OK, so Engines for me are not much trouble, but when it comes to transmissions and rear ends, I am fairly new. My dad left me a 79 a couple years ago when he passed. He put quite a bit of money into it. Its an L-48 with the 3 speed auto. The problem is, I do quite a bit of highway driving, and at 65mph (average speed limit here) I am pushing just over 3k rpm. I would simply just slow it down some, however, at 65, I am still almost getting run off the road by the local drivers doing 75+.

So I am curious to know what are my options for lowering my RPMs. The only thing I can really think of is swapping the transmission out for one with overdrive. Which, I am somewhat heasitant to do, because my dad had the Turbo 350 trans worked through, fully rebuilt and beefed up.

So my question is, are there any other options besides a transmission swap to lower my highway cruising RPMs?

If not, what kind of money are we talking about for a transmission swap, and what do you recommend?

 

3 things that will change your rpm at highway speed, tire height, trans gearing, and rear gearing. I suggest an overdrive trans. You can change the rear gearing, but will sacrifice performance for a better cruising gear.

I had a 4 speed, switched from a 3.08 rear to a 3.70 rear for more performance. My rpms were at 3100 at 60 in 4th gear with the 3.70 rear. I swapped the 4 speed trans for a T56 6 speed. Now my rpms are about 2200 in 5th gear and 1550 in 6th gear at 60. I'm not sure what the cost of an auto overdrive swap might be, maybe others can chime in.

 

I have a '79 too. Best mod I ever made was the 700R4. 2nd cousin rebuilt it for $700. Bowtieoverdrives has the TV cable needed. Now my '79 loves the hwy. Bonus points: first gear on the 700R4 is steeper than the th350 so your rear end will think is has 4.56 gears!

 

2004R would be another option, direct bolt in replacement for the 350

 

Your three options are tranny w/ overdrive, lower gears (3.08), or drive slower.
I'd pick the overdrive tranny myself.

 

If you don't really get into the acceleration "thing", just change the gearing in the rear end. That is much simpler than changing out the transmission and will [effectively] give you the same result with highway driving.

If you have a 3.55 rear gear, going to a 3.08 will drop the rpms to less than 2500 on the highway...very comfortable for you and the engine.

For a good, local repair shop, that's not a tough task.

 

I am starting to lean more and more towards a tranny swap. I am guessing a 200r4 swap is pretty strait forward, but can it handle 400+ HP? I know the 700r4 is a damn tough transmission, but I will have to do a search and see what is involved in that. Cost is also a big issue right now, but I am thinking if I can drop the cruising RPMs, I will get better fuel economy too.

 

If you can find a 200R4 that came from a later model turbo Buick or other higher performance rear wheel drive vehicle then you have a pretty strong version to start with. One from a base engine will not have the better internal pieces but will do fine behind an L-48 that is treated well. I'm doing that swap now and other than the cable mentioned above it is a pretty simple change and you will find creative ways to handle the shift indicator in the console. There are plenty of forum threads about the "how-to" about it. Just search "200r4" and you'll find all you need to know. I'm personally looking for the highway mileage boost for my '80.

 

The 700r4 is awesome with the super low 1st gear and a nice overdrive. The best part is you can use an 82 vette as a template .
The 82 vette's run a 2:78 rear just to give you an idea of how low the 1st gear is....

 

I like the idea of a 200r4 because it is much easier to put in. How long did it take to do a tranny swap for those of you who have done it?

 

I plan to install the 200-4R in my '73. There are a lot of aftermarket companies building these transmissions to handle much more than 400 HP so that shouldn't be a concern. BowTie Overdrives is one company.

The 200-4R has a smaller transition from 1st to 2nd which some people like better (2.74 1st to 1.57 2nd vs 3.06 1st to 1.62 2nd for the 700R4). The 700R4 has a much more aggressive 1st gear. The 200-4R has a slightly better overdrive (0.67 vs 0.70 for the 700R4).

Although I haven't found any numbers to go with the explanation, it seems the consensus is that the 200-4R is both lighter weight and consumes less power overall than the 700R4 which gets more of the available HP to the rear wheels. I know my TH400 is a power hog so I am looking forward to some improvement in this area.

Most of the experts say to shoot for an overall 1st rear ratio of 10:1 but not higher (Transmission 1st gear x rear axle ratio). The 700R4 exceeds the 10:1 by quite a bit with 3.36 rear gears so I would probably need to run 3.08s with it but that takes the engine out of the cams recommended rpm range at highway speeds. I can easily run 3.55s with the 200-4R and still turn 2200 RPM at 75 MPH. I could probably even run 3.70s if 10.14:1 1st gear overall isn't too much (not sure).

If I was just running around town, the 700R4 might be a little more fun but when throwing highway driving into the mix, it seems clear the 200-4R is a better choice for me.

DC

 

Well here is the kicker. I like that acceleration thing. I like to get into it once in a while. And the engine is putting out 400+ right now. So I wonder if the 700r4 would be a better choice. Also, I remember dad saying something about putting taller gears in for better mileage. Just gotta figure out what those gears are.

 

5 speed manual

 

once the 2004r is rebuilt with the upgraded parts they will handle 500 plus all day long. they are very easy to rebuild also, no special tools required and plenty of tech info.
my 2004R cruises at 1800-2100rpms at 65 mph with 27 inch rear tires and will do burnouts in the first three gears...gets 20-22 mpg if I keep my foot .... outofit'

 

Alaskaman, what kind of rear gears are you using? I have been tooling around with some gearing calculators, and I am starting to think my dad put 4.10 gears in this thing....

 

3.08

 

Also, I am still reasearching, but I was told that if I use a 700r4 from a 92ish or newer chevy 1500, I will not need a TV cable, because that is electronic on these?

sorry, I is are trans noobz

 

electronic will mean a puter or an adaption, more spendy, TV cable isnt an issue once you understand it...and very simple
if it is an issue, you can convert to a constant pressure , then all the cable does is set shift points

 

I have the 700R4 in my '79. Probably the best thing I have done to the car. First is a little deep though with 3:55 gears. But 2000 rpm at 65 mph all day is nice!

The swap is not hard. Just do your homework and make sure you have all the parts before you put the car in the air, and you should be able to get it done in a couple of weeks.

Now I'm going to swap my 700 out for a 5 spd stick because I really miss the manual trans.

 

Having a TV cable is not a problem, once it is set-up properly. And, for your car, either a 200-4R or 700R4 trans will do the job well, once they are built to handle the power you will be putting through it. There are several good suppliers that will sell you a complete package to convert from stock to their tranny, etc.

One thing you should know: the low gear with either of those transmissions will be really LOW, since you are marrying the O/D tranny with a 3.55 rear end. It will be great for the highway; but at WOT, it will only be in 1st gear for about 1-2 seconds.

I still think the easiest way for you to go is to swap out the rear gear to a 3.08. Just my .02....

Good luck!