I have a 75 coupe with a 383 bolted up to a TH400 transmission. I believe my car has the 3.36 rear gear ratio. I would like a little more low end, but really do not want to increase my cruising rpms. I have thought about switching over to a 5speed manual, but right now I just dont want to put that much money into the conversion. I have other things higher on my priority list. I then came across the idea of replacing the low gear planetary set in the transmission. As I understand it this would give me a lower first and second gear but would leave 3rd gear unchanged. I have freshed up all of the drivetrain with the exception of the 400, so doing this at the same time would seem like a great idea. I have done some reading on the internet, and lets face it, you can find problems with anything if you read enough on the internet Some of the questions raised were the possiblity of excessive noise depending on the gears and how they are cut? and also the quality/durability of some of these sets. My car is probably pushing 400ish horses. I mostly run around on the street and use street tires. However, this is not to say it might not see a trip to the drag strip now and again. I would love to hear comments, thoughts on the topic, especially anyone who has actually done this.
Thanks

 

Why not just keep those stock TH-400 2.48/1.48 gears and go with a tight 10 inch torque converter? You'll get much improved acceleration in 1st gear, but very little loss of efficiency at cruise if it is built as a tight converter.

__________________
1978 Corvette
406 SBC AFR heads Lunati roller Quadrajet TH-350 3.08
11.59 sec / 118.9 mph 1/4 mile

 

When I first put the motor in, I had a converter built for the car based on motor,tranny, rearend, cam, weight, tires,etc. The shop suggested I go loose but I insisted they build it tight and they did, that was ok. I ended up having to go back into the motor so I decided to pull the converter and had them loosen it up. The shop is very well respected and I suppose I should have listened to the builder to start with. I am much happier with the combonation now. I would still like some more bottom end though and thus the interest in the planetary set.

 

I have no info on the 2.75 gear set but I was also pretty interested in it when building my motor/tranny....>my motor guy said not to mess with the gears and spend the money on the converter....He did recommend going to a "straight cut" planetary for the tranny that you can get out of Motorhome and some 1 ton T400s. He said less side loading in the tranny therefore less power loss and more strength. However I did not go with the straight cut because I didn't want to hear the noise associated with them in 1st gear....

I do agree about the converter....Even if your converter is good now, it can be better. The converter will effectively multiply the torque way more then going up on the rear gear ratio....and I would think DEFINITELY more then just increasing the 1st gear ratio.

If your tranny is out for a rebuild, what the hell, try the 2.75 gear set but if the tranny is still in and working fine, I doubt the gains would be worth it.....If you got 1-2 tenths at the strip, I would be suprised. Although I would definetly take a couple tenths reduction myself...

 

The best alternative for your problem would be to install a 700R4 transmission in place of the THM400. The 700R4 is an overdrive transmission (4 speed with .67:1 final ratio), plus it has lockup for even better efficiency in upper ranges. You could go to a 3.73 rear end and still run the highways at not much over 2000 rpm.

 

I am happy with my converter at this point, which is why I was thinking of the gears. I was warned about the gear noise and that was one of my concerns. The 400 is actually still doing well at this point though it does like to shift from first to second early if you let it shift by its self, but I still intend to pull it and freshen it up.
As for the OD tranny,,,,uggg. I know there are people out there that can build them but I had a horrible time with the 200r4 in my Grand National and went through multiple shops each with multiple rebuilds trying to get the thing right. I can only speak for me, but I like a manual and if I were to drop that kind of money, I would convert the car to a manual before sticking in a OD auto.
The hard time I had on my Grand National is the biggest reason I went back old school and picked up the Corvette. I have to say I have not regretted it once. I love my Vette.

 

Yeah if you had hell with a 200r4, you will really hate the 700s.....up to about 400 crank hp, a well built one will hold up...more power then that and you will need a killer 200r4....

I was going to go the 200r4 route because I drive mine so much but...after talking to many people, searching the net etc, I decided to rebuild my original T400 and just let it ride with 3.08 gears. Even after spending $1200 beefing up my T400, I still managed to break it. It was repaired under warranty and so far, so good. I have been beating the hell out of it and no slipping. The key is making sure the kickdown is working....manually down shifting it to 2nd from a 60mph roll and stomping the gas at the same time is apparently hard on the band and 2nd gear intermediate clutches.

Now I have my kickdown working and calibrated my governor for automatic redline upshifts and I have no slipping, clunking...nothing...smooth and fast. I definitely recommend setting the governor up to your desired shift points.

I am about to rebuild my IRS and go up to a 3.36 gear....it will kind of suck on the highway but I need a little more hit off the line. I 60' 1.61s with 3.08s and I need 1.50s. My converter is already as loose as I am going so.....hopefully a little gear will help.

I am sticking with my T400 for sure now....now way a 200r4 will hold up for me...

 

I have to tell you this got a laugh out of me. I somehow get the feeling that I might not drive mine as hard as you do yours. I think my worst habit was interstate ramps but with the stock oil pan I was occasionaly uncovering the oil pickup..oops. That also got me fussed at by the engine builder