Racing Beat Aluminum Flywheel

The 12.5 lb. Aluminum Flywheel from Racing Beat comes with a counterweight and hardware. I purchased mine from MazdaTrix. The flywheel itself weighs just 8.2 lbs., and the total including the counterweight and hardware is 12.8 pounds. My stock flywheel weighs 20.2 lbs. after being resurfaced once or twice. The lightened flywheel is designed to minimize rotational inertia where the stock flywheel is designed to maximize it. Because of this, I would imagine that the lightened flywheel has less than half of the rotational inertia of the stock one, despite being more than half the weight.

Subjectively, the car is great to drive with the light flywheel. Acceleration in low gears seems much improved and it is easier to downshift because it does matter that much if you match the revs. I have not any problems stalling. I think I do rev it up a little higher to start from a standing stop, but the engine's new willingness to rev more than makes up for it and the driving experience in this situation is about the same as stock.

There was a lot of discussion on the RX-7 Mailing List about the performance benefits of a lightened flywheel. I decided to do some calculations to see what kind of difference it would make. This calculation is just an approximation, but I feel that it is a reasonable one. There are factors unaccounted for, but I believe I captured the major factors and that the smaller ones tend to cancel each other out. Here is what I did:

I used the CarTest vehicle performance simulation program to determine starting and ending RPM for each gear, and the time required to rev through that RPM range.
I calculated the torque required to accelerate each the stock and lightened flywheels through the RPM ranges and time constraints determined in the first step. I do not know the actual rotational inertias for the flywheels, but I just estimated them using discs of 14 and 25 pounds or something like that. I used Interactive Physics to simulate a constant torque on a rotating disc of various weights.
I used the torque values from step 2 to calculate how much HP is lost in each gear at 6500 RPM. I came up with about 50, 16, 5, and 2 (from memory) for first through fourth gears.

So, it does make a big difference in low gears and makes less of a difference in higher gears. 50 HP in first is a huge difference and you can definitely feel it while driving. 2 HP in fourth gear is nothing.

There is one performance drawback with the light flywheel -- it is not a great mod for drag racing. Revving up the engine before a drag race launch stores a bunch of energy, and the lighter flywheel does not store as much energy and must be revved up higher for a good launch. It also makes the car idle a little rougher, and should wear the clutch faster. None of these are significant to me, and I highly recommend a light flywheel to anyone but a die-hard drag racer.

Pictures

Here is the front (note the replaceable friction surface, available from Racing Beat for about $70)...

clutch side

...the back...
engine side

...with the counterweight (which I think is a Mazda auto trans part)...

with counterweight

...here is the whole kit, including flywheel, counterweight, and all the necessary hardware...

the whole kit