[gearmesh, inc.]

An electronic shift kit can only go so far. It can't give you any more maximum line pressure than what the factory tune can give you, but it can "inflate" the curve across the midrange up to that point. This has the effect of all your part throttle shifts stiffening up noticeably, but not much change in the feel of your WOT shifts.

When you put a physical shift kit in the valvebody, you get into stiffening up the accumulators with stronger springs and/or travel limiters to give you a firmer shift quality, even at WOT. A handheld tuner or program can't do this. In addition to stiffer accumulator springs and spacers, holes that feed various clutches are opened up some to get a quicker shift engagement, too. In high horsepower applications, I have personally even gone one step beyond the HD-2 Transgo kit instructions by blocking off the accumulators alltogether when used with a higher stall converter. I currently have a customer with a Transgo AOD-HP shift kit in his old style AOD with blocked accumulators and good internals holding 600-650 horse at the flywheel. This is about the limit on pump gas and a production block anyway, and any more horsepower beyond that needs to step up to a Lentech valvebody.

Line pressure can be increased by changing out your EPC solenoid in your 4R70W trans to one from a 5.4 truck. The car EPC solenoid will only take your line pressure up to about 190-200 psi in drive, and the truck one will give you to about 220 psi of clutch and band holding power. I have found that some of the ~ 2000 and up Mustangs have the 5.4 EPC solenoid in them from the factory. If you have a tan or yellow connector one, it is the 5.4 version.

I, like MonteC, am suspect of some of the hole sizes in the separator plate that Jerry's mod calls for. I have customers with the Transgo HD2 shift kit and they are very happy with the way their transmissions shift. Transgo even makes a kick ass shift kit for the GM 4L60-E transmission, too. I have a customer holding 650 horsepower with this shift kit and upgraded internal hard parts. Trust me, Transgo shift kits can hold the power of anything pump gas powered that you can throw at them, considering you upgrade some internal parts in the rotating assembly to support your horsepower level.