Lightened flywheel and driveshaft

[CobraStang]

OK, so a lightened flywheel costs 1 PIP, even if you go with the Dyno Plot option. I've been told that a chassis dyno will actually measure more HP when a lightened flywheel is installed. I've sent a couple emails to get some feedback from clutch manufacturers on this.

Since I would be taking a hit on the Dyno plot as well as the extra PIP, isn't that double damage? It's not like there's any sense in geting the dyno done, and then changing the flywheel...that's more work and expense, since the engine was already apart.

Also, once you're up one class, the driveshaft doesn't cost any PIP's. I expect that would also have an effect on the chassis dyno numbers.

Has anyone else seen increased dyno numbers because of lightweight components?

[Dave]

Russ, the reason we "double hit" you for a lightened flywheel or lightened crankshaft is that the increase in horsepower from these mods doesn't capture the whole picture. Sure you'll gain a bit of horsepower on the dyno from these mods, but it's minimal. The real gains come from the fact that when you reduce rotating mass the effective weight reduction increases as you go through the gears.

To illustrate, I'm going to be lazy and use James Mewett's '89 Mazda RX-7 Turbo II, which has an OE flywheel that weighs 22 lbs and for which there is available an aftermarket aluminum flywheel that weighs 12 lbs, because he's already done the math for me.

"Average" gear on track is 3rd, where the OE flywheel appears to the car to
weigh 109 lbs. The aluminum flywheel appears to weigh 59 lbs in 3rd. This
takes into account both rotational and translational inertia.

On the dyno in 4th gear, taking only rotational inertia into account, the OE
flywheel appears to weigh 45 lbs while the aluminum flywheel weighs 25 lbs.
Thus the dyno only sees a 20 lbs benefit when in reality you're actually achieving a 50 lbs benefit at the track in third gear (the gear you spend the most time in). Because of this fact, we've chosen to exclude lightening of rotating mass from the dyno plot process even though many competitors will dyno their cars with these mods in place (including myself). As you say, it's not practical to remove these before/after dyno testing.

There are also race driving benefits to a lightened flywheel outside of the dynos vision, namely being able to keep the revs up more between shifts. So at the end of the day I think 1 PIP is a fair value for a lightened flywheel even though a part of its benefit is captured by the dyno.

Cheers,
Dave