Richiec77

iTrader: (3)

What would be the effect of decresing the diameter of the exhuast say 4' or so from the collector on a set of headers?

My line of throught was triggered when reading thru some STS Turbo threads. The whole arguement on the STS is that it is considered less efficient then a front mount due to heat loss, which decreases pressure available to drive the turbine.

If you were to taper the exhaust piping down from say 3" to 2.75" or 2.5", what effect would it have on the overall system (from an exhaust point of view)?

Richiec77

iTrader: (3)

https://ls1tech.com/forums/showthrea...&highlight=sts

I've also read thru this post, and it helps a bit, but doesn't take this idea into play. I'm thinking Ventury effect would help keep velocity up even though density has decreased (gas density has of coursed increased due to cooling) as far as the over all volume of gas is concerned.

MadBill

First, backpressure is bad! Always. There is no upside to it.
However, until the exhaust gasses reach the muffler(s), the issue is pressure wave tuning, more than back pressure, and the diameters of primary pipes and collectors can be selected via various formulas, common practice, etc. From the mufflers on, pipe size and muffler construction needs to be chosen to minimize back pressure, especially with a long duration cam. Typically this means the tailpipes can (but don't have to) be maybe a half inch smaller than the collector, as they are just carrying a fairly steady flow of gasses.

H8 LUZN

** edit; assuming this is concerning a turbo system**

it doesnt matter what shape the pipe is, becuase the gas already lost that much energy that can never be recovered. All you can do is optimize the energy you have left. but pipe shape, turbo size, etc.. is how you can do that..

But what would help is to have a very low thermal conductivity on the pipe material and also have a large wall thickness. That would probably help more than pipe shape. (depending on your design of course..)

P Mack

iTrader: (6)

There's really no point to using a smaller pipe unless it's for clearance. Velocity is good for header primaries, merge collectors, x and y pipes. But concerning the rear part of the exhaust, more velocity = more back pressure. One reason is more velocity = more frictional losses. The other reason is the static pressure at the exhaust tips has to equal atmospheric pressure.

The only thing you'd gain from having more velocity at the exhaust tips is a tiny amount of forward thrust.

Adrenaline_Z

I'm not following this reply PMack;

Bernoulli states that a fast moving gas in a pipe produces lower pressure.

Keeping in mind that the pressure within the exhaust is never static, maintaining
a lower average pressure, and timing the negative pulse back to the exhaust
port allows for more gas to leave the chamber at calculated RPM inetervals.

When I read this thread, it seems most are thinking exhaust pressure is
a specific value across the engine's operating range...which is not the case.

I'm not sure what this means either. Do you mean when the engine is off?

P Mack

iTrader: (6)

Bernoilli's equation is based on a couple of assumptions, one of which is frictionless flow. That doesn't mean it doesn't apply, but you do have to realize that friction lowers pressure while lowering velocity, the opposite of the venturi effect.

If the exhaust had more pressure than the atmosphere it's dumping into, it would speed up until the pressures became equal. The only way the pressures are not equal is if the exhaust reaches the speed of sound at some point.

So how is it possible that there is backpressure if the pressure at the tips is equal to atmospheric? The reason is friction causes a pressure drop, which makes the pressure upstream higher than the pressure at the tips. More friction equals more pressure loss, which means for the same pressure at the tips there is more pressure upstream.

So how does exhaust pipe diameter figure in to all this? A smaller diameter pipe means the exhaust velocity is faster. Which means there are more losses due to friction. Which means more pressure loss, which means more back pressure.

92RS98TA

The only time i found back pressure to be good was when i over cammed an engine. The car actually went faster with full exhaust than just open full lenght headers. My theory: with too large of a cam the exhaust valve timming was too early and was actually letting cylinder pressure from the power stroke out too early.

oange ss

iTrader: (50)

friction would decrease velocity therefore increasing pressure, you cant have one without the other....maintaing pipe size to increase velocity is good thinking in theory but you'll reach a point where the restiction increase would negate any velocity gains

Adrenaline_Z

Friction would raise the pressure in the pipe. A slow moving fluid, or gas
due to increased friction would then also create a high pressure zone in
the pipe.

Exhaust gas may, or may not have more pressure than the atmosphere.
It depends on what point in time you are sampling. Exhaust pressure is always
fluctuating, you cannot simply state that a certain set up will yield "x pressure".

There is a sinusoidal

Not sure what you are stating here?

Again, the pressure in the pipe is always changing. It is a wave, or a pulse
that travels along the pipe. The pulse at time1 may be high at the port,
but could be low at time2 when reaching the collector.

Backpressure is not a scientific unit of measure. There is no benefit of having
a restrictive exhaust as Madbill states.

P Mack

iTrader: (6)

You guys really need to pick up a fluid dynamics book. Friction causes pressure downstream to be lower than upstream.

Adrenaline_Z, you are right pressures are always changing, but that doesn't mean rules like conservation of mass, momentum & energy go out the window.

Adrenaline_Z

PMack, I understand that surface drag will cause a loss of energy. Fluids and
gas cannot flow on the surface of an object; and I also understand the velocity
gradient where the least amount of drag occurs as the fluid/gas reaches the
center of the pipe.

The statements you made are typical of a mass in motion. However, the point
I would like to clarify is:

- that fast moving exhaust gasses create low pressure in the exhaust system to allow charge to leave the cylinder(s). Gas moves from high pressure
areas to low pressure areas.

Having a slow moving gas in the exhaust system creates a higher pressure.

Somebody09

The effect is a really annoying goddamn vibration at around 600 RPM that's louder than my exhaust when I floor it. It resonates throughout the entire car and makes my ears want to bleed... At least that was my experience with SLP's long tubes, their 3" Y-pipe, and my stock catback. (for those who wonder why anyone would do that, think dual cutouts in an attempt to be sleeper-like) I installed their dual/dual catback in an attempt to eliminate the vibrations, it only mitigated. Time for new headers...

white2001s10

iTrader: (4)

You would increase gas speed at the turbine for quicker spooling but the energy isn't free. It will cost you in added backpressure at the valve to do this.
It's probably not worth the effort.

P Mack

iTrader: (6)

Imagine a simple pipe where gas enters at point A at a certain pressure, moves through the length of the pipe, and exits at point B into the atmosphere. And lets say the static pressure at point A is 20 psi and at point B it must be atmospheric (lets say 15 psi). As the gas flows from A to B, friction lowers the pressure from 20 to 15 psi.

Now lets say you shrink the area of the pipe, but keep the same massflow. Now the velocity is faster, meaning there is more momentum lost to friction, so now you get more than a 5 psi pressure drop from point A to point B, lets say 7 psi. Since atmospheric pressure is still 15 psi, now you have 22 psi at point A.

This is way oversimplified since massflow would change along with a change in backpressure, the flow is not steady, there is heat loss at the same time, etc, etc. But none of those things change the simple fact that friction creates pressure loss, and more flow area minimizes pressure loss which in turn minimizes pressure upstream.

Adrenaline_Z

We're talking about two different things.

This should put us on the same page:

http://home.earthlink.net/~mmc1919/venturi.html

P Mack

iTrader: (6)

I understand Bernoulli already. I don't think you're applying it correctly though. Let me give you another example. This time without friction to illustrate bernoulli. With a regular 3" pipe and no friction, the gas at point A will be at the same pressure as point B (lets say 14.7 psi). And also lets say the mach number in the pipe is 0.2. Now compare that to a pipe that is 3" at point A and tapers down to 2.5" at point B. Now if the mach number is 0.2 at point A, it is around 0.30 at point B. And since the pressure at point B is still 14.7 psi, the pressure at point A is now 15.2 psi. Big picture, tapering the exhaust caused more pressure upstream.

Adrenaline_Z

You're right, I was interpretting your backpressure theory in general.

I have a pet peeve with that word and many people use it incorrectly.
When referring to the tapered pipe, I can understand how you are applying
the 'term' backpressure.

WHen Madbill was speaking of highpressure/backpressure in the exhaust,
he was writing about pressure being high and that is what I was feeding from.

Sorry about the mix up!

P Mack

iTrader: (6)

I was about to say we need a definition for backpressure so we know we're talking about the same thing. It could really be measured anywhere.

white2001s10

iTrader: (4)

You lost me with this.

How are you defining "pressure loss"?
I'm reading it as a drop in pressure which makes no sense at all.