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not enough vacuum to shift new T350 transmission

23K views 11 replies 7 participants last post by  BuzzLOL 
#1 ·
I have a 37 Chrysler with a flat head 6 cyl motor, I just installed a standard fresh rebuilt turbo 350 transmission with an adapter kit from Milcap.
The problem im having is my flathead 6 dosent produce enough vacuum to shift the transmission properly...Im hoping some one has the magic answer...
 
#4 ·
Sorry, not going to work. The TH350 and TH400 use manifold vacuum as a sort of throttle position sensor. High vacuum means light throttle so the modulator allows an early upshift. Low vacuum means W.O.T., so the modulator delays upshift. A vacuum pump gives you high vacuum (and early upshift) all the time.

You have four options.

The first is to replace the TH350 with a trans that uses a throttle valve cable instead of vacuum, like a 700R4 or a 200-4R.

The second is to convert to a fully manual valve body and shift manually.

The third is to convert the vacuum modulator to a cable-operated modulator. This eliminates the need for manifold vacuum by mechanically operating the modulator from the throttle (essentially what the 700R4 does).



TH350 TH400 Cable Operated Modulator 350 400 | eBay

The fourth is to locate a vacuum regulator valve used on one of the Oldsmobile diesels. This is a valve that modulates vacuum signal in response to throttle position, essentially mimicking the manifold vacuum vs. throttle position curve that a gasoline engine would have. Olds used this to modulate the vacuum signal from the engine-driven vacuum pump to properly operate the TH350 in those cars, as you are trying to do.
 
#10 ·
. I don't think the problem is vacuum, but RPMs... the trans upshifts as RPMs increase... and downshifts for passing with low or no vacuum at WOT...


. Obviously, low or no vacuum is no problem to achieve... but your flathead may be lacking RPMs like a SBC does easily...


. May need lighter springs in the governor to achieve lower RPM shift points... and/or a low vacuum modulator as well...
.
 
#11 ·
. I don't think the problem is vacuum, but RPMs... the trans upshifts as RPMs increase... and downshifts for passing with low or no vacuum at WOT...

.
It's both RPMs and vacuum. That's why the TH350 has both a governor AND a vacuum modulator. The modulator uses vacuum as a throttle position sensor. Light throttle = high vacuum, so the modulator raises the line pressure for early upshifts. Heavy throttle = low vacuum, so the modulator delays the line pressure rise and thus delays upshifts. If you don't believe this, simply disconnect and plug the line to the vac modulator and drive the car. It won't upshift ever except at W.O.T. (when the governor RPM finally overrides the vac modulator). If the engine doesn't generate enough vacuum at light throttle, the modulator thinks the throttle is W.O.T. and delays upshifts.
 
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