Stillen Muffler pressure drop measurments.

Today I was able to run a pressure drop test on the Stillen Catback muffler. As some of you may have already seen in prior posts I cut off the Stillen muffler of the Stillen cat back system for muffler interchangeability.

See Gurgen’s PE TT dyno link for more details of the specific mod.
https://g35driver.com/forums/showfla...sed&sb=5&part=


I ended up putting flange stubs at the end of the cat back system so I could switch out different mufflers easily. The objective was (is) to find a muffler that would sound better and reduce back pressure to any extent possible. But while I had the exhaust system all taken apart and on the work bench, I added a pressure tap to the pipes just upstream of the muffler section for taking pressure measurements.

Although I haven’t connected the high flow MAGNAFLOW 14211 straight thru mufflers yet, I was able to take a few measurements today and I obtained some interesting and somewhat unexpected results.

Pressure drop across straight pipes:
Measurements were taken in 4th gear @ 6.5K RPM. As you can guess, the pressure drop across the straight pipe is negligible. Delta P = 0.05” Hg or 0.0245 psid.

Pressure drop across the Stillen muffler:
Measurements were taken in 4th gear @ 6.5K RPM. Pressure drop across the Stillen muffler was substantially higher (even higher than I expected). Delta P was 4.85” Hg or 2.382 psid.

2.382 psid is rather high. Gurgen and I previously noticed a design deficiency with the Stillen muffler, but when you consider this to be one of the highest performance catback systems available producing ~22 additional horses 4.85” Hg is still somewhat high.

A while back I remember seeing one of Q45tech’s links showing approximately 2HP is lost for every inch of Hg back pressure. If this is the case the Stillen muffler is dropping approximately 9.7 HP. Conversely, the straight pipes should be gaining 9.7 HP.

If the effect is linearly added to the baseline HP gain of the Stillen cat back, the total yield of a Stillen catback modified with either straight pipes or a pair of straight flow through mufflers, it should yield a total of ~ 31 extra ponies. (or+45%)
Not exactly chump change….. and this is on a NA engine.

With Turbos, the effect of back pressure is much more pronounced and the gains could potentially be twice as much! (Hear that Gurgen?) This suggests Turbo users really should consider straight flow through muffler designs. (but you already know this.)

For straight pipe sound go to:
http://www.gurgen.com/G35Pics/Hydrazine Exhaust Videos/

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Anyways, my next objective is to put pressure taps all over the stock exhaust system I have sitting in my garage and re install it for testing. This way the entire stock exhaust system can be mapped for pressure drop so we can finally bypass all the marketing BS and find out where the largest choke points really are.

Is it in the OEM Y pipe?
The OEM resonator?
The OEM muffler?
How about the OEM cats?

Until they are measured we can only speculate.



2004 G35 6MT Blk/Blk Sedan Stillen Air, Stillen Exhaust

 

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2004 G35 6MT Blk/Blk Sedan Stillen Air, Stillen Exhaust

362339-Pressure tap.bmp.jpg

 

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2004 G35 6MT Blk/Blk Sedan Stillen Air, Stillen Exhaust

362340-ready for installation.bmp.jpg

 

My back pressure number only apply if measured at the O2 sensor flange [after/at] exhaust manifold collector.

The farther you are away from exhaust valve the less it matters...........dynamic volume of exhaust system.

Try a Dynomax Stainless UltraFlow straight thru muffler the lowest of any I have ever measured!

 

Hey Q45,

This is generally true. =P I hope to get a set of resonated test pipes. I will weld pressure taps on them before installation. This should provide the most conclusive overall pressure measuring conditions. Having a pressure tap just upstream of the muffler is a good place to measure muffler pressure drop for evaluation purposes.

That along with a couple other measurment locations, I could put together a flow model to simulate the effect of changing different parts.

Being furthest away from the valve will always be technicaly true but in the case of the Stillen it has little effect.
True, large pressure drops at the end of a flow system provide pressure recoveries toward the inlet of a flow system....Particularly with compressible flow. In the case of the Stillen I think the pressure drop at the muffler most likely has a real effect. I say this because I see no significant pressure drop mechanisms anywhere in the Stillen system except for the muffler. The muffler is the only part producing meaningful pressure drop. All the pipes are very large diameter. The X pipe is so large that that it shouldn't produce any meaningfull drop. The resonators are large in diameter and straight thru so they won't really produce anything either. If you had seen the internal flow path of the Stillen muffler you would readily agree it would create the strong majority of pressure drop. So what you see at the muffler is what you get at the catback inlet.

Conversly, I can't agree with you on the Dynamic volume arguement:
1) The volume is static.
2) The added volume provides more of a true RMS measurment.

As an example, for pulsating compresible flow, it takes more work to pump a fluid at say a maximum 3psi delta P with a large static volume than it does to pump same pulsating flow at a maximum 3 psi delta P with a small static volume.

Having a large volume provides more of a constant value of pressure drop. If the same peak pressure is obtained from a smaller volume the overall RMS value is lower.

In other words, droping a constant 3 psi through a large ullage at the muffler can affect performance more than droping a peak of 3 psi just upstream of the very low volume cats.


2004 G35 6MT Blk/Blk Sedan Stillen Air, Stillen Exhaust