TOMR what size crossover are you using? Have you ever used a 2.5 or 3 inch to fix back pressure issues? Also what are you suing to measure the back pressure? Always great advice from TOMR. Thank you for the replys
I had a complex wastegate issue I just fixed. I'll get into that, but I never would have found the issue without data logging hot side pressure and looking at how that compared to dome pressure and dome pressure solenoid switching times.
I still have the back pressure issue. At 21 psi boost my data log shows my hot side was 42 psi and the dyno said about 1100 RWHP.
I run a 10 psi boost spring and apply dome pressure with CO2 to get target boost. I use the Holley HP internal controller, generally ramping dome CO2 to to get 22 psi dome CO2 which makes 26 psi boost and 55 psi hot side.
This is what I missed that might help other people. Part of the reason I missed this was I started to believe my engine load affected boost. Before I could log hot side pressure, I just believed my boost would wobble around and be a 3-4 psi lower in low because of engine load. I thought, based on what I read on forums, my 4.10 gear and 6500 stall just wasn't loading the engine, because on shift and toward the end boost would creep up to 26-27 psi when it was only 20 in all of low gear.
You know, you read that stuff a turbo needs a 3.08 gear often enough and start to believe it.
When I finally looked at my data, I saw the hot side pressure average (not pulses) was jumping all around. That made me look at dome, and the dome was jumping all around. That made me look at the solenoids, and the solenoids were charging and bleeding the dome. This was doing two things. My launch might hit at 10 psi or 18 psi, and my boost would wobble, build different in different gears, and drift a couple psi.
Turned out my exhaust pulses pushing on the gate valve rattled the diaphram, and that translated into very fast pressure pulses on the dome pressure sensor. The pulses were at the exhaust pitch, around 400 pulses per second. The Holley only randomly samples the sensor, because it is busy doing lots of tasks. If it sampled the sensor on a peak, it would think the dome was way too high. It would command a bleed and reduce CO2. Then it would read too low, and command more CO2. This would rapidly open and shut the waste gate, causing erratic hot side pressure.
Because of inertia in the turbo, the quick pulses would smooth out. Since the pulse rate changed with RPM in gears, things got more stable with the slow increase when in high. This made my gate stay closed better, although still chattering, in high.
Holley had recommended a dome sensor PID setting of 0 on D term. D is the response smoothing. I bumped it up to 30 and all of that went away. Now my boost is just within 1 psi of whatever I dial. I can dial 20 psi on the line, and it is 20. I can dial 10 psi on the line, 20 psi starting in .2 seconds, and it is right at 20 psi in .4 seconds and stable. Now I can set it for 21 psi after launch and it goes to 21 psi and sits there in low and high.
The data log shows steady average dome pressure and steady hot side average pressure now, with normal exhaust pulse peaks. I still have excessive pressure, but it is stable.
The odd thing about this is after fixing this, the car is smoother and faster. With 10 psi launch and 21 psi all down track, it ran .2 seconds and 4 MPH faster then with 26 psi down track and the low gear variation. The average power must be way up.
None of this speaks well for me. I should have done this a year ago, when I first switched to the turbo. It took me exactly one year to get off my chair.