425in67

Hi I was wondering if anyone tried or knows of different transmissions that will fit in behind a 425?I like what I have and it works well but thought of an overdrive for long highway trips.
thanks

joe_padavano

The 200-4R is the only bolt-in but it will require modifications to survive behind a 425.

425in67

Thanks Joe for the info and for your reply

2blu442

If you have time use the search option and read through other threads on this topic. Some people have used an adapter plate to install a 700R transmission. Most people have had the 200-R4 beefed up to withstand the torque of a big block. I've read the Buick guys are the ones who first started building 200-R4 transmissions to take more abuse. Chip has a reputation for making these transmissions really strong. Another condsideration is what kind of rear end gears do you have. I believe the 200-R4 overdrive is a 0.67 so if you already have highway gears in the rear end you might not be running the engine in it's most efficient rpm band. All things to consider before breaking out the check book! John

hotrodpc

IMO, I'd stick with the 400. Sure you can build a 200-4 or adapt a 700-r4 that has a .70 OD but both of those OD transmissions are going to need some beefing up to handle a 425. It won't be cheap either. And as 2blu says, you're likely not going to benefit form the OD gear Chances are you have 3.08 rear gears. I wouldn't run an OD unless I had 3.73 or lower gears in the rear.
What is your purpose of wanting something different than the 400?

rustyroger

If you are looking for fuel economy the 200r4 has much in its favour, the th400 is a very strong transmission and is designed to work with GM big blocks, however it absorbs a lot of power which hurts gas mileage.
The 2004r is much more efficient in this respect but will need strengthening to live with BB torque unless you can lay your hands on one from a Buick Grand National.
How worthwhile it would be depends on how many miles you intend to travel, over time the money laid out on changing the transmission will be recouped at the gas pump. But if your annual mileage is small it may take years or you may never even reach break even point.
Getting the overall gearing right for your application is another good point, as a rule of thumb I would aim for the engine to be running at peak torque at your desired cruising speed with the overdrive engaged.
Just my opinion, it's how I would approach the issue.