Backpressure measurement
#1
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.
#2
TECH Addict
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.
#3
TECH Regular
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.
PS- mine would peg the 30psi gauge at 5lbs or so with the wastegate open.
#5
10 Second Club
iTrader: (22)
Join Date: Mar 2006
Location: Northwest side of Chicago
Posts: 3,677
Likes: 0
Received 2 Likes
on
2 Posts
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.
#6
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; 11-23-2015 at 07:08 AM.
#7
TECH Veteran
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
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
#9
9 Second Club
So does your pressure gauge ever read zero ?
#11
TECH Senior Member
iTrader: (23)
Join Date: Jul 2004
Location: Houston, TX
Posts: 9,853
Likes: 0
Received 0 Likes
on
0 Posts
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.
#12
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.
#19
8 Second Club
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.
2:1 is fine I saw almost 2.8 at 26lbs with the small s475 on my 5.3.
#20
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.