Originally posted by SCT Guy
Having helped design and worked on the trans that's in your GT, probably working on it before most on this board had drivers licenses, and being the Jerry of the Jerry Mod, I think I'm qualified to pipe in here and add my 2 cents....
I dont' care who recommends what, TransGo=bad. Here is why.
Once upon a time, oh 10 years or so ago, a higher up at the Blue Oval mandated to us that we make the transmission shift out of manual low gear into 2nd even if you didn't move the shifter. Kicking and screaming we complied. Starting in the late 90's we were able to change this back due to early retirement packages.
Along comes TransGo trying to be hero. They are going to change a whole bunch of stuff in the valve body to prevent the trans shifing out of manual low. All the shift valves get replaced, the solenoid pressure regulator valve, the manual valve, etc. All of this is done in an attempt to prevent it from shifting out of manual low and a few things like that. They are trying to take the control away from the EEC and put it in the trans. If you want to do this, buy a manual valvebody and be done with it.
Here's the problem.
TransGo doesn't realize there are things called production tolerances and stack ups. About one in every 20 TransGo kits won't shift. I have talked to some people on this board that have had this problem. Why doesn't it shift? They cannot account for all the variances in the valve body when trying to do something mechanically that should be done electronically. This is bad, very, very bad. A 5% failure rate is out of control. How lucky do you feel?
Next issue with TransGo. They put a TV pressure blow off valve in the valve body right under the main regulator valve in the cover plate on the bottom of the trans. They claim this is to prevent TV pressure spikes which occur in the trans. I bet I have personally done the post morteums on 500 trans's and looked over a few thousand others. I have taken more pressure data than can be imagined. Guess what? I have NEVER seen an excessive TV pressure spike, ever. The main reason I think they did this is because again, they are trying to change something that should be changed electronically. They change the main regulator valve spring to have a "flatter" line pressure curve based on TV pressure. The problem with changing that spring, you make so much line pressure that you will explode the clutch cylinders. So they add this blow off valve to keep TV pressure to some set limit. Two issues here. One, this should be done electronically by asking for more pressure (there are reasons why this is bad, but I'll get to that later) and not trying to override the electronics. Two, if you ever take the pan off of a trans, even one that functions well, you'll see friction material in the pan, totally normal. What happens when one of these little pieces of material lodge in the blow off valve and have it bleeding off because it doesn't seat? I can answer that, you get your trans rebuilt.
While this may be enough to convince you, I'll give you one more.
Anyone out there with a transgo kit notice that when you are in 1st gear and mash it, about half way through low gear the car seems to really take off (this would be mainly on blower cars, not N/A cars), almost like a turbo kicking in? Ever wonder what this is? It's the freaking trans slipping from the shift kit.
There is a circuit in the valve body to feed the torque converter. Under certain conditions, high pressure demand, lower RPM (<3000) the production valve body will reduce the flow to the converter circuit to keep pressure to the clutches. TransGo gets rid of this nice feature. So you feed the converter circuit full bore all the time. While this is great if you have enough pump flow, but you don't until you really get the pump spinning. So what happens? The line pressure, hence the pressure to the clutches, drops and is not what it should be. In fact, in many cases it drops enough to make the forward clutch slip.
Do you know nothing in the valve body causes shudder? It's a breakdown of the oil/friction interface in the converter causing a stick/slip condition? How do you fix this in the valve body? You rape the converter clutch circuit to slam it on. That way when shudder does happen the friction is so fried you need a new friction disc. You can lock the converter faster electronically. Change your trans fluid ever 21k like you should and you'll never have shudder.
Still want a TransGo? If you think PI knows more than I do then go ahead and get one. I didn't see any of them sitting next to me in the early 90's when we were designing this.
In all honesty it's not that hard to do. You remove two springs and drill about 4-5 holes, that's it.
I've driven my car pretty hard sometimes (even before the shift kit) and never had any problems. The one thing that I've always done since day one is let off the throttle completely (or use VERY light throttle) when shifting into O/D. I also force the car to downshift on it's own first (with the throttle) on a downshift BEFORE I lock O/D out. Do you ever do either one of those?
Those can be fitted to a 4.6 motor i think? 
