Benefits of 2.5" diff back - Page 4 - Perth-WRX.com

WRXTASY · #31 20-09-2007, 05:23 PM

Quote:

Originally Posted by tuna

datalog exhaust manifold pressure.

using a varex muffler, i can definately confirm that you WILL loose efficency as soon as you begin restricting anything below 3".

and yes, i have the data and results.

I will agree with that statement but also consider...

Quote:

The gains are made from reducing fresh charge lost out the turbo at low rpm, it becomes very complex as it revolves around reflection waves, which some might not believe when turbo changers are involved.

Many engine manufactures try many things to reduce this, controlling inlet and exhaust cams is one way, Honda played with exhaust valves in the exhaust to reduce flow at low rpm, some play with variable exhaust scrolls in turbo's, everything is a compromise, tuning all your parts to suit your targets is the key.

Stace did you log below 4k as the butterfly closed and became restrictive on the Varex ?
This is where the gains are made with the 2.5" diff back.
They are all pretty much the same up top on a proper 2.5 muffler (Not Varex)

Also all the graphs i have seen with a 2.5" diff back have boost being brought on 100-200RPM earlier compared to a 3".

J

Kato · #32 20-09-2007, 05:52 PM

Quote:

Originally Posted by WRXTASY

Stace did you log below 4k as the butterfly closed and became restrictive on the Varex ?

He already said it was at 3,750rpm

Quote:

Originally Posted by tuna

... data logged at 200 samples per second, barometric at 100.5kpa (stable) exhaust at a consistent temp at 450 degrees celcious (stable), 3750 rpm (stable), consistent 29.5c inlet temps.

tuna · #33 20-09-2007, 05:53 PM

i held the steady state on the fwy at 3750 (100km/h) to get the on/off/on/off results to see the restriction.

its dead noticable anyhow, at 3" the exhaust system could maintain a partial vacuum (-0.02 kpa against barometric, meaning that scavenging was working) as soon as i close the varex, the pressure build up reached 0.12kpa above barometric, which is back pressure against the turbo and not improving flow. google bernoulli's principles...

WRXTASY · #34 20-09-2007, 06:06 PM

Bernoulli's Theorem:

General Concept:
The Bernoulli effect is simply a result of the conservation of energy. The work done on a fluid (a fluid is a liquid or a gas), the pressure times the volume, is equal to the change in kinetic energy of the fluid.

General Facts:
Where there is increased flow in a fluid, you will find decreased pressure.

Where there is slow flow in a fluid, you will find increased pressure.

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tuna · #35 20-09-2007, 06:44 PM

increase in pressure = not cool!

foxey · #36 20-09-2007, 06:48 PM

But as the gas cools it slows down, which equals an increased pressure?

Kato · #37 20-09-2007, 06:57 PM

To be perfectly honest, I think you guys are starting to take this waaaay too seriously.

If you are going to look at how the gas cools, why not look at pipe wall frictions, insulations, turbulent/laminar flows, losses in bends, losses in joints, etc etc etc etc etc.

The simple theory is a 3" pipe will have less back pressure than a 2.5" pipe. Measuring the backpressure will then have to take into consideration all the other factors of the exhaust. This is why there is the rule of thumb that a 2.5" is good up to XXX hp and 3" up to XXX hp, 3.5" up to XXX hp etc. All that is saying that as the power goes up the exhaust diameter has to increase to deal with the extra gases.

Changing a section of exhaust from 3" to 2.5" (i.e. diff back) may yield fractionally earlier torque, but big whoop. I'm sure changing the dump pipe alone to something of a much greater design will achieve better results.

Heck removing the 90 degree bends that some people have in their exhaust setups will achieve greater results than changing the diameter of the diff back section.

WRXTASY · #38 20-09-2007, 07:08 PM

Kato, yes agree with all of the above - especially with picking the correct diameter of exhaust to match your application and expected HP output

I not throwing out Stace's logging and reading of increased backpressures using the Varex with the butterfly valve either.

It just that the design of the butterfly valve in the Varex may not be causing the same high pressure exhaust gas waves reflections back up through the turbine blades into the combustion cylinders just before exhaust valve closure, as it does on a proper 2.5" diff back.

These high pressure exhaust gas waves are produced by the convergent section in the 3" to 2.5" stepdown pipe section of the exhaust.

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At the end of the day its a pretty black art getting an optimum exhaust system for your setup and application.

Tweak it as a last resort when you have done everthing else!!

J

maddog · #39 20-09-2007, 07:18 PM

Quote:

Originally Posted by rspct

Yeh, real accurate results there haha

. I agree that it would not be in the order of 1 psi difference... It is only a 1-2 m section of exhaust with half an inch difference...

The cat and number of elbows will have the largest effect on back pressure, if the diff back is only section you are changing the diameter.

The most important message here is that it has nothing to do with "maintaining the gas velocity as the exhaust gas cools". There are so many fundamental things wrong with that statement, bernouli would be turning in his grave.

big deal, the result is it does something (increases velocity) that helps deliver more torque in the lower rpm range for a given power range. Is that more correct...btw who the fuck is Bernouli

and what does he drive

Tuna, do you even monitor individual cylinder EGT's or A/F ratios?

tuna · #40 20-09-2007, 08:20 PM

Quote:

Originally Posted by maddog

big deal, the point is it provides more back pressure and that helps deliver more torque in the lower rpm range for a given power range. Is that more correct...btw who the fuck is Bernouli

and what does he drive

Tuna, do you even monitor individual cylinder EGT's or A/F ratios?

my perrin headers i have already had modifed for 5x k-types plus left/right bank lambda.

the additional equipment has been purchased to facilitate this (e888+ktypes), which complements the existing single EGT, EMAP and wideband sensors.

anyhow, more on my stock fuel rail/reg/avcs setup later- its off topic!