shift rpm 1981
Thread Starter
Advanced
Joined: Dec 2011
Posts: 89
Likes: 4
shift rpm 1981
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 ?
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 ?
Last edited by Diablozx9; Mar 25, 2012 at 06:42 PM. Reason: more specific
Burning Brakes
Joined: Jun 2010
Posts: 1,165
Likes: 93
From: Bellmore NY
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.
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.
Burning Brakes
Joined: Jun 2010
Posts: 1,165
Likes: 93
From: Bellmore NY
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.
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.
Burning Brakes
Joined: Aug 2006
Posts: 1,058
Likes: 27
From: Eastern North Carolina
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.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Advanced
Joined: Dec 2011
Posts: 89
Likes: 4
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) ?
If I would like to raise the shift point, at full throttle, what would be the easiest way (other than manual shifting) ?
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
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
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
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) ?
If I would like to raise the shift point, at full throttle, what would be the easiest way (other than manual shifting) ?
http://forums.corvetteforum.com/c3-t...ft-points.html
Keep the shiny side up!
Scott
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
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.
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
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.
Thread Starter
Advanced
Joined: Dec 2011
Posts: 89
Likes: 4
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.
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.
Burning Brakes
Joined: May 2008
Posts: 1,116
Likes: 5
From: Middleburg Florida
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...
This is out of the 1981 service manual. If you don't have one, you should really, really consider getting one...
Thread Starter
Advanced
Joined: Dec 2011
Posts: 89
Likes: 4
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 ?
However, why would it shift at different RPMs for 1-2 and 2-3 shift ?
