Direct Drive Automatics

[luxeryvic]

ive been reading up a bit on trannys and how to make em fast for future projests and i came upon a direct drive and manual valve bodis for powerglides in circle track racing y isnt this around as muchin drag racing? wat are the advantages/disadvantages of it?

[manicmechanix]

For drag racing the torque converter acts like an extra low gear, so it's an advantage to keep it. Also the race down the 1/4 mile is constant full throttle and there's no real need for manual control of shifting.

[luxeryvic]

ya but with direct drive and manual there is less power loss in the trannyi know it is the same almost as a manual tranny but easier, wiood it not be as fast or faster if shifted right because you dont have to clutch it? also can the same be done on turbo 350s or 400s???

[manicmechanix]

Originally Posted by luxeryvic

ya but with direct drive and manual there is less power loss in the trannyi know it is the same almost as a manual tranny but easier, wiood it not be as fast or faster if shifted right because you dont have to clutch it? also can the same be done on turbo 350s or 400s???

The power loss in a drag race is made up for with the torque converter's torque mulitplication and the consistincy of an automatic's launch and shifting. Consistincy and torque is the name of the game in drag racing. And since there's only going to be one shift with a powerglide the shift can set so fast there's no need to manually shift. You could pick up a little with a manual in a drag race but the inconsistency probably wouldn't be worth the trouble. I don't see any reason why I Turbo 350 or 400 couldn't have a full manual valve body.

Even in road racing some of the advance manual transmission have automatic clutching and shifting. I don't think a manual automatic makes much sense for road racing. A 3 or 4 speed manual transmission would give you more advantages.

[luxeryvic]

well wat about a turbo 350 or 400 there 3 speed so wood they not be any better then the glide? and without the converter wieght wood it not take off a bit better??? im just askin cuz im new to it and thinkin about it but again ive never drivin one and it wood be my first real drag car so times arent that important i was kinda thinkin it would be more fun to rip thru the gears i was thnkin bout a manual but it looks like it wood bee cheaper and easier to convert an auto

[luxeryvic]

also could you not have a better take off becase you can lauch it at wat ev rpm? without the wieght of a converter? or are there converteres that do it better?

[manicmechanix]

I think you would want to pick your torque converter to stall around your engine torque peak or at least where you engines cam starts to make power. I think for drag racing it would be best to use a normal working TH350 with a torque converter, or 400 if you are running a big displacement engine. A powerglide is used for cars under 2500lbs with huge engines. Like you said it would be expensive to use a manual transmission in a high torque application.

[luxeryvic]

ok so i was thinkin of doin like a chevette with a big block 454 if i can, so then a powerglide wood be best?

[manicmechanix]

Yeah I think if you have a big block in a very light car, under 2300-2500lbs, a Powerglide would be faster for drag racing.

[MagicRat]

Originally Posted by manicmechanix

Yeah I think if you have a big block in a very light car, under 2300-2500lbs, a Powerglide would be faster for drag racing.

A few years ago, I read that the key component for trans selection is weight.

If your drag car, with the driver and fuel on board weighs less that 3200 lbs, then a PG has the potential for turning the lowest ET.

If the car is heavier than 3200 lbs, then a 3 spd auto has greater potential than a PG, providing its built properly.

[luxeryvic]

ok so then wat is the difference between th350 and the th400 is the 400 just built stronger? which one is faster?

[manicmechanix]

Originally Posted by luxeryvic

ok so then wat is the difference between th350 and the th400 is the 400 just built stronger? which one is faster?

Yes the TH400 is buillt stronger and is more durable. If I remember correctly the gear ratios for the TH400 are 2.48, 1.48 and 1, and for the TH350 2.52, 1.52 and 1. Not much difference. The 350 would generally be a tad quicker, but probably not much.