Originally Posted by Loki (Post 4780092)

There are 2 counterweights: one at the front of the engine, one at the back. In stock form, the flywheel is the counterweight ( you will notice it's not symmetric. When installing an aftermarket flywheel, which are symmetric, you need to add thr counterweight. What they actually do I'm not exactly sure, but suspect they cut down on harmonic vibrations in the eccentric shaft. Regardless, you need the counterweight to keep everything balanced. Note that while the stock flywheel has the counterweight mass along the circumference, the separate counterweight brings that mass toward the center.

Think again about machining the flywheel, because it will be going 9,000 rpm about 8 inches from your right ankle ;)

There are lighter options than 11lbs. An aluminium flywheel from, I think, Fidanza, is 8.5lbs + counterweight.

Originally Posted by Mr X (Post 4780084)

I have always been a big fan of lightweight after-market flywheels. I drive aggressively and race, so I don't mind a few more revs to get it going in trade for the quicker response.

So, when I got the RX8, one of the first things I did was consider an after-market flywheel.

But after market flywheels for RX8's are different than many other cars. They all seem to need an expensive counterbalance.

The first thing that confuses me is the shape of the counterbalances. They are all half circle shaped. This seems like a tremendous way to throw everything off-balance. What are they counterbalancing?

Then comes the weight. A decent chromoly flywheel is about 11 lbs, compared to the stock flywheel which is about 16.8 lbs. 5.8 lbs is a generous savings. But that excludes the counterweight. The counterweight is 4 lbs, so the actual savings is only 1.8 lbs. This seems like a minor savings to adopt an oddball counterweight arrangement.

I could probably have 1-2 lbs shaved off the stock flywheel and end up with no net weight savings.

I realize the weight moves from the outer edge to much closer to the center (which is certainly worth something), but I am still trying to process this.

Can someone give me a mini lesson in the pros and cons of a lightweight flywheel in an RX8, and explain these crazy counterweights?

Thanks!