I have a 1971 LS5 454 coupe with a factory 4 speed. It’s a numbers matching original car that I’m taking the factory drivetrain out of for safe keeping. I would like to install a transmission with more gears, as even equipped with the optional “highway” 3.08 rear, it does 3,200 rpm at 80 mph.

What choices do I have of manual transmissions that will fit the stock welded crossmember frame? (A 5 speed would be good, a 6 speed even better.) The transmission needs to be able to withstand the torque of a warmed over BB, and require no irreversible modifications to the car to install. I have read quite a few discussions about swapping in different 5 & 6 speed transmissions into C3s, but I’m not entirely sure which ones meet my requirement to keep the car so it can be converted back to stock configuration.

 

Get a Tremec TKO 5 speed, they're rated to handle 600 ft/lbs of torque in stock form. I just read an article about them in the new car craft magazine, it sounds like its what you're looking for.

 

I have done this swap on multiple C3 cars. The TKO 5 speed will fit in with the welded cross member. I have a 6 speed in mine now but no way will it fit with the welded in member. There are a number of "kits" that are available for the TKO. I have used SST many times and they have a good product and good customer service. Some others have brought other kits to me also and I have had OK results with them. They all use the TKO transmission. I would recommend the TKO 600 if you are putting power to it. What rear gear are you going to be using.

 

as even equipped with the optional “highway” 3.08 rear, it does 3,200 rpm at 80 mph.

I am thinking he has the 3.08?

 

Yes but is he going to change it. I did see he has a 3.08 but my question was is he keeping this setup.

 

I put a TKO500 5 speed in my Vette got it from Silver Sport Transmission ,if you give them your diff ratio they will supply a transmission to match.

Absolutly the best mod I have made to my Vette

 

If you are using a warmed over 454 I would consider the .64 5th gear. But before you decide use an online calculator to determine your 5th gear rpms at your typical cruising speeds and your configuration. My ZZ454 pulls fine at 1500 rpm and 50mph. Even 45 mph is not a problem. I have 3.55 gears and 27" tire height.

My previous engine was a bit hotter 454 and 5th gear was not really usable until over 60 mph and then the engine wasn't really turning high enough. By 65-70 it was ok. We do a lot of driving on two lane country and mountain roads so the milder ZZ454 cam and low end torque is a better match to the .64 5th gear.

 

That's the key question. I was hoping he saw the posts and would fill in the answer.
Yes, I should of phrased that a little better, sorry.

 

Thanks for the replies. Having looked around, the Tremec TKO 500 was what I was thinking. Obviously a six speed would be nice, but I have never come across one that would fit a welded crossmember C3 without surgery, which isn’t an option in this case.

I’m still in the rough sketch stage of figuring out what the drivetrain configuration will be. I have never been bothered by the 3.08 rear behind the LS5 454, as that setup already delivers enough torque to break loose the rear tires. I’m looking at installing an oval port aluminum head 454 or 502 Chevy crate engine, with headers and possibly fuel injection. I’ll obviously need an L-88 style hood, as the regular big block hood seriously limits intake options. A crate engine like that will have even greater low end torque than the LS5.

I plan to use the car for spirited street driving, occasional runs down the 1/4 mile and even possibly some track days. So, I would like it to be a good all-round performer. As a rule of thumb, with a high torque V8 I try to keep the cruise rpm between 1,500 and 2,000, and obviously downshift when I need more power. Most of the highways around me have 70 mph speed limits, making 80 mph about maximum you can cruise at. Spinning a 454+cid engine at 3,200 rpm while cruising is a bit excessive. The TKO 500 with the 0.64 5th gear would turn 2,050 rpm at 80. (I have been spoiled by my C7’s Tremec 6070 7 speed, which cruises at 1,650 at 80 with a 3.42 rear.)

Even just changing to the ‘71 standard 3.36 rear, the 5 speed with the 0.64 5th gear would then turn 2,240 rpm at 80. If I go to a lower rear ratio, it will only spin even faster, so I believe I’ll stay with the 3.08 for a workable compromise between low end grunt, and top end cruising. (These rpms are based on 245/60/15 tires, which likely won’t be the size I’ll end up running, but the numbers should still be close.) If I need to drive at 45, then 4th gear is 1,800 rpm which is fine. The switch to 5th would take place above 60 mph, as the rpms at that speed would be 2,200 in 4th and 1,535 in 5th.

Assuming a 5,500 rpm redline, with the TKO 500 and a 3.08 rear, it would reach 45 mph at 5,200 rpm in 1st, and then drop to 3,400 rpm shifting into 2nd. This is almost exactly jumping from max HP down to max torque, which is ideal. While some may say that is not enough torque multiplication in 1st gear, even with the BB & A/C the car only weighs 3,450 lbs. (I weighed it.) and it’s still enough torque to exceed the traction limits of any street tire that will fit in a stock ‘71 C3 wheel well. Any more torque will literally go up in smoke, and just make the car more squirrelly and difficult to control.

I have owned the car since 1986, and it’s definitely a memory mobile. When I bought it, the national 55 mph speed limit was still in effect, so engine speed at 80 mph was not an issue. Also, a ‘71 numbers matching LS5 4 speed C3 with leather & A/C was’t the gem it is today. The C4 had recently debuted, and all C3s were quite out of style. I bought it based on the performance, not looks, paying $10,000 with 35K miles on it. On top of normal maintenance, in the early ‘90s I rebuilt the suspension using all urethane bushings, and lowered the car 1” front and rear. For a long time, it was about the best handling car you could get with a big V8.

Built in December of 1970, the car is now closing in on 50 years old, and has some birdcage rust, so it’s time to give it a full body-off overhaul, putting it back together close to original configuration, other than some non-permanent updates and upgrades. When I eventually get too old for racing around. I’ll put the original drivetrain back and have a nice collector car. In the meantime it will be a blast to drive. There is something very visceral about a C3, big block, stick shift Corvette, that makes for a quite different exhilarating experience than you get from driving a modern Corvette.

 

Sounds like you have a good plan there. One thing I would caution on the 3.08 is not the ratio but the condition of it. If it is the original rear and you plan on "spirited" or a run or two down the strip it may come apart. These are not known in stock configuration to taje a lot of shock load or abuse with the kind of power you are going to be running. I lunched one with only about 450 HP and street tires on the strip. It was a stock rebuild. Some are lasting behind some big motors but I think they are driven easily. Also half shafts and ujoints need to be in good shape. The BB will find the weak link. So build accordingly.

 

Yes, I‘m aware that serious drag racers often put solid rear axles in C3s due to the independent rear being a weak link. Surprisingly, this car still has the original posi rear and trailing arm wheel bearings, with the only maintenance being changing the diff fluid and U-joints. I’m not counting on these parts lasting any longer, and will rebuild them, but it is an amazing testament to how rough and rugged the original components can be. Not everyone is as fortunate though. (The rear drivetrain is just one in a long list of reasons that this car is due for an overhaul.)

I think pulling the factory rear, and storing it along with the rest of the original drivetrain, then building an equivalent replacement with beefed up components is a good idea. This way, I get a stronger rear drivetrain that, if it does happen to fail, at least I haven’t blown up correct original parts.

With sturdily built half shafts, driveshaft and a rear differential, it should function reliably, with the limit on tire size acting as a safety valve. It’s when you start hooking up with slicks on hard launches that the failure rate of driveline components goes way up. This is also why I’m sticking with oval port heads, and not trying to build an all out monster motor. When I go to put the car back to original condition, I don’t want to find the frame is all cracked and twisted. 500 HP and 500 lbs ft of torque are plenty of power for having loads of fun, yet stay within reasonable limits of what the chassis, a TKO 500 and heavy duty driveline parts can handle.

Configured like this, it should easily deliver far more performance and reliability than most cars its age, and even perform respectably in comparison to modern cars.

Of course I’ll be back with updates and pictures as this project unfolds, and I’ll probably be looking for more advice on certain decisions. At least now I know what transmission I’ll be using. Thanks for everyone’s input.

 

Stock differentials ranged from outright junk to decent performing units, different years had different issues. Biggest factor is usage and power. Building an engine with more power then the drivetrain was designed for and then abusing the car will break parts every time.

Some things to consider, times are changing. Getting 5 cut USA made gears sets is pretty much gone now. Getting the tried and true HP diff parts will also be an issue now. There are only a few companies offering 10 bolt gear sets for the 63-79's and only one USA made company. Vendors rebox from those companies, for those that didn't know that. Those that pay attention can see some guys building their own HP diff setups, some companies came out with newer kits the past year or so as well. Bottom line is the price tag will be higher than ever and real good parts may be going away. A good example are the good Tom's 10-17 x30 spline heat treated spiders. He ran out years ago, the manufacturer stopped making them and while some are still around the real deal one he used for 40 years are gone. I was able to stock up on those when things started to change years ago and still have some so those looking to build a strong 10 or 12 bolt IRS should really do their homework today.

Good luck

 

Did the higher horsepower (i.e. L88) Corvette rear ends have anything different done to them internally than the run-of-the-mill small block? I always thought they were all pretty much the same center section (but I'm not 100% sure on that), maybe somebody can shed some light on that? If they were all the same then I suppose it would be an issue of poor assembly and/or poor batch of parts? Thanks for any help on that subject.

 

Agree. Blueprint/rebuild of the diff is good insurance.

 

They were said to have shot peened cases but other then that I doubt it. I rebuilt one of the original (5) - 63 Grand Sport Diff's and the original posi case was long gone from abusive use. Today a properly polished, tuned, and cryo'd case will hold up well, but anything will break given enough abuse. I did see oil coolers on the Grand Sport and also 63 Z06 I built, the Grand Sport also had a deflector installed inside. They were all 17 spline axle setups but the axles were hardened and a good part.