Is it ok to Use a Trubo 350 Tranny?

Question

I have a 88 cadillac with 200-r4 & I'm thinking of using my buddys th350 tranny with a chevy 350 motor. I think my rear end is a 2.56 & I dont want my rpms spiraling out of controll on the highway. I'm not an extreme speeder but want reasonable highway speed/economy at around 70-75mph. So how would this combo porform compared to a 4-speed 700-R4 overdrive tranny.

Answer 1

I own a transmission shop, and I don't think you will like this combination unless you are going to a 350 lock up tranny. First gear will be too high, and it will struggle to take off from stand still, and it will bring the RPM's way up at highway speeds. So with this in view, you may want to consider going with a 200R4, a 700R4, or something that has lock up, and a low first gear. Look at your 200R4 you have, you may be supprised to find it just might bolt right up to the 350. Some will, some won't. Measure from center to center on the top bell housing bolts, and from center to center on the dowl pins, and if its the same, you are in business. Jack up the rear of the car, put it on jack stands, and make a mark with chalk on both tires, and a spot on the driveshaft. Use three people to do this. One person to watch the driveshaft, while two people turn the wheels in the same direction, at the same speed for one full turn. If your drive shaft goes 2 and 1/2 turns to one turn of the wheels, then you have a 2.56:1 geared rearend. You probably have a 2.72:1 which will be 2 and 3/4 turns on the drive shaft will equal 1 turn of the wheels. This is not rocket science, but it will get you close, or you can get the numbers off the axel tube on the drivers side, and a dealer can tell you what it is. Actually some of the websites that sell rear ends has a chart to tell you what they mean. The most common is the 2.72:1 ratio in yours.
Part 2:
Sorry but Paul is wrong! No standard transmission that does not have lock-up and over drive can even come close to the low rpm's you get out of these transmissions. A standard transmission will have about 1,000 rpm's more at 60 mph than an overdrive with lock up at the same speed on the freeway. 3rd gear on a 3 speed is the same as direct drive. Now when you use the 700 3rd gear is direct drive also, but you still have 4th gear and lock-up to go. See what I mean?
Glad to help out, Good Luck!!!

Answer 2

Paul is right. If it's got 2.56 gears or similar, you will never have to worry about a three speed transmission. Just run it now down the highway in drive (not OD), and it'll give you an idea. If it's got a tach, add about 1/3 of the OD highway RPM. This will get you close, as I believe the 200R4 has a 30 or 32% OD. I had an old Ford LTD with a 400/FMX and 2.76 rears. It was extremely smooth at 100 MPH (and still not pulling much RPM). Keep in mind, a 2.56 gear with no OD would be about the same as running a 3.70 with a 30% OD.

The RPMs added to the engine with the OD and lock up converter is a percentage of the 1:1 RPMs, not a constant of 1000. If you are only pulling 1500 RPMs at 60 with 2.56 gears and a 32% OD, then it's only going to add about 700 RPM and make the total about 2200, plus a little difference for the lock up. Still not much. It's not like you have 3.73 or 4.10 gears. The final drive reduction from the engine to the tires is what matters. It doesn't matter how it gets there. The point is, it's not going to be running hard without OD with those rear gears. This is not only automotive knowledge, but basic math! Numbers do not lie!

I hope this helps out!

Answer 3

GM put a 200-r4 in a Cadillac?

Answer 4

With a 2:56, you will never worry about freeway RPM's with ANY trans.

Answer 5

Just google "tranny". You will be surprised at the information you get.