Vacuum source for modulator on turbo with THM350

[Freddie's Buick]

I've got a hot air turbo with a THM-350. Can someone give me an alternate source of vacuum on the intake for my modulator?

 

[bluettype]

you're not going to get vacum when it's boosting, they do make a mechanical modulator. check raptor transmissions. i'm getting ready to do the swap but i went with a manual vb.

 

[Under Pressure]

You need to put in a 1 way valve inline and that's iffy at best. we had 1 car it worked on and 1 that didnt.

 

[Freddie's Buick]

You need to put in a 1 way valve inline and that's iffy at best. we had 1 car it worked on and 1 that didnt.

What is that??

 

[SSedan64]

One way check valve

It will open from vacuum, but boost pressure can't push it open the oposite direction.

 

[KevinB]

It's a hotair, right? Since the throttle body is before the turbo, isn't there vacuum there?

 

[1of1547]

You need to put in a 1 way valve inline and that's iffy at best. we had 1 car it worked on and 1 that didnt.

I had the same problem when I converted to a Turbo 350. Older Ford trucks are a good source for the valves. I finally just got fed up with trying to reinvent the wheel and went with a reverse manual.

 

[Freddie's Buick]

I had the same problem when I converted to a Turbo 350. Older Ford trucks are a good source for the valves. I finally just got fed up with trying to reinvent the wheel and went with a reverse manual.

This is all Chinese to me. Throw a dog a bone here. If it works....then mo info on this set-up if you can help.

By the way, my TV cable is too short now. Does anyone know if any GM's came with longer ones?

 

[Freddie's Buick]

It's a hotair, right? Since the throttle body is before the turbo, isn't there vacuum there?

You're right.....

 

[turbomatt]

cable modulator

Would this help?

http://www.turbobuick.com/forums/transmission-talk/247973-th-400-cable-operated-modulator.html

 

[1of1547]

This is all Chinese to me. Throw a dog a bone here. If it works....then mo info on this set-up if you can help.

By the way, my TV cable is too short now. Does anyone know if any GM's came with longer ones?

Sorry, Older Ford trucks have a one way valve for vacuum. I tried using a couple of the valves right in front my vacuum modulator in the transmission. This is the hard line that went from the intake manifold down to the transmission modulator. I had a lot of problems with the turbo blowing the rubber hose off of the modulator when the car went into boost. Even with a zip tie to hold it on. When I got everything working with the vacuum modulator the transmission shifted soft and often was a little unpredictable at what RPM that it would shift at. I finally got sick of messing with it and went to a manual valve body.

If you go with a manual valve body you don't have to worry about installing the vacuum line for the modulator or the passing gear kick down cable. You can get either a forward (PRN321) or reverse (PRN123)manual valve body. I've also heard that you can do a manual style modulator valve. But if memory serves me right a manual valve body and manual modulator valve were pretty much the same price.

 

[turbofish38]

Many years ago I ran the 350 in my GN. I used the vacuum modulator from a carb turbo application and I used a TV cable from a 78-81 vintage carb turbo.No check valve needed on that one. And of course those pieces have been long discontinued from the General. I custom taylored the weights and springs in the govenor so it would shift at 5300. I was all set for bracket racing anyway.

Strange that your TV cable wont work. Maybe one from a SBC truck application?

Edit: For a vacuum source I used to same line that supplied vacuum for my cruise and HVAC which is what must of us SFI guys also use as a source for the boost gauge. Aint there a port on the plenum?

 

[Freddie's Buick]

....Strange that your TV cable wont work. Maybe one from a SBC truck application?

Edit: For a vacuum source I used to same line that supplied vacuum for my cruise and HVAC which is what must of us SFI guys also use as a source for the boost gauge. Aint there a port on the plenum?

Hey what's up Eric. The OEM cable would work only that it's too short. I'll measure it later. I estimate it's about 18 to 20 inches short.

Hey, I do have several ports on the plenum by the carb base. Can I use those as a source of vacuum?