I am thinking more seriously about my transmission choice for my build. It' a 70 Electra with 2.93 rear gears. The motor will push about 600 hp/torque at the crank and I'm curious to know what trans you guys would use. The motor will have a roller cam similar to a 413 grind and thinking about a 2000-2500 stall converter. Overdrive would be almost useless even at interstate speed. Th350 or th400 is what I'm thinking about. I like the 350 because of less parasitic loss, but would it hold up to 600ft lbs in a big electra with 2.93 gears?
if the car came with a Th400 I'd go with that, if it came with the th350 I'd go that way. Either can be assembled with readily available parts to stand that power. When you get into swapping stuff and the hassle that brings I wouldn't want that.
I would do a 400, it should live behind that with just a few basic mods,( dual feed, better int. sprag, shift kit tailored to your liking, ) I would use high energy clutches and call it good. A 350 will take some more money to get to live
I would never put a TH350 behind 600 hp. A swap from TH350 to TH400 is not very difficult, once the details are understood. I tried the low gear sets decades ago. They worked initially, but after high road miles they caused several failures. I found switch pitch did better anyway, so my low gear sets are retired and will be for sale. Bruce Roe
Yeah, the TH350 behind 600 HP isn't so much of a concern as much as the 600 ft lbs of torque the engine has is. With the power output of your engine, you shouldn't worry so much about "parasitic" loss so much as you should worry about reliability. The HP loss between the different transmissions with your engine would be very minimal at the track, fuel economy wise or seat of the pants. I went from a powerglide to a 700R4 and went .2 faster with my 60 ft time AND picked up 2 mile per hour, all while going .4 faster in the QM with no other changes! The 700 is suppose to use much more HP than a powerglide AND I got better MPG to top it off! so.............? With an overdrive you want to shoot for a 1st gear multiplication ratio of at least 9.5:1, that is the 1st gear of the trans times the rear gear ratio so the engine won't lug in overdrive. For examples; 700R4 with a 3.06 1st gear needs close to a 3.08:1 1st gear to get the 30% overdrive to be able to shift into O/D by 45 MPH. 3.06 X 3.08 = 9.4248:1 1st gear multiplication ratio. While the 200R4 needs at least 3.42:1 rear gear ratio. 3.42 X 2.74 = 9.3708:1 1st gear multiplication ratio. While not quite @ the 9.5:1, they are very close for the ratios that are available. Each one would give you a final drive gear of, for the 700........... .70 X 3.08 = 2.156 final drive ratio. The 200 would give you..................... .67 X 3.42 = 2.2914:1 final drive ratio. So with the torque of the BBB 455 platform the 700R4 would be a good choice to be able to hit O/D @ 45 MPH even with the 2.93:1, but at your level would be pushing the limit on a built one. That leaves you with the 4L80E that the 9.5:1 rule doesn't really apply because it is basically a TH400 with O/D. Has the same 2.48:1 1st gear as the TH400 but has a 4 gear that is .75:1 so 25% O/D that would make your final drive ratio; 2.93 X .75 = 2.1975:1 So in between the 200 and 700 with the above mentioned rear gear ratios. The bad part about the 4L80E is that in needs an expensive electronic controller for it to shift properly.
Out of curiosity, what failed in the low gear sets? I have been running a 3:1 for 25 years and have installed a few 2.75s with no issues. None of these have high miles as they are hot rods and muscle cars not daily drivers. Thanks Brent
