Diablozx9

What RPM should my 1981 Corvette auto be shifting at (what was the factory shift point)
for full throttle shifts ?

I ask because I really don't believe the 4000 rpm shifts its making now are normal.
Shouldn't it be closer to the factory indicated red line ?

Diablozx9

anybody with a 1980 - 1982 auto please ?

Big Block Dave

Is it shifting 4K as a part throttle shift, or is this full throttle.

Im guessing since this is a new issue, that it is part throttle shifting. I'm not a transmission expert, but I have broken quite a few. The th350 operates on vacuum....check the simplest things first, like the vacuum line going to the modulator for a breakage. If this is intact, perhaps your modulator is in need of replacement.

Diablozx9

new car and new concern.
It is shifting at 4k rpm at full throttle.
Is that normal (seems low to me) ?

I dont know where this vacuum line is.
Any hints ?

BTW - thanks for replying.

7T1vette

It may need a TV cable adjustment. Or, if that tranny has been rebuilt (or exchanged), the governor that came with it could be wrong for the car.

Big Block Dave

If its an 81 Im going to take a stab at it that there is a th350 in there. I think the kickdown cable adjustment only effects downshifting.

If someone swapped in a 700r4, then a tv cable adjustment would do the trick.

Considering that your motor probably peaks around 4k, Its probably appropriate for it to shift at that rpm at full throttle.

The vacuum modulator is on the passenger side of the trans tailshaft, and receives vacuum signal via plumbing coming from the intake manifold...usually right behind the carb and in front of the distributor.

7T1vette

I believe the TH-350 in the '81 had a TV cable rather than a vacuum modulator. If so, it affects much more than just initiating the WOT downshift.

Tim81

The 81 has a TH-350C with a vacuum module for controling shifts. It also has a kick down cable but no TV cable. Can't say for sure what the shift rpm should be but mine is around 4k.

Diablozx9

Thanks everyone.. If Tim81 also sees 4k rpm shifts I will assume that is fairly normal. The engine is not stock and has a hotter Cam, Intake, Exhaust. I also believe this is the original transmission but all I know for sure is that it is a three speed auto.

If I would like to raise the shift point, at full throttle, what would be the easiest way (other than manual shifting) ?

scottyp99

Oh, it's a brand new '81 Corvette? I didn't think they were making them anymore, ever since about......ohhhhh..........1981!!!!!:lol :

Ha! Just yankin' yer chain, that's all! One thing to remember here is that it's an old car, and the tach may not be representing the true rpm accurately. Your car may be shifting at higher than 4k rpm. These old C3 tachs are known for not being all that accurate. Just something to think about,


Keep the shiny side up!
Scott

scottyp99

This thread talks about raising the shift points on a th350.

http://forums.corvetteforum.com/c3-t...ft-points.html


Keep the shiny side up!
Scott

BKbroiler

The reason for my low shift point problem with a TH350 at WOT was the position of the detent cable bracket changed when I switched manifolds. As a baseline, since you are new to the car, you need to release the detent cable in the bracket by prying up the release with a screwdriver. Then hold the throttle wide open (engine off) and let the cable adjust to that position, then clamp the release tab down again. That will affect your WOT shifts, hopefully higher.

qwank

My '81 shifts over 5000 rpm. Not sure exactly where, but it's not anywhere near 4000. I'm in the middle of a manual trans swap, so I can't drive it and tell you for sure right now. But I would check the vacuum modulator first and the line going to it. the line comes off a vacuum fitting right behind the carb on the manifold itself. it should have 2 ports on it, one going to your headlights, and the other to the modulator on the right side of the engine. there is a short rubber line, and then its a steel line all the way to the right rear of the trans, then another short rubber line, then the modulator. Do a google search on how to adjust the modulator if you find the vacuum line is ok.

Diablozx9

Thanks all for replying... Lots to look into here.

Diablozx9

OK,, Just got more confused.
I drove the car again (spring so, cant drive every day) and the shift point is different
on the 1-2 shift (3600 rpm) and 2-3 shift (4600 rpm).

This is messed up,,

I think the down shift cable is adjusted correctly.
It has a CAM, would that be affecting vacuum enough to do this ?

The thing runs FAST when manually shifted later Help me correct.

damoroso

It's either the detent cable or the vacuum modulator itself, most likely the cable adjustment. The cam has nothing to do with it. There's a specifc procedure for adjusting the cable. To adjust the detent cable; disconnect the cable from the carb, depress and hold the metal lock tab, move the slider back through the fitting (away from the carburetor) until it bottoms out against the fitting. Release the lock tab, connect the cable to the carb. Open the carb fully (make sure the choke is OPEN) pulling the cable to the correct adjustment.

This is out of the 1981 service manual. If you don't have one, you should really, really consider getting one...

Diablozx9

Thanks. I will try again but, I did adjust the cable this weekend and I believe it is adjusted correctly. I will have to look at the vacuume modulator.
However, why would it shift at different RPMs for 1-2 and 2-3 shift ?

Diablozx9

anyone?