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
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.
NOTE; It is entirely feasible to use the pre-Verto flywheel and clutch assembly with the later pre-engaged (integral solenoid) type starter motor. The best way to accomplish satisfactory fitment is to fit the narrow ring gear of either the Verto flywheel (part no. PSF10003, 129 teeth). However, the standard pre-Verto ring gears (107 teeth) will also work OK with the pre-engaged starter, albeit somewhat noisily that will shorten ring gear and starter bendix life. Where the standard, wide ring gear (0.50-in wide) is fitted, a 0.125-in spacer MUST be fitted between the starter and the transfer gear case to prevent the starter bendix from being permanently engaged. Washers to the value of 0.125-in will be Ok for a short period, a proper, full spacer plate duplicating the starter mounting plate is necessary to ensure long life of the starter. Failure to do this will cause extensive damage to ring gear and starter at the least. The thin ring gear (0.345-in wide, part no.12G2613) such as used on the ultra light and steel lightweight flywheels is used, no spacers are required.