Moment Of Inertia - Reduce MOI & Gain Horsepower

Forty-eight years of racing, many in a professional capacity of some type, have taught me many things. An important point that comes to mind is that it's always easier to lose power than it is to gain it. The most power you can expect at the rear wheels of your race car-in a perfect racer's world-is whatever it makes in the cylinders. But we don't live in a perfect world, so losses from the engine to the rear wheels are to be expected. A great key to success is to minimize those losses. We are all aware that friction in the gearbox and final drive can sap power, but there is an even more insidious form of power loss. In engineering terms, it's called MOI.

Exactly what is MOI? Well lack of it is actually the main ingredient that gives an 800hp Sprint Car its whiplash throttle response. Of course, that does not exactly define MOI, but if you gave the answer as "Moment Of Inertia," you can score yourself 10 out of 10.

Now that we've given it a label, let's look into exactly what an object's Moment of Inertia is. In the context we are dealing with here, it is 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 that needs 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 that is all you need to know, fortunately, are not at all difficult to understand. Figure 1 should put you clearly in the picture. What we see 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. That's because its MOI is lower.

The key issue toward understanding both the concept and consequences of MOI reduction is that mass and the radius at which it operates are our enemies toward accelerating our cars faster.

The concept for this article began developing at the Performance Racing Institute trade show in Orlando last year. During a conversation with the guys at Quarter Master, they revealed that they had a new low-MOI multi-plate clutch and asked if we would like to try it in Nick Losito's Late Model stocker, which Nick races at Hickory (North Carolina) Motor Speedway. The car already had a lightweight clutch and flywheel assembly, but it was nowhere near as light as the new Quarter Master clutch and flywheel we were about to use as a replacement.

Since the Late Model regulations at Hickory call for a crate motor, engine output across the field was near flat, so any means of increasing rear wheel horsepower was welcome. From there, the conversation with the Quarter Master guys went on to a discussion of how little the benefits of reduced MOI had been quantified in tech features. Sure, most racers appreciate that lighter rotating parts can bring about better acceleration, but few appreciate the truly staggering effect that unwanted mass, acting at an unnecessarily large radius, has on the rear wheel horsepower of a car.

Since I had a lot of data from tests personally conducted on the subject, I decided I could at least make an effort toward setting the record straight.

To get you up to speed on this subject, we need to start by covering the ways and means that successful race car builders optimize the transmission of cylinder-developed power to driving-wheel power. To do this, we need to start at the engine and work our way back through the drivetrain to the driving wheels.

The first components in the chain of power transmission from the cylinders to the rear wheels are the pistons and rods. Although they reciprocate, the motion generates the same effect as flywheel mass. Here's how it works. Starting our analogy with the piston halfway up the bore, we find that as the piston and rod assembly slows down toward TDC, its kinetic energy is absorbed into the crank, increasing crank speed. When leaving TDC to go down the bore, the piston and rod assembly speeds up and absorbs energy from the crank, slowing crank speed. In other words, this assembly acts as if it is a small flywheel with fluctuating mass. Not only do we need to reduce piston and rod weight to relieve the crank of unnecessary reciprocating loads, but we also need to reduce its flywheel effect.