With regard to "Large Pipe Diameter releaving backpressure"

I disagree based on the text that I have read, and some real world examples
used to explain exhaust tuning.

There is a nice explanation of this in, "Scientific Design of Exhaust and Intake Systems",
as well as some less complex articles on Magnaflow's website.

Here is a quoted paragraph from their site:

http://www.magnaflow.com/07techtips/faq/question10.asp

It is a fine line to reduce backpressure while maintaining good exhaust velocity. It is not about getting the biggest pipe, it is about getting a more efficient pipe diameter while maintaining exhaust velocity.

http://www.magnaflow.com

 

I think your post is valid, but sort of off-point. No one is saying the "the best exhaust system is always the one with the least back pressure." More like "whenever possible, back pressure should be eliminated." That's provided it doesn't interfere with some other desirable characteristic of the system. On a race car, the main goal is exhaust scavenging. To do that effectively, some back pressure is inevitable. If the only design criteria was back pressure, race cars wouldn't even have headers. The least back pressure for a given cylinder would be just dumping the exhaust port into the atmosphere.

Rich

 

from a little differant perspective. i have a 230-236-112 xe esct... long tubes and true duals out the back with bullits. it was so loud the interior was fallling apart sounded bad so i went on a search for something better. i found a flowmaster 430408 i think is the part number. its a single chamber 40 series. has better throttle responce revs smoother and quicker, and is noticably quicker.

i know its not scientific, but its what i experianced. and if the size is an issue this muffler is only 4" tall and 8" wide, i think 10" long case.

 

"I think your post is valid, but sort of off-point. No one is saying the "the best exhaust system is always the one with the least back pressure." More like "whenever possible, back pressure should be eliminated."

Agree

"That's provided it doesn't interfere with some other desirable characteristic of the system. On a race car, the main goal is exhaust scavenging."

Agree

"To do that effectively, some back pressure is inevitable."

Disagree. Backpressure is inevitable because the pipe dimensions
are static and can only be resonant at one specific RPM, and efficiency
falls off to some degree above and below that specific point.

IOW: The tuned pipe should target the engines peak HP RPM as the
resonant point with dimensions to get best power over a window of
about 2500 RPM.

The most waste removed from the chamber occurs when backpressue is lowest in the exhaust system at that point in time

"If the only design criteria was back pressure, race cars wouldn't even have headers. The least back pressure for a given cylinder would be just dumping the exhaust port into the atmosphere."

Disagree. There must be some sort of length to the port to tune the exhaust
pusles. I would also equate this to a 'pitot tube' for intake tuning which improves
the flow into the runner.

The exhaust gasses exiting the port would also benefit from a tuned length of pipe.

Of course, I'm speaking from retained information, not experience...but I have
to start somewhere!

Looking forward to a friendly technical debate!

 

To big a pipe and you create backpressure. This is a fact . Example : If you had a 6 cylinder stock and you put in a 3 1/2 dual exhaust system you would lose alot of power due to backpressure created by running that too BIG a pipe. Good exhaust velocity would not be there and the flow would reverse. Part throttle would feel sluggish!

 

An effective exhaust system should have "negative back pressure" (ie vacuum) during the relevant portion of the combustion cycle (at the end of the exhaust stroke/beginning of the intake stroke). That's why a setup with a (good) exhaust makes more power than no exhaust system. So, back pressure IS good as long as it's negative pressure!

The less positive the pressure seen by the exhaust port during the period of an open exhaust valve the better. To accomplish the most negative/least positive pressure you need an exhaust system. Any exhaust sytem will create back pressure during most of the combustion cycle, that's why I said that some back pressure is inevitable with an exhaust system. You just want the system tuned so that it does no harm (by occuring when the exhuast valve is closed for that particular pipe).

Rich

 

What some people here are trying to get across is , that you never want "more" back pressure introduced into an exhaust system. Some comments here are completely wrong. Another example of a person not understanding any part of exhaust systems would be if they made a comment "hey , going to a 3 inch dual exhaust setup would be to much because there wouldn't be enough back pressure" (I heard this at exhaust shop) . Now if this person had substituted "exhaust velocity" in place of "back pressure" then he would have been correct.

 

An engine that has a cam cut with large lobe sepperation would need back pressure to hold the gasses in the comb. chamber, though. Right? How much is just right? Headers w/ cats and H/F "cat-back" ? I have heard an old racer say that he draws a line down the side of his collecter on the headders (open headers, 502) with a croyon or something like it. He takes it for a run down the track and where the line melts is where he cuts the collector...Is that accurate?

 

As far as the line with the crayon , it is not reliable.

 

the only time I want extra backpressure in my exhaust is if it caused by an exhaust driven supercharger

 

Am I the only one who hasn't entered the twilight zone here?

Backpressure is positive pressure created in the exhaust system. A small pipe is very restrictive, thus causing pressure buildup in the header making it more difficult to pump the exhaust out the engine.

Build a 722 cid big block and install a single 2" exhaust and the pressure buildup in the header would be huge. Thus, too small a pipe creates backpressure. Install a big pipe, and the backpressure goes away.

Mike

 

I never said a to small exhaust pipe size could not choke a high hp engine. Once again what I am saying is that too big a pipe can and will hurt performance. The key words are "TO BIG". If you get the optimum pipe size it will have good exhaust velocity .

 

I think the problem is that the discussion is using a generic term like "exhaust system", without looking at the function of each discrete part of the total system... primary, collector, separation and tailpipe. Each has its own function and design parameters. Once you get past the collector, its all about minimizing pressure losses in the system. Maintaining velocity is no longer an issue. I think Rich put it best.

 

Let's get everyone on the same page. It seems like we're all trying to get
this across in our own analogy and nobody is understanding.

This is a summary of what I have taken from the text, "Scientific Design of
Exhaust and Intake Systems"

Instant in time A:

Exhaust stroke, 5000 RPM. Exhaust valve is fully open. Intake valve is fully
closed. Piston is about midway up the exhaust stroke.

Exhaust gas is being forced into the header primary at a certain velocity.

Exhaust gas is forward moving via positive piston/cylinder pressure.
A pressure wave is created equal in length to the period of time the exhaust
valve is open (duration @ 5000 RPM)

The pressure at the exhaust port is - 5 PSI.

The pressure behind the intake valve (closed) is 2 PSI

The pressure in the cylinder is + 50 PSI


Instant in time B

Overlap period, 5000 RPM. Exhaust valve is partially open. Intake valve is partially open. Piston is at TDC and virtually stopped (for a period of time
@ 5000 RPM)

Exhaust gas continues to move forward under it's own kinetic energy, but
has slowed down due to less help from the piston.

The cylinder pressure has decrease to possibly - 2 PSI from low pressure
area created by the exiting exhaust gas.

Scavenging is now happening

The intake charge begins to move into the cylinder (from high pressure area
to low pressure area)

The pressure at the exhaust port is - 3 PSI.

The pressure at the intake port is 4 PSI

Instant in time C

Intake stroke, 5000 RPM. Exhaust valve is fully closed. Intake valve is fully
open. Piston is mid way down the bore.

Exhaust gas continues to move forward under it's own kinetic energy, but
has slowed becasue of a closed exhaust valve (possibly creating a vacuum effect)

The cylinder pressure has decrease to possibly - 40 PSI from low pressure
area created by piston moving toward BDC.

The intake charge begins to move into the cylinder (from high pressure area
to low pressure area) at an increased rate. The intake port pressure may be
-10 PSI

A pressure wave is now being created in the intake port from the valve open
duration @ 5000 RPM

Repeat the above holding the same RPM
If the intake runners and exhaust primaries are at a tuned length, the reflected
pressure waves will be in phase with the next cycle's pressure wave to increase
cylinder filling.

If the intake runners and exhaust primaries are not at a tuned length for
5000 RPM, OR the RPM changes, reflected pressure waves will be
out of phase to a certain degree which will reduce cylinder filling.

Correct me where I'm wrong.

 

zero', post 29,
define 'tuned lenght'.

My understanding is, the primary pipes of any
'normal type' header is much too short,
30 inches, not even close.

30 inches, might be OK for a 1 cid model aircraft motor.