Flywheel & Pressure plate - Effects of lightening the assembly.

22 November 2000

How does this affect performance? Not as many folk believe, that’s for sure. For a start, lightweight flywheel/clutch assemblies don’t necessarily give rough running at idle, particularly where a performance cam is used. Nor do they make the engine produce MORE power. BUT they do make a difference to the ACCELERATIVE performance of the car - and that is what we’re most interested in most of the time! Basically, the engine sees the car as a weight to move, via the gearbox.

The combustion pressures created by your common or garden suck-push-bang-blow engine have to accelerate not only the mass of the car as a whole, but the mass of the engine internals too. However, the engine can only accelerate the car at a certain rate with what power is left over after the engine internals have consumed their share. The lighter the rotating and reciprocating parts are made the less power is consumed by them, leaving more to actually accelerate the car. I’m going to ignore the reciprocating stuff here (rods, pistons, etc.), as it is more complicated to determine their effects.

To over simplify things, the gearbox is no more than a complex lever. Taking off in first gear, the engine doesn’t see the total mass of the car. It really sees the total mass divided by the total gear ratio. To illustrate, a Mini is roughly 1400lb without driver (we’ll do this by remote control - I don’t want any hate mail by different ‘weight’ factions). Take a first gear ratio from a standard gearbox of 3.33, and multiply by final drive of said gearbox of 3.44. The over all ratio is 11.45. Now, the engine actually sees the weight of the car (1400lb) divided by the overall ratio in first gear (11.45) - so that’s only 122-odd pounds. BUT it also sees the extra weight of the flywheel/clutch assembly and a few other drive train components - typically around 40lb. It now sees a total of around 162lb. Lightening the flywheel/clutch assembly by 10lb means the engine sees 6% less mass to accelerate. A considerable difference compared to the overall mass!! Of course this will deteriorate as you go up through the gears. This neatly illustrates the fact that no amount of lightening will make an iota of difference to the top speed. It can also be seen that where more modified, higher-revving engines are used with lower ratios to achieve maximum performance, the greater effect the lightening has. Thus the faster the car will accelerate.

The formula to assess the effect of lightening the flywheel per pound in conjunction with various gear ratios, final drive ratios, and wheel/tyre sizes is:

0.5 x n2 x r2 + R2

R2

n = Total gear ratio (gear ratio x final drive ratio)

r = Radius of gyration (approx. 3.75” on a Mini)

R = Radius of wheel/tyre used

Calculating the radius of gyration of other components, such as the pressure plate, will allow further ‘button punching’ for further assessment.

Incidentally, balancing the crank and flywheel assembly will not improve the rpm capability of the engine at all. Not unless you have some particularly bothersome vibration. Neither will it give any power increases. ALL it will do in reality is give a smoother running engine. That’s it. So if the budget is tight for a road engine rebuild, your money may well be better spent elsewhere, particularly where the components used are as they were fitted at the factory from the same engine.

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