Mr X

OK, so this is perhaps a slight variation on the questions that have been asked before regarding this (yes, I've used the search). I'd put this in the performance section, but I am not allowed to yet.

Discussions of the RX7 vs RX8 transmissions seem to stop at a generic statement about strength ("RX7 is stronger"), or an equally generic statement about quality ("RX8 2nd gen is better").

I'd like to discuss a little more in depth.

Some specifics...

- My car is an '04 Series 1 with a (believed to be) blown transmission.

- I am building a car primarily for low budget racing. Primarily autocross- with a little bit of drag racing. (I don't care about theoretical top end, cruising rpm's, or fuel economy)

- I have a free RX7 5 speed laying on the floor in my shop.

- I am also buying a good RX8 transmission, and can swap it in whenever I want to.

- My car will be 500# lighter than stock, have a lightened flywheel, a racing clutch, and nitrous.


The default RX7 transmission discussed seems to be the FD, or the turbo variant (cauz everyone knows turboz make it stronger, right?? )

However, the trans I am considering is the '87-88 FC 5-spd non-turbo version.

The reasons:

- Free. That's a realistic consideration (though I don't want to make a permanent mistake)

- Lighter weight. It is 27.2 lbs lighter than the RX8 6 spd.

- Strong?? Well, I actually don't know anything about this. Case strength probably doesn't matter at all. It's not a turbo, but maybe the gears internally are actually similar in strength anyway?? Maybe not?? Anyone know??

- Gearing- Isn't this the real reason for a transmission? The RX8 gearing is 3.76/ 2.269/ 1.645/ 1.187/ 1.00/ .843. The RX7 gearing is 3.475/ 2.002/ 1.366/ 1.00/ .697. I will certainly loose fuel economy and top end, but (considering my intended use) I think I would find much better acceleration with those gears. (and maybe can find a 4.77 rear end).

I am concerned, however, that I will have a harder time keeping the car in the narrow powerband of the renesis with fewer gears.

Thoughts?

Mr X

Just to clarify...

I understand the RX7 FC trans is not the optimum for the racing I am doing.

The game I am playing (the $2016 Challenge) is always a series of compromises. The car must be built for $2016 or less. That stresses a racing budget, no matter what you do.

A free transmission means that I can spend money in other ways to make the car the best possible. If I spend $1000 on a trans, I can't do anything else to make it race worthy.

So, if I can swap final drive, or drop tire sizes to correct for gearing problems, in the big picture it is worth it.

I realize this is not the type of advice you would normally give in the best interest of building a great car. That's OK. I am looking for compromise solutions.

Thanks!

BigBadChris

iTrader: (4)

Cons

The rx7 transmission cannot and will not bolt up to the renesis. The bell housing bolt patterns are not compatible. The 5 speed JDM / Euro market manual is similar to the rx7 unit, and it will bolt up to any rx8.

The second gen six speed is regarded as stronger (the synchros in the gen one wore out quickly). The second gen will also bolt up, but needs a minor wiring tweak.

Mr X

Hmm... that would definitely cause a problem!

You sure? I haven't laid them side by side yet- I was just working off info I had found (which obviously disagrees with your info). Heard it was a direct bolt-in.

It's a new platform to me, so I claim no knowledge here. I appreciate your input.

BigBadChris

iTrader: (4)

My mechanic has built rx7s and rx8s. He is the one who convinced me to use a five speed JDM. I am very confident that the rx7 unit won't mount up.

Mr X

BigBadChris: I'm not picking a fight, but in the RX7 world, the renesis has been used quite a few times with the FC trans. There are more popular swaps, but the renesis has been done.

So, I will keep you posted as I learn more, and when I get the chance to actually lay them side by side.

What did the 5 spd JDM trans do for you? What are the advantages? What are the ratios?

BigBadChris

iTrader: (4)

I own it, but it isn't installed yet.

From what I have been told, the swap can't be done because the bolt patterns are different. Have I tried it myself? No.

Best of luck to you

dannobre

iTrader: (3)

You can bolt an FC tranny to the Renesis and it users the same type clutch and starter. ..the problems are the way it is mounted. The RX8 uses a PPF like an FD. FC will need mounts for tranny and diff. .and a new drive shaft. And the shifter location is different. Rx8 5 peed is basically FD guts and ratios...but it uses a push clutch and the proper mounts and same shaft.

Mr X

Hmm... Does that mean I could swap the tailpiece and clutch slave cylinder from the RX8 and use the RX8 PPF? How about a Miata tailpiece?

Just thinking...

dannobre

iTrader: (3)

Nope....afraid not 😊 That would be way too simple 😊

I would try and find a 5 speed...simplicity is a good thing 😊

Mr X

I am assuming you mean an RX8 5 spd, right?

Looks like I've got some experimenting to do!

dannobre

iTrader: (3)

Or find a 6 speed at a wrecker....

That way you don't have to redo the drive shaft and figure out a way to mount the FC tranny.

Sometimes Free gets really expensive

Mr X

You are absolutely correct.

FWiW, "free" to me is not that I am being a cheapskate, or can't afford it. "Free" (as defined by the rules of the $2016 Challenge) is a part of a STRATEGY for competing.

It's not something most people understand unless they have competed in this event.

I actually know a guy who bought a Kirkey aluminum race seat and used it as a pattern to build his own out of scrap aluminum so he could put it in the car for "free".

Making unusual choices that take stupid amounts of labor to save a few dollars enables spending budget dollars to accomplish other things.

I know people who have over 4000 hours invested in a car that cost less than $1000. I'd say that kind of "free" is ridiculously expensive.

Labor and creativity are the currency of the $20XX Challenge.

nycgps

iTrader: (1)

Get a JDM 5 Sp tranny, it bolts right up to the Renesis, uses Rx7 FD's internal, so it can take quite a lot of power (but it also inherited some rx7 fd transmission known issues)

I have one u can have for cheap, but I'm not in the states right now (overseas) so that might be a problem.

Mr X

I'll be happy to discuss it further when you can.

What are the gear ratios for the JDM 5spd?