Stock Exhaust Back Pressure???

BuyAmericanIron · March 13, 2016

Tried something today out of curiosity. Pulled a rear O2 sensor and installed an exhaust back pressure gauge for testing. Truck is a 2016 Sierra Denali 6.2L. Here are the findings:

Stock exhaust

30 mph cruising, .50 ~ .80 psi back pressure
55 mph cruising, .50 ~ .80 psi back pressure
WOT in 3rd gear right before shifting at 5500 rpm, 5.5 ~ 5.8 psi back pressure

Stock exhaust with flapper bolted open

30 mph cruising, 0 psi
55 mph cruising, 0 psi
WOT in 3rd gear right before shifting at 5500 rpm, 4.8 ~ 5.1 psi back pressure

Magnaflow 15318 cat back exhaust

30 mph cruising, 0 psi
55 mph cruising, 0 psi
WOT in 3rd gear right before shifting at 5500 rpm, 0 ~ .1 psi back pressure. (the needle was fluctuating a bit but barely moving off of zero)

I thought this was very interesting and wanted to pass on what was learned. Now the questions. How much HP do you lose with 5.5 psi back pressure? How much fuel economy does .5 to 1.0 psi cost? When accelerating with the stock exhaust at WOT, the pressure would pretty much follow the tach. I.E. 3000 rpm, 3 psi, 4000 rpm, 4 psi, 5000 rpm 5 psi. It may have been in my head, but the engine seemed to run more freely with the Magnaflow and zero back pressure at any RPM.

GMC207 · March 13, 2016

I don't know much about this stuff but would be curious to find out.

muddkatt · March 13, 2016

The GM Borla exhaust claims a 10HP gain(GM makes the claim, not Borla), and no mention of increased fuel mileage. Only way to compare that I can think of is test the back pressure of the GM Borla system or put your truck on a dyno.

Jon A · March 13, 2016

Tried something today out of curiosity. Pulled a rear O2 sensor and installed an exhaust back pressure gauge for testing. Truck is a 2016 Sierra Denali 6.2L. Here are the findings:

Stock exhaust.....

Excellent data collection! It seems to be in vogue these days for people to say various mods "don't do anything" or "are for looks/sound only" etc. Instrumented data collection always trumps internet speculation....

If you search through enough SAE papers you can likely find equations/example data that will correlate backpressure to power for a given set of circumstances. I can't tell you exactly off the top of my head what it would calculate out to be, but with 5 psi going to zero it's certainly not nothing.

07Softail · March 15, 2016

Didn't Black Bear do a test between Performance and stock mufflers and found no gains on the dyno? Someone did at one time not sure who.

RT

m3n00b · March 15, 2016

Awesome stuff OP!!

Jon A · March 15, 2016

Didn't Black Bear do a test between Performance and stock mufflers and found no gains on the dyno? Someone did at one time not sure who.

Old tests on different engines (exhaust and intakes) are not really applicable to the new engines. Items that pose little restriction on a 300 HP engine can become big restrictions on a 420 HP engine.

07Softail · March 15, 2016

Pretty sure it was on the 14's when they first came out.

RT

TJay74 · March 16, 2016

Pretty sure it was on the 14's when they first came out.

RT

It was on the previous generation trucks.

glcbn · March 16, 2016

Those numbers make me feel good that I've had my flapper clamped open since the thread about it's usefulness good or bad first appeared on these forums. Granted, it doesn't help with WOT, but I rarely ever do that anyway.

Jon A · March 16, 2016

Pretty sure it was on the 14's when they first came out.

Do you have a link, I don't think I saw it. I do know the CAI test everybody quotes here all the time was done on an older, much less powerful engine.

07Softail · March 16, 2016

I do not have a link. I can't remember what forum I saw it on.

RT

glcbn · March 16, 2016

Do you have a link, I don't think I saw it. I do know the CAI test everybody quotes here all the time was done on an older, much less powerful engine.

Is this the one you are referring to?

http://www.gm-trucks.com/forums/topic/145676-bbp-independent-intake-test-results/page-1

I searched for exhaust tests by BBP, but couldn't come up with any on the Ecotec.

Jon A · March 16, 2016

Yup. There's nothing wrong with the test (although many misinterpret the results), I give Kudos to BB for doing it, it's just that on an engine that makes 100 more HP and all the parts being tested are somewhat different, different results can be expected.

07Softail · March 17, 2016

I would venture to guess that the damn valve after the muffler would be the only obstruction causing the minimal BP evident in the testing. Even bolted open there is still that weight on the blade itself that still blocks a bit of flow imo. Can these gauges be rented at an auto store if so I will perform this test being that I will be bolting up some headers soon with the stock exhaust and will be removing the valve completely and I will do a before headers, after headers, with valve, without valve for comparison.

RT