69-chvl

iTrader: (11)

I tapped into the drivers side exhaust manifold and hooked-up a sensor and recorded some figures. When the boost gauge zero's out, I'm already at 15psi BP. That in an of itself I thought was interesting. At at my boost setting of 13.5psi, I got 26psi of BP. So about 2:1. Does this seem high or present an issue for a street car? Wondering if this is why the last couple times at the track the car didnt pick-up a whole lot at ~16psi.

This is on an LQ9 with the s475/t4/1.1 83mm turbine, 3.5" downpipe to a 2.5" dual exh system with straight through muffs.

3 window

iTrader: (1)

Not sure about the backpressure numbers being typical or not but I do know that Jarrett (stock48) had the same turbo only with a 1.25 AR and he saw less gain after 16-17 psi. He thought that the turbo was pumping a lot of heat at that point. I do know that he has a 4" exhaust system on his Nova, so that's a little different than yours and he may not have been seeing the same back pressure numbers as you. So all in all, his issue was possibly heat, while yours could be back pressure.

chiaj144

iTrader: (4)

I had similar back pressure with my sled last year when the boost read zero. It wouldn't make any power at all, slower than stock. My exhaust housing was defective or not machined right. I put a known good one on it and back pressure dropped big time, sled made great power. My test were with a short 3" downpipe only. Did you test post turbo? Maybe your exhaust is contributing to the pre turbo numbers.
PS- mine would peg the 30psi gauge at 5lbs or so with the wastegate open.

69-chvl

iTrader: (11)

Just tested pre-turbo. I was going to disconnect the exhaust system at some point to see if theres any change. I was just wondering if my #'s now are concerning.

SPRAYED 01

iTrader: (22)

I have done a lot of back pressure reading so I am familiar.

I have to say the 15psi bp with no boost is weird.

The 26 with 13psi in the intake is within reason for a t4 on a big motor. I would disconnect the exhaust and run a wide open down pipe and retest.

69-chvl

iTrader: (11)

Yeah I dont u/s the backpressure at 0psi....seems like it backs-up and needs to be squeezed through the turbine.

I'm going to try with the air filter and exhaust disconnected just to see what effect they have - if any.

This is only with 13.5 psi and under 6000 rpms

forcd ind

iTrader: (7)

Interesting post, never gave much thought to back pressure-I have an S475, T4, not sure but I think a 92MM turbine (i'll ck it when I pull the engine) I have an LQ4 with Stocks 218 cam, 4" exhaust all the way to the rear bumper with 2 straight thru mufflers-I have to find a way to meas. it.
Just a thought, could you expect some BP with no boost because there isn't a lot of velocity to push the exhaust out? not sure how that works

Guard dog

Does the turbine spin freely?

stevieturbo

So does your pressure gauge ever read zero ?

69-chvl

iTrader: (11)

You mean the backpressure gauge? Seems to be a zero while idling...havent really paid attention to it while driving normally, probably should though.

GMCtrk

iTrader: (23)

I would just turn the boost up and go. the baby 475 did around 17 psi on my setup before pumping nothing but hot air, but was good enough to trap 130. I'm now running the big 480 t6 and same wastegate spring it's making 21 psi and I picked up 4 mph in the 1/8th with problems at the track.

69-chvl

iTrader: (11)

Sweet gain!!!

T.Fiddler

What are you using for the sensor?

Busa_rob

iTrader: (1)

2:1 or less is pretty common, and ideal.

69-chvl

iTrader: (11)

Using a 100psi stainless sensor made for Autometer. The datalog pick-up a pretty severe pulse so I have to filter the data - ALOT but it seems to jive.

Busa_rob

iTrader: (1)

That's the standard 100psi SSI sensor everyone takes, puts their label on, and charges twice the amount.

T.Fiddler

Thanks
Nice pictures!

93camaro_zzz

iTrader: (13)

Haha, sounds about right.

Forcefed86

iTrader: (4)

Auto meter sensor is crap IMO. Are u data logging w it or just reading a gauge? I hooked the same sensor up to my MS3. Ran shop air to it w 2 known good gauges. I had to skew the voltage vs press table to get to read accurate. On vaca now can post numbers when I get back.

2:1 is fine I saw almost 2.8 at 26lbs with the small s475 on my 5.3.

69-chvl

iTrader: (11)

I checked the accuracy of the sensor by hooking up the sense to my fuel pressure line and they read the same so I'm fairly confident the reading is relatively accurate. My concern was whether or not 26 PSI of back pressure was high for only 13 PSI of boost but 2:1 is 2:1. Still perplexing is the back pressure at zero boost. I guess all that exhaust still has to get through the teeny turbines have some back pressure is to be expected – I think.