The only two things I don't like about my '79 seem related to Jimmy Carter.

First that 85mph speedo. At least I can change that easily. The other is the three-speed automatic with no lock-up that I believe was geared for 55 mph highway speed. While I don't use it on the highway often, the RPMs it's turning at 70 mph or so are ridiculous.

Two part question here: Is there any reasonable way to change the transmission to a four-speed automatic (true four speed)? As my car is highly original if the tranny can be changed would such be reasonably reversible?

 

Do a search in the forum for 200r4 and 700r4 swaps. Several members have done the swap with varying methods, costs, and results. These are non-computer controlled, so the swap is a little more straightforward than using a newer 4L60 or 4L80. I've got a 4 speed, but just like 3rd gear in your automatic, 4th gear is 1:1 on the T-10 so highway RPMs are high.

 

I'd already seen that the 4-speed manual had the same 1-1 final ratio so even if I wanted to shift in my hilly town that loves to put stops at the crest of steep hills it wouldn't make any real difference.

Thanks for the model number references as it gives me a great starting point yet from what little I've already read about the subject it seems that I either get into the "highly modified = highly troublesome" thing or have to spend more than the car is reasonably worth. I'm highly aware of the deal breaking computer control required of truly modern transmissions.

 

I've done a few of the 2004R swaps, it's pretty straight forward. What's your power estimate and what is the rear end ratio?

 

Including mine I went with a 3.70 rear end. Most go with 3.55 with the 2004R. If I had to do it again, I would go 4.11 The OD trans is far better than the TH350 I had in there before.

 

I converted my 81 to an overdrive trans 9 years ago ... the 700r4 is a great combo with 3.73 gears... at 80 mph down the Interstate, my engine is turning a mere 2550 RPM

 

I did the th400 from a junk yard because they are indestructible and a new cross member because the 400 is 4" longer then the 350 witch cost more then the transmission and i couldn't be happier

 

700R4 in my 78 on the highway I average 19 mpg. The car is also so much nicer to drive when the engine isn't spinning at 3500 rpm

 

I owned a '79 that had a 140mph speedo. It was an L-82 4 speed though

 

There was an early and a late '79. I also have an early L48 with a 140 speedo.

 

My 77 has a 92 model 700R4 with 373 rear...works great. No real issues in putting in the car...changed shift indicator in console and looks original.

 

Despite true dual exhaust and possibly a different cam it's a stock L82 so definitely less than 300HP. Given the engine RPM at highway speed, 99.9% it has the stock 3.55:1 rear gearing.

 

I know. Mine is quite late production. I have the 85 MPH speedo but still have the three-compartment rear storage likely because the green interior color was the least common.

 

Something to be aware of is the total cost of the swap not just the price of the 4sp trans. Many are surprised to find the cost of a trans swap doubles once they consider a new torque converter, trans mount, drive shaft (or at least shorted and balanced), shifter,TV cable and I'm sure other items also. Most will need a transmission lift also to do the job.

Hope this can help.

 

I'm saving my pennies for a th200r4 myself.

'81 modified Vette with a th350 and a 3:54 rear gear. I'd like to be able to go 80 mph on the toll road and save some fuel as well.

 

As most have said the 200r4 is the easiest way to get OD. BUT, if you have a shop cut, weld and balance the drive shaft and get a diff yoke you can do a 700r4 swap that will give you OD, but also a super low 1st gear, that along with changing the cam from 2 degrees retarded to 4 or 6 degees advanced will give you a really fun 1st gear that can actually do a decent burnout. (Unlike my stock 79 L82 with the TH350 that refuses to do a burnout.)


200r4 ==easier and cheaper for sure.
700r4 ==more fun AND with the OD fuel economy savings

Like others have said, you're NOT going to get your money back in fuel economy savings; I take this to mean "you might as well get more fun factor out of it so you get your money's worth".



Adam

 

I've read about the 700r4 swaps, but here's what I know about the 2004r swap. If you now have a th350, your driveshaft yoke will work in the 200 or 700. Driveshaft from a vette with 400 or 350 will work for a 200. Original cross member will work with the 2004r, no mods other than unbolting the th350 mount bracket from it. Original rear trans mount will work fine. Original shifter will work ok, but you won't be able to manually pull it down to 1st. You can grind out the gate in the original shifter to get 1st. You can buy the shiftworks mod kit for your shifter, it comes with a new detent gate and indicator lense as well as cable mount bracket around $100. Along with your trans you will need dipstick and tube, correct speedo gear, some version of converter lock up, either as complicated or simple as you like ranging in price from $20 to $200. Trans will usually come with a torque converter, make sure it matches your engine performance. Stock L82, you will probably want 1800-2400 rpm stall. Additional trans cooler is a personal choice, not needed for normal or even spirited driving. You will also need to get an adapter bracket to hook the tv cable to the carb linkage, tv made easy is a good solution although there are less expensive solutions that work very well. I've actually hooked the tv cable in the same place the th350 kickdown cable was and it works but not ideal.

With a 355 gear, I cruise 80 mph at around 2200 rpm, 20mpg.

 

This is exactly what I found when I did my 200-4R. Moved on to a T56