Moment Of Inertia - Reduce MOI & Gain Horsepower

The green line represents... 

   

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The green line represents a step test that holds the engine at a steady speed to take a reading. Because there is no acceleration involved, the flywheel weight made no difference. This was not the case at a 600 rpm/sec acceleration rate in which the 10-pound flywheel made almost 22 hp more than the 30-pound one. (Fig. 2)

The next item in the power transmission train is the crankshaft. Its heavy weight works against it, but, in part, this is offset by the fact that the mass involved is at a much smaller radius than, say, with a flywheel. Nonetheless, cranks have a considerable MOI factor. This is why manufacturers making cranks for top teams go to great lengths to minimize the MOI. Dropping the Moment of Inertia of a crank by 15-20 percent does not seem like much, but, as we will see, it's worth a lot more than you might think.

Up until this point, we have looked at reductions in MOI that are significant not only because they reduce MOI, but also because they almost always reduce windage as well (because they are within the engine's crankcase). This is a bonus, but the effect that the mass of the flywheel and clutch has on MOI makes the engine internals a secondary factor by comparison. The reason for this, assuming an OE flywheel and clutch assembly is a starting point, is that we are not only dealing with a great deal of mass, but also the fact that this mass is operating at a large radius. Indeed, there is so much of both of these ingredients that the negative effects on output, as we shall now see, is nothing short of dramatic.

When Super Flow 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 Super Flow did something with its dynos that none of those others did (at least not at a price that anyone outside of GM or the likes could afford). Namely, a Super Flow dyno permitted 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.

Here, we see the results of... 

   

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Here, we see the results of an accelerated test from 60 to 80 mph at a rate about equal to what we would expect on a moderately banked 3/8-mile oval. Note that the lighter flywheel allowed the delivery of an extra 9-10 hp to the rear wheels.

I was so enamored with the capabilities of the then-new 801 Super Flow that I bit the bullet and bought one. What this dyno allowed me to do for the first time was test the differences produced by rotational assemblies of high and low MOI. There was some controversy as to the worth of efforts directed toward reduced MOI, so, to settle any arguments, I got right on with the job of putting some numbers into the deal.

A call to Red Roberts at McCloud Industries resulted in the delivery of two test flywheels. The heavier of the two weighed about 30 pounds and the second about 10 pounds. The effects these flywheels might have on output were tested on a relatively mundane small-block Chevy representative of many circle track budget classes. This dyno mule cranked out a nominal 375 horses. Tests were done in steady state, plus acceleration rates of 600 rpm per second and 300 rpm per second. These acceleration rates approximated engine acceleration for a Street or Super Street stocker from about 20 to 45 mph and 40 to 90 mph.

The test flywheels were of differing designs. Although the lighter flywheel had considerably less MOI, I never got around to establishing just how much less. But, as can be seen from Figure 1, the dyno results show a big difference in output under accelerating conditions. The faster the acceleration rate, the greater the advantage of the lower MOI flywheel. This means a short-track car makes greater use of reduced MOI compared to a superspeedway car.

Our data acquisition system... 

   

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Our data acquisition system is the basic Racepak. Seen here is the brain of the system.

From the curves in Figure 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.

Let's move on to Figure 3, which isolates the significant numbers-namely the power increases involved. The gains achieved by dropping the flywheel weight are, at the 600 rpm per second acceleration rate, probably a lot bigger than you might imagine. However, an acceleration rate of 600 rpm per second is not a realistic figure for anything but a high-dollar, Pro class circle track car. Those numbers are there to show just how big a difference there is during rapid acceleration. The 300-rpm figures, though, are significant to most of us. Although affected by the banking angle, these numbers are fairly representative of a track of a little over 3/8 mile.

Fig. 3