I have a 1972 Cutlass Supreme (original owner) that has the following:

• Original 455 motor rebuilt by Jasper Engines that has less than 1500 miles on it. The engine has an Edlebrock aluminum 455 performer olds intake manifold,1.6 ratio Magnum Roller Rockers, High energy hydraulic lifters, 750 cfm 4 barrel summit carburetor ,6A MSD ignition, cam(but I do not know the size.) It doesn't sound like a large one.
• Original th400 3 speed transmission that has not been rebuilt in over 35 years. It has a minor leak due to seals that have to be replaced.
• Replaced 14 inch factory rims with 22 inch offset rims (car show purposes)
• Positive traction

Here's the issue. I was not getting the power I wanted on takeoff (with and without factory rims and tires) and my rear end had a roaring sound above 50 mph, so I had the differential rebuilt last week, and I went from the factory 2.73 gears to 3.73 gears. The wheel bearings were replaced as well . The roaring is gone, and I LOVE the power on takeoff. However, when I reach 55 to 60 mph, the power felt like it was gone. I also noticed excess heat coming from the engine/transmission after driving for approximately 10 minutes at 65 mph. The vehicle is not a daily driver, and I do not travel further than an hour and a half on occasions for car shows. I would like to cruise at 75 to 80 mph without the RPMs shooting through the roof. Other than the minor decrease in gas mileage, I would like to keep the 3.73 gears. Going 55 mph on a highway in a 72 Olds cutlass is hard for me to do (need for speed lol). I am asking the Oldsmobile experts for your advice on how I can accomplish restoring highway power with lower RPMs? I read many reviews on the 700r4 transmission, and I'm thinking this will help lower my RPMs at higher speeds without losing front end power. Experienced advice wanted.

 

Do you know what RPM the engine is turning at your highway speeds? My car with 3.23 rear gears and short 245-60-14 tires would turn around 3200 RPM at 65-70ish MPH, and I did that all the time, in 110º Arizona heat with the AC on, with no issues.

So my advice is don't worry about "RPM shooting through the roof" and enjoy the car. That's what folks did back in the '70s without any worries.

 

You might get more responses by moving this thread to something like the general decision, race forum or engines, this forum doesn't get a lot of traffic.

Not my era of expertise but gear ratios and RPM's are always a compromise, one will effect the other. Unless you swap in a over drive I can't see you getting happy. I would think with 22's it would be higher geared unless the wagon wheels overall are shorter in height compared to 14- 15 or 17inch tires.....Tedd

 

Thanks, Fun71 for your reply. I do not have a tach, so I cannot tell you the exact rpms. I can hear and feel the difference at 60 mph. I do not want to ruin the engine or transmission from running the rpms at a high level.

 

Hello, Ted. The 700rf transmission has an overdrive as well. Thanks for the advice on moving the thread. Now I need to figure out how.

 

As said previously, engines and transmissions didn't get ruined back in the 60s-70s due to driving on the highway.

 

In all honesty, a engine in good mechanical condition with a sufficient cooling system will run 3200-3500 rpm all day, everyday, with no problem. If it weren’t for modern overdrive transmissions we wouldn’t know any difference. We have been spoiled by today’s whisper quiet drivetrain.

Since you say the existing transmission is tired and leaking, now is the time to “fix” your concern, whatever that may be. The cheapest/easiest is to just rebuild the 400 and deal with the cruise RPM. If you decide you just can’t tolerate the engine speed on the highway, your choices are: regear the differential (you will loose that get up and go feeling) taller tires (unless your running really small tires now, probably won’t make much of a change) or swap in a overdrive.

The easiest swap is a 2004R. No trans crossmember work (which means your existing exhaust will fit) you just need to do some minor modifications to the parking brake cables, and a driveshaft.

The next easiest swap is a 700. The crossmember will need to be moved back, a driveshaft made, since the 700 was never offered in the BOP bellhousing bolt pattern, an adapter plate will be needed. In either case, the 2004R/700 trans needs a TV cable, and needs to be installed and adjusted correctly. And depending on your power goals, either of these transmissions will need to be built with some internal upgrades to live behind a 455.

The next option is the 4L80. A junkyard 4L80 in good condition with a shift kit will easily handle just about any stout 455 with ease. Unfortunately, it requires a computer to control it, it will require driveshaft/crossmember/etc modifications to make it all work.


The last option is the most expensive. Get your existing 400 rebuilt, and install a Gear vendor. It’s a add-on transmission that installs behind your existing 400, and provides a overdrive. You still need driveshaft modifications, and it requires some minor transmission tunnel clearance work. The advantage of the gear vendor is it’s stronger than the transmission in front of it. Unless you run it out of fluid, I’m your not going to tear it up.

 

This really isn’t true though. Highways used to be predominantly two lane and speed limits were generally lower.

I drove across country last year with 3.73s and 26” tall tires and I was nearly a road hazard in some stretches out west. 3000 RPM to do 60 mph isn’t great. Trying to pull 3700-4000 RPM to do the speed limit? Forget it. You’ve got to build everything else around it to maintain that engine speed for any length of time.

 

As I initially said, I have no issues driving my car on the interstate with 75 MPH posted speed limits, and I live "out west". But if a person wants to keep up with traffic on the highway and turn low RPM, then pick one of the options Matt posted and crack open the checkbook to make it happen.

 

1) you’re running 3.23s. I’m running 3.73s. 15% difference. You doing 75 is equivalent RPM to me running 65. Me running 75 is equivalent RPM to you running 87. That’s a big difference.

2) I did break out the pocketbook and am installing a Tremec Magnum. As I said...I’ve run this on a multi day road trip and see no reason whatsoever to run those RPMs. My DD Miata turns less RPM with a tiny 4-banger. No point in a big motor if you’re just going to wring it out in my opinion.

 

^^^THIS^^^

Your experience seems pretty normal changing from 2.73 to 3.73 gears. As others said you might consider an overdrive transmission (200-4R) or a Gear Vendors unit.

BTW, I will move the thread to the General Forum. It gets the most traffic.

 

A built 455 with 3.23s will be fun on takeoff and cruise at 3200 at 70. Good radiator, no issue.

Please get a tach running and make sure you have all three gears and get back to us with rpm at speed data to help more.

 

Thank you Matt69olds for your advice! I do have a brand new 3 row radiator that was installed a few months ago, so the cooling is pretty good. I like how the 2004r option. I will research all suggestions you mentioned.

 

@ Koda, I plan on getting a tach soon, so everyone can get a clearer understanding. Thanks

 

Update! My tach readings are 45mph -2000rpms, and 70 mph - 3200 rpm. I hope this helps your suggestions for me.