Vac modulator advise
Thread Starter
Burning Brakes
Joined: Oct 2011
Posts: 956
Likes: 86
From: McDonough Georgia
Vac modulator advise
I have just changed oil/filter, and adjusted my TH 400 on 1974, LS4, 454.
The car runs fine except to me, the shifting seems a little late and sloppy.
I've been reading up on the adjustable vacuum modulators and would like some recommendations on these.
Apparently, there are red stripes, red/black stripes, purple stripes, and no stripes. They seem to range in price from $15.00 to $50.00.
Anyway, I would like to get one of the adjustable ones in order to simply firm up the shifts, make it shift a little sooner, and use the manual 1,2,3 occasionally.
The TCS is disconnected, and I don't race or anything like that. Just like to get on it a bit from time to time when the wife and I go for ice cream.
Any recommendations?
The car runs fine except to me, the shifting seems a little late and sloppy.
I've been reading up on the adjustable vacuum modulators and would like some recommendations on these.
Apparently, there are red stripes, red/black stripes, purple stripes, and no stripes. They seem to range in price from $15.00 to $50.00.
Anyway, I would like to get one of the adjustable ones in order to simply firm up the shifts, make it shift a little sooner, and use the manual 1,2,3 occasionally.
The TCS is disconnected, and I don't race or anything like that. Just like to get on it a bit from time to time when the wife and I go for ice cream.
Any recommendations?
Drifting
Joined: Aug 2019
Posts: 1,510
Likes: 498
Are you sure the Modulator is not adjustable now ? You can look inside them or stick a pocket screw driver inside the vacuum fitting if you feel the screw head give it a 1/2 turn counter clockwise
stripe is spring pressure inside the modulator , don't remember which is which but bigger camshaft like the lighter spring do to less vacuum available from the engine/camshaft combo
stripe is spring pressure inside the modulator , don't remember which is which but bigger camshaft like the lighter spring do to less vacuum available from the engine/camshaft combo
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
I don't believe a Modulator will affect shift firmness, only the time it shifts, sooner or later by vacuum.
To get firmer shifts you need to drop the tranny pan and replace the valve body. These change the fluid dynamics.
B & M has a shift kit that allows you to hold the trans in any gear at any RPM. (manual) and allows downshifting.
But the kit also allows normal shifting in "D".
These up-grade kits are firm, tire chirping, no hesitation shifts.
To get firmer shifts you need to drop the tranny pan and replace the valve body. These change the fluid dynamics.
B & M has a shift kit that allows you to hold the trans in any gear at any RPM. (manual) and allows downshifting.
But the kit also allows normal shifting in "D".
These up-grade kits are firm, tire chirping, no hesitation shifts.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
how many miles?
sounds like a worn tranny.
car shifting like usual when you got it?
a shift kit may help.
if the mod holds vacuum, should be right for your car.
Race Director
Joined: Jan 2007
Posts: 10,683
Likes: 696
From: Danville Illinois
2025 Corvette of the Year Finalist - Modified
2024 C3 of the Year Winner - Modified
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2019 C3 of Year Finalist (appearance mods)
We did a B&M shift kit 30 some years ago, well worth the money.
The kit gave you the options of heavy duty shifting, street/strip, off road or strip only it think. Gave nice crisp ***** on our tb400, also went with extra deep pan.
The kit gave you the options of heavy duty shifting, street/strip, off road or strip only it think. Gave nice crisp ***** on our tb400, also went with extra deep pan.
Thread Starter
Burning Brakes
Joined: Oct 2011
Posts: 956
Likes: 86
From: McDonough Georgia
The car has 90 k on it. All original. Faithfully maintained.
I just rebuilt the steering column and shifter in the car with new spring and neutral safety switch. As part of that process adjusted the shift so everything lines up and works smoothly. It does.
I'm pretty sure the modulator is stock, and wish I had replaced it when I had it in the air.....but hindsight being what it is. It has always been a little emission controlled shifty if you know what I mean.
I would like to try an adjustable, but now i'm not sure of the spring size, which way to turn the screw and such.
I just rebuilt the steering column and shifter in the car with new spring and neutral safety switch. As part of that process adjusted the shift so everything lines up and works smoothly. It does.
I'm pretty sure the modulator is stock, and wish I had replaced it when I had it in the air.....but hindsight being what it is. It has always been a little emission controlled shifty if you know what I mean.
I would like to try an adjustable, but now i'm not sure of the spring size, which way to turn the screw and such.
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If your car is for general use (not racing), just replace the modulator with one from your local car parts store. They work fine for normal use. Don't worry about the 'adjustability' issue.
If you need performance-correct shift points and red-line RPM's, ignore this advice and do research to find sources for precision stuff (including governor unit).
If you need performance-correct shift points and red-line RPM's, ignore this advice and do research to find sources for precision stuff (including governor unit).