Exhaust Pressure Testing

[80FoxCoupe]

Great info @engineermike

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[GregO]

The sound is beautiful but surprisingly loud considering the car has 2 cats and 3 mufflers.

Awesome test results and your willingness to share.
I'm going to guess the dB level is partially due to the modified catalytic mantle outlet. The sound now has a decreased path of resistance with your new 4"X3" Eccentrics.
You didn't mention or I missed this, do you have a crossover tube someplace ?

 

[engineermike]

You didn't mention or I missed this, do you have a crossover tube someplace ?

The stainlessbros muffler shares the same packed chamber, so I figured that would at least partially act as a crossover. There is not a dedicated one though.

 

[GregO]

The stainlessbros muffler shares the same packed chamber, so I figured that would at least partially act as a crossover. There is not a dedicated one though.

A size on size or one size diameter smaller crossover will make some magic happen, both dB's and extended packing life.
I wonder if a crossover would change the sample port readings ? It's obvious it would, the question becomes what sample port is impacted the most ?
One thing's for sure, those True X's kill flow.

 

[18psi]

Fascinating read, thank you so very much for such thorough documentation.
Can you confirm, have you actually measured power before/after or are the expected gains (I've seen everything from 11hp-30hp mentioned in this thread If I'm not mistaken, for the 650-750 power level) are just calculations?

I had a whipple stg2 gt with a completely stock exhaust system and always wondered how much power I gave up due to that.
Currently have a paxton on e85 making 740 with long tubes and very open exhaust system and tempted to actually go backwards: put stock exhaust back on the car.
Doing research on just how much power I should expect to lose from the added restriction and found this awesome thread. Would you estimate the loss to be somewhere in the 30-40hp range?
Sorry for the off topic question and again thanks for this thread, following with great interest.

 

[engineermike]

I have dyno numbers but I’m afraid it really just muddies the water. Different day, different dyno, different gear, plus there are some tuning considerations (regarding knock and blow-through effects on lambda) that I think further skew the results. Raw numbers, 15 hp for the 5 psi reduction holds pretty true at the same corrected air flow and spark timing. I *should* be able to bump up the boost and timing a bit and realize a little more, as a result of lower backpressure. In many combinations this will happen automatically.

I can’t say what your combo would do switching to stock. My results seem to imply that the piping diameter is the biggest problem with the stock system, so if yours is still 2.5” or even partly 2.5” then the backpressure probably isnt dramatically different from stock. You’d need to measure it to know for sure though. The stock system I believe imposes over 15 psi.

 

[18psi]

Ok last question (maybe)
I got inspired by this thread and just measured my whole system: 3" from long tubes all the way to the mufflers. Steps down to 2.5 at the mufflers. Does this basically ruin the rest of the system? I know I know, only way to know is testing but I'd still love to hear a hypothesis.

 

[Andy13186]

^ I would switch that out to 3", also I wouldnt consider those mufflers lol

 

[mangosmoothie]

cool thread. Right now I have kooks -> 2.25 neckdown (I'm assuming it's 2.25 because it's factory connection compatible elbow and uses factory clamp) -> 3" mid pipe -> reducer to factory axleback..

I don't like the volume so I'm switching to a FRPP touring cat back with the resonator. I was worried going from the 3"section down to 2.5" would make a difference. This thread gives me confidence it won't do much, the real bottleneck is that elbow connection.

 

[VictorH]

Very fascinating thread. I wonder if we are over-simplifying this issue. Not a criticism but just pointing out it's complicated. We know from head porting that you can increase flow into head with larger ports, so by that logic you would just want the largest ports possible, but you'd find you're down HP and driveability. What you lose when you increase flow just by opening things up is velocity and velocity is important for both the inertia of the column of air as well as better mixture and turbulence in the combustion chamber.
Maybe none of that is true for an exhaust system, but you are almost certainly slowing down velocity in the exhaust system. Maybe this has no consequence whatsoever, however it's something to think about. If the goal is maximizing power output, at some point you would need to test or find a surrogate for power output with these changes.

 

[engineermike]

Blanket conclusion: If you have a forced induction coyote, anything under 3” diameter dual exhaust is hurting power.

@VictorH, velocity in the intake port helps with cylinder turbulence, which increases burn rate in the cylinder. Velocity can also be beneficial to ram-tuning in both the intake runners and exhaust primaries. However, I am talking about average pressure in the collector so this is past the location where ram-tuning might help.

 

[Angrey]

Ok last question (maybe)
I got inspired by this thread and just measured my whole system: 3" from long tubes all the way to the mufflers. Steps down to 2.5 at the mufflers. Does this basically ruin the rest of the system? I know I know, only way to know is testing but I'd still love to hear a hypothesis.

In a word, yes. Although gaseous flow isn't exactly the same as liquid, it's still fluid flow and you might have giant components in some or most of the system, but if it has to constrict down to any smaller portion, you're largely limited by that restriction. Having 3" everywhere else helps for overall flow and reducing losses (and reducing pressure) but if you have a traffic jam just before the mufflers, it's going to have detrimental effects.

You might have an 8" water main coming into your house, but if you have a 3/8" line to your bathroom, you're going to have wimpy flows out of your shower.

 

[18psi]

Agreed. However, the part I'm focusing on though is the fact that exhaust cools down by the time it gets that far back, and therefore it's volume decreases. This is very different from fluid.

I'm planning to test this and (hopefully) contribute to this thread at some point. Measure as is, and then with those two 2.5 "bottlenecks" removed. Lastly vs stock exhaust (which I'm picking up today).

 

[cbrtrx]

Agreed. However, the part I'm focusing on though is the fact that exhaust cools down by the time it gets that far back, and therefore it's volume decreases. This is very different from fluid.

I'm planning to test this and (hopefully) contribute to this thread at some point. Measure as is, and then with those two 2.5 "bottlenecks" removed. Lastly vs stock exhaust (which I'm picking up today).

You are correct having a smaller diameter that far down stream is not that "detrimental" at all.

 

[Angrey]

Agreed. However, the part I'm focusing on though is the fact that exhaust cools down by the time it gets that far back, and therefore it's volume decreases. This is very different from fluid.

I'm planning to test this and (hopefully) contribute to this thread at some point. Measure as is, and then with those two 2.5 "bottlenecks" removed. Lastly vs stock exhaust (which I'm picking up today).

It may lessen the issue, but it's not going to remove the fact that it's going from 3" to a 2.5" which is about a 31% drop in cross sectional area.

The "good news" is that it's right at the muffler, which depending on the internal muffler design could mean a restriction anyway (meaning you could up that section to 3" and it might not make a big difference if you don't have straight through muffler design.

For incompressible flow, the general rule of thumb is that if you double the pipe diameter, you quadruple the flow.

Just as a general concept, that means you're 16.667% reduction in diameter is going to result in a flow factor (at the same pressure) that's probably significant, like 50% reduction in flow+.

But again, it doesn't do much good to knock down one dam if there's another right behind it.

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