Jeff20B

iTrader: (13)

I just gained access to enough clutch parts to do my REPU and my S model. Two RB light steel flywheels, two street strip pressure plates and one stock 225mm disc with almost no use (over 8.7mm thick). I'll pick up an HD disc (no springy marcels between friction surfaces so it grabs harder than stock like Steve uses) or another stock disc.

There is a catch.



One flywheel is the early type with no clearanced sections in the upper lip. it is only compatible with 215mm pressure plates. If you try to use a 225mm pressure plate, it will work for a little while until the rivit heads come in contact with the upper lip. Then it's slip city.

Can you tell which is which from the picture?

twinkletoes

iTrader: (17)

I'm pretty sure the one on the right is only compatible with 215mm pressure plates?

Jeff20B

iTrader: (13)

Of course I'm talking about the flywheel on the right. With this type of flywheel, you must use a 215mm pressure plate. The rivits are up a few mm higher and will actually work.

Kinda sucks that you can only use the less total grip capability of 215mm components though. However, you have a choice between a 215mm disc or a 225mm disc in this situation. How is this possible? It is possible because the flywheel surface is flat out to 225mm and then it starts to curve up. This means you can use a 215mm pressure plate with a 225mm disc and get up to 220mm of grip!! Wows!

Now for a demonstration.

You take the unclearanced RB flywheel


and a 225mm disc


here is a typical 215mm pressure plate (don't look at the rust)


and here it is bolted down


the 5mm excess is clearly visible, but not detrimental


You can also see that the rivit is plenty high above the flywheel lip. This setup will run for years and will offer slightly more grip than a fully 215mm setup, and slightly less total grip than a fully 225mm setup. A compromise that works.

Jeff20B

iTrader: (13)

Now that you've seen it is possible to use a 215mm pressure plate with a 225mm disc on RB light steel flywheels, my next concern is which vehicle the 215mm pressure plate and unclearanced flywheel should go into, and which vehicle should get the fully 225mm components.

The REPU is getting a supercharged 13B and will be expected to tow and haul heavy loads. My friend's REPU is setup like this with an S5 T2 engine, 7" Camden SC with a -8 pulley, Holley 600, RB 225mm compatible flywheel, 225mm HD disc and 225mm street strip pressure plate. The HD disc is a little grabby, so you have to pay attention whenever you drive it (can get tiring) but it has plenty of grip for towing and has worked fine for a few years now.

I'd like to get away with a stock disc (pictured above ) if I can. If not abused, I believe the stock disc will hold up for many years as well. What do you think? As for the pressure plate size, 215mm or 225mm, I'm not making a decision yet because I need to factor in the S model's eventual setup.

The S model will start out with a '74 ported 12A that may get a Sterling (depends on the funds). Then it will get a mild ported 20B. I already have an idea of how it willl run and drive thanks to my friend's FB which had a streetported 4 port 13B, 5" Camden with an edelbrock 600 Performer Series, early RB aluminum flywheel which is ONLY compatible with 215mm discs as the steel friction surface ONLY extends out to 215mm! Plus a 215mm street strip pressure plate.

The 215mm disc and pressure plate had plenty of grip for such a small light car. Of course the power output was anywhere from 170 to 190 HP and whatever the torque is. This SC powered FB could break 'em loose in gear and was a lot of fun (on stock tires). I'm sure the S model will be even more tail happy so I have to figure that into my choices. We're talking about 300HP at the wheels and around 200 foot pounds of torque. Serious overkill for an FB... or is it? All I know is I don't need to go over 300 HP in this chassis. I'd also like to leave the tires stockish so I don't break rear ends and trannies.

If my friend's stock FB drivetrain could withstand an SC, I bet mine could withstand a 20B as long as perhaps the clutch is the weak link. I seem to recall peejay mentioning something about using the clutch as the slip point to keep from breaking stuff. Something's gotta give. If not the clutch, than hopefully the tires. If neither one is willing, something metal (expensive) is.

Jeff20B

iTrader: (13)

It's a tough decision. On one hand the REPU needs a strong clutch for towing etc, but the 20B needs a strong clutch because it's a torque monster. Is the difference between 215 and 225 that great? Especially when using a 225mm disc?

How much grip does the 20B really need? I'm almost willing to use the 215mm pressure plate and an HD disc in the S model and call it good. This is mainly because it is a small light car after all and should have plenty of grip and burst strength for the chassis. This allows the fully 225mm components to go into the REPU where I feel they are needed most for reliability and towing ability. Plus if the 20B decides to break something or slip the clutch, at least I can upgrade to a T2 tranny and 240mm parts then, while keeping peace of mind in the REPU.

Yeah, I'd rather keep the overkill parts in the REPU where it's been proven to work, and the perhaps slightly less capable parts in the lighter chassis where if something does go wrong, upgrading is less of a hassle and has been covered on the forum. Imagine getting a T2 tranny in an REPU. It was enough of a hassle upgrading from a 4 speed to a 5 speed. Plus it's already a rib case with super strong gears, but weak synchros so no fast shifts. See where I'm going with this? If I have to do any tranny upgrades, I'd rather have them be in a 1st gen chassis.

Here is the list of parts I'm going to use.

REPU: 225mm SS pressure plate, stock 225mm disc, unclearanced (early) RB light steel flywheel

S model: 215mm SS pressure plate, 225mm HD disc, clearanced (newer) RB light steel flywheel

Can anyone talk me out of it? Any holes in my logic? I'm willing to entertain alternatives within my budget. Thanks.

twinkletoes

iTrader: (17)

I'm with you. I think it's a smart decision. I think your drivetrain(in the 1st gen) will hold up for awhile, but not for a very long time. I would get at least 205s put on the wheels also, but I get your point.

nick812

Very interesting jeff I was just getting ready to pm you about a flywheel question and I seen this tread I was hoping it would answer my questions but it didnt. But very interesting the difference between the 2 flywheels.

Siraniko

iTrader: (13)

slipping clutch

Jeff20B

iTrader: (13)

I made a minor mistake up above and no one noticed. The 225mm pressure plate goes with the clearanced flywheel.

wackyracer, that's an interesting observation. The flywheel was used in the white REPU with a 215mm HD disc and the 215mm SS pressure plate mentioned above. You say it was slipping? From what I can remember it grabbed hard. This was helped by the ultra low 4.625 diff ratio and some small wide 14" tires. Anyway that's how all those heat spots formed.

Maybe the PO had to slip it a bit sometimes to get it going? Or maybe 215mm is just too small for an REPU. Mazda did put an early form of a 225mm clutch in the REPU when all the other rotary models had 215mm. The parts are incompatible with the other models; you either use all REPU clutch components or you don't.

So I think I will use the 225mm SS pressure plate, stock 225mm disc and clearanced (newer) flywheel in the REPU and it should be just fine. Decent pedal effort and decent grip.

As for the S model, I'm ok using the heat spotted flywheel with the 215mm SS pressure plate again, only this time the chassis will be lots lighter and it will get a 225mm HD disc for that important extra 5mm of grip. And if the 20B overpowers it, I can upgrade to something else. I'll have plenty of time to use it during engine break in and it will be fine with the 12A.

dbragg

iTrader: (7)

can you do me a favor and post the other side of a RB light weight steal flywheel. i need to see if the one in my car is or not wuthout dropping the tranny.

B6T

iTrader: (2)

Why don't you just take it to a machinist and have them machine the lip off on a lathe? I don't know if its possible given the geometry, but it's an idea.

Jeff20B

iTrader: (13)

I have no pictures handy of the backside of an RB flywheel, but I do have some pictures of a superlight steel flywheel.

The front (or back lol), so you get an idea of what I'm talking about. These only came in 215mm and are super thin. Notice it even has a little clearancing notches although it isn't really needed with 215mm pressure plates as far as I know.



Check out how thin this thing is!



One more pic kind of showing the back side.



If yours has thick starter teeth, it's RB. If small and thin, it's this super light aftermarket one.

Jeff20B

iTrader: (13)

The steel has been hardened, so it wouldn't be a nice easy cut like on the newer flywheel, which I suspect they machined it before the hardening process. Plus the material removed would have to be the same weight at each spot. It would suck to induce an imbalance.

When it's all said and done, the total cost would probably come close to a new flywheel, so might as well just get a new one.

Plus I might end up going with an aluminum flywheel for the 20B anyway since the engine's extra rotating weight and extra low end torque would negate any negative effects for drivability with such a light flywheel, methinks. I know PercentSevenC loves the superlight flywheel in his FB with the recently installed R5 13B. It was great with the 12A too. Weight is 8 pounds like an aluminum flywheel (not counting the counterweight's weight). So at least I know what to expect. It's not that hard to drive.

Oh, here's one more picture of the superlight flywheel for your viewing pleasure.

stilettoman

iTrader: (1)

We had a ported 13b in our REPU and originally used a light RB flywheel. It worked great in my GLC and in a 1st Gen RX-7, but we did not like it with the extra weight of the REPU. When we redid the engine we used a heavier flywheel and it seemed to work much better. If my memory is correct, we used a GSL-SE flywheel, which is a bit lighter than most of the earlier ones.

If you have not used a light flywheel in a REPU, I would recommend you give this some thought.

Jeff20B

iTrader: (13)

Thanks for that. I know what you're talking about, please understand. I have a freshly resurfaced GSL-SE flywheel sitting here that was slated to go into the REPU, but then a 5" Camden SC came along so I had to reevaluate my plan and consider the best clutch components for the vehicles I have here that need clutch jobs and/or fresh rebuilds that need clutch components.

In my brother's RX-4 wagon, I did a quick rebuild of the engine to get it going and the first flywheel we tried was the stock 215mm 30 pounder and stock pressure plate. Then I did another engine rebuild using lots of new parts in preperation for the SC and this time went with a GSL-SE flywheel, HD disc and 225mm SS pressure plate. That was a major improvement. It was imediately apparent how much better it was. This was while NA before the SC went on. Then with the SC, the extra rotating weight felt like the old 30 pound stocker was back on it. We realized we should have gone with a light steel at least.

Anyway you're right about the GSL-SE flywheel in a heavier car, like the RX-4 (with hideously tall diff gearing of 3.636 in 1978) because after I removed the SC last month and stuck a good old Hitachi back on there, the flywheel felt perfect again.

Since the SC is going into the REPU, the SC's extra rotating weight will add to the light steel flywheel and make it feel more like a GSL-SE flywhyeel. That's what I intend to do. The other GSL-SE flywheel I have sitting here will go into my Cosmo with a stock disc and a medium strength pressure plate (DK/Daikin/Exedy with light colored rivits instead of black; I just installed one the other day in a beautiful silver '83 with my nice '74 ported 12A and modded Nikki and it had decent pedal effort; not too hard and not oo soft). The Cosmo also has a disgustingly tall geared rearend at 3.636 and even taller 14" tires. The GSL-SE flywheel should be just about perfect. The engine has '74 spec ports and just needs paint to match the tranny and a few other items. It should look good and perform well.

GSL-SE flywheel is like 27 pounds, but less weight is concentrated at the outer edge. It's thinner than the '74-'80 30 pound flywheels.

j9fd3s

iTrader: (3)

as an aside, RB uses a stock mazda ring gear, we've been to a couple of their open houses and they've got crates of em.