Transmissions and DrivetrainNeed help with your trans? Problems with your axle?
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Are there still any aftermarket kits available to replace the vacuum modulator for when there isn't enough vacuum to correctly operate it?
What is the minimal amount of vacuum required to use the aftermarket adjustable vacuum modulators?
Finally, if a full manual valve-body is installed, is a vacuum modulator still required?
If you don't have a manual valve body, you still need the modulator to upshift the transmission.
The modulator needs up to 16 in Hg to operate. If your vaccum signal is too low, the transmission won't shift properly.
A properly operating modulator will start moving the plunger at 12-18 in Hg and be fully retracted before 20 in Hg. If your engine only makes 10 in Hg at an idle, the plunger will never move and the transmission won't upshift a part throttle. An adjustable modulator allows you to change the spring preload. Turning the adjuster out lowers the spring pressure allowing earlier shifts. Turning the screw in increases spring preload causing later shifts but it still needs a good vaccum source to operate.
The shifts vary depending on vehicle weight and diff gearing. From a dead stop, minimum throttle upshifts should occur at 6-19 mph for 1-2 and 14-30 mph for 2-3.
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If you don't have a manual valve body, you still need the modulator to upshift the transmission.
The modulator needs up to 16 in Hg to operate. If your vaccum signal is too low, the transmission won't shift properly.
A properly operating modulator will start moving the plunger at 12-18 in Hg and be fully retracted before 20 in Hg. If your engine only makes 10 in Hg at an idle, the plunger will never move and the transmission won't upshift a part throttle.
So, I would need to find an alternative way to operate the modulator or get a full manual valve-body...
Quote:
Originally Posted by Stephen 87 IROC
An adjustable modulator allows you to change the spring preload. Turning the adjuster out lowers the spring pressure allowing earlier shifts. Turning the screw in increases spring preload causing later shifts but it still needs a good vaccum source to operate.
The shifts vary depending on vehicle weight and diff gearing. From a dead stop, minimum throttle upshifts should occur at 6-19 mph for 1-2 and 14-30 mph for 2-3.
I know that much, but can an adjustable vacuum modulator compensate for an engine that only makes 10 in. HG at idle?
There were many Hi-Perfomance vehicles form GM in the late 1960's and early 1970's, that had TH400's with vacuum modulators that worked fine. Most engines had 112 - 114 degree lobe centerline camshafts, that still gave a little vacuum at idle. Remember, big camshafts start to pull good vacuum at around apx. 2,500 rpm. Try the black stripe or the adjustable vacuum modulator that was called for in the mid 1970's. (I don't have my parts book in front of me, so I am relying on faded memories, age will do thst) I have not had big camshafts cause me any big problems here when building TH305-400's years ago. Vacuum modualtors are very easy on the front pump, and sealing rings. Full line pressure all of the time is very hard on parts, causes haerd shifts, is hard on the driveline, etc.
I have a cam with a 110* lobe centerline.
IIRC I have one of the large, black can type canisters, and one that has a yellow stripe. I'm almost sure the yellow stripe one isn't adjustable, but I'll have to check the large black one.
So a full manual valvebody causes constant full line pressure?
I'm just trying to figure this problem out before I install this engine or transmission into my Camaro.
The big black canister type were not adjustable, or at least not easily. You will need an adjustable one. The closer the lobe centers, the less vacuum at an idle. Still by apx. 2,500 rpm, you will start pulling very good vacuum.
