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Discussion Starter #1
Hi all, I'm getting ready to put my 69 Mustang back on the road with a built 306 roller motor. I want to go with a 3" exhaust, but I don't want it to be a total race car. Any recommendations on what mufflers are semi streetable in that size? Or am I just better off with 2.5"? I'm not planning on spraying the car, but a stroker may be in my future and I hate doing things twice. Whatever mufflers I put on will have dumps since there is no room for tail pipes because of the fuel system. Resonance is the biggest issue as I almost don't care how loud it is outside the car, as long as I can keep the interior noise liveable. I was thinking maybe a 3" Super Turbo or a Borla maybe? Any help would be appreciated. Thanks!

Dave
 

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I have the dynomax 3 inch racing mufflers on a 514 and they are on the loud side
 

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A big key to getting rid of resonance is to run pipes off the mufflers that exit in front of the rear wheels and clear the body by at least a inch or two. If the exhaust doesn't clear the body resonance will be far worse. Also, when buying mufflers make sure that the 3" mufflers you choose have 3" tubing inside them. Many 3" mufflers have the same 2 1/2" tubing inside them as their smaller mufflers and negate any advantage that the 3" piping has given you. Choosing exhaust is a lot different that choosing cams and intakes though. When choosing cams and intakes you always need to compromise. More cam will make more power but will almost always reduce low end torque. However with a exhaust system it's different. Once you're past the collectors, more exhaust flow always means more power and torque at all RPM levels. So this is a case where bigger is always better if you can stand the noise. Go with a 3" system and good low restriction mufflers and you won't regret it.
 

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Discussion Starter #4
Yeah, that's what I figured in regards to exhaust size. I know that if I do a 2.5in system I'll always be wanting to upgrade to a 3in, especially if I build a stroker for it. As it is, the motor is a 10.5:1 306 w/ AFR 185cc heads, Comp 282HR cam, Vic. Jr., and a 750HP holley so I was guessing a 2.5in would be maxed out while a 3in would give me room to grow. How about the Hooker SuperComp mufflers? I know they make those in a 3in, has anyone had any experience with them? Thanks for the help!

Dave
 

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I like Edelbrock mufflers. They sound great. Take a look at them at thier site. I use the RPM 'Heavy Duty' series (part #5525). These are fully welded stainless steel. They are also very small for a 3" muffler and fit cramped spaces. (mustangs)


http://www.edelbrock.com/automotive/mufflers.html

Good Luck!


_________________
Mike Burch, 66 mustang real street
302 4-speed 289 heads, 10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367

<font size=-1>[ This Message was edited by: n2omike on 3/12/02 5:26am ]</font>
 

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Exhaust science is a most interesting subject, and widely misunderstood and misapplied. When you buy a carb, do you ask what the venturi sizes are? When you buy heads, do you ask how many cross-sectional square inches the port has? No! You ask how much air they FLOW.

When we choose an exhaust system, we should think the same way. The engine can't "see" pipe or muffler size; but it responds immediately to changes in flow.

Various mufflers of different brands flow quite differently in the same "sizes." Nearly all companies have the flow numbers available; there have been tests done and published in the magazines as well. Make sure the pressure drop is specified in the flow figures, so you know you're comparing apples to apples. Mufflers should be tested at low pressure drop, like 1.5 inches mercury.

What you want to do is size the entire exhaust system to have no more than 0.2 psi of backpressure in the system at maximum engine speed. A guideline to attain that goal is to allow 2.2 cfm exhaust flow per engine horsepower. So a 400 horsepower engine x 2.2 cfm = 880 cfm total exhaust system flow. This will put you within 1% of what the engine will produce with open headers.

Selecting pipe size is easy. With mandrel bends (no "squashed" sections in the pipe), a pipe system will flow well over 100 cfm per square inch of cross-sectional area. (An 18 inch section of pipe measures about 115 cfm at 1.5 inches of mercury). So a 2.5 inch pipe system will easily flow 500 cfm. With a dual exhaust system, that's 1000+ cfm, enough to handle the healthiest of naturally aspirated, streetable small blocks. More pipe won't make measureably more power.

The trick comes in selecting the muffler. Once you've selected a suitable pipe size, you need to be sure that the muffler isn't "putting a cork" in the system. Check the flow numbers on the muffler and make sure that they will deliver total flow that allows 2.2 cfm per engine horsepower. Some 2.5" mufflers will do that handily; others won't. If you wanted to use a particular brand and type of muffler whose flow was less than desired, you could always step up to the next larger MUFFLER size, and leave the pipe sizes the same; just adapt at the muffler connections.

More muffler flow and pipe size than needed just costs more and makes more noise. Do your homework and get it right the first time. Less noise, less cost, max power.

Use an H-pipe or preferably an X-pipe with any setup. They always mitigate resonance and noise, usually help power, and never hurt power. No down side.

Steve Amos
 

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You can estimate with reasonable accuracy how much air a given diameter of exhaust pipe will flow at a realistic pressure factor by calculating its cross-sectional area. Area is equal to pi x radius(squared).

As mentioned, a short, straight pipe will flow about 115 cfm per square inch of cross-section area at 1.5" of mercury pressure drop.

So, let's apply the formula for area to your exhaust pipe. if you have a pipe with 2.5 inches inside diameter, the radius is half of that, or 1.25 inches. Pi is about 3.14, so the formula for a 2.5 inch exhaust pipe would be: 3.14 x (1.25 x 1.25) = 4.9 square inches of cross-sectional area. Multiplying 4.9 x 115 cfm/inch gives you 563.5 cfm flow capacity. I'd recommend lowering that flow/inch factor to about 100 (instead of 115) to allow for a longer pipe with a couple of bends in it. In that case, you'd find a 2.5" pipe system flowing about 500 cfm per side.

If you haven't dynoed your engine and don't know the flywheel horsepower (to do the 2.2 cfm per horsepower calculation), use the flow bench numbers from your exhaust port x number of cylinders. Your exhaust system won't have to move more air than your heads will flow.

Steve Amos
 

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If you haven't dynoed your engine and don't know the flywheel horsepower (to do the 2.2 cfm per horsepower calculation), use the flow bench numbers from your exhaust port x number of cylinders. Your exhaust system won't have to move more air than your heads will flow.
These flowbench numbers are measured at only 28 inches of water. Those gases are being pushed out of the cylinder under much higher pressure than that.

If we were dealing with a smooth, steady flow of exhaust, we could try and do some simple math, but in reality... we are also dealing with high pressure pulses, in addition to mass flow. It's why seemingly restrictive Flowmaster mufflers seem to work as well as they do.

You'll hurt an engine's performance way more by using pipe that is too small than too large. When in doubt, go larger.


Cross section of 3.0" pipe = 7.07 square in.
Cross section of 2.5" pipe = 4.91 square in.

I really like the performance and deep sound of the 3" pipe on the mustang. The larger pipe doesn't really have a louder sound, but a deeper one. It sounds good. It's just hard to do tailpipes for most cars.

No matter what pipe size is used, do NOT use muffler shop tailpipes. They are kinked all to hell in making those tight bends up over the axle. Flowmaster makes mandrel bent tailpipes for most cars out there, and most only cost around $99 a pair. It's the best hundred bucks you'll spend on the car. (assuming tailpipes are going to be used)

Good Luck!
 

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Discussion Starter #10
Hi, I would like to thank you all for the insight, but I'm still a little unsure what's out there. I have a set of 2.5in Edelbrock's on my Lincoln with an X-pipe and don't like their sound with the X. They are far better with an H instead, but I was looking forward to putting an X on the stang. What about the borla XR-1 mufflers? How loud are they? Any insight is appreciated. And I am going to go 3" on this car, btw. Thanks!

Dave
 

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Previous post:

"These flowbench numbers are measured at only 28 inches of water. Those gases are being pushed out of the cylinder under much higher pressure than that.

If we were dealing with a smooth, steady flow of exhaust, we could try and do some simple math, but in reality... we are also dealing with high pressure pulses, in addition to mass flow."


The above is absolutely true; we are dealing with pulses instead of steady flow... however, believe it or not, dyno tests have shown that if you match your exhaust side head flow to exhaust system flow, you'll hit within 1% of open pipe capability. (Actually, this is completely true only as long as you have a significant diameter change, or expansion box, at the proper point in the system to define the ideal header collector length. Those pressure pulses need to be reflected on a harmonic that induces a negative pressure effect during the valve overlap phase. The more overlap you run in your cam grind, the more important this becomes).

The reason the math works out is that although head flow is only measured at 28 inches of water pressure drop, it is a constant measurement. The much higher pressure occurs in very short spikes, followed by a negative pressure wave in tuned exhaust systems. Simply stated, the flow averages out and the math works.

The one place you could get off track by calculating exhaust flow requirements based on exhaust port flow is when using power adders. With a supercharger, nitrous, etc. you will far exceed the net mean effective cylinder pressure that is possible with a naturally aspirated motor. That means the exhaust port will likely flow more total mass than it would in a N.A. motor.

Morale of the story: only use the exhaust port flow method to calculate exhaust system requirements for a naturally aspirated motor. For a power adder set-up, go back to the 2.2 cfm per horsepower calculation, and you're home free again.

Be sure to follow N2OMike's advice on mandrel bent pipes; you might as well shove potates in the tailpipe as use some of the "squashed" stuff that non-performance oriented muffler shops produce. It's cheaper and faster, but doesn't work.

Steve Amos
 

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P.S: If you LIKE the sound of 3" pipes and mufflers, they'll fit under your car, and you've got the bucks...GO FOR IT! One of the joys of our hobby is doing things because we want to, not having to follow someone else's rules. Viva la difference!
 

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All that theory sounds great, but the fact remains that when you flow even a very good 2 1/2 inch full length dual exhaust system you'll get numbers in the 450-600 cfm range at 28" of water which is good for about 275 HP at best. Flowing the mufflers by themselves will usually give you between 280 and 380 cfm each at the most which would be sufficient for up to 350 HP if their wasn't much pipe and no bends before or after them. Those CFM numbers correspond well with dyno tests as well. On the dyno, a 450 HP engine will definetly benefit when changing from a 2 1/2" to a 3" exhaust system.
 
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