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Just looked up the ratios in my T56.....2.29, 1.60, 1.21, 1.00, 0.85, 0.76
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If my memory is correct, that gear set originally was developed by Tremec for either a Viper or a Corvette using a 6060 gearbox, which is the OEM equivalent of the T-56 Magnum. Rockland then developed an uber-strong Tranzilla version of the gears made out of 9310 nickel steel, which I think is what you have.
I helped a friend do a custom T-56 for his SCCA FR500C (I believe). The transmission was stunning on track compared to the standard 2.66 first-gear box. The price for the gears was also stunning. I don't remember the exact number anymore, but I do remember you could buy an entire standard T56 for about the cost of just the gear set. |
I have had both Tremec ODs. The .68OD was OK in ERA 755 with a VERY mild stroked 428. My current car, ERA 838 has the .81OD with a mild 496. Both cars had 3.54 rears and 295/50/15 tires.
838 wouldn't have been happy with the .68. The engine doesn't want to pull below 1800rpm and that would have made a .68OD only useful on the interstate and worthless on state highways. The .81 is great for long trips, especially on 2 lane highways. On the interstate it does spin up a bit at 80mph, but most of our trips are on state highways. .81 with 3.54 rear is a great choice for a Cobra. John |
I"ll throw my $.02 in ... I have 427W with 600 hp, 600 tq, TKO600 w/ 0.82 OD, 3.50:1 gears, 295/50/15 tires.
At 80mph i'm running about 2900 rpm, if memory serves. Its not pleasant on interstate driving and I get 12mpg if I'm lucky. Really wish I had gone with taller 5th gear. Anyone want to swap this TKO600 close ratio with a TKX w/0.64 ? |
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If your engine demands 2600 RPM there's no question you want the .81 for street driving. Don't even think about it anymore. :cool:
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But you still might find the mid to upper 60's to be troublesome. With your setup, fourth gear needs 2800 RPM to hit 65MPH, using a 26.8" rear tire, which is what my MT 295/50 tires are. A shift to fifth at 65MPH drops your revs to 2300RPM, which your engine might not like.... I think I did that math right. The truth is, you really won't know for sure until you have the engine nicely broken in and tuned to perfection.
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@ 2800 RPM 4th will cruise @ 72 mph and 5th is 89 @2500 (minimum for happy engine) 4th is 65 mph and 5th is 80 mph, I'm sure on a good tune I can go down to 2400 RPM to cruise @ 76 mph. Plus I'm using the programmable Progressive distributor so I can tune on the fly, that is if I know what I am doing LOL So I think the .81 is the way to go |
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http://www.mtfca.com/discus/messages/179374/246981.jpg |
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