Muffler flow test

[jdean]

This site has some interesting flow info on straight pipes and various popular mufflers.

Also, he says he will flow test your mufflers if you pay shipping to/from.

I was surprised at how little the straight-thru type mufflers flowed.

http://www.geocities.com/broaderperformance/page2.html

John

[750hp]

John - thanks for the link! I read an interesting article over here where they flow tested a number of mufflers on a Superflow flow bench, then ran the mufflers on a 550hp, 7000rpm small block. The highest flowing mufflers on the flow bench placed about 4th and 5th for power production on the engine dyno..... Why? No idea!!!

[Richard Hudgins]

Hi Craig,

I think that just looking at flow rates for mufflers will not give you much indication on their performance on a given engine.

The most critical characteristic of an exhaust system are its pulse lengths. (Note: we will consider it a given that adequate flow area is available for the system)

The many variations in muffler design will give you wildly different pulse lengths, but will have comparable flow numbers.

The effects of these variations will give you very different power curves, mixture curves, and total HP numbers.

There is also the consideration of heat retention. The specific heat is very critical to the acoustic properties of a system. (IE: positive and negative pulse speed and pressures.)

For instance, the standard headers and sidepipes on the JBL are designed to produce maximum negative pulse strength at 4500 rpm on a 351.

By changing the secondary pipe (Sidepipe) pulse lengths, one can move the maximum negative pulse event up to about 7500. For anything higher than this, the whole system timing will have to be changed. (IE: primaries.)

In other words, exhaust design is rather complex and many factors need to be examined in addition to flow.

(Of course, all of this changes with a turbocharger. So anything that I said above does not apply to your car. Except for specific heat.)

[niles]

John:

Don't understand your comment; the straight 2 1/4 and 2 1/2 in pipes outflowed all the mufflers; in certain cases near 100 cfm.

Richard:
Help me out here; I was of the understanding that the neg. pulse was generated where the header pipe dumped into the collector and was reflected back toward the source and that was a functions of pipe dia and length; there are std. formula for setting this; normally around 38 -42 inches is typical. Will coating, which increased heat, raise the Mach no. and thus require a longer pipe for scavaging for the same rpm?
I didn't know the side pipe(muffler) lenght had much effect on overall exhaust performance other than back pressure. Could you elaborate on how/what the side pipes contribute to the overall exhaust design in tech terms? Also there is a general comment that Cobra style 4 into 1 pipes/mufflers are a poor design and waste alot of power. I don't understand or believe this common folk story. Any comment/observations?

thanks

gn

[Richard Hudgins]

GN,

Let me try to answer to the best of my limited knowledge.

1. The negative pulses are a result of rapid pressure change (read expansion) from the primary into the secondary. Every time that a positive pulse (High pressure) meets a expansion point, a negative is created. Therefore, when the pulse reaches the collector a negative pulse is generated. However, there is still considerable pressure and therein energy in the initial positive pulse. When this continuing pulse reaches the sidepipe (which typically has a greater area than the collector) another negative pulse is created. This positive pulse will still have considerable energy after this event and will produce another negative pulse at the exit point to atmosphere.

The timing of these pulses are very critical to cylinder scavenging and tuning as I am sure you know. We used to go to a lot of trouble with anti-reversion schemes etc. to place the negative pulse X distance prior to the exhaust valve face to maximize the resulting positive pulse and to minimize mixture dilution due to the creation of this positive pulse.

2. You are correct. There are standard equations for determining the correct primary and secondary lengths for the exhaust system. A typical one used for our type of engines (Low rpm, large bore) is swept area X 2 = primary pipe volume. Or for a 351 43.875 X 2 = 87.75 Therein, with a 1.75 primary the total length should be 36.5625 to the collector. (note: this needs to include the port area as well) This volume will give you good results in the 4>6000 range. (Of course, now you have to factor in the collector and sidepipe volumes to match pulse timing.)

3. Yes, temp changes mach number. Also, the higher the temp the higher the energy contained in the positive pulse. Therein, the strenght of the negative pulse is greater. Each time there is a expansion in the system, the energy of the pulse is reduced from cooling. Therefore, the higher the specific heat at each expansion event the stronger the pulses. I have seen very positive results with ceramic coatings as it pertains to power production.

4. The design of the sidepipes on these cars is basically good. But, they are seldom sized to the engines installed due to the wide variety of engine choice and visual contraints.

Now, we need someone who really knows exhaust design to give us good information.

[jdean]

Niles,

Really what I meant was not expecting that there are mufflers that flow as well or better than a 2 1/4" straight pipe. I was surprised to see it possible.

The curious thing is how much more 1/4" in pipe diameter can flow. I'm sure it makes perfect sense mathimatically, but still seems like a larger difference that I would have guessed.

John

[flipper35]

Years back when the Trans-Am cars were made to run mufflers on certain tracks, they found they were making more power with and ran mufflers on all the tracks, required or not. I think nitro burning cars are one of the few instances where flow is paramount.