Rear Wheel Power Increases - The Spin Zone

Have you ever been to a dirt track and watched a World of Outlaws winged sprinter race? No? Then that is most certainly your loss. What you are missing is truly spectacular racing and the chance to see a vehicle with a very high degree of optimization of moment of inertia. Listen to one of these 400-inch, 800hp injected alcohol monsters rev up, and the greased-lightening throttle response leaves the impression that it defies the laws of physics. It sounds as though it's digital (a step function) rather than analog (a progressively increasing function). In other words, it sounds as if it goes instantly from 1,200 to 7,500 rpm. This eardrum-fraying whiplash lets you know right away this is not an engine for the faint-hearted.

Sprinter-like throttle response is a target I strive for (to a varying degree) with every engine I build. Although having an effective engine spec in terms of induction, compression, cam, and ignition is half the battle, it's important to realize that it really is just that--half the battle. The very overlooked other half is moment of inertia.

Let's cut any pretense at suspense; exactly what is moment of inertia? As per Sir Isaac Newton's Third Law of Motion, every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. The moment of inertia of an object is, in the context we are dealing with here, its resistance to rotational acceleration. For example, a big, heavy flywheel will take more engine torque to accelerate it up to speed than a smaller, lighter one. But there's more to it than just the amount of mass to be accelerated. The radius at which the mass resides also has a major influence on how rapidly an object can be rotationally accelerated. The basics (and fortunately, that is all you need to know) of what we're leading up to here are not at all difficult to understand. Fig. 1 should put you clearly in the picture. What we see here are two flywheels of the same weight. The significant difference is that one has the weight concentrated toward the outside of the flywheel, the other toward the center. The one with the mass in the center will accelerate far quicker for a given applied torque than the one with the mass at the outside.

The fact is, both mass and the radius at which it operates are our enemy. Understanding the concept and consequences of inertia reduction is the key to unlocking the existing power in your engine. What you probably don't appreciate is the truly staggering effect the unwanted mass, acting at an unnecessarily large radius, has on the rear-wheel horsepower of a car. This is what we intend to set straight, by covering eight areas successful race-car builders optimize in the transmission of power between the engine and the drive wheels. To do this, let's start at the engine and work our way back through the drivetrain to the driving wheels.

FIG. 1 Although both of these flywheels weigh the same, the lower one will spin up easier because the mass is located nearer the center, giving it a lower moment of inertia.

Three Key Items: Crank, Rods, and Pistons
When I was a kid, I often wondered where the energy of the piston and connecting rod's mid-stroke speed went at each end of the stroke. Although not intuitive, the answer is that in slowing the piston and rod assembly down, the energy is absorbed into the crank (which tends to increase crank speed). When speeding up, the piston and rod assembly absorbs energy from the crank (which slows crank speed). In other words, this assembly acts as if it is a small flywheel with fluctuating mass. This means not only do we need to reduce piston and rod weight to relieve the crank of unnecessary reciprocating loads, but also to reduce its flywheel effect.

Cutting piston and rod weight has a positive knock-on effect on the crank design. When the piston and rod assembly becomes lighter, the amount of counterbalance mass needed on the crank becomes less and usually by a bigger amount. Not only does this reduce flywheel effect, but it also cuts windage. As to how much crank windage reduction is worth, it's not something for which I have a simple answer because I have not really built a 9,500-rpm engine with an ugly, high-windage crank to find out what a low-windage one will do in its place. What I can tell you is that the result of my flat-versus-aero counterweight crank test (a Scat crank in an engine operating in the 7,200-rpm range) revealed almost a 10hp increase. Cutting piston mass so that a lighter and more aerodynamic crank can be used means an increase in static horsepower and a decrease in power absorbed by having to accelerate that mass. We have a good idea of what can be had by improved windage, so now is the time to look at the proportions of what may be gained by reducing the flywheel effect of the rotating assembly.

This detailed $220 Scat aero crank has over .25 inch more stroke, but weighs 1.5 pounds less than a stock cast crank, partly because of the hollow big-end journals. Even with the longer stroke, moment of inertia remains about stock.

Item Number 4: Measuring the "Flywheel Effect"
When a large dyno manufacturer got into the business of dynamometers back in the early '80s, I was already a dyno-testing veteran of some 20 years. At this point, I already had experience with all the major brands of dynamometers. But this company did something with its dynos none of those others did (at least not at a price that anyone outside of GM or the like could afford), namely, to allow the engine to be tested at various user-defined rpm acceleration rates. This allowed the simulation of engine operation in any gear from First to High. I was so enamored with the capabilities of the then-new dyno that I bought one of the first ones. What this dyno allowed me to do for the first time was to test the differences produced by rotational assemblies of high and low moments of inertia. I already knew that "low" was better by a bunch, but up until this stage could not put any figures on it. Now, using a relatively mundane 350 small-block Chevy, things were about to change.

A call to Red Roberts at McCloud Industries got the ball rolling. Red sent over a couple of flywheels, one about 30 pounds, and the other about 10 pounds. The effects that these flywheels would have on output were tested on a nominally 375-horse mule motor. Tests were done in three modes: steady state, plus acceleration rates of 600 rpm-per-second and 300 rpm-per-second. These acceleration rates approximated engine acceleration in Second and Fourth gear of a typical five-speed gearbox.

This forged, internally balanced, Lunati 4-inch stroke small-block Chevy crank with its hollow big-end journals is affordable for the serious engine builder and weighs just less than a stock 350 crank.

Because the flywheels were of a different design, the moments of inertia did not directly transpose at the same ratio as the weights did. All I knew for sure here was that the 10-pound flywheel had a considerably lower moment of inertia than the 30-pound one. Fig. 2 shows the test results.

From the curves in Fig. 2, it is easy to see that cutting the flywheel effect allows much more of the available steady-state horsepower to be accessed during acceleration. The chart in Fig. 3 shows the gains at 300 rpm/sec (simulating Fourth gear) and 600 rpm/sec (simulating Second gear) when the 30-pound flywheel was replaced with a 10-pound item.

Don't put any more rod into your engine than necessary. These detailed budget lightweight 4340 rods from Scat serve well in engines up to about 525 hp.

These tests definitely point toward a lighter moment of inertia being better for performance, but are there downsides? The principle reason the factory puts heavier flywheels on is for idle quality. The flywheel's mass smoothes out the engine's compression and firing impulses. Here, the crank slowing caused by the compression stroke is a significant deal. Fortunately, when we install a longer-period cam, the amount of flywheel effect required to smooth out the compression stroke is reduced. An example here gives an idea of the significance of the required flywheel/clutch rotating weight in relation to the cam used. The engine in question is a 13:1 compression (it runs 100-octane fuel) F.A.S.T.-injected, street-driven 440-inch small-block Chevy. It pumps out right around 700 hp and 600 lb-ft. This prodigious output is transmitted to the rearend via a super-light Fidanza flywheel and a small-diameter twin-plate clutch (good for about 900 hp) from Clutch Masters. Because of the 297/304 degrees of off-the-seat duration, the engine idles at a steady 700 rpm with a clutch/flywheel inertia about 75 percent less than stock. Remember that Sprinter throttle response mentioned earlier? This engine, due to the small-diameter clutch and the lightweight flywheel, has got it by the boatload.

So far, the situation looks good for a low-inertia clutch/flywheel assembly, but the dyno tests only point us in a direction toward more driving wheel output. What they don't take into account is the fact that these flywheel tests were run only with the inertia of the engine's rotating mass and the dyno's absorber mass. In the real world, cars have clutch, transmission, and wheel inertia to deal with, as well as launches from the start line, which can rely greatly on stored kinetic energy. It is possible for this--especially for the drag racer--to skew the picture somewhat. The question is, which way is the skew?

The CP Cup Car piston (right) is a complex high-tech forging with a lot of internal machining. It's light, and you will pay an appropriate price. Not quite as light, but a lot less money, is the new KB forged piston. It's worth checking out if you are on a tight budget.

The Low/High Moment of Inertia Flywheel Controversy
A lot of successful "old-school" racers claim best e.t.'s are made by harnessing the considerable kinetic energy potential of a heavy clutch and flywheel assembly for a harder launch. The theory is simple: First, pump horsepower into the flywheel prior to leaving the line by revving 500 to 700 rpm past peak power. At this point, pull rpm back by clutch control to about peak torque as the car is launched then retrieves much of this stored energy. There is no doubt this technique delivers more "available power" for the most critical part of any fast quarter-mile pass, namely the start. On the downside, the flywheel will, during the time the rpm climbs back up to the shift point, absorb torque from the engine. As the shift is made, the rpm is once again pulled down and in the process energy in the clutch/flywheel assembly is transferred to the car. Up to this point, the theory sounds good, but to transmit the entire inertial energy content from the higher rpm to the lower rpm requires that neither the tires nor the clutch slip. If any slip occurs within the system, it means that some kinetic energy has been converted to heat rather than forward motion of the car.Proponents for light clutch/flywheel assemblies point to the fact that there is more available power when the engine has less rotating mass to accelerate. The lower the gears involved, the more rapidly the rotating parts are accelerating. This, as the graph in Fig. 4 shows, results in a greater amount of power being absorbed during acceleration. This means after the clutch is fully engaged, the clutch/flywheel absorbs the greatest amount of power (and a heavier one moreso) in First gear right when all the power possible is needed to launch the car. The real-world difference in output between First and Fourth gear for a drag-racing factory-stock 5.0 Mustang test car was a mind-boggling 140 lb-ft and 85 hp!Such a huge loss begs the question as to why this occurs. In the case of the test vehicle in question, we find that although the car itself may have only accelerated to about 35 mph in First gear, the outside diameter of the flywheel actually hit over 220 mph. On the road, its mass not only has to be accelerated forward, but also rotationally. Cutting such losses means, for road-race situations, there is potential for more rear-wheel power without actually having to modify the engine for more output, but for the drag racer, the situation is not quite so clear-cut.

Success on the strip requires a fast launch for low 60-foot times. For a road racer, the start (though hardly immaterial) is far less important, as any advantage a heavy clutch/flywheel has is cancelled out upon arrival at the first corner. From the exit of the first corner on, the lowest inertia (lightest rotating parts) is absolutely the way to go. But for drag racing, the case for lighter flywheels with a low moment of inertia is still a topic of some debate.

FIG. 2The green line is a steady-state test, and since the engine is not accelerating, the flywheel weight made no difference. This was not the case at the 600 rpm/sec acceleration rate (simulating Second gear) where the 10-pound flywheel made almost 22 hp more than the 30-pound one.

The Big Plan
Knowing my keen interest in inertia testing, Chris Jewell of Competition Clutch gamely volunteered to organize some track and dyno time. Accomplices here were Robin Lawrence with what is now his ex-12.5-second factory-stock 5.0, and Craig Baldwin with a Vortech-blown NMRA Real Street 10.5-second car.The plan was to test a stock-weight flywheel (23.75 pounds and a moment of inertia of 1.614 in-lb-sec2 against a lightweight Fidanza flywheel at 10.8 pounds and a moment of inertia of 0.771 in-lb-sec2. This represented a weight and moment of inertia reduction right around 55 percent. This was deemed a sufficient difference to easily show differences in our before and after tests.

Dyno Test and Results
The first move was to put Robin Lawrence's factory-stock 5.0 Mustang on a Dynojet chassis dyno. The test procedure involved rolling smoothly from low rpm to full throttle in First gear. At the shift-light prompt, Second gear was grabbed as per a live dragstrip run. At the 2-3 shift point, the test was terminated. Instead of rpm, the power figures were measured against speed and time. This would allow us to see both the "in-gear" power absorption of the accelerated rotating parts and whatever portion of returned stored energy that might appear as additional rear-wheel horsepower during the shift.

The only place to be sure our theory and dyno testing produce the expected results is to do real-world testing. With a pair of fast 5.0s, that's just what was done.

Before and after dyno test comparisons on both test vehicles (Fig. 5 and 6) clearly show that reducing the rotating mass helps horsepower during the "in-gear" accelerating phase. The dyno tests also show flywheel energy being reintroduced into the drivetrain/wheels during the shift (the area between the red and blue curves as indicated by the green arrows in Fig. 5 and 6). Here, the heavier flywheel produced higher figures during that transitional shift period. After the shift, when the clutch was fully engaged, the lighter rotating assemblies once again showed their superiority.

The tests with the Vortech-supercharged 5.0 showed similar trends to the factory-stock 5.0, but in different proportions. Check out the curves in Fig. 6. Because this Vortech-equipped engine was of far greater output (especially in terms of lb-ft), the test had to be done in Second and Third gears. Per the factory-stock 5.0, the lighter flywheel delivered more power.

FIG. 4Here is the output of a factory-stock 5.0 drag racer as measured in First and Fourth gears. The staggering 140 lb-ft and 85hp difference between the two is due to the power absorbed by the moment of inertia of the entire rotating mass from crank to wheels. Reducing the inertia can cut these losses considerably.

The shift light prompted the shift to be made, but because the lighter parts accelerated faster; an identical response time meant the rpm went a little higher between shifts. This makes the comparison during the shift phase (green arrow area) a little difficult to make. The time difference meant the shift with the lighter parts was some 150 rpm higher than with the heavy parts. Although this was not intentional, this test does start to show another factor in the equation, namely the effect increased rpm has on the energy available at the shift point. Check the shape of the curves and the distance apart in the region indicated by arrows in both Fig. 5 and 6. There is a much bigger area on the factory-stock 5.0 than on the Vortech-blown 5.0. This bigger area represents the difference in energy being paid back to the driving wheels during the shift. The amount of energy involved is proportional to the mass but also proportional to the square of the rpm, i.e., rpm X rpm. With the Vortech-blown car, the additional 150 rpm and the resulting 5 percent more energy before the shift with the lighter rotating mass added what appeared to be almost enough additional energy to compensate for the lower moment of inertia of the lighter flywheel involved.

We have established three facts from the dyno tests:

* With full clutch engagement, a low moment of inertia clutch/flywheel assembly produces more driving wheel horsepower.
* Assuming the same shift rpm during the shift phase, a high moment of inertia clutch/flywheel can return more stored energy than the low moment of inertia setup can.
* If more rpm is used with the low moment of inertia clutch/flywheel, the difference in stored energy can be totally compensated for.

At this point it is apparent that though some questions have been answered, we are still left with the most important one: Does the stored energy of the heavier setup, which is available for the launch and each subsequent upward shift, compensate for the reduced output during full clutch engagement periods? The most critical part of a fast quarter-mile time is the launch. Here, the extra energy of a heavy flywheel is available with no prior penalty. This makes the answer to "heavy or light" anything but obvious in the case of the lower-output factory-stock machine. The Vortech-blown 5.0 though is much nearer a clear-cut case of light is better. Why? Because we can see from the dyno tests that a small increase in launch rpm will cancel out any advantage the greater rotating mass may have had on the start line. At least in the case of the factory stock car, it now looks like time to go to the track and settle this debate.