escalating the opportunities for the unscrupulous or incompetent to fleece those with the need for speed: Install this electronic doohickey, double your power. Car guys needed a cost-effective, repeatable B.S. meter every bit as much as bikers. Dobeck hired his dad and put several engineers on the project to handle critical design issues and the team constructed the original Dynojet 248A using two 48-inch-diameter, 1,200-pound rollers, later increased to 1,600 pounds.
When it came time to market the new car dyno, Dobeck realized that although his company was big-time in the motorcycle universe, no one in the door-slammer crowd had ever heard of him. So he went on the road again. The import crowd embraced the new Dynojet first, since they were the victims of a lot of bogus power claims from unscrupulous manufacturers. Then Dobeck visited some of the bigger aftermarket companies. The Dynojet often brought bad news to hot rodders and manufacturers-now everyone on the street knew exactly how much power the parts were worth. But the good news was, in the right hands, the dyno could find "free power" through tuning 8 out of 10 times.
With the automotive aftermarket sold on his Dynojet, Dobeck wanted to relax. By 1996, he was running on fumes and on the road way too much working like a madman. "I had no normal life," he says. An investor group was looking at buying the company, but it was on the fence, so he chased after a NASCAR licensing agreement. Back in the trenches he went, this time to offer his Dynojet to the NASCAR teams in North Carolina. They bit, and after a while, NASCAR agreed to his humble terms and made it "The Official Dynamometer of NASCAR" for three years. The NASCAR teams bought dynos, and Dynojet designed fabulous NASCAR chassis-dyno rooms that purportedly generated six times the revenues of the dyno itself. At that point, he sold the company for six million dollars in cash.
Over the course of his 27 years of work, Dobeck helped make hot rodding more honest. Performance consumers now expect to know dyno results for speed parts, and dyno tuning and development has become essential for serious racers and hot rodders. Chassis dynos from Mustang, Superflow, and others now provide an alternative to Dynojet, but Dobeck's little bike dyno is the one that started it all.
What's he doing now? Dynojet was recently sold again, terminating Dobeck's non-compete clause, so he's back at it with Dobeck Performance. He reassembled technical talent from the old Dynojet days and has created a handheld gas analyzer (The Sniffer) and a computer interceptor that allows fuel tuning in an EFI car or bike (The Fuel Nanny). He's also looking at a new chassis dyno based on proprietary patented inertial and torque measurement technology. Meanwhile, he's on the road, as always. "Again, I did the routine that works: I put myself right out there in the pits, at the track level, playing around."

Tuning 4 Link Rear Suspensions for the Drag Strip

Tuning 4 link rear suspensions can make a car launch quicker and use less horsepower to do it. Which makes you faster, so you win, get famous, and live happy ever after...
So, let's get into it and find out how to do it...
Note: You need at least 45% of the car's weight on the rear wheels to make a 4 link suspension work properly.
Also, get some graph paper. It helps to draw out what we are doing here so you can measure and be able to set up your 4 link the easy way. You don't have to be an artist, just measure your car, height and length of the 4 links, etc. and transfer them to the paper.
Ok, first we need to know what an 'Instant Center' is.
Imagine a line going through the lower control arm extending towards the front of the car. Now imagine a line going through the upper control arm extending until it intersects the lower line. Where this intersects is called the 'Instant Center'.

Where the 'Instant Center' is located in the car compared to the 'Center of Gravity' of the car is what determines how the suspension acts on the car to launch it.
What you're shooting for is a position that will apply just enough force to the tires to make them grab without spinning and the rest of the force will launch the car.
Next we will figure out where to place the 'Instant Center'.
But first we have to locate our '100% Anti-Squat Line'.
Huh?
Theoretically, if the 'Instant Center' falls on the '100% Anti-Squat Line', the rear of the car will neither lift or squat, it will just be pushed foward. That's a good thing...
So let's find it... Imagine a point on the center of the rear tire contact patch. Now imagine a point on the center of the front tire contact patch.
Now take a line straight up from the front contact patch, and have it intersect the height of the center of gravity.
Note: If you need to know the center of gravity of your car, click here: How to find your Center of Gravity

Ok, now imagine a line from that point to the rear tire contact patch. This is your '100% Anti-Squat Line'.

Now then, let's figure out what all this means...
If your 'Instant Center' falls on your '100% Anti-Squat Line', then think of your car having as having a 'neutral' setting. The rear of the car shouldn't lift or squat, and all the horsepower should launch the car.

Now that's in a perfect world... There are lots of variables that affect that, so your car may want a little above the line, or a little below. This is where testing comes in. We'll get to that in a minute.
Now if your 'Instant Center' falls BELOW your '100% Anti-Squat Line', you will have less than 100% Anti-Squat. The rear of your car will squat and the suspension will 'lift' lightening the rear wheels. There won't be enough force applied to the rear tires and the tires will spin.

Now if your 'Instant Center' falls ABOVE your '100% Anti-Squat Line', you will have more than 100% Anti-Squat. The rear of the car will lift and the rear tires will be forced down, sometimes violently if it's too much. This does give more traction, but it's wasting horsepower doing it...
Sometimes high horsepower cars that have too much 'Anti-Squat' will force the tires down very hard for the first several feet but then start to unload the tires and spin when the chassis starts to settle.

Now that we know how it works, let's set it up!
The way most people go about tuning 4 link suspensions is to get it in the general position they figured out on paper. Now experiment by launching the car and having someone watch or videotape it. Watch what the rear of the car and the tires do.
Now you can see what happens and adjust from there...
You're shooting for the tires to hook up without slipping, and for the car to launch foward without it lifting or squatting.
If the rear of the car squats, you want to raise the 'Instant Center'.
If the rear of the car lifts, you want to lower the 'Instant Center'.
Note: A fully-adjustable 4 link suspension allows you to move the 'Instant Center' not only up or down, but also forward or back by increasing or decreasing the angle between the bars.
If your car wheel stands too much, you can angle the 4 link bars closer together at the front to move the 'Instant Center' closer to the rear of the car.
If your rear tires are being forced down too violently you can angle the 4 link bars farther apart at the front to move the 'Instant Center' closer to the front of the car.
Now let's talk about 'Pre-Load'.
You know when you're at the track and see a car launch and it tries to pull the left front wheel up? What you can't see is it's also pulling the right rear tire up, causing it to lose traction. It's more common on large vehicles with lots of horsepower.
What's happening is the torque from the engine and driveshaft is trying to move the car, and the car is twisting around it. It's actually much more technical, but you get the idea...
You don't want this to happen.
That's when you want to use some 'Pre-Load'. If your 4 link bars are adjustable for length, you can do it there.
The first thing you want to do is make sure the upper bars are the same length and that the lower bars are the same length. This is critical.
After you get the bars the same length you need to put them back in and get the car square front to back and corner to corner.
Now the right upper bar sets any type of preload you want. It is the only bar that you adjust! Leave the other three alone.
'Pre-Load' helps offset the torque from the engine. You will usually shorten the top right bar.
Shortening the upper right bar adds weight to the right rear tire. A little does a lot. Try one 'flat' (on the adustment nut) at a time and keep track of how much you adjusted and the results. One-quarter turn can add as much as 50 pounds.
If you want to check your 4 link setup, go to a chassis shop or find someone with portable race scales. Have them put the car on the scales weigh the car with the 4 link disconnected. Then connect the 4 link, preload it and weigh again. Then you will be able to tell if each side is preloaded the same. Alot of tuning can be done this way.

Nitrous Oxide

Nitrous Oxide

Nitrous oxide ( N2O ) contains 33 vol% of oxygen, consequently the combustion
chamber is filled with less useless nitrogen. It is also metered in as a
liquid, which can cool the incoming charge further, thus effectively
increasing the charge density. With all that oxygen, a lot more fuel can
be squashed into the combustion chamber. The advantage of nitrous oxide is
that it has a flame speed, when burned with hydrocarbon and alcohol fuels,
that can be handled by current IC engines, consequently the power is
delivered in an orderly fashion, but rapidly. The same is not true for
pure oxygen combustion with hydrocarbons, so leave that oxygen cylinder on
the gas axe alone :-). Nitrous oxide has also been readily available at a
reasonable price, and is popular as a fast way to increase power in racing
engines. The following data are for common premixed flames [131].
             
                               Temperature     Flame Speed 
  Fuel         Oxidant            ( C )           ( m/s )           
Acetylene        Air               2400         1.60 - 2.70
   "         Nitrous Oxide         2800             2.60
   "            Oxygen             3140         8.00 - 24.80
Hydrogen         Air               2050         3.24 - 4.40
   "         Nitrous Oxide         2690             3.90
   "            Oxygen             2660         9.00 - 36.80
Propane          Air               1925             0.45
Natural Gas      Air               1950             0.39

Formula 1 1987

From Honda Civic to Formula 1 winner.                   


There are many variables that will determine the power output of an engine.
High on the list will be the ability of the fuel to burn evenly without
knock. No matter how clever the engine, the engine power output limit is
determined by the fuel it is designed to use, not the amount of oxygen
stuffed into the cylinder and compressed. Modern engines designs and
gasolines are intended to reduce the emission of undesirable exhaust
pollutants, consequently engine performance is mainly constrained by the
fuel available.

Honda Civic uses 91 RON fuel, but the Honda Formula 1 turbocharged 1.5
litre engine was only permitted to operate on 102 Research Octane fuel, and
had limits placed on the amount of fuel it could use during a race, the
maximum boost of the turbochargers was specified, as was an additional
40kg penalty weight. Standard 102 RON gasoline would be about 96 (R+M)/2 if
sold as a pump gasoline. The normally-aspirated 3.0 litre engines could use
unlimited amounts of 102RON fuel. The F1 race duration is 305 km or 2 hours,
and it's perhaps worth remembering that Indy cars then ran at 7.3 psi boost.

Engine                 Standard         Formula One     Formula One
Year                     1986              1987            1989
Size                   1.5 litre         1.5 litre       1.5 litre
Cylinders                 4                 6               6
Aspiration              normal            turbo           turbo
Maximum Boost             -               58 psi          36.3 psi          
Maximum Fuel              -              200 litres      150 litres 
Fuel                    91 RON           102 RON         102 RON
Horsepower @ rpm      92 @ 6000         994 @ 12000     610 @ 12500
Torque (lb-ft @ rpm)  89 @ 4500         490 @  9750     280 @ 10000
  

The details of the transition from Standard to Formula 1, without
considering engine materials, are:-

1. Replace the exhaust system. HP and torque both climb to 100.
2. Double the rpm while improving breathing, you now have 200hp
   but still only about 100lb-ft of torque.
3. Boost it to 58psi - which equals four such engines, so you have
   1000hp and 500lb-ft of torque.

Simple?, not with 102 RON fuel, the engine/fuel combination would knock 
the engine into pieces, so....

4. Lower the compression ratio to 7.4:1, and the higher rpm is a
   big advantage - there is much less time for the end gases to
   ignite and cause detonation.
5. Optimise engine design. 80 degree bank angles V for aerodynamic
   reasons, and go to six cylinders = V-6
6. Cool the air. The compression of 70F air at 14.7psi to 72.7psi
   raises its temperature to 377F. The turbos churn the air, and
   although they are about 75% efficient, the air is now at 479F.
   The huge intercoolers could reduce the air to 97F, but that
   was too low to properly vaporise the fuel.
7. Bypass the intercoolers to maintain 104F.
8. Change the air-fuel ratio to 23% richer than stoichiometric
   to reduce combustion temperature.
9. Change to 84:16 toluene/heptane fuel - which complies with the
   102 RON requirement, but is harder to vaporise.
10.Add sophisticated electronic timing and engine management controls
   to ensure reliable combustion with no detonation.

You now have a six-cylinder, 1.5 litre, 1000hp Honda Civic.

For subsequent years the restrictions were even more severe, 150 litres
and 36.3 maximum boost, in a still vain attempt to give the 3 litre,
normally-aspirated engines a chance. Obviously Honda took advantage
of the reduced boost by increasing CR to 9.4:1, and only going to 15%
rich air-fuel ratio. They then developed an economy mode that involved
heating the liquid fuel to 180F to improve vaporisation, and increased
the air temp to 158F, and leaned out the air-fuel ratio to just 2% rich.
The engine output dropped to 610hp @ 12,500 ( from  685hp @ 12,500 and
about 312 lbs-ft of torque @ 10,000 rpm ), but 32% of the energy in
the fuel was converted to mechanical work. The engine still had crisp
throttle response, and still beat the normally aspirated engines that
did not have the fuel limitation. So turbos were banned. No other
F1 racing engine has ever come close to converting 32% of the fuel
energy into work [136].

In 1995 the FIA listed a detailed series of acceptable ranges for
typical components in racing fuels for events such as F1 races, along
with the introduction of detailed chromatographic "fingerprinting" of
the hydrocarbon profile of the fuel [137]. This was necessary to prevent
novel formulations of fuels, such as produced by Honda for their turbos.

Injector Firing Angle

 

Injector Firing Angle

 

The Injector firing angle is the angle at which the injector will SHUT after the injection even has occured
(relitive to a full camshaft revolution).
The injection angle depends on things such as camshaft profile, intake manifold design, fuel type, injector location as well as other external factors.
Injection angle is also used in certain applications to cool in th...e inlet valves.
When tuning the end of injection angle (with the exception of above) you want to make sure all injected fuel is taken in by the motor
and not "blown" back up the inlet manifold.

In practice the easiest way to do this is by tuning your engine to a known AFR. Next, adjust the injector firing angle until the engine becomes as rich as possible, now we know that the engine is burning the maximum amount of injected fuel. Do this for each RPM site you require.
You will notice that the engine will be slightly more responsive at lower RPM with the correct injection angle.
Please note, if the engine is not tuned to accpetable mixtures (rich and lean spots all over the place) that injection angle will not make much of a diffrence and you should spend more time on the tune.

It depends on where you injectors are located. Stock values represent stock ports and injector locations,
and you also need to consider the secondary injectors which have an offset angle and are fired earlier than primaries.
Injectors mounted further than stock need to be advanced, especially at lower rpm areas.
Bigger ports will need to fire a little more retarded, down to around 50-60 at idle.

We have different requirements for different engine conditions.
For example at very low engine speeds you probably want to fire the fuel just after the intake valve opens,
this will ensure that a high percentage of the fuel delivered as it gets caught up in the moving air stream
(because the engine speed is low the valve is open for alot longer than the injector is open so this is easily achieved).

At part throttle and higher RPM it is often of benefit to spray the fuel onto a closed intake valve.
...because the valve is hot the fuel will hit the valve and evaporate giving a better air/fuel distribution due to the smaller droplet size of fuel
(ie its now evaporated) - this of course assumes that the injector actually sprays fuel directed at the valve,
if you have a throttle body injection setup then there is much less benefit to this.

Of course at high duty cycles there is much less importance.

Bore washing is rarely a problem under any condition apart from startup because of the heat in the cylinder walls
(once the engine is warm the temperature of the liner is well higher than the fuels evaporation temperature so bore washing wont occur -the fuel just evaporates).
This is exactly why you would want to fire the fuel into the moving airstream on startup and low engine speeds.
Get the fuel droplets suspended in the airstream and move them into the combustion chamber,
if you spray fuel into the valve when the engine is cold it will liquify,
pool and then run into the combustion chamber - this is where bore washing occurs.

Know your rod bolt

Know your rod bolt

This write-up is not intended to be a chapter out of an Engineering Design Book. That would be way too long, way too involved, and way too boring for most folks here to have any interest in. Instead, this is just a general overview of how connecting rod bolts compare, and what we REALLY NEED in our motors.
Yield Strength = the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape and size when the applied stress is removed. Once the yield point is passed, the deformation will be permanent, which is considered a “failed” condition for a bolt. So, the bolt must be discarded.
Tensile Strength = the maximum stress that a material can withstand while being stretched or pulled, without starting to neck down and ultimately breaking.
----------------
First let’s look at some typical strength values of various bolts, to get a general feel for how they compare.

Grade 2 hardware store general purpose bolt:
Yield strength = 55,000 psi
Tensile strength = 74,000 psi
Cost = a few cents each
--------
Grade 5 hardware store general purpose bolt:
Yield strength = 85,000 psi
Tensile strength = 120,000 psi
Cost = a few cents each
--------
Grade 8 hardware store general purpose bolt:
Yield strength = 120,000 psi
Tensile strength = 150,000 psi
Cost = a few cents each
---------
ARP 8740 chrome moly “connecting rod” bolt:
Yield strength = 180,000 psi
Tensile strength = 200,000 psi
Cost = $120.00USD per set of 16, or about $8.00USD each.
---------
ARP 2000 “connecting rod” bolt:
Yield strength = 180,000 psi
Tensile strength = 220,000 psi
ARP 2000 rod bolt material has twice the fatigue life of 8740 chrome moly rod bolt material.
Cost = $200.00USD per set of 16, or about $13.00USD each.
---------
ARP L19 “connecting rod” bolt:
Yield strength = 200,000 psi
Tensile strength = 260,000 psi
ARP L19 rod bolt material is subject to hydrogen embrittlement, and stress corrosion. It also cannot be exposed to any moisture, including sweat and/or condensation.
Cost = $200.00USD per set of 16, or about $13.00USD each.
---------
ARP Custom Age 625+ “connecting rod” bolt:
Yield strength = 235,000 psi
Tensile strength = 260,000 psi
ARP Custom Age 625+ rod bolt material has nearly 3 ½ times the fatigue life of the ARP 3.5 rod bolt material.
Cost = $600.00USD per set of 16, or about $38.00USD "EACH".
----------
ARP 3.5 “connecting rod” bolt:
Yield strength = 220,000 psi
Tensile strength = 260,000 psi
Cost = $855.00USD per set of 16, or about $53.00USD "EACH"!!!
So, as you can see above, hardware store general purpose bolts are considerably weaker than “purpose built” connecting rod bolts. And we won’t even bother getting into the differences in fatigue life. Suffice it to say, we CANNOT use general purpose hardware store bolts in our connecting rods.
--------------
A connecting rod bolt’s maximum tension loads are determined by the mass of the parts involved, the rod length, the stroke length, and the max rpm. That’s it. It has absolutely nothing what so ever to do with the amount of HP being made. The max tension loads on the rod bolts will never change, no matter if you add Nitrous, a Turbo, or a Blower to an engine, as long as the short block and redline don’t change. That max tension loading occurs at TDC on the exhaust stroke as the mass involved is brought to a dead stop, and has its direction reversed. In order to change the max tension loading on the rod bolts, you’d have to change the short block configuration and/or the redline. And vacuum pulling on the rod bolts when chopping the throttle at high rpm, is not a concern. Because those affects don't even begin to build until well past TDC, which of course is "AFTER" the mass of the parts involved has already been brought to a stop, and their direction reversed.
The rod’s big end “clamp-up preload” provided by stretching/torquing the rod bolts, must always be HIGHER than the “cyclic tension load” applied to the bolts at TDC exhaust, in order to prevent rod bolt failure. And the larger the difference between the preload and the cyclic load, the better. Precision detailed "Strength Analysis" calculations can be performed using sound Engineering principles, to determine the “Margin of Safety” (MOS) between the “cyclic tension loading” and the “clamp-up preload”, to make sure you have a sufficient MOS for the engine to be reliable. I’ll spare you all the involved and complicated math, and just show you the results.
Before we go on, first a comment on “cap screw” rod bolt sizes. Your rod bolts are NOT the size you think they are. If you run 3/8” rod bolts, only the threads are 3/8”. But, the part of the bolt that matters regarding the stretch, is the shank. And the main length of the shank is only 5/16”, not the 3/8” you might have thought. And if you run 7/16” rod bolts, the threads are 7/16”, but main length of the shank is only 3/8”. So, where you are most concerned, the bolts are one size SMALLER than you thought.
And if that isn’t enough detail, you must also consider, in addition to the main section of the shank, the other diameters involved which come from the radius transition between the threads and the shank, the radius transition between the shank and the shoulder right under the bolt head flange, and that shoulder itself right under the bolt head flange. The bolts stretch the whole length between the threads and the bolt head flange. And all those individual sections contribute to the total stretch by different amounts.
So, the rod bolt “Strength Analysis” must take into account all those various diameters, as well as the length of each of those diameters. Because the stretch has to be calculated for each individual section of the shank between the threads and the bolt head flange. If this is not done correctly, the “Strength Analysis” results will simply end up being wrong and worthless. But, for the results shown below, all those details were carefully worked out for those portions of the “Strength Analysis”. So, the answers below are all accurate.
Rod bolt "Strength Analysis" performed on known real world Street Hotrods, Street/Strip cars and Sportsman Drag cars, being operated at their typical maximum rpm, indicates the following:
• An engine with a max rpm rod bolt MOS of around 125% or higher, results in the engine being as safe and reliable as a stock grocery getter, or in other words essentially bullet proof. This is our design target when planning a new build. Having a MOS higher than this can’t hurt of course, but in terms of strength requirements, there is really no added value for doing that. However, a higher MOS can help with rod bolt fatigue life, if that is critical for a particular application. More on fatigue life later.
• If you are a little more aggressive, and run a maximum rpm rod bolt MOS between 100% and 125% only “on occasion”, which limits the number of cycles at this higher stress level, you will still generally be able to keep the engine together.
• But, if you were to run a typical maximum rpm rod bolt MOS under 100%, your rod bolts will be expected to fail prematurely.
As mentioned above in the definition of Yield Strength, we CANNOT stretch our rod bolts beyond the yield point. Because once the yield point is passed, it is considered a “failed” condition for a bolt, and the bolt must be discarded. So, a typical conservative Engineering approach in most general applications is to use a preload clamp-up of about 75% of yield. That way you have a good range between the installed preload and the yield point, in case the bolts get stressed even more during operational use. However, typical engine connecting rod bolt preload clamp-up in most reliable engines, can vary from a low of about 60% of yield to a high of about 90% of yield, with 75% of yield, the sweet spot you might say, right in the middle.
Since rod bolt stretch specs have generally become the standard in High Performance engine builds, the stretch called for is more often around 90% of the yield point. Stretching to this higher percentage of yield, is used to maximize preload clamp-up, in an effort to try and help minimize rod big end distortion at high rpm, which can cause additional undesirable rod bolt bending that would add to the bolt stress.
So, this high level of stretch is a good idea from that standpoint, but at the same time, you are left with a smaller range between the installed preload clamp-up and the yield point. But, this common 90% of yield has worked out quite well in the real world for Hotrods, Street/Strip cars, and Sportsman Drag cars. Even though there is less range between the installed preload clamp-up and the yield point, the yield point in properly selected rod bolts is not typically reached in actual operation, so all is good.
You may also have noticed that through all this discussion of rod bolt strength, there has been no mention at all of rod bolt tensile strength. That’s because we CANNOT go beyond the yield strength which is reached well “BELOW” the tensile strength. So, what good is tensile strength then? For a large number of steels, there is a direct correlation between tensile strength and fatigue life. Normally, as tensile strength increases, the fatigue life increases. So, while tensile strength does not come into play during rod bolt "Strength Analysis", it is a factor in rod bolt fatigue life.
Rod bolt fatigue life is important to Road Racers because of the number of cycles they see. And rod bolt fatigue life is absolutely critical for Endurance Racers like NASCAR. And NASCAR teams do an incredible job managing the fatigue life of their rod bolts. But, for our Hotrods, Street/Strip cars and Sportsman Drag cars, rod bolt fatigue life isn’t typically a big concern, if the motors are built with the correct rod bolts in the first place. That is because these bolts won’t typically see enough cycles in their lifetime to cause a failure due to fatigue. But, with that said, it is still a good idea to keep fatigue life in the back of your mind, when it comes to choosing your rod bolts. It can be a tie breaker, in the event that multiple rod bolts are being considered for a certain build. More on that below.
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Even though there are various companies that offer rod bolts, below we will compare 5 different rod bolts offered by Industry leader ARP.
So, let’s take a look at a typical 540ci BBC motor, running steel rods with 7/16 “cap screw” rod bolts, and uses 7,500 rpm as its typical maximum, which results in a cyclic tension load on each rod bolt that = 7,280 lbs or about 3.6 tons:
• For general reference, let’s first take a look at rods installed the old school traditional way, here using ARP 2000 rod bolts that are torqued to about 75 ft lbs with original ARP moly lube.
Bolt stretch is about .005”, which = 76% of yield strength
Clamp-up preload on each rod bolt = 16,531 lbs or about 8.3 tons
Margin of Safety (MOS) for this setup = 127%, which meets our MOS design target for being safe, reliable and essentially bullet proof.
Now, for the rest of the rod bolts we’ll be looking at, we’ll preload them to the more common higher percentage of yield strength, which is typical of the stretch called for these days.
• Using ARP 8740 chrome moly rod bolts (this has the same yield strength as ARP 2000)
Bolt stretch = .006” which = 90% of yield strength
Clamp-up preload on each rod bolt = 19,686 lbs or about 9.8 tons
Margin of Safety (MOS) = 170%
• Using ARP 2000 rod bolts (this has the same yield strength as 8740 chrome moly)
Bolt stretch = .006” which = 90% of yield strength
Clamp-up preload on each rod bolt = 19,686 lbs or about 9.8 tons
Margin of Safety (MOS) = 170%
• Using ARP L19 rod bolts
Bolt stretch = .0066” which = 90% of yield strength
Clamp-up preload on each rod bolt = 21,655 lbs or about 10.8 tons
 Margin of Safety (MOS) = 197%
• Using ARP Custom Age 625+ rod bolts
Bolt stretch = .0078” which = 90% of yield strength
Clamp-up preload on each rod bolt = 25,445 lbs or about 12.7 tons
Margin of Safety (MOS) = 250%
• Using ARP 3.5 rod bolts
Bolt stretch = .0073” which = 90% of yield strength
Clamp-up preload on each rod bolt = 23,821 lbs or about 11.9 tons
Margin of Safety (MOS) = 227%
As you can see above in all 6 examples, whether torqued the traditional way to a lower stretch value, or stretched to the more recently called for higher percentage of yield value, all these rod bolts are above the minimum 125% MOS target for safety and reliability. Therefore, all these configurations would operate without issue, just like a stock grocery getter. So, if a builder chooses any of these bolts or stretch values between the 127% and the 250% "Margins of Safety" above, he could NOT go wrong, no matter how much HP the motor makes. Remember that HP has NOTHING to do with the max tension loads on rod bolts.
Since most Hotrods, Street/Strip cars, and Sportsman Drag cars, with their lower number of cycles, can live almost indefinitely with some of the more affordable mainstream rod bolts above, it’s rather hard to make a case for using the much more expensive and higher strength 625+ or 3.5 bolts, even if they do have higher fatigue life values.
BOTTOM LINE
So then, all we REALLY NEED, from a conservative Engineering standpoint, is to at least reach the 125% MOS target for safety and reliability, no matter how much HP is being made. And anything above that 125% is fine, but not necessary.
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But, things aren’t always wine and roses, because some engines will NOT stay together and live like the well built configurations above. I've done "failed" rod bolt "Strength Analysis" on two smaller very high revving engines, after the fact, to take a look at why they failed. One blew-up catastrophically when a rod bolt broke, costing its owner 20 grand. And the other engine was found to have rod bolts stretched beyond the yield point, during a teardown for other reasons. So, its fuse had been lit, but fortunately it was caught just in the nick of time before they let go, saving its owner a ton of money and agony.
In both cases, the rod bolt "Strength Analysis" revealed that they had been built wrong, and that they were well BELOW 100% MOS, which predicts premature rod bolt failure. One had only a 67% MOS and the other had only an 86% MOS. If rod bolt "Strength Analysis" had been performed before these engines were built, during the planning stages, then all that grief and cost could have been avoided. They have since been rebuilt much stronger, with MOS values now well ABOVE that 125% safe target. And they have now been raced for some time without issue.
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SUMMARY
• ARP 8740 chrome moly rod bolt - a strong affordable rod bolt, but it has only a moderate fatigue life, which makes the ARP 2000 rod bolt which is in the same general price range, a much better choice since it has twice the fatigue life.
• ARP 2000 rod bolt - considering how good its strength and fatigue life are, this rod bolt is an excellent choice for most Hotrods, Street/Strip cars, and Sportsman Drag cars.
• ARP L19 rod bolt - the strength and fatigue life increases this bolt provides over the ARP 2000 are not significant enough to overcome the concerns the L19 has with hydrogen embrittlement, stress corrosion, and the fact that it CANNOT be exposed to any moisture, including sweat and/or condensation. Don’t forget that every engine forms condensation inside, at every cold start-up. Plus, oil rises to the top of, and floats on water because of density differences, which can leave portions of the rod bolts exposed to water even after the engine is built. Therefore, it is best to avoid the L19 rod bolt altogether, especially since the ARP 2000 rod bolt already provides way more than enough strength and fatigue life than is typically required by most Hotrods, Street/Strip cars, and Sportsman Drag cars. So, there simply is no good reason to select the ARP L19 rod bolt. If you are currently running L19 bolts, I’d suggest you consider replacing them with different bolts the next time you have the motor apart.
• ARP Custom Age 625+ rod bolt - a very pricey bolt, but with its excellent strength and its impressive fatigue life, this bolt is one of the very best rod bolts on the market.
• ARP 3.5 rod bolt - this bolt has excellent strength, but its staggering cost is 43% HIGHER than the 625+ bolt, yet the 625+ bolt is superior to the 3.5 bolt in virtually every way. So, there is no good reason to select the 3.5 bolt either.
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CONCLUSION and RECOMMENDATION
Of the 5 rod bolts above:
• The ARP 2000 rod bolt is an excellent value, considering how good its strength and fatigue life are. And it should be considered the rod bolt of choice for most Hotrods, Street/Strip cars, and Sportsman Drag cars, no matter how much HP they make. And this is why you most often see quality aftermarket rods come with these bolts.
• ARP Custom Age 625+ rod bolt has a price that is not for the faint of wallet, but it should be considered the rod bolt of choice for very high revving engines, road race engines, and endurance engines, which require the utmost in rod bolt strength and/or fatigue life.

Spark Timing

 

Spark Timing Myths Debunked


A widely-held myth is that maximum advance always means maximum power. Here’s what’s wrong with this thinking:


The spark plug ignites the mixture and the fire starts burning. The speed of this flame front depends on the mixture, this means how many air and fuel molecules are packed together in the combustion chamber. The closer they are packed together in the same volume, the easier it is for the fire to jump from one set of molecules to the other. The burning speed is also dependent on the air-fuel-ratio. At about 12.5 to 13 air-fuel-ratio the mixture burns fastest. A leaner mixture than that burns slower. A richer mixture also burns slower. That's why the maximum power mixture is at the fastest burn speed. It takes some time for this flame front to consume all the fuel in the combustion chamber. As it burns, the pressure and temperature in the cylinder increases. This pressure peaks at some point after TDC. Many experiments have shown that the optimum position for this pressure peak is about 15 to 20 degrees after TDC. The exact location of the optimum pressure peak is actually independent of engine load or RPM, but dependent on engine geometry.


Typically all the mixture is burned before about 70 deg ATDC. But because the mixture density and AFR in the engine change all the time, the fire has to be ignited just at the right time to get the peak pressure at the optimal point. As the engine speed increases, you need to ignite the mixture in the combustion chamber earlier because there is less time between spark and optimum peak pressure angle. If the mixture density is changed due to for example boost or higher compression ratio, the spark has to be ignited later to hit the same optimal point.


If the mixture is ignited to early, the piston is still moving up towards TDC as the pressure from the burning mixture builds. This has several effects:
The pressure buildup before TDC tries to turn the engine backward, costing power.
The point where the pressure in the cylinder peaks is much closer to TDC, with the result of less mechanical leverage on the crankshaft (less power) and also causes MUCH higher pressure peaks and temperatures, leading to knock.


Many people with aftermarket turbos don't change the spark advance very much, believing that earlier spark creates more power. To combat knock they make the mixture richer. All that happens really then is that the mixture burns slower and therefore hits the peak pressure closer to the right point. This of course reaffirms the belief that the richer mixture creates more power. In reality the flame front speed was adjusted to get the right peak pressure point. The same result (with more power, less emissions and less fuel consumption) could be achieved by leaving the mixture at the leaner optimum and retarding the ignition more instead.


Turbo charging or increasing the compression ratio changes the mixture density (more air and fuel molecules are packed together). This increases the peak pressure and temperature. The pressure and temperature can get so high that the remaining unburned mixture ignites by itself at the hottest part in the combustion chamber. This self-ignition happens explosively and is called 'knock'. All engines knock somewhat. If there is very little unburned mixture remaining when it self-ignites, the explosion of that small amount does not cause any problems because it can't create a large, sharp pressure peak. Igniting the mixture later (retarding) causes the peak pressure to be much lower and cures the knock.


The advances in power of modern engines, despite the lower quality of gasoline today, comes partially from improvements in combustion chamber and spark plug location. Modern engines are optimized so that the flame front has the least distance to travel and consumes the mixture as fast as possible. An already burned mixture can no longer explode and therefore higher compression ratios are possible with lower octane fuel. Some race or high performance engines actually have 2 or three spark plugs to ignite the mixture from multiple points. This is done so that the actual burn time is faster with multiple flame fronts. Again, this is to consume the mixture faster without giving it a chance to self-ignite.


Higher octane fuel is more resistant to self-ignition. It takes a higher temperature and pressure to cause it to burn by itself. That's why race fuels are used for engines with high compression or boost. Lead additives have been used, and are still used to raise the self-ignition threshhold of gasoline, but lead is toxic and therefore no longer used for pump-gas. Of course a blown engine is toxic to your wallet.