Mods, please move this thread if it is inappropriate for the Advanced Tech area, but I didn't figure there'd be a lot of responses generated in LT1 Tech because most people wouldn't bother switching from a 6-speed to a 5-speed.

Also, sorry for the length...

My 396 LT1 is almost done and I've already got a McLeod Street Twin and had a '95 T56, but planned to use a '93 "G92" (2.97:1 first gear) box instead because the gear ratios looked better in the computer sims that I've run. The main problem is that the car has a 4.10:1 diff and the only "taller" option is the 3.90:1 out of the automatic car ('95 RX-7). Obviously a 4.10:1 diff isn't going to help much on street tires in a 2,750 lb. car with over 550 lb-ft. of torque.

With a T56, any T56, and the 4.10 gears, 1st and 2nd gear are probably write-offs past 2,500 rpm or so and 3rd is probably iffy at best. With a maximum tire width of P295 in the back without major modification to the car, I'm not going to get any more rubber on the road. With 400+ lb-ft. of torque at 2k rpm and up, and 4.10s in the back, I forsee replacing tires often in my future.

If I go to the "middle" range of differential gears (3.23-3.42:1) then I still have problems with 1st and 2nd, but the car will probably hook up in 3rd, and 6th gear is usable as a cruising gear. The problem is that the guys with this kind of power on the Z06 Vette forum are having no luck with traction in 1st and 2nd with 3.42:1 differentials. The Z07 conversions that MTI turns out just fries the tires, and will light up P345s (mini-tubbed) in the back pretty easily. I want to be able to use more of my power on the street, if at all possible.

So if I dip into the 2.xx:1 range, things start to look much better, but at that point, I'm dragging around an extra gear (6th) which doesn't do me much good until well after 90 mph, and even 4th gear runs out at nearly 200 mph, so what's the point of packing around the extra weight and an unusable gear? 5th gear would be my "highway cruising" gear. With that in mind, I've started looking at the Tremec TKO II as an alternative to using a T56.

What interested me about the TKO II was that I have heard that John Moss used one in the 572 CID ZL1 Camaro he built. The transmission is rated at 525 lb-ft. of torque, so it's plenty strong. The gear ratios are pretty much ideal for what I'm trying to do, and coupled with a 2.90 diff, would give me final drive ratios that might get me a chance of traction in 1st gear and probably hook up good in 2nd. It's also 27 lbs. lighter (95 lbs. vs 122 lbs. dry) than a T56, and I'm not carrying around an "extra" gear that I can't really use.

I contacted Forte's about the TKO II and my setup, but haven't gotten a response yet, so I was wondering if anyone here knew anything about this transmission. It apparently has a 26T spline input shaft like the LT1 T56, but I'm not sure that it's compatible with the Street Twin that I already have. Anyone know? They don't list a bellhousing depth with the transmission either, and I thought it might afford me the chance to use an SFI-approved bellhousing, "just in case", which would be a plus. And they don't mention whether or not it uses an electronic vehicle speed sender (VSS) which I'll need to interface with my recalibrated speedometer. If anyone knows the answer to any of these questions, I'd really appreciate the help.

Also, while I've got your attention, the TKO II with a 2.90:1 differential is just slightly higher in final drive than the "standard" '93 T56 (3.36:1 first gear) with the 2.73:1 differential. Does anyone here know if the cars that had the 2.73 gears had any problems with acceleration or launching? Anyone know of anyone who has an F-body with 2.73 gears and a high horsepower stroker or supercharged engine? I have to believe that if Chevy set up the car that way, even for only one model year, that they must have known what they were doing. With 2.80 gears, the TKO II would be nearly identical to the gear combinations used in the '93s with the standard T56 (1st through 5th).

Anyway, my main goal is to try to hook up as much power as possible on the street, which is where I'll be using the car 99% of the time. I won't be running slicks, and it's not important to me to make the car launch hard at the strip. What I'm looking for is some way to put as much power to the ground as possible on the street, with the amount of tire that I have available, and short of a (possibly overly interfering) aftermarket traction control system like the RaceLogic, changing the gearing is the best way I know how to do this, unless someone has a better suggestion. The car sure doesn't need a 4.10:1 diff now.

Any advice from gearing experts or from anyone who has experience with higher horsepower setups in the '93 2.73 cars, or anyone who knows anything about GM bellhousing setup and clutch selection with the TKO would be greatly appreciated.

Thanks in advance!

 

Your fine Jim
And its nice to see your project almost done, keep us updated (pics if possible )

 

Jim, I remember a couple of years ago there was a car in GMHTP that had 600+ RWHP with a 93 T56 and 2.73s. They didn't mention a problem, and if your car won't see sticky tires, I don't see there being that much stress on the transmission. Another route would be a 3.23:1 with the 93 T56. That would give you somewhat of a cruising gear, 6th should be around 75 - 80 @ 2000 rpms. It would at least keep you out of the higher rpms when not racing. Then again, I don't know what your redline is, and it may put you into 2nd gear before 60mph. I know I would want at LEAST 60 in first with a car like yours. McLaren killer! Hmm, hadn't thought of it till now, but what is the Cd of the RX-7 and frontal area? You might definitely be builing a car to go head to head with a McLaren on acceleration AND top speed! WOW!!!

 

My redline is arbitrarily 8,000 rpm, and my shift point will probably be about 7,500. The engine is still making 637 horsepower out at 7,400 and just starting to drop off. Cd of the 3rd gen. RX-7 is 0.29, and frontal area is 19.3 sq. ft.

 

Never mind, folks. Jordon has described to me the joys of living with big cam surge.

Sounds like the shorter the rear gear the better off I'll be for "normal" driving, because it'll get the car into higher rpms at low vehicle speeds and reduce the "mechanical bull" effect. I'll just have to get good at modulating the throttle, or find out how effective the RaceLogic traction control system really is for WOT use.

Thanks Jordon for all your help!