71Cutlass.

I might go ahead and grab a TH400 for my 1971 cutlass with a Rocket 350. If I buy a BOP TH400, will there be any modification required to bolt it up to my Rocket 350? Thanks.

oldcutlass

You'll need a driveshaft and yoke, a kickdown switch, and Ebrake cables because you'll have to move the transmission xmember.

71Cutlass.

Cool, thank you for the help!

Sugar Bear

Any particular reason your considering that? A TH350 is a very capable transmission behind a small block and is lighter. Someone please confirm/refute, I think the TH350 also has a lower first gear ratio for better acceleration???

71Cutlass.

well my current th350 is bad, and it’s cheaper to just buy a new one. I was under the impression that a th400 had better acceleration, but maybe that’s wrong?

Sugar Bear

TH 400 is very robust but a lot more robust than needed for most small blocks. Let's see what others say about the gear ratios as I'm not certain.

Good luck!!!

oldcutlass

Yes, the T350 has a better 1st gear. Drive is the same 1:1 ratio in both transmissions. I would just go with another T350 or rebuild yours for a stockish SBO.

Sugar Bear

If replacing the transmission I assume you'll be replacing the torque converter. Figure out your rear axle ratio and anything you know about your engine so that members with experience can guide you on the optimal converter stall speed. Don't rely on whatever "comes with it" as you may be needlessly leaving some performance on the table at not much expen$e.

Flush the cooler and lines, it's critical.

Good luck!!!

Fun71

Yep 2.52:1 vs 2.48:1.

bccan

I don’t know where you’re located but there are 2 pretty fresh TH350s in “parts for sale” classifieds. If either works geographically for you, that might be expedient and economical.

joe_padavano

2.48:1 vs 2.52:1 will barely be noticeable.

BangScreech4-4-2

Whatever is gained from the marginally lower first gear will be lost to increased weight and pumping losses. Unless your 350 is built to generate ungodly amounts of torque, the TH350 will likely better serve your purposes and won't require any modifications.

There's a reason you don't see this swap very often.

joe_padavano

The TH400 has a HIGHER first gear (2.48 vs 2.52) so acceleration will be marginally worse. As I noted above, you'll be hard pressed to feel or measure the difference, which is 1.5%.

BangScreech4-4-2

I knew that. I was just testing you.

matt69olds

Supposedly, all things being equal, a car with a 400 trans will run about a tenth quicker with a 350 trans. This is due to much lighter internals, the overall lower weight of the 350, and due to the 400 consuming more powe just to operate.

matt69olds

Supposedly, all things being equal, a car with a 400 trans will run about a tenth quicker with a 350 trans. This is due to much lighter internals, the overall lower weight of the 350, and due to the 400 consuming more powe just to operate.

You would need a VERY stout 350 engine to stress a properly built TH350 trans

Greg Rogers

I put a 455 from a 69 Toro in a 73 Omega in the early 80's. I was looking for a Turbo 400 to go behind it, and working on figuring out the modifications needed, etc. A friend had a salvage yard and said why don't you try a Turbo 350? He sold me one out of a 76 Cutlass or something, not rebuilt about 80,000 miles on it. Never did go back and install a T400. Withstood a lot of spirited driving behind that 455. Turbo 350's are pretty tough.

olds 307 and 403

As said, a properly built TH350 should hold up, every shop can build them. I bought a nice 76 Cutlass for $50, it was parked with 120,000 miles due to the TH350 failing. Our 75 also died, both behind a whooping 165 hp. Nothing special with GM's mushy calibration. You have 3.42 gears, correct? Seriously consider a mildly built TH2004R, it will drop highway rpm by 1000 rpm with lock up. It also has a 2.74 first gear with a good gear spread. Pick a 2000+ stall with your cam, no matter what trans you pick.

CANADIANOLDS

The 400T is about 10 lbs heavier dry.

it’s takes a bit more fluid.

the 400 eats up roughly 10 more hp than the 350

the 350 also has a steeper second gear so you gain a bit of overall ratio in 1st and 2nd

oldcutlass

The butt dyno is a highly sensitive finely tuned instrument...

joe_padavano

The TH400 gear ratios are 2.48, 1.48, and 1:1
The TH350 gear ratios are 2.52, 1.52, and 1:1

The difference in 1.6% in first and 2.6% in second. You will not be able to feel that, no matter how finely tuned your butt dyno is.

matt69olds

I seriously doubt it would even show up on a chassis dyno. Those figures are within the e we indie of repeatability.

The difference in weight and power consumption would probably show up on a timeslip, but not the “butt-o-meter”. While I’m sure it’s be done, I’d be curious to see a back to back chassis dyno test.

CANADIANOLDS

My butt has nothing to do with it. The car will ET quicker. It’s my eyes looking at the time slip that will see it.

redoldsman

If I were the OP I wouldn't worry about the 1.5% or my butt dyno. The cost of the installation will be a few hundred bucks more unless he gets the parts for free or really cheap. If he is planning on a bad *** 455 later there is no reason to go with the turbo 400.

BangScreech4-4-2

FTFY ... that's what you meant, isn't it?