Plz Help with Trans Gearing

[8litercobra]

Just looked up the ratios in my T56.....2.29, 1.60, 1.21, 1.00, 0.85, 0.76

[Davidless]

Originally Posted by 8litercobra

Just looked up the ratios in my T56.....2.29, 1.60, 1.21, 1.00, 0.85, 0.76

I agree that looks perfect, too bad the Tkx doesn’t come in the 6 spd flavor

[eschaider]

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.

[Grubby]

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

[bwcobra15]

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 ?

[patrickt]

Originally Posted by bwcobra15

Really wish I had gone with taller 5th gear.

Anyone want to swap this TKO600 close ratio with a TKX w/0.64 ?

davidless, it's actually pretty simple, if you fall in to the jhv48 boat, go one way, if you're in the bwcobra15 boat, go the other. I'm in the bwcobra15 camp, but on the few times when I am on the highway, and the traffic speeds are constantly varying between 55MPH and 75MPH, .64 can be a nuisance because 1600 RPM is too low for 5th gear and 3500 RPM is too high for 4th gear, so you're shifting back and forth. What I have generally done in that situation is to put it in 4th and rev around 2800 which only takes it up to about 62MPH. But if I had to do that day in and day out, that would not be acceptable.

[Davidless]

Originally Posted by Grubby

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

Thanks for that info, my builder said the engine I have likes 2500 to 2600 rpm minimum so a .81 would put me at 80 to 83 mph. I guess that's a no brainer then.

[patrickt]

If your engine demands 2600 RPM there's no question you want the .81 for street driving. Don't even think about it anymore.

[patrickt]

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.

[Davidless]

Originally Posted by patrickt

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.

Based on 18" wheels my calculations are a bit different

@ 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

[patrickt]

Originally Posted by Davidless

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

Here are the instructions: