Welcome to the Corvette Forums at the Corvette Action Center!

Which auto trans in 78 SA?



Can you folks tell me which automatic trans I have in my 1978 L48 Silver Anniversary and what do you recommend to improve the performance of this transmission? Which shift kit? What fluid? Etc?


It's a TH350. Save your nickels and leave it alone until you can do this: A lot of folks are replacing them with 700R4s, which are 4-spd. autos. One day, I'd kinda like to do that myself.



I'm actually going to have mine rebuilt and put back in. I don't take long enough drives where gas mileage is a concern to warrant the cost of a new 700R4 and modifications.

Don't power-flush your TH350. Lots of posts around regarding this issue. It can spell the death-knell for your old tranny.

Years ago I (yes, me!) installed a B&M shift kit into the TH350 of my '74 Camaro. The process was easy and I was pleased with the performance. It has been so long, I don't know any numbers or name of the kit. Heidi

Big Fish

The TH350's seem to get a bad rap, but they will easily handle whatever that L-48 engine can deliver. I would recommend doing an overhaul with new clutch packs, and install a Transgo shift kit. You can choose to stay with fully automatic or manual shifting. You could also install an adjustable vacuum modulator. Stick with the GM recommended fluid, and install a high performance filter, should be good to go! You might want to replace the torque converter with a mild stall speed unit. You could install the Transgo kit while the trans is in the car, just follow directions carefully.


I would steer clear of the TransGo shift kit. I have one in my old Dodge Cummins and it shifts a little too hard. If the car were used on the strip regularly, I would say go for it. Reminds me of the 62 fuelie my buddy had when we were in college. He replaced the four-speed with a B&M Hydrostick; it shifted so hard he got rid of the car! Naturally he wishes now that he had kept the car and put the four-speed back in, but who knew then?


In deciding on a shift kit, you need to remember two things. The kit adds pressure, which results in more heat to achieve those crisp shifts. Therefore, if you are not rebuilding the transmission, the seals that may be marginal will be prone to leakage. If you take the time to choose the proper torque convertor as a modification, you won't really need the shift kit.

Also, don't mix different transmission fluids. You have to remove the torque convertor to drain all the old fluid completely, which requires the removal of the tranny itself. If you can do this, I recommend Ford Type F. It has additives that firm up the shift, giving you the feel of a more performance oriented shift than the GM alternative.


Big Fish

I might be wrong here, but I don't believe the shift kit adds pressure, as pressure is a function of the pump. I believe the shift kit simply allows more volume of fluid to be passed through the valve body, resulting in faster shifting, also resulting in less heat due to less plate friction during "stock" shifting. I would agree that tired seals in the transmission would probably take a beating from faster shifting. The addition of a stall speed converter would generate more heat, but might also tend to soften up the shifting somewhat when you are not really on the throttle. If someone can advise me if I'm wrong I would like to know? :confused


Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors


MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!



Advertise with the Corvette Action Center!

Double Your Chances!


Top Bottom