I'm contemplating using either a Powerglide or TH400 transmissing in a 2800 lb street car with a stout big block.
I've seen cars at this power and weight run good with either transmission.
But if a powerglide car can run good with this combo; could a TH 400 be used as a two speed at the track, then use third as an overdrive on the street.
Powerglide gear ratios are 1.76 and 1.00, a 57% spread.
TH 400 gear ratios are 2.48 and 1.48, a 60% spread, plus a 1.00 third gear.
Since the gear spreads between first and second are close in the two transmissions, I could make the overall result similar by changing ring and pinions.
With a 4.56 ring and pinion behind the Powerglide, the final ratios are 8.03 in first and 4.56 in second.
With a 3.23 ring and pinion behind a TH400, the final ratios are 8.01 in first and 4.78 in second, and then also have the 3.23 ratio for cruising.
What do you think?

If this won't work, maybe I should look into a 4L80E or Gear Vendors overdrive, and if they can handle 1000hp+ .

what gear are running in the rear now?

Your math makes sense, which is exactly why Hydromatic created the th350 and 400.

A good plan! :thumbsup:

I plan on building three future projects in the same fashion.

1. Completion of my 67 C-10 shop truck with a 500 HP big block 462/TH400 combo and 3.08 rear gears. :D

2. A street rod (hopefully a 41 Willis) with a high torque big block 472/TH400 combo and 2.73 rear gears. :D

3. And last but not least: My 67 Chevelle with a 625+ HP 496/TH400 combo running an S-60 with 3.54 gears. :D

We are running a 2.56-1 rearend behind a turbo 350. Feels just like a powerglide with OD. Of course, in a 3500 pound vehicle with a mild 350, there is not much performance. Not totally dead but no real get up and go. A light car and/or lots of horsepower would work fine. Ours was built to cruise and out on the highways it purrs along very nicely.

My SS 454 was fun for a little while with 2.56's
Ran a best of a 12.99 and it ran consistent 13.2-13.3 on drag radials.
Drove it everywhere.

Although its a manual I have a similar deal going on my my camaro. Its a 383 with vortec heads and an XE-262 camshaft. 2.41 gears in the differential, 4-speed saginaw tranny with the low first gear (3.3 I think). I ran a 13.8@106mph, going through the traps in third gear. I'm going to switch the tranny for a muncie with a 2.2 first gear and switch the differential gears to compensate, I'm still not sure if it will be faster.

Buy a copy of the Mr. Gasket/Motion sim. program DRAG from Summit for probably less than $50 and play w/ it to see how the TRENDS go!! Use a copy of D2k to input your AGAIN sim. power curves into it.......................

I found out that a custom geared Muncie would gain me a schosh in the quarter!!

pdq67

Anybody have an estimate about the efficiency difference between running the Powerglide vs. the TH 400 in first and second gear only? How much would be left on the table for mph and ET in a low to mid 9 second car?