there shouldnt be Much lag on this GT25 if any at all. this thing should spool fast. and Branden is right, there are ways to "correct" the lag of turbos: free up the intake and exhaust tract, port/gasket match the manifold and O2 housing/turbine elbow. the problem is that the American driving public wants a car that will get up and go when you step on the gas; instant torque. thats just one thing that is not a trait of a turbocharger, it will always have some degree of lag. it can be minimized by using a small AR ratio turbine housing and higher compression to placate the driving public. this is bad for us Tuners though. it puts a limit on how much boost we can safely run. And i dont know about you, but i want to run 93 octane all the time, which really will limite the amoutn of boost.
Anyhow, this problem sounds like its related to the o2 sensor feedback in the ECU. The ECU has two modes of operation for the O2 sensor: open loop and closed loop. Closed loop is what causes the blinky light on your AF guage to swing back and forth, the O2 voltages are cycling. This is when the ECU is using the O2 sensor for feedback and tries to gain a fuel mixture as close to Stoich. as possible for best fuel mileage. The other mode is closed loop, when it Stops cycling back and forth, and the O2 voltage is rather steady. This is when the ECU disregards the O2 sensor feedback, and then begins to run off of preprogrammed fuel maps inside the ECU.
Usually, the change over from Open and closed loop is related to 2 things: Throttle postion and RPM. For instance, on our DSM, anything more than like 50% throttle or somethign liek that, it goes open loop. and anything above 4000 rpm, it goes open loop, regardless of throttle position. So there may be a dead spot or a glitch when the ECU transitions from closed loop to open loop. its probably the reason why some MSP tend to run lean at partial throttle; because the Throttle isnt opend enough to cause the ECU to go open loop, and the ECU is still trying to gain a stoich mixture even though the turbo is creating postive manifold pressure. if this is the case, then you will have quite a bit of part throttle knock and get timing pull. Usually a safe A/F ratio in a turbo application is 12.5:1. Stoich is 14.7:1, or too lean for high load conditions. 11:1 is REally rich, and very safe for a turbo application, but its cooling the EGT's too much and doenst create as much cylinder pressure as it possibly could...so leaning it out a little would create a little more power, at the risk of incresing knock.
so, to sum it up: ECU and O2 sensor problems...i think
Branden feel free to step in
But I still beat him with 3 wheel drive
