Phish806

iTrader: (5)

Now the setup i have is a lq4, ls3 topend,cam and a s475 with a 83 trim turbine. I had back pressure issues with the previous turbo i was running (Comp 8079...imagine that) so i swapped it out to the S475. I understand that the 83 is a little small for a 6.0 still but i am not looking for high boost or numbers. After all its a stock Lq4 bottom end. The most It would ever see is 15 psi.

Moving on, i got the turbo installed and logged back pressure ad its still high. were talking 3:1 and at sometimes spiking 4:1 on 5 psi. I tried removing the exhaust thinking its a restriction and just running the downpipe. It made little differnce. Maybe dropped it a pound or two, but still crazy high.

Another thing that i found odd, is that when its coming into boost, you will see around 0.2 lbs of boost and already have around 5psi of back pressure in the exhaust manifold. This doenst seem right to me, but maybe its normal?

I am really out of ideas now. My last one would be that the intercooler is causing a restriction??

I also understand that i am doing pulls on low boost... around 3-5 psi. Would the back pressure keep climbing as i turn the boost up or would it keep itself in check? I know that backpressure is supposed to get worse as psi increases, but at this point, i am doubting myself at every turn.

PS... were talking pre turbo pressue. The sensor gets its reading from the drivers side manifold where the stock o2 sensor would be.

If anyone has any ideas of help i would greatly appriciate it. thanks.

JoeNova

Which turbine housing? What A/R?

What cam?
What size downpipe?

SPRAYED 01

iTrader: (22)

Could be a boost leak. Your comparing the difference in the psi in the intake compared to exhaust. What if the intake psi was off because of a good size boost leak?

Phish806

iTrader: (5)

t4 1.10 ar

cam is a 223/231 .615 .598 on a 115+5
Downpipe is a 3.5" downpipe

full exhaust would be the downpipe going to behind the trans splitting into two 2.5" duals and going out though a "straight through" muffler in each


Yes, it could be a boos leak, but it would hav eot be quite large. I will have to boost leak test it again. However i did when the comp was on the car, and all checked out fine.

JoeNova

Your turbine is small, but still not small enough to cause that much backpressure on low boost. I typically try to stay away from smaller turbines anytime I go with an aftermarket cam, especially one that size because backpressure can be more of a problem.

Check for boost leaks. Where are you getting your boost reading from?

Phish806

iTrader: (5)

The boost reading is coming from one of the ports behind the TB on the passanger side of the intake. It was drilled and tapped and now goes from an NPT to an AN fitting. Then the hose is going to the map sensor mounted on the firewall.

slowride

iTrader: (3)

I think he is asking where you are measuring the back pressure. Sometimes certain locations might distort the numbers due to being in a tight bend etc

Phish806

iTrader: (5)

I am coming off the drivers manifod where the factory 02 sensor would be. You can see the aluminum line i have coming off the manifold in the pic

slowride

iTrader: (3)

Not sure how well that location would be. In test cells we would always be a few inches from the turbine housing but that will require drilling.

SPRAYED 01

iTrader: (22)

All my testing was done tapped very close to the turbo flange.

Phish806

iTrader: (5)

I was told that any place in the manifold would give you a correct pressure reading. Basically, the hotside would be the same pressure no matter where the sensor was. Now, is this wrong?

Phish806

iTrader: (5)

Did a boost leak test. All i had leaking was a tiny tiny leak at the IAC valve. Other then that it was solid. I was actually pretty impressed. Now i am unsure of a direction to go...

fastbarry

try reading back pressure at or right near the turbine flange.
it appears that you are reading from between the third and fourth port in the exhaust manifold?

pressure waves near the exhaust port can exceed 1000 psi when the exhaust valve opens. how far is the gauge port from an exhaust valve? i believe this may be skewing the readings.

i would move where you are taking your reading and see if you observe the same or different drive pressure numbers while changing nothing else.

rotary1307cc

iTrader: (5)

Do you have a rag in the IC? Lol

rotary1307cc

iTrader: (5)

Your pickup for the pressure is fine and fumctional

Phish806

iTrader: (5)

Then what could be my issue.... only thing i can think of is how i built the crossover and maybe where it merges together is messed up. But i know its not bad.... i will try and find a pic

Phish806

iTrader: (5)

i couldnt find a decent pic. there are some in the build thread. Basically a ton of pics on the first page that you guys can check out.

https://ls1tech.com/forums/forced-in...o-lsx-rx7.html

forcd ind

iTrader: (7)

Curious, being new to turbos, how do you know when you have a BP problem, is there a certain # you look for-I would think with a turbo all
that exhaust going thru a small opening would create some BP
Thanks

69-chvl

iTrader: (11)

So when you guys are measuring backpressure, one bank's readings is all that's needed? For a divided system too? I figured the turbine housing would of given a more accurate reading as the banks are combined at that point.

LJMSJohn

Yea... i wouldnt be getting pressure readings in the manifold.... id be near the turbine entry....