I've been sorting out the various bugs on my single turbo kit for a while now and have gotten it to the point that it's pulling pretty hard and is reasonably quick given the lack of dyno time (tuning with a wideband on the street).

I'm using a T78, which, according to many, is inherently laggy and one needs to use the shifter in order to get the most out of this turbo on the street. I had mine upgraded using what was supposed to be, a more efficient compressor that would produce more cfm per psi of boost AND less lag. I'm not so sure a LARGER compressor could produce less lag however.

Thing is, I'm not getting significant amounts of boost before 4500rpm. Granted, I have a stock motor, stock ports, but this thing really does not have great response. If I'm in say third gear, and floor it at 2K rpm, I'll get 2psi of boost, which climbs slowly to 4-5psi by 4k and then WHAM! it takes off and shoots the car forward. The car doesn't FEEL any slower below 4500rpm than the stock sequentials that I was running previous to the T78. Rev matching and downshifting from say 5th to third and flooring it produces pretty instant response however.

ASSUMING that the stock GReddy spring is 10psi, I noticed that the adjustment screw is ALL the way out. I'm using an AVC-R to help boost. Playing with the duty cycles helped a bit, but did not make a huge difference. My question is this; does using the adjustment screw only affect peak boost and not response? I want to run 14-16psi daily on pump. Should I adjust the screw in a bit or keep it adjusted all the way out and let the AVC-R do it's thing? I am wondering if the "soft" adjustment is allowing the wastegate to open earlier than it should, despite the AVC-R. Any help appreciated.
Michel

 

Oh, one other question...the turbine side is the stock GReddy turbine. GReddy has several rear exhaust housings to choose from. Would going to a smaller rear housing help increase my spool-up times? Also, would such a modification requre machining to the turbine housing or is it a direct bolt-in?
Michel

 

With the boost controller off (running on the wastegate alone) what boost are you seeing?

 

Haven't tried that one yet :-) I'll do it today. I did screw the adjustment in yesterday. I didn't really notice any difference. I know that the previous owner ran 15psi using a profec B.
Michel

 

Michel,

It will at least tell you something about the wastegate...running on it alone.

Tuning a stock port/block T78 today...Profec B equipped. I'll let you know what I see on that wastegate.

Tony

 

Ok, with the boost controller off, I'm seeing .53kg/cm2 or about 7psi. So...the stock spring in a T78 is a 7 to 15psi spring. Should I just wind this thing all the way in? I guess that would be the next step. I can't see there being any difference between adjusting the 7psi spring in or swapping in a 15psi base. Other than I'll be able to run more than 15psi later on....
Michel

 

So I wind the adjustment screw almost all the way in..with the boost controller off I'm seeing .65kg/cm2. Not much of an increase. Sign of a bad wastegate or just too weak of a spring? With the boost controller on, I can easily see well over 1.0kg/cm2 depending on adjustment. Is the weak spring increasing the turbo's lag?
Michel