Opinions on lightweight flywheel for an RX7?

The clutch in my '93 FD is slipping on hard 1-2 shifts, looks like it's time to replace it.  (Only 61K on the car, so the previous owner must not have been very good with a manual transmission.)  It's a street car, so I'm planning on an Exedy stage 1 clutch.

The question is the flywheel.  The stock one is way too heavy (supposedly 19 pounds).  Racing Beat offers a 12 pound and an 8 pound, both of which require a 4 pound external counterweight that's built-in to the stock one.  They caution that the 8 pound might not be great for street use, but google search results suggest that many people think it's fine.  Anyone here tried it?

For reference, I have a 9.5 pound flywheel on my Miata and have never found it to be an issue at all.  That said, the FD is a heavier car, has less torque, less rotating mass in the motor, and wider gear spacing, so...

Usually clutches first start to slip when shifting into the 2 top gears. Perhaps something else is going on.

I've swapped in many lightweight rotary flywheels and everyone of them introduced a gearbox "moan". That might not be a concern, but you should be aware of it.

I have a SR Motorsports 9lb aluminum flywheel, in a bridge ported N/A RX-7.  (In full disclosure, I have also run it in street ported nonturbo T2 engine, half bridge nonturbo T2 engine, and street ported 6 port engine form, in that order, before my current full bridge old-school 4 port engine)

There are zero downsides except for cost.

Do it.  Do it.  Do it do it do it do it do it.

My next step is a flexplate and a 5.5" clutch setup to eliminate even more engine inertia.

It's not so much the weight that matters, it's how close to center it is. It would be possible to have two flywheels of the same mass behave very differently if one had much more of the weight at the outer edge and another with more weight towards the middle. How heavy it "feels" to the engine has more to do with moment of inertia than mass.

I have limited experience here, but a Renesis' flywheel weighs ~17 lbs and the rotors weigh ~0.4 less combined compared to a FD.  That combined with possible throttle body improvements gives it a nice revy feel to it compared to the S4 TII I had prior which had >11lbs more weight in the flywheel and rotors combined.  I'd be somewhat concerned that a 2lbs reduction in rotating mass might not be enough, although as RX8driver points out where the mass is located matters too.  I drove a Miata with a very lightweight flywheel on the street and didn't like it.  The fact that you already do might suggest you would be okay with a light one on the FD.

Your FD has less torque than your Miata?   :)

Knurled - what was the purpose of the n/a TII engine?

You should be okay with a mild clutch.  I think where these really get tricky is paired with a single plate pucked clutch that's grabby. My S13 had a Tilton 7.25" cerametallic twin with a really light low inertia flywheel (7 lbs, mass pretty central) and even that was street drivable because you can slip those discs a little.  

The clutch in my FD engages at the very top, it's been showing signs of being near the end of its life since I bought the car about 5K miles ago.  I haven't tried the tests in 5th gear because the car is geared so high that 5K RPM in 5th is like 120 mph. :)  When I first got it, a fast 1-2 shift would chirp the rear tires, now it just results in a bit of clutch slip.

Yeah, rotational moment of inertia is what matters.  I don't know how to compare that, because there's also a big component in the form of the engine's mass itself, and the two engines are so different.  There's no question that the stock flywheel has to go, the only question is 12 vs 8.

ChasH:  I'm not sure what a gearbox "moan" is?  Can you elaborate a bit?  Is it a constant noise?

Knurled:  Does that 9 pounds include the counterweight?

Snrub:  Off-boost at clutch-engagement kinds of speeds (say 1500 RPM), I'm pretty sure a rotary makes less torque than a BP.  At higher revs, well, my Miata isn't entirely stock... :)  (red is Miata, blue is FD)

Knurled. said:

I have a SR Motorsports 9lb aluminum flywheel, in a bridge ported N/A RX-7.  (In full disclosure, I have also run it in street ported nonturbo T2 engine, half bridge nonturbo T2 engine, and street ported 6 port engine form, in that order, before my current full bridge old-school 4 port engine)

 

There are zero downsides except for cost.

 

Do it.  Do it.  Do it do it do it do it do it.

 

My next step is a flexplate and a 5.5" clutch setup to eliminate even more engine inertia.

Yes! with a capital YAAAAAASSSSSSSSSSSSSSSSSSSSSSSSSSSSSS. what he said, however he is using the NA flywheel which is ~1" smaller diameter and is lightweight. Do not go with a lightened steel one just go straight for an aluminum one. the RB one is good but it has some weird spacer thing which I'm not fond of, the fidanza I had was of sub par quality so I returned it and now I have (I think) an ACT which works great.

the FC turbo/FD/Rx8 share the same flywheel just buy the specific rear counterweight which I think are the same from S5-S7 turbo and rx8 but the jury may still be out on that... but you can buy whatever flywheel is the cheaper as they are the same.

Every rotary wakes up with a light flywheel, as long as you don't go full race (5.5") it will be easy to drive on the street even with a stage 1 clutch its pretty tame.

and typically the weight is measured without counterweight.

codrus said:

ChasH:  I'm not sure what a gearbox "moan" is?  Can you elaborate a bit?  Is it a constant noise?

 

The noise is somewhat like a whine but lower in pitch. If your car makes more noise than normal you may not notice it. 

Snrub said:

Knurled - what was the purpose of the n/a TII engine?

Four port engines, especially Turbo II and 13B-RE, make a much better foundation for a N/A build.  The best would be FD center housing and 13B-RE end housings, but you can't find those engines hardly anywhere anymore.  (The 13B-RE is the engine that looks like 2/3rds of a 20B)

In reply to codrus :

There are no downsides just do it

Interesting dyno, thanks for sharing!  I hadn't considered that a not in boost 1.8L miata would make more *very* low end torque than a not in boost rotary.

Knurled. said:

Four port engines, especially Turbo II and 13B-RE, make a much better foundation for a N/A build.  The best would be FD center housing and 13B-RE end housings, but you can't find those engines hardly anywhere anymore.  (The 13B-RE is the engine that looks like 2/3rds of a 20B)

Based on what you're saying, presumably you mean that you used TII housings for their greater available room for porting with n/a rotors?

In reply to Snrub :

Opposite.  Not "more" so much as "better shaped".  You could make 6 port end housings work well if you spent a lot of hours on the milling machine and used a lot of epoxy/brazing rod to make them look more like 4 port units.  They have too much surface area and they close waaaaay too late.

My bridge port closes something like 15 degrees earlier than a 6 port's stock closing line and the power is still climbing long past the point where I feel comfortable revving it.  And it is based on '74-85 type geometry, to get the port runners as large as a Turbo II's stock runner, I'd have to port it out to air everywhere...

The thing that sucks about the turbo engines is the manifolds are garbage.  My favorite engine of all time was the Turbo II block with a 6-port's intake manifold on it.  Woke the engine right up compared to the turbo manifold.  More power down low, in the middle, and it gained another thousand RPM of powerband on top too.

Heh, so I went to the SR Motorsports web site to look at their 9.5lb aluminum flywheel (listed for an RX8, but from what you guys say it should work on my FD, right?) and they say:

" ***Our flywheels are not hard to drive like some of the lighter weight race only units.(The 8 pound racing flywheels that our competitors offer are too light for a Mazda RX-8 .Too hard to drive without slipping the clutch too much) "

The amount of clutch slippage required with a lightweight flywheel vs. a heavy one is "none at all".

Maybe if you have a horrible driving technique where you let out the clutch, and then accelerate.  Of course you will have a bad time with that.  If you meet the clutch with the accelerator, there is zero difference in how much you slip the clutch.  It is far easier to drive an aluminum flywheel than it is to drive a puck clutch with a stock heavy flywheel.

I will have to admit to two downsides.  It seems to be easier to overheat the clutch if you are doing a lot of drag launches in a row (like five runs in five-six minutes).  And if you have a raunchy ported engine, you can't cruise at low RPM without feeling all of the drivetrain lash as the engine bucks.  But you can barely do that with a 30lb flywheel, as well, so big berkin' deal.

I'm not saying I agree with it, I just found the text amusing. :)

No porting on my FD (stock motor), and I'm not doing drag races with it, so yeah.

I had a lightweight aluminum flywheel from Racing Beat on my 83 RX-7 for a couple of years. I hated it! Pulled it out, and sold it. It was great for acceleration, but tough to drive, especially taking off from a stop. Now it wasn't a bad ass FD with power, but I really didn't like having to take off from a stop like I was some F&F kid. Much prefer the stock "heavy" flywheel on a rotary. I love the maybe 30 pounder on my REPU.  But then again I'm old now. YMMV.

I put an ACT "pro-lite" flywheel on my FC when I did a TII trans swap. I didn't find it to be too light at all, and I liked that it was one piece of steel rather than several pieces bolted together, plus that outer edge has minimal material for lower moment of intertia.

Thanks for the input everyone -- I went with the 8 pound aluminum one, finished amassing the parts & tools today.  I think I've got everything I need. :)

Holy crap that is a lot of stuff!

In reply to Knurled. :

While you're in there might as well... 

clutchsmoke said:

In reply to Knurled. :

While you're in there might as well... 

Clutch, pressure plate, flywheel, counterweight, pilot & throwout bearings, tranny seals, rear seal, a few gaskets, clutch hydraulics, and an assortment of tools. :)