Hi Guys,

Having trouble making up my mind on this one, figured I would see what input everyone had. I am finally getting started installing my 20B in my FD. When it was in the FC, it was pretty unmanageable, and also it killed transmissions. At the time i bought a ZF 6 speed transmission, which appears to be a good match for my torque goals (~550 ft-lbs, measured at the wheels), and capable of handling high RPM better than the T56 everyone likes to use. I then realized that the FC was never going to be driveable with 600+whp, and bought an FD.

My initial intention for the FD was to fab a rear subframe that would accept a transaxle from a C5 vette or a 911. Now that I started mocking up the engine fitment, I realized that to put the engine where I want it, I will have to modify the firewall a little bit anyway. This brings up the question of whether it is worth it or not.

Anyway, here is my conundrum:

Option 1)
20B mounted just behind the PS rack (with custom oil pan). Firewall will be slightly trimmed, HVAC system will still fit. Make a rear subframe to mount a transaxle with a custom torque tube up to the engine. This will place the forward/aft CG location just behind the driver's seat (best guess). This will also create a fairly large PMOI to contend with.

Advantages:
CG location puts more weight on rear wheels where I need it
Takes care of diff upgrade
Only minor body changes needed
easier turbo location

Disadvantages:
Possible PMOI increase could make the car harder to control
Need to source parts
Potential headaches with drivetrain lineup and flexure
All work needs to be completed at once

Option 2)
Mount the 20B about 3" farther back, and fab a bellhousing to use my ZF6. This will put the shifter roughly in the stock location if I fab a linkage. I will not be able to use the stock HVAC system, and will have to make major firewall modifications. This will put the CG just a bit in front of the driver's seat. PMOI will be lower.

Advantages:
Already have the transmission
Can run with stock diff until it breaks
Car should be more settled
Easier mounting of 20B
Less driveline hassles

Disadvantages:
CG further forward means less rear traction
Will have to fab a new rear subframe for stronger diff eventually anyway
Major chopping of firewall and tunnel required
Cannot use stock HVAC system, will have to source aftermarket one or adapt junkyard part
I might have to fab a new turbo manifold (not a big deal, just a pain in the butt)



Just to make it more clear, this will be a street/track all around crazy/fun car. I also have an Elise and a 911. The Elise is damn near perfectly balanced, but I would prefer a bit more forward CG and a bit more oversteer as it makes the car more fun to drive. The 911 annoys the crap out of me, because it understeers into turns, but then tries to snap oversteer at the exit. I don't imagine that the RX7 will ever be this much rear biased, but it does prove the point that rear bias and high PMOI sucks. I will not be drag racing this car (not much anyway), but I don't want it to smoke the tires at 100MPH like the FC did, so I need as much rear grip as possible.

Anyway, sorry for the long-winded post, and let me know what your opinions are.

Pat

 

Which ZF-6 transmission were you planning on using? The one I'm familiar with is the one on Duramax And Powerstroke diesels which don't rev much over 3k. My family owns an F350 with that transmission, and it definitely belongs in a truck. Pretty clunky, even with the 3' shifter. Also, it's more like a 5 speed with an extra low first.

The rear transaxle would be a cool project, but would have a lot more fab work and material costs like custom cvs and axles, torque tube and driveshaft, ect.
Why not spend your money on a built T56 and a reinforced rear end

 

Rear transaxles are non-trivial because of the torque tube. And making room for the transaxle, if any part of the rear suspension interferes...

My OPINION is screw 6-speed transmissions, you have a big engine with a broad powerband, you shouldn't need more than five. Lots of gears are a crutch for an awful powerband.

Heck, the only reason the Corvette has a 6-speed auto is because of marketing, the 4-speed got better performance and fuel economy because the engine didn't really care and the transmission was simpler/lighter/less drag.

 

it is the ZF6 from a c4 vette. similar weight to the FD trans, better ratios, much stronger (allegedly).

peejay, I am not doing this for the 6-speed, I am doing it because the stock transmission is not even close to strong enough, and because the way in which I install the engine in the car depends on transmission selection.

The torque tube is not a big deal as it could be integrated into the FD truss, and there is no suspension interference.

I am leaning towards the other option though, as I have found that engine placement with the ZF can still be pretty close to where I want it to be, and I am hoping this will still put enough weight in the rear that any issues can be solved with bigger tires.