Re: Might convert to a 5/6 speed in the gt
Durability is always going to be an issue with pretty much any manual transmission.
Shock Loading is the problem.
An automatic doing a brake launch has alot of torque built up in the driveline (working against the brakes), so when you finally let go of the brakes and nail the throttle (maybe in the opposite order) you don't suddenly have 1500 ft-lbs of torque hitting the output shaft. Instead, you already have a bunch of power applied, you've taken all the slack out of the driveline, and you're just adding more power. Plus, the torque convertor is a fluid clutch, it can't "grab" so to speak.
With a manual, you're sitting there with the clutch in, waiting to go, trying to figure out how to make boost with no load (stutter box or whatever) and then you drop the clutch on the poor thing. Goes from no load to full load in a blink of an eye (ok, maybe 2 blinks). Problem is there may be slack in the drivetrain, and you take up that slack all of a sudden (like taking up the slack in a chain all at once). That kind of thing usually results in broken parts, because the loading is just so severe. Add to the drivetrain slack the fact that the clutch is a mechanical coupling (with additional shock factors) and probably inconsistent application (that'd be cockpit error) and there is no good way to guarantee that the trans won't break parts.
Not to disagree with George at RSGear, but the 27 spline shaft really isn't the problem (or shouldn't be). The powerglide runs 27 spline shafts and lives just fine behind big blocks in a multitude of applications.
If you put the trans behind a 500 to 700 hp race motor (like the SCCA and Grand Sport applications), wind it up to where the motor makes power (call it 4000+ rpm) and drop the clutch for a standing start, you certainly run the risk of breaking stuff.
The clutch in a manual transmission application is kinda like a fuse. I think the selection of the appropriate clutch (and pressure plate since they work together as a package) will be critical. You need to have one that will grab consistently and not too quickly. You don't need lightswitch action (on or off), you need some slip to minimize the shock loading. You need a clutch that can do that and not wear out in 2 months. And you need one that can hang on to the amount of power you're going to make. Most of the race clutches (the little multidisk setups that are used for reduced rotational inertia and ground clearance) grab pretty quick and I don't think they like to slip, and probably aren't a good choice.
And ultimately, whether the trans and clutch survive will come down to your driving technique.
So, it all comes down to what do YOU want to do. Obviously the 700R4 has some durability problems. The T56 may not be all that much better, depending upon your usage. For the dragstrip, I think a TH400 or 4L80E would be a much better choice (depending upon the need for street cruising ablitity). But for a road course / pro-solo / SCCA corner burner, a manual trans might be worth the effort. Then all you have to do is figure out whether the T56 is enough trans or not (or whether you've got to go to a Jerico 4 speed or something like that to get the durability.)
'JustDreamin'
