Opaco

I bought a 1979 c3 corvette 2 months ago I’m new to it and just learning new things everyday

my question is does it have a fourth gear if so how do I make it switch to it cuz I can’t even go past 50 miles without going through high revs which also causes overheating, the shifter says PRN321 which got me wondering if it even has a fourth gear or is there something wrong with it,

any help is greatly appreciated thanks

corvgreg

No 4th gear(OD) in a 1979. In fact they went to a 3.55 rear gear to help acceleration but at the expense of freeway rpms.
Either swap in a common 3.08 rear or put in a 4 speed automatic with OD.

Opaco

thanks for your reply could I ask how do I even get started in that?

drwet

Search this forum. Plenty of information about transmission swaps. The popular ones are the 200-4R, the 700R4, and a number of 5-speed manuals like the Tremec TKO 500 and 600 or the new TKX.

Bikespace

What RPM are you spinning on the highway? My 79 L48 Auto could go all day without overheating, at least a highway speeds. Are you sure you are in 3rd gear (1:1)? 3 is D on your TH350.

Other quick things to check:
Timing, radiator cap, and make sure the lower radiator hose is not collapsing (original ones have a spring inside to keep them open when the water pump is sucking coolant from the radiator).

Opaco

thanks for your reply

i don’t exactly know what rpms I’m at since the gauge is not working properly but it’s definitely high which isn’t big of a problem until it starts overheating and yes I’m 100% sure I’m in 3rd

ill check on the lower radiator hose as you advised and update on any improvements

Opaco

thanks for the reply

what auto trans with 4 or 5 speed would you recommend I get for it since I’m not that experienced on them

kanvasman

Agree. Check all of that stuff before you go changing transmissions. Nothing wrong with changing ( I put a 200R4 in my 78) but after all that work you still might have an overheating issue if it is not properly diagnosed first. Welcome aboard.

Opaco

I agree with you, thanks for all the positivity this community really is nice

just in case tho what automatic transmission would you recommend for me to swap it with like at-least 4 speed

kanvasman

You will get lots of opinions. I went with the 200R4 because it has an overdrive (4th gear) and it is the same dimensions as the trans you have now, which means the original driveshaft fits, no shortening required. Can't recall if it is the same yoke or not, but the trans itself was a direct bolt in.

Opaco

looks about the best option the lesser the changes I have to make the happier I am, also if you can recall if I get one what else would I need to order to make it work?

SH-60B

Yes, it's the same yoke. Same crossmember too, just change the mount bracket.

SH-60B

There is an air dam under the front that helps direct air thru the radiator at highway speeds. The tend to get hot if that's missing.

Opaco

do you have a link for the mount I should get?

if so could you kindly provide it, thanks

Opaco

Hey thanks for the reply

the air dam is definitely there but it still manages to overheat despite that

Bikespace

Another common cause of overheating is timing. If you want to tackle that, too, pick up a digital timing light, like the Innova 5568, which will display RPM.

Perhaps you can rig it such that you can do a test drive while you can see it. If nothing else, it can ground-truth your tachometer, should you try to fix that.

Info kits for your car can be found at the GM Heritage website:
https://www.gm.com/heritage/archive/...formation-kits

And you can buy a copy of the original dealer invoice that will tell you how the car was delivered, and to what dealer. A lot may have changed since then (including the differential ratio), but it is a nifty piece of information.
https://www.alliedvaughn.com/service...ce-order-form/

kanvasman

One more overheating issue at highway speeds. If everything else checks out, including timing and radiator hoses, I had a 1992 300ZX that would gradually overheat on the highway. After much checking and going over things, it ended up that the radiator was partially blocked. Flow was fine driving around town but on the highway it just could not keep up. Replaced the radiator and problem solved. Maybe not a common issue, lots of smaller things to check out but something to consider. As for my 200R4 you will need a TVR cable to hook the trans to your carb setup. Think of it as a kick down cable sort of, At WOT it makes sure things are working together correctly. This is a critical adjustment, not difficult to do, but if not done correctly you will burn the trans up by the time you get around the block. I had my trans built here by a local guy who knew his way around it and he helped educate my on the TVR cable.
As for the mount, if I recall on my 78, the mount was the same but it was relocated. There was a bracket on my crossmember that had the mount attached to it for the original trans. . With the 200R4 I got rid of the bracket and the trans mount attached directly to the crossmember. That was the only difference. Dipstick tube is on the right side. I also added a cooler, bypassing the radiator. That is another debate here on the forum but for another day.

carriljc

A 200-4R sits on top of the crossmember with a regular tranny mount. If your 79 has a 2-hole crossmember it would be similar to what mine was. But really, you oughta just get an aftermarket crossmember that allows the exhaust to pass under. I would have been better buying an aftermarket crossmember after all the mods I have done to mine to make it removable and then to allow the exhaust to pass-under--- plenty strong but looks like crap. Here is what mine looks like right now with the 200-4R installed:

C3 4ME

If your car is overheating with a 3 speed trans, you need to fix that problem FIRST before you go replacing the transmission. These cars came from the factory with a 3 speed automatic and did not overheat. While adding a 4 speed automatic is not a bad idea, that will not be the resolution to your overheating issue.