Buick 215 Heavy Flywheel - note the inertia ring! - 32 pounds total

Introduction to Flywheels

This article appeared in The British V8 Newsletter - Volume X, Issue 1 - January 2002

by: Dan Masters

Sure, we all know what a flywheel is - it's that heavy round thing with teeth on it that fits on the end of the crankshaft, engages the starter, and holds the clutch. Yeah, but what is a flywheel?

Basically, a flywheel is an energy storage device; you have to put energy into it to make it spin, and you have to take energy out of it to make it stop. In a vacuum (no wind resistance) and with perfect bearings (no friction), a flywheel would continue to spin forever, once put into motion, as there is nothing there to take energy out of it.

Try this: Take your kids to the park, and put them on the merry-go-round. Gather up all the kids you can find, and their parents, and really load it up. Now, try to make it go around. Hard, isn't it? You may have to get help from your friends to get it really spinning. Once you get it started, try to stop it. Very difficult. When you do get it stopped, have everyone get off, and try again to get it started spinning. Without the extra weight (mass), you'll find that it's much easier to spin. Much easier to stop too.

Ok, so what does all this have to do with the flywheel in your car? Consider the following two scenarios:

A) You're on your way home from the local home improvement store in your pick-up truck, loaded down with lumber, cinder blocks, mortar, etc. - more than you really should be carrying in that truck. You're sitting at a stoplight, and when the light turns green, you try to take off. With all that load in the truck, your poor little wheezer engine just doesn't have the necessary oomph (torque) to pull you off the line. In order to get moving, you have to really rev the engine up to get it into its maximum torque rpm range, and slip the clutch until you're finally moving. In this situation, you could really use all the help you can get in the form of stored energy from your flywheel.

Here, you'd like to have the heaviest flywheel you can get - the heavier the flywheel, the more stored energy.

B) You're running an F1 car, under the yellow, in the final laps of the race, following the pace car at what seems to be an excruciatingly slow speed. When the flag goes to green, you want to get off the line as fast as you can, out accelerating your competition. You have your car in the proper gear to put the engine in the rpm range for maximum torque, you have plenty of torque, and your car weighs next to nothing. In this case, you do not want to spend any of your engine's precious energy spinning up a heavy flywheel. You want all of that energy directed to the rear wheels. A heavy flywheel here would be an unnecessary burden. In fact, in this case, no flywheel at all would be the preferred condition.

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This then defines the prime criteria for flywheel weight considerations:

The LIGHTER the car, and/or the MORE TORQUE the engine produces, the LIGHTER the flywheel needs to be.

The HEAVIER the car, and/or the LESS TORQUE the engine produces, the HEAVIER the flywheel needs to be.

The flywheel also performs another function as well, one of smoothing the engine's power pulses. As the engine turns, each cylinder goes through two distinct phases - the compression stroke and the power stroke. On the power stroke, the piston is driving the crankshaft. On the compression stroke, the crankshaft is driving the piston. Thus, for every other revolution, the crank alternates between "being twisted" and "twisting." The flywheel absorbs energy on the "being twisted" phase, and then returns the energy on the "twisting" phase, helping to smooth the engine pulsations. On a single cylinder engine, this pulse damping is of significance. On a one hundred cylinder engine, the pulses would be distributed so evenly that the flywheel damping would not be needed. For this reason, a heavy flywheel would be of less benefit to a V8 than it would be to a four cylinder engine.

The damping effect of the flywheel is also assisted by the damping action of the harmonic balancer. In addition to the "flywheel" effect of the relatively heavy damper, the elastic material between the inner and outer portion of the harmonic balancer adds to the smoothing effect. This elastic material absorbs some of the "being twisted" forces, and gives back during the "twisting" phase.

So, what is a good weight for a flywheel in a British V8 conversion? Well, most of the British cars we're concerned with here weigh around 2500 pounds or less. That's relatively light weight, as cars go. If the V8 engine we're installing doesn't have pretty good torque, we wouldn't be putting in the car in the first place, so we can count on good torque values. Looking through the various flywheel vender catalogs, we find flywheel weights ranging from 15 to 50 pounds.

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Ford Motor Company uses a 40 pound flywheel in their stock cars equipped with a 302ci V8. For their high performance crate engine, with torque values around 350lbft, they supply 27 pound flywheels. Generally, they expect these engines to be put into cars weighing over 3000 pounds, so the 27 pound figure would seem to be the upper limit for our applications, assuming a fairly low torque motor.

For a really hot engine, with stump pulling torque, a 15 pound flywheel would be about right. Ted Lathrop is running a 15 pound flywheel in his 350 Chevy powered TR6, and it feels to me to be just about right. His car weighs less than 2500 pounds, and that engine should produce in excess of 350lbft of torque. I have driven his car, and it is just about perfect. Believe me, getting off the line is NO PROBLEM! Nor is there any significant vibration. For lightly-modified BOP/Rover 215ci engines, which are quite often used in MGB-V8 swaps, perhaps a 22 pounder would be more appropriate.

That heavy round thing with teeth on it isn't the only flywheel you have on your car - you have four more, one on each corner of your car! Your tires and wheels are also flywheels. Just like all flywheels, the heavier the tire/wheel combo, the harder it is to get it spinning, and the harder it is to get it to stop. Unlike the engine flywheel, though, there is NO purpose to having extra weight on the tires and wheels. The ideal weight here is ZERO!

Besides the flywheel effect, extra weight here is just that much more mass to get moving off the line, and that much more mass to stop. Not only does the weight effect acceleration and braking, wheel and tire weight has a tremendous impact on handling as well. The heavier the tire/wheel combo, the harder it is for the suspension system to control the tire/wheel movement, and the harder it is to keep the tire in correct contact with the road. There is, then, a triple whammy to pay for extra weight on your tires and wheels. Ample justification, I think, to spring the big bucks on a set of super lightweight aluminum or magnesium wheels!

Additional Note: (added with internet publication in March 2007.) Early in this article Dan suggested an experiment where you spin neighborhood kids on a merry-go-round. While you're playing with the kids, why not take the opportunity to make an additional observation and also teach the kids a fun, interesting and relevant scientific principal? Have the kids sit near the center of the merry-go-round, and you'll note that it's relatively easy to spin. Have them move to the outer edge, and it gets harder to spin. Amazingly, if the kids move outward on the merry-go-round as it spins the merry-go-round will actually slow down, and if they move inward it will actually speed up (in terms of RPM). The part of this experiment that's relevant to flywheels is that a spinning merry-go-round with kids concentrated around its outer edges actually has significantly more rotational inertia than one with kids evenly distributed over its surface.

Cutting to the chase, WEIGHT ISN'T THE ONLY CONSIDERATION WHEN COMPARING TWO FLYWHEELS. Diameter and distribution of mass relative to the axis are important too. A flywheel of relatively constant thickness from center to edge is far less effective at storing energy than a flywheel of equal weight that has a significant portion of its weight in a ring, outboard of the pressure plate, such as the "heavy" version of a Buick 215 flywheel. Another way to look at this is that cutting a pound of iron off the outer ring of a "heavy" Buick 215 flywheel will have more effect on performance than cutting two pounds off its face.

In addition to weight, anyone who makes or sells aftermarket performance flywheels should also be able to tell you their respective "moments of inertia". You should ask for this information... even though you're unlikely to get any response but a blank stare, because we deserve to be given enough information by parts vendors to make informed decisions. Weight alone is not enough information to compare the performance of two flywheels if they have different cross-sections.

A fuller explanation of moment of inertia and related engineering principals that apply to sports car design and performance is planned for a future issue of the newsletter.

For more information on flywheels, please also see:
Buick and Oldsmobile 215 Flywheels, by Kurt Schley (Vol 13, Iss 3)
Buick/Olds 215 Flywheels, by Kurt Schley (Vol 12, Iss 1)

Disclaimer: This page was researched and written by Dan Masters. The "additional note" was written by Curtis Jacobson. Views expressed are those of the authors, and are provided without warrantee or guarantee. Apply at your own risk.

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