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Stock Exhaust Back Pressure???

BuyAmericanIron

By BuyAmericanIron
in Engines & Drivetrain

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BuyAmericanIron Enthusiast

BuyAmericanIron

Posted
Posted

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.

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BuyAmericanIron
BuyAmericanIron

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

Jon A
Jon A

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 interne

Victory14red
Victory14red

The following is copied from http://www.dsmtuners.com/threads/exhaust-the-straight-scoop-on-backpressure.168578/ Granted this is a completely generalized answer not in any way specific to our trucks

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GMC207 Enthusiast

GMC207

Posted
Posted

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

muddkatt Senior Enthusiast

muddkatt

Posted
Posted

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 Senior Enthusiast

Jon A

Posted
Posted

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.

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07Softail Senior Enthusiast

07Softail

Posted
Posted

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

Jon A Senior Enthusiast

Jon A

Posted
Posted

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.

TJay74 Senior Enthusiast

TJay74

Posted
Posted

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

 

RT

 

It was on the previous generation trucks.

glcbn Enthusiast

glcbn

Posted
Posted

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 Senior Enthusiast

Jon A

Posted
Posted

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.

glcbn Enthusiast

glcbn

Posted
Posted

 

 

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 Senior Enthusiast

Jon A

Posted
Posted

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 Senior Enthusiast

07Softail

Posted
Posted

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

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