An engineer would probably know this best.

Comparing exhausts. Alright, mufflex 4" is way big, bigger than most any car would need except turbo/sup...doesnt hurt, but not necessarily needed...ok?

Well then as a layperson I would think that a set of 2" duals would be the same amount of overkill as a 4" mufflex, but I just know that cant be true.

So what is going on here, can someone explain the relation between single pipe size compared to dual pipe size and where they equal flow?

 

Leaving the physics out of it, you can get a good idea figuring the cross section. It works out that a 4" tube has a cross section of 50.24" while a 2" pipe has a cross section of 12.56". It would take four 2" pipes to have the same cross section, and the laminar restriction (for lack of a better word) is much greater because of the increased surface area. To get the same area as a 4" single, you would need almost two 3" tubes.

People who are jealous of Mustang guys that can easily run a 2.5" dual setup don't consider that two 2.5" pipes have a combined CSA of 39.25, which is about equal to one 3.5"- commonly available..

It seems to me (complete layman) that the exhaust system in the 3rd and 4th gens might be a good design?

 

I think part of the answer is volume of a tube ( Pi * radius^2 * h)

Interference tuning vs. individual bank exhaust.
A Y-pipe and single tube exhaust will degrade the collector's ability
to tune pressure pulses; where seperate banks can focus on pipe
length for proper tuning.


Reversion vs. Scavenge
A large pipe will create higher pressure in the exhaust causing a
reverse flow of charge - mainly at low RPM, resulting in loss of torque.
Smaller diameter pipe will allow exhaust gas velocity to build sooner to help scavenge the chambers.

That's my 0.01 cent

It will be interesting to see what the real Engineers have to add.

 

How do you figure cross section?

That sounds right then since 2-2" pipes is barely half the CSA of a single 4". But like zero said...tuning probably has a lot to do with it, so we will need an engineer with a few fluids classes under his belt, as well as knowledge of race engines to be able to answer the question completely.

 

Area of the circle created by the inner wall surface.

2 pipes x 2" pipe cross-section = total cross section
2 pipes x (Pi * 1"^2) = total cross section
6.28 square inches = total cross section

4" pipe cross-section = total cross section
Pi * 2"^2 = total cross section
Pi * 4 sqaure inches = total cross section
12.56 square inches = total cross section (100% larger)

Quote:

That sounds right then since 2-2" pipes is barely half the CSA of a single 4". But like zero said...tuning probably has a lot to do with it, so we will need an engineer with a few fluids classes under his belt, as well as knowledge of race engines to be able to answer the question completely.

Actually it's exactly half, by cross-section... but talking into account surface-fouling in fluid flow through a pipe, the effective Inner diameters are a bit less. For a dual-exhaust system with the same cross-section as a single-pipe design (say a 4" pipe vs. a dual 2 7/8" design), the flow rates would actually be a bit lower in the dual system.... and the weight of the tubing would be greater.

 

You should really use the inside diameter to figure cross section or "flow area". A close approximation for exhaust systems is an .050 wall thickness, so subtract 2 x .050 or .100 from the outside tube size to get the inside diameter ID).

A 4.0 in. tube has ID of 3.90 or 11.95 sq. in.
A 2.0 in. tube has ID of 1.90 or 2.84. 4 x 2.84 = 11.36

FWIW, circular area is easy to figure on a calculator.
A=PI x radius x radius, but it also = PI x diameter x diameter divided by 4, because radius = diameter/2.

Anyhow, PI/4 =.7854 which are the numbers if you go clockwise in the upper left corner of a keypad.

So area = diameter x diameter x .7854. Many simple calculators will square a number if you key in the number and hit "x' then "="

Example: Here's the keystrokes"

1.9X=X.7854 I got 2.835294 on my $2 calculator. 2.84 is close enough.

As steve said, the total surface area of the inside of the pipes has an effect because there is little/no flow right at the surface. To compare surface area, use the circumference of the ID s. That is PI times diameter. If you approximate PI with 3.14, you are accurate to about 1/100 of an inch, so circumference = 3.14 x ID

1 4 in. tube; 3.14 X 3.9 = 12.25 in.
4 2 in. tubes = 4 X 3.14 X 1.9 = 23.86 in. or about 1.95 times as much. As was said, this area reduces the flow capacity of the 4 2 in. tubes.

Look at 2.5 duals: (2.4 ID)
Area of 2 pipes = 2 X 2.4 X 2.4 X .7854 =9.05 sq in.
Area of a 3.5 (3.4 ID) pipe = 9.07 sq in. with about 1/2 the surface area.

Look at 3 inch header collectors dumping into a 4 inch pipe. Flow area of the 4 in is about 90% or two 3 inch pipes, but with about 2/3 of the surface area. Sounds about right to me.

My $.02

 

So basically what youre saying is, strictly based on surface area and CSA...that a 4" single would be more flow than you could almost ever need (save a few people). Unless you went to at least 3" duals?

What about exhaust scavenging? Since duals/bullets/dumps are basically a straight out flow instead of a lot of bends. I guess to take full acount of what were speaking of youd have to figure that the 4" is a full catback and a muffler, so you can take that weight into acct. and the added surface area of a full length of 4" pipe.

 

It probably depends on hp the engine makes which is pretty much proportional to exhaust flow (or vice versa )

If you are designing a race-only system, by all means choose the end of the system where the engine wants it and where the rules require it.

Cup cars are an interesting example. From the overturned ones I've seen, most folks bring the header collectors to an X cross-over then out both sides thru flattened pipes which are maybe 1-1/4 (or less) high and more than 12 inches wide. Of course they probably(?) dump into a low pressure area on the side of the car or use the airflow to cause an "extractor" effect.

As to collector length, the first big change the collectors see is the X. The collector length to there appears to be 2-3 times where it probably should be for tuning, but as Larry Meaux said in "The Myth" thread, multiples of that optimum length work well.

On a street car with big hp, I'd try to minimize back pressure in the exhaust after the collectors. Cats would be one source of BP, and muffler(s) are probably next. Because exhaust gasses are continually cooling as they travel thru the system (as much as 1000 F), the cfm or flow continually decreases along the system. So it would seem logical to place the muffler as far downstream as possible. Besides, with a visible rear mounted muffler you can put a 6 inch "coffee can" boom tube on it!

Bends are big restrictions to flow but not wave tuning. Packaging determines how many bends you are forced to have. I would tend to keep the pipe a little larger if there were more bends. So, 4 inches is probably not overkill for some full systems, even if it seems like it based on collector size and fwhp.

Putting pressure taps along the system and measuring BP during chassis dyno pulls is a fine idea. If I saw less than 1 psi on a street system that my ears could live with, I'd probably be happy. If you were to put pressure taps just in front of the mufflers on a 450+ rwhp F-bodies you might be surprised at how much BP some folks have, and how little others have. BP behind muffler(s) should be about zero, BTW.

 

I have a pretty healthy 357 (450+ hp) with 1.75"/3" Hookers into a Mufflex Y into a 3.5" catback with a dynomax 21" muffler 3.5" straight through. Am anxious to see if I can stand the noise does that sound like a reasonable setup from a design standpoint?

 

From a previous discussion w/ Flowmaster, I got some guidelines to street performance sizing when asking about using dual 3" exhaust. I was told that dual 3" (or single 4") was not necessary until 450hp.

From conversation, I'll group sizing into 3 categories:

twin 2.25" or single 3" up to approx. 300hp for best performance
twin 2.5" or single 3.5" to approx. 400hp
twin 3" or a single 4" suggested over 450hp

With a properly sized header/collector, a larger than required exhaust does not have a negative effect on performance. With stock manifolds or shorty headers, a big exhaust can have an negative effect as the exhaust is helping to control scavenging.

I've seen open shorty headers (no y-pipe) go slower at the track compared to good full exhaust (3"single no cat) on a mid 13sec LT1 Camaro even with retuning.

I've also seen long tubes on an 11sec LT1 not make a noticable difference in track times with open headers vs. a dual 3" exhaust.

Lastly I have seen a LS1 w/ SLP longtubes, 3" y-pipe LS1 w/ Mufflex 4" pick up .1 sec by replacing the SLP 3" collector w/ a 4" collector to match the Mufflex. That 6" length of single 3" pipe restriction equates to an approximate 10hp loss on a 12 sec car.

Obviously too small does hurt power..... & larger is generally louder just as equivalent area duals are louder vs. a big single due to the firing pulse spacing.

Hope this helps.

 

I guess Im mainly curious because Im trying to balance the two. I dont want any added attention from the popos, i have enough already. And according to Joe, Ill hopefully be close to the 500rwtq mark, so I need something that will flow.

I was thinking dual 2.5", but that seems to small now, so likely I will go dual 3" dumped before the axle.

Anybody have experience with a few different bullet style mufflers? That can give me a route to go for a nice mellow deep tone? I dont mind screaming under WOT, but a nice mellow idle would be nice, to go with the single cylinder at a time firing sound

 

Sounds like everyone hit on the main points already, but I'd like to stress something:

Both mufflers Mufflex offers are CRAP!

Example 1: My old car gained 40 rwhp when I replaced a 3" Flowmaster 2 chamber with an SLP muffler.

Example 2: Thunder just dyno'd an LS1 with heads and cam as well as a Mufflex 4" with Spintech. The car gained 30 rwhp when removing the Spintech.

Example 3: My friend removed his 3.5" Mufflex Spintech and gained 4 mph at the track.

Example 4: My car, which pulls 133 mph, gained squat, zero, nothing when I totally removed the 3" Cat-Back Hooker Aerochamber.

Mike

 

Not arguing...but seems hard to believe any full exhaust would be enough for a big head/cam/blower car?

 

Yes, jon, it surprised a few of us.

The dyno backed up the track results, though. It gained about 3 rwhp when removing the whole 3" cat-back.

I have a theory about the engine being over-exhausted. If you remove backpressure from an engine and it doesn't gain, then the exhaust ports, exhaust cam lobe, and headers may be working too well and overscavenging the cylinder.

Mike

 

Well if the heads/headers are overscavenging the exhaust...then would that exhaust need to go somewhere? I believe you, its just hard to believe an engine like that wouldnt have WAY too much exhaust for anything smaller than 3.5" or 3" duals