Backpressure measurement
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
Backpressure measurement 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.
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.
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.
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.
PS- mine would peg the 30psi gauge at 5lbs or so with the wastegate open.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
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.
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.
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.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
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
Last edited by 69-chvl; Nov 23, 2015 at 07:08 AM.
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
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
Trending Topics
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
9 Second Club
Joined: Nov 2003
Posts: 13,616
Likes: 185
From: Norn Iron
So does your pressure gauge ever read zero ?
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
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.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
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.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
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.
2:1 is fine I saw almost 2.8 at 26lbs with the small s475 on my 5.3.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
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.
2:1 is fine I saw almost 2.8 at 26lbs with the small s475 on my 5.3.