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Old 09-21-2009, 01:52 PM
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Blower transmission shifts soft at low RPM's

I'm helping a buddy with a new Weiand 671 blower on a 350 sbc and had some automatic transmission questions. He has a B&M TH350 with a BDS "transmission pressure diversion valve" installed inline before the vacuum modulator. Under 3/4 throttle and no boost, shifting 1-2 manually and the transmission shifts sluggishly soft. Under full throttle and with boost increasing, shifting 1-2 manually and the transmission shifts quick and chirps tires. We isolated the problem as an issue with boost or the BDS "pressure diversion valve". The BDS valve flows vacuum to the transmission vacuum modulator at all times, but once it senses boost the BDS valve blocks the vacuum signal and the vacuum modualtor increases line pressure accordingly. Because my buddy wants firm shifting all the time, regardless of boost, we thought of several to ways to alleviate the issue.

1) Install smaller blower pulleys so boost comes in sooner at a lower RPM
2) Delete the BDS "diversion valve" and get vacuum from above the blower and below the carbs
3) Install a full manual valve body...last choice as this is a street car

Any ideas as we are open all suggestions????
Thank you!!
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Old 09-21-2009, 05:31 PM
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is there any adjustment on the diversion valve? it seems like its allowing to much vacuum. if that be the case, and its adjustable, try taking away some vacuum at idle.
if not, then maybe try turning in the modulator a couple turns. the shifts will be more aggressive than they were already @ WOT, but theyll firm up the lighter throttle shifts.
how much vacuum is the modulator getting where the hose goes into the modulator? the thought of having the wrong modulator could be a possibility.
also some spring changes on the governor with the modulator adjustment or change could be all that you need.
if anything, try running off the full vacuum port on the carb and see how it does. at least youll know.
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Old 09-21-2009, 06:29 PM
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Quote:
Originally Posted by tnsmith10
is there any adjustment on the diversion valve? it seems like its allowing to much vacuum. if that be the case, and its adjustable, try taking away some vacuum at idle.
if not, then maybe try turning in the modulator a couple turns. the shifts will be more aggressive than they were already @ WOT, but theyll firm up the lighter throttle shifts.
how much vacuum is the modulator getting where the hose goes into the modulator? the thought of having the wrong modulator could be a possibility.
also some spring changes on the governor with the modulator adjustment or change could be all that you need.
if anything, try running off the full vacuum port on the carb and see how it does. at least youll know.
thanks tnsmith10 !!
Unfortunately the BDS valve is non-adjustable, and we tried two new modulators with no luck. One modulator had the red stripe and the other was for big cams with low vacuum. So we left the "big cam" modulator in there.
I also spoke with a local "hot rodder" that suggested selling the BDS valve and run a vacuum line to a port under the carbs & above the blower. Then turn the screw in the vacuum modulaotr all the way in. Anyone try this???
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Old 09-21-2009, 07:41 PM
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Quote:
Originally Posted by tubbedz
thanks tnsmith10 !!
Unfortunately the BDS valve is non-adjustable, and we tried two new modulators with no luck. One modulator had the red stripe and the other was for big cams with low vacuum. So we left the "big cam" modulator in there.
I also spoke with a local "hot rodder" that suggested selling the BDS valve and run a vacuum line to a port under the carbs & above the blower. Then turn the screw in the vacuum modulaotr all the way in. Anyone try this???

that is the way I have run the vacuum in my super charged applications.
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Old 09-22-2009, 12:55 AM
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Quote:
Originally Posted by Crosley
that is the way I have run the vacuum in my super charged applications.
Thanks Crosley!!

We'll try it this weekend, as this is easy to do and doesn't cost anything. We'll report back with the results.
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Old 09-22-2009, 10:33 AM
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I know this doesn't solve your particular problem, but just as a point of reference, I have the BDS PDV on my 6-71-huffed SBC (1:1 drive) and TCI Streetfighter TH350, and mine shifts exactly as expected both in timing and firmness of shifts. I am using the modulator that came on the tranny from TCI.
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Old 09-22-2009, 11:06 AM
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Quote:
Originally Posted by XNTRCI-T
I know this doesn't solve your particular problem, but just as a point of reference, I have the BDS PDV on my 6-71-huffed SBC (1:1 drive) and TCI Streetfighter TH350, and mine shifts exactly as expected both in timing and firmness of shifts. I am using the modulator that came on the tranny from TCI.
Thanks XNTRCI-T !!
We were thinking along the same line and maybe try smaller pulleys so the boost will kick in sooner. Which would hopefully "trick" the BDS valve to cut the vacuum to the trans sooner, so that pressure in the trans will rise faster. Out of curiosity what rpm does your boost start at??
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Old 09-22-2009, 11:29 AM
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Quote:
Originally Posted by tubbedz
...what rpm does your boost start at??
Well, I'm afraid I can't answer that! I haven't really paid much attention to my boost gauge. I have only floored the loud pedal a couple of times, and I was so busy keeping the car pointed in the right direction, I didn't think to look at the boost gauge or tach! I think the only way I'm going to be able to develop a boost profile is to strap my hot rod to a chassis dyno. Here it is...

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Old 09-25-2009, 04:50 AM
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Well......we got in contact with BDS to get a definite answer. According to BDS, the PDV will only send a 0=vacuum signal to the transmission modulator when there is excessive boost. So accelerating from a stop and shifting before boost builds will not produce a snappy, quick shift. We will always experience a sluggish, slow shift unless there is boost.

So this weekend we will remove the BDS PDV and connect a vacuum tube from the modulator to the vacuum nipple below the carburetor and above the blower. If this does not work, we will ultimately have to install smaller pulleys so that boost will build up sooner.

Wish me lotsa luck, I hope I can solve my buddies' dilema.
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Old 09-25-2009, 07:03 PM
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Hmmm, I don't understand that response from BDS. When shifting, my tranny gives me a swift kick in the pants at any throttle setting off idle, and chirps the tires at anything above about 1/4. I'll have to put a vacuum gauge on the line to the modulator to see what it's doing.

I'm thinking your problem is not the PDV, but the tranny... either the modulator or the shift kit.

You're going to find with the blower that there is always high vacuum below the carb. The blower is always sucking more air than atmospheric pressure, which is what a normally-aspirated intake would see. So for any given throttle setting, there will always be more vacuum under the carb with a blower than without.

CAVEAT... I'm writing this after my second Jack Daniel's, so it may not make sense. I'll edit tomorrow as necessary...

Last edited by XNTRCI-T; 09-25-2009 at 07:10 PM.
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Old 09-28-2009, 06:37 PM
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............I'm thinking your problem is not the PDV, but the tranny... either the modulator or the shift kit.

The modulator is new and we replaced it twice to rule it out. We first tried a stock modulator then a "red stripe" modulator then finally a "red/black" stripe modulator, with no change. The transmission came out of a working vehicle with a new Transgo 1&2 shift reprogramming kit. It has red Alto clutches, hardened drums, hardened shaft, and 36-element sprag. The fluid is perfectly red and not burnt. Without the blower installed, the TH350 shifts firm and fierce all the time with long burnouts in between gears.

This weekend we removed the PDV valve and ran the modulator hose below the carb & above the blower. Same outcome....soft shifting. That night we went to a HOT ROD "cruise night" on Saturday and it was packed with over 150 rods. We talked to everyone with a blower that had an automatic transmission and retained all it's automatic features. Guess what......all the "rodders" with "blown" cars told us the same thing. Their "blown" cars shift soft at part throttle as the modulator sees vacuum but as soon as boost kicks in their shifts get firm.

We gave up "chasing out tail" trying to figure this out. If anyone has any other suggestions, we will just go full manual and install a "full manual" valve body........case closed. THe BDS PDV will be on Ebay.
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Old 09-28-2009, 08:39 PM
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I no just about nothing about trans but I do have a blown small block Chevy and a turbo 400, what you just stated is true for me too.

Now heres a question, trans shift harder when there is no vacuum present right? I installed a shift kit once in a turbo 350 and it came with a plug you could install instead of using a vacuum modulator. Is that an option for you? This would mean hard shifts all the time as fluid pressure would be at max all the time, but maybe that would be hard on the trans?

Ok guys please comment, like I said I have no real idea what im talking about lol

Duke
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Old 09-29-2009, 12:26 AM
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Found a fix !!!!!!!

It's a mechanical transmission modulator !!!! The turbo buick guys use it!!!

http://www.turbobuick.com/forums/tra...d-i-think.html

Problem sovled, firm shifting all the time and we can retain all the automatic features in "D" !!

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Old 09-29-2009, 09:45 PM
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They are moving the site. So what is the part number and where can I get one??
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Old 09-30-2009, 01:56 AM
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Quote:
Originally Posted by bentwings
They are moving the site. So what is the part number and where can I get one??
I found two on-line transmission shops that have the mechanical modulator well stocked. You will also need to purchase a 700R4 TV cable to actuate the modulator which is linked to your carburator linkage. Similar to a kickdown cable.

http://www.transmissioncenter.net/35...erformance.htm
"#4X. 350 / 400 Cable Operated Modulator, no vacuum needed. This works well for superchargers and on diesel motors without a vacuum regulator. This modulator uses a 700R4 throttle valve cable." - PATC transmissions

AND

http://www.transmissionpartsusa.com/...=609-00000040M
"40M TH400 transmission mechanical modulator" - Transmission Parts USA
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