Stock exhaust back pressure

Kjewer1 · Apr 12, 2016

Has anyone here measured the back pressure on the stock exhaust? I tested my car today and was surprised by the results. 14 psi by redline. Dropping the cat back yielded the same result, so it's in the downpipe. Considering the expansion ratio this turbine must run, that seems like too much back pressure to be deliberate (been suspicious of this cat since the blown motor pump a few quarts of oil through it last year). Should be able to easily triple that in the turbine housing. If I get motivated tomorrow I'm going to move the sensor to the turbine housing and see what's going on there. If anyone else has tested this I'd love to hear the results.

Tucsonturbos · Apr 13, 2016

Kjewer1 said:
Has anyone here measured the back pressure on the stock exhaust? I tested my car today and was surprised by the results. 14 psi by redline. Dropping the cat back yielded the same result, so it's in the downpipe. Considering the expansion ratio this turbine must run, that seems like too much back pressure to be deliberate (been suspicious of this cat since the blown motor pump a few quarts of oil through it last year). Should be able to easily triple that in the turbine housing. If I get motivated tomorrow I'm going to move the sensor to the turbine housing and see what's going on there. If anyone else has tested this I'd love to hear the results.

This Could Be interesting! Side note, I run a muffler and resonator delete (single exit) with stock downpipe and gained roughly 1.5psi. Can't wait to see if any results

Kjewer1 · Apr 13, 2016

I checked on turbine back pressure tonight. Back pressure peaked at 42 psi, no matter what I did. Not at all what I expected. With a measured 15 psi in the downpipe that puts turbine ER around 2:1, which is really impressive since compressor PR is around 2.5:1. This tells me there must be more left in the turbo, clearly I'm slacking.

Fast64ranchero · Apr 14, 2016

42, I was expecting alot more, good info, thx for sharing

Kjewer1 · Apr 14, 2016

I was expecting that much with zero back pressure from the exhaust. Still seems crazy to me after sleeping on it. Got a downpipe coming tomorrow, we'll see what that does to both pressures.

RubyRed15 · Apr 14, 2016

How do you measure this? Pressure gauge in the O2 bung?

Edkiefer · Apr 14, 2016

14lb sound way high for pressure measured at down pipe.
I would of guessed a few lbs max for whole exhaust.

Could it be your cat is damaged, did you smoke oil at anytime ?

Kjewer1 · Apr 14, 2016

The whole reason for this test was suspecting the cat was damaged. It was on the car when the motor blew last year and put a bunch of oil and water through it while it was nice and hot on a pass. Exhaust is pretty much the only bolt on I haven't done yet, and I'm not surprised that it's holding me back, just surprised by how much.

I was hoping someone might have tested this on a "good" stock exhaust for comparison but I seem to be alone in this so far.

I drilled at tapped the down pipe, both O2 sensors are still in place so I can make pulls with no change to the tune or anything. For turbine back pressure I tapped the turbine housing. When I get the new downpipe here I'll weld a proper bung to it. Normally I'd be running pressure sensors to a logger of some kind, but I started with a spare boost gauge to save time just to get a ball park idea of where it is.

Edkiefer · Apr 14, 2016

Kjewer1 said:
The whole reason for this test was suspecting the cat was damaged. It was on the car when the motor blew last year and put a bunch of oil and water through it while it was nice and hot on a pass. Exhaust is pretty much the only bolt on I haven't done yet, and I'm not surprised that it's holding me back, just surprised by how much. I was hoping someone might have tested this on a "good" stock exhaust for comparison but I seem to be alone in this so far.

I drilled at tapped the down pipe, both O2 sensors are still in place so I can make pulls with no change to the tune or anything. For turbine back pressure I tapped the turbine housing. When I get the new downpipe here I'll weld a proper bung to it. Normally I'd be running pressure sensors to a logger of some kind, but I started with a spare boost gauge to save time just to get a ball park idea of where it is.

I can't help on eco boost, but i have done that tests to other vehicles and like I said few lb is a lot IMO.

I was in same spot as you one time with my XR4ti, with a borla cat back exhaust, larger down-pipe but stock cat (a replacement).I never measured above 1lb or so with 17bls boost, if my memory is right.

When the cat gets damaged the exhaust note changes too, to a more woosh sound, hard to explain but just think of bad restriction,

After a while the honeycomb will melt and then break up, so if you get rattles hitting DP, its definitely bad.

Kjewer1 · Apr 14, 2016

The cat seems to be physically intact, but who knows what condition it is in. I'll take a closer look when I get it out. Either way I'm sure this is the biggest restriction currently on the car. Not exactly a huge surprise, but good to see numbers put to it.

The airflow through the motor seems to be about all the turbo is capable of, which is another reason that I'm surprised total back pressure is so low. At any rate, if that is accurate, the only real gains we might stand to pick up should be in the form of reduced pumping losses. In the past I've been able to ballpark pumping losses using differential cylinder pressure, I may try it this time too and see if the numbers make any sense, or correlate to any gains we see. It's just math, but it can sometimes prove interesting.

ypena02 · Apr 14, 2016

What is the max lbs/min you're seeing out of the stock turbo and at what psi?

Edkiefer · Apr 14, 2016

Kjewer1 said:
The cat seems to be physically intact, but who knows what condition it is in. I'll take a closer look when I get it out. Either way I'm sure this is the biggest restriction currently on the car. Not exactly a huge surprise, but good to see numbers put to it.

The airflow through the motor seems to be about all the turbo is capable of, which is another reason that I'm surprised total back pressure is so low. At any rate, if that is accurate, the only real gains we might stand to pick up should be in the form of reduced pumping losses. In the past I've been able to ballpark pumping losses using differential cylinder pressure, I may try it this time too and see if the numbers make any sense, or correlate to any gains we see. It's just math, but it can sometimes prove interesting.

Yeh, maybe one of the tuners or exhaust makers will chime in on your data.

For sure cat is biggest restriction ,does the car perform ok ?
If you were able to free up even half of that pressure I am sure you feel it, but again ECU would adjust as far as boost but still i think you'd notice it a lot.

Kjewer1 · Apr 14, 2016

I'm asking the turbo for all the boost it will make up top, but limiting it to around 23 psi peak to keep this gobshyte fuel system happy. Airflow is in the mid 40s. Boost at the top of the rpm range did drop a few psi after porting the head, which is just another indicator that the comp wheel doesn't have anything left under these conditions.

Edit> Looking at the last log I took, 7k rpm is 43 lbs/min and 17-18 psi.

ypena02 · Apr 14, 2016

Not bad at all for a stock turbo, thanks for the info!

Kjewer1 · Apr 14, 2016

It seems to be similar in capacity to an EVO3 16g, obviously, but if these back pressure numbers are right, the turbine side is a vast improvement. When I ran 10.31 at ~130 on an EVO3 turbo in my Talon, airflow was 42 lbs/min at 19-20 psi at redline, but back pressure was 40-80 psi depending on what I was doing. It ran the best at ~45 psi back pressure. That would have been with 0 psi at the turbine outlet of course, the "exhaust" was a single 3" bend. So that shows the difference in turbine expansion ratios. Also noteworthy, no difference whatsoever between that 3" bend and a 2.5". 3" is always the standard, and there's nothing wrong with going bigger, but I've been able to put 103 lbs/min through a 3" downpipe for reference.

Stock exhaust back pressure

Kjewer1

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Tucsonturbos

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Kjewer1

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Fast64ranchero

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Kjewer1

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RubyRed15

Ok, so it's blue.

Edkiefer

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Kjewer1

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Edkiefer

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Kjewer1

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ypena02

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Edkiefer

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Kjewer1

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