[Guest]

Okay...It all started with just a cam swap, now my edge 3000 is on the way with a transgo kit, now Iam reading about the line pressure vacuum control. For my street application is it needed? I will have the trans out when I do the t/c and transgo so it would be a perfect time to do it.
My setup will be XE230/236 cam, pocket ported heads, AS&M headers w/exports, borla exhaust, 3.73 gears, edge 3000 2.45 STR, Herter tune with 6000 shift points. Problem is all these mods are adding up on my budget cam swap! One mod always leads to another!! Will my trans last longer if I spend another $90.00 on the vacuum mod?

Oh buy the way... I am not optimizing this car for drag strip usage, Yes I should have went with a smaller cam if daily driver was the goal but its the cam I got so its going in. I am willing to give up some 60ft and ET so thats why the lower shift points and stall rpm.

 

[Guest]

Yes, do the vac mod. My stock GM SRTA trannys has the vac mod and the shift kit and has survived the brute force of the 383. Spend the $90 and save your tranny....er... make it last longer.

 

[Guest]

Where did you get the modulator for $90?
Mike

 

[Guest]

I was looking at the one On the Teamtripp site. Unless that is something different.... I was assuming that is the one?

 

[Guest]

$90 seems about right. I might have paid a bit more.. $100 but its in the ballpark.

 

[Guest]

Just make sure who ever does it knows what they are doing. I have my trans done and I had nothing but problems. I had to find another trans that did not have the valve body played with. I hear its the way to go, but I got burned.

 

[Guest]

Go to http://www.700r4l60e.com/store/customer/product.php?productid=160&cat=52&page=1 this should get you what you want.

 

[Guest]

Can anybody explain how vacuum line pressure modulation works? I know that lower vacuum -> higher pressure and that an upshift would occur after vacuum increases a certain amount....But I don't understand how the tranny shifts without knowing throttle position.

 

[Guest]

Kevin.. the PCM still controls the shift points and only the shift points after the vac mod (speed vs TPS table). The TRANNY no longer uses the PCM to rely on line pressure.

 

[Guest]

Ok, that makes sense.

Now how does an old school vacuum modulated tranny, like the TH400, shift itself?

 

[Guest]

Why does it matter? On the older non-electronic trannies it is a PITA to fine tune shift points, leaving control of those to the pcm is a very good thing not only for speed but drivability as well.

 

[Guest]

Originally posted by 96capriceMGR:
Why does it matter?

Oh I see, because you don't know the answer, it must not matter.

Well, I'd like to learn, and I suspect at least one person on this board knows how it works.

Maybe it makes sense...if there is high vacuum, it'll upshift early because the governor pressure will easily overcome [what used to be TV pressure]. Then if you dip more into the throttle, the vacuum goes down, and it takes a higher governor speed to overcome [what used to be TV pressure]. Maybe that's all it is.

 

[Guest]

It is not that I don't know the answer I have something of an understanding. My answer was because many people who would ask that question in a thread like this are then going to look to making their tranny shift like that, as I already covered that would be a bad thing. People are scared of the electronics, opti strange WP etc even though they are good things. The vacuum modulator is a rare case where going back the the old way of doing things has benifit.

Here is a page explaining how a tranny works, better than me trying to explain it.

 

[Guest]

Originally posted by Kevin Moore:
Now how does an old school vacuum modulated tranny, like the TH400, shift itself?

There is a TV cable that goes to the throttle body. You can use a 93 f-body tb to do that.

IMO, a manual valve body is a better way to go for strength reasons.

 

[Guest]

Originally posted by 96capriceMGR:
The vacuum modulator is a rare case where going back the the old way of doing things has benifit.

Not all the tranny builders (I'm talking about reputable ones) agree on that. There is also a risk of the vacuum line popping off or welding itself shut on a header.

 

[Guest]

Originally posted by Ram Air 9C1:
There is a TV cable that goes to the throttle body. You can use a 93 f-body tb to do that.

IMO, a manual valve body is a better way to go for strength reasons.

I thought the TH400 didn't have a TV cable. The TH350 does, 700R4/4L60, 2004R, and others do. But I thought the 400 was JUST vacuum. I think I nailed it in my little spiel above though.

 

[Guest]

Umm, TV cable and what the original TH400 and 350s had are different things. Offhand the 400 had the vacuum modulator and a electrical kickdown switch at the throttle linkage. The 350 had the vacuum modulator and a mechanical kickdown lever hooked to the throttle linkage. Both worked together to firm up the shift and downshift the trans into a lower gear under full throttle. The 400 vacuum modulator back then was adjustable whereas the 350 wasnt.
This was what I remember up til the 70s when it may have changed.

 

[Guest]

PCM controled pressure: The PCM uses MAF sensor, Throttle Position Sensor, MAP sensor and other factors to "calculate" how much load it "thinks" the engine is under. It then uses that "calculated" load to commande Pressure solenoid Duty Cycle. The Duty Cycle of a solenoid dictates how much fluid can flow through it, therby adjusting line pressure (there is more to it than that, but you get the point).

Modulator controlled pressure: Manifold Vacuum is a very good indicator of engine vacuum. The higher the vacuum, the less load and of course, the lower the vacuum, the more load. The Modulator regulates transmission line pressure based solely on engine vacuum (load) and repsonse is instant. As soon as you go WOT, manifold vacuum drops to zero, making line pressure go to max.

Which is better? Hard to say. Its a good mod to do, but for daily driven vehicles, I don't usually use them unless requested or I feel the need for it.

 

[Guest]

As soon as you start getting into the throttle the vacuum drops, and the same as when you let off of the throttle, vacuum rises. This relates in terms of line pressure the same way, instant. It is "not" moving backward, it is that there are "proven' ways hard to beat. Yes, it would be much easier to do it the factory way, but until someone "matches" the vacuum modulator in its results "electronicly", there will still be the need to use the "modulator".

 

[Guest]

I've been watching this thread and want to know,with all the opinions here is it an improvement or not ?
I'm not looking for the car to be a race car just better than factory with a spirited driver behind the wheel.