How much exhaust pressure is there behind the muffler?
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How much exhaust pressure is there behind the muffler? I'm sure a few of you guys have placed pressure and/or flow meters before and after a muffler in order to determine it's restrictiveness. My question is how much pressure is there downstream of a typical muffler at WOT (i.e. on a stock exhaust, the 2 pipes that lead form the muffler to the tips)? Is there almost none since it's basically just an open pipe with a large restriction upstream, or is it still pretty significant?
The reason I ask is because I've spent dozens of hours and a good chunk of change fabbing up a true 4" exhaust system with great flow characteristics in mind. The thing is I got CME tips for Christmas and I really want to put them on, but the inlet to the tips are only 2.5" instead of the 3" I have coming back from my muffler. Would this reduction in tube diameter be enough measureably reduce flow? It would just be so stupid to spend so much time and so much money on a bitchin 4" exhaust system, only to have it necked down to 2.5" in the last one and a half inches.
The reason I ask is because I've spent dozens of hours and a good chunk of change fabbing up a true 4" exhaust system with great flow characteristics in mind. The thing is I got CME tips for Christmas and I really want to put them on, but the inlet to the tips are only 2.5" instead of the 3" I have coming back from my muffler. Would this reduction in tube diameter be enough measureably reduce flow? It would just be so stupid to spend so much time and so much money on a bitchin 4" exhaust system, only to have it necked down to 2.5" in the last one and a half inches.
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Is the 4 inch system one pipe after the "Y" ?. If so, 2 x 3 inch muffler outlets will be enough flow area, but anything you do to restrict those 3 inch tailpipes will increase backpressure. Will it cause significant hp loss? I guess it depends on your total hp. Your 4 inch system should support well over 500 rwhp. More than likely the muffler is the biggest rerstriction if you have that much hp. What muffler?
You might fab up the system and dyno the car back-to-back with and without the tips. That should tell you if you want to use them or not.
You might fab up the system and dyno the car back-to-back with and without the tips. That should tell you if you want to use them or not.
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David Vizard says that exhaust tubing will flow 115 CFM per square inch @ 1.5" Hg., and that it takes 2.2 CFM per HP to approach zero backpressure. Therefore, a pair of 2 -1/2" tailpipes would flow 1.25 x 1.25 x 3.14 x 115 = 564 CFM and would support 256 HP each with virtually no power loss.
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Originally Posted by MadBill
David Vizard says that exhaust tubing will flow 115 CFM per square inch @ 1.5" Hg., and that it takes 2.2 CFM per HP to approach zero backpressure. Therefore, a pair of 2 -1/2" tailpipes would flow 1.25 x 1.25 x 3.14 x 115 = 564 CFM and would support 256 HP each with virtually no power loss.
Of course we could get into gasses cooling across the muffler, and the effects on flow, but that's probably unnecessary. Many mufflers won't flow what the 4 inch pipe flows anyway. Choose your mufflers as carefully as you choose your wife.
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OldSS- I'm still in the process of fine tuning the cam so I haven't dynoed it yet, but I'm thinking flywheel should be greater than 600HP. People with similar setups are getting between 410-450. I'm hoping for around 420. Factoring in a 15% drivetrain loss and a 150HP shot of nitrous that's somewhere in the neighborhood of 633HP.
I guess what I should be asking myself is if my 4" Spintech muffler can outflow 2x2.5" straight pipes. If it can't, this discussion is moot. Spintech claims a 2.5" inlet muffler of the same type can flow 170CFM @ 5" of water. How well that scales up to 4" inlet and 28" of water, I don't know.
Now that I'm thinking about it I think I'm just going to modify the tips so they're 3" inlet. If I do it any other way it'll be nagging me in the back of my mind every time I drive the car.
I guess what I should be asking myself is if my 4" Spintech muffler can outflow 2x2.5" straight pipes. If it can't, this discussion is moot. Spintech claims a 2.5" inlet muffler of the same type can flow 170CFM @ 5" of water. How well that scales up to 4" inlet and 28" of water, I don't know.
Now that I'm thinking about it I think I'm just going to modify the tips so they're 3" inlet. If I do it any other way it'll be nagging me in the back of my mind every time I drive the car.
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Originally Posted by Zeus
OldSS- I'm still in the process of fine tuning the cam so I haven't dynoed it yet, but I'm thinking flywheel should be greater than 600HP. People with similar setups are getting between 410-450. I'm hoping for around 420. Factoring in a 15% drivetrain loss and a 150HP shot of nitrous that's somewhere in the neighborhood of 633HP.
I guess what I should be asking myself is if my 4" Spintech muffler can outflow 2x2.5" straight pipes. If it can't, this discussion is moot. Spintech claims a 2.5" inlet muffler of the same type can flow 170CFM @ 5" of water. How well that scales up to 4" inlet and 28" of water, I don't know.
Now that I'm thinking about it I think I'm just going to modify the tips so they're 3" inlet. If I do it any other way it'll be nagging me in the back of my mind every time I drive the car.
I guess what I should be asking myself is if my 4" Spintech muffler can outflow 2x2.5" straight pipes. If it can't, this discussion is moot. Spintech claims a 2.5" inlet muffler of the same type can flow 170CFM @ 5" of water. How well that scales up to 4" inlet and 28" of water, I don't know.
Now that I'm thinking about it I think I'm just going to modify the tips so they're 3" inlet. If I do it any other way it'll be nagging me in the back of my mind every time I drive the car.
Last edited by Old SStroker; Jan 11, 2006 at 07:06 AM.
Originally Posted by MadBill
David Vizard says that exhaust tubing will flow 115 CFM per square inch @ 1.5" Hg., and that it takes 2.2 CFM per HP to approach zero backpressure. Therefore, a pair of 2 -1/2" tailpipes would flow 1.25 x 1.25 x 3.14 x 115 = 564 CFM and would support 256 HP each with virtually no power loss.
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Originally Posted by Ari G
And what about mandrel bent pipes woulden´t they cause a huge reduction in flow?
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Originally Posted by Zeus
Why is this? Is it just the fact that they are bends or is it something about mandrel bends in particular?
Mandrel are better then normal because the diameter is the same but bent in a machine the diameter gets smaller that causes high reduction in exh.flow
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mandrel bends are much less restrictive than the crimped bend, out-of-the-box pipes that muffler shops use and overcharge for. Just try to have as few bends and as small of bends as possible.
Hotter air flows slower? I wouldnt see why. Hotter air would be less dense, so it would seem like if the same amount (mass) of air is going to pass through a pipe, hotter air would have to go 'faster' through the pipe than colder denser air.
Hotter air flows slower? I wouldnt see why. Hotter air would be less dense, so it would seem like if the same amount (mass) of air is going to pass through a pipe, hotter air would have to go 'faster' through the pipe than colder denser air.
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In the article, Vizard qualifies these recommendations as applying to normally aspirated engines with long duration (>290 advertised duration) cams. He actually says: "..a length of straight pipe the length of a typical muffler...", so depending on how long the system is and how tight and numerous the bends, a little 'insurance' re diameter might be in order.
Zeus: I don't know the flow of a 4" SpinTech, but in the same article, a 4" oval Borla flowed 1,100 CFM and a 2.5" 'Flowmaster street muffler' flowed 290.
Also, flow increases as the square root of pressure difference, so using 1.5" Hg (~20.5" H2O) instead of 5" H2O would give a flow of: sq. rt. of (20.5/5) or ~ 2 x 170 = 340. By area, a 4" ST should be ~ 880 CFM, good for only 400 HP.
Zeus: I don't know the flow of a 4" SpinTech, but in the same article, a 4" oval Borla flowed 1,100 CFM and a 2.5" 'Flowmaster street muffler' flowed 290.
Also, flow increases as the square root of pressure difference, so using 1.5" Hg (~20.5" H2O) instead of 5" H2O would give a flow of: sq. rt. of (20.5/5) or ~ 2 x 170 = 340. By area, a 4" ST should be ~ 880 CFM, good for only 400 HP.
Originally Posted by MadBill
In the article, Vizard qualifies these recommendations as applying to normally aspirated engines with long duration (>290 advertised duration) cams. He actually says: "..a length of straight pipe the length of a typical muffler...", so depending on how long the system is and how tight and numerous the bends, a little 'insurance' re diameter might be in order.
Zeus: I don't know the flow of a 4" SpinTech, but in the same article, a 4" oval Borla flowed 1,100 CFM and a 2.5" 'Flowmaster street muffler' flowed 290.
Also, flow increases as the square root of pressure difference, so using 1.5" Hg (~20.5" H2O) instead of 5" H2O would give a flow of: sq. rt. of (20.5/5) or ~ 2 x 170 = 340. By area, a 4" ST should be ~ 880 CFM, good for only 400 HP.
Zeus: I don't know the flow of a 4" SpinTech, but in the same article, a 4" oval Borla flowed 1,100 CFM and a 2.5" 'Flowmaster street muffler' flowed 290.
Also, flow increases as the square root of pressure difference, so using 1.5" Hg (~20.5" H2O) instead of 5" H2O would give a flow of: sq. rt. of (20.5/5) or ~ 2 x 170 = 340. By area, a 4" ST should be ~ 880 CFM, good for only 400 HP.
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Hotter gas will be flowing faster.
Do we know the context of Vizard's statements? Unless we know otherwise I'll assume he was talking about flow at the front of the car. ie. in the headers where the gasses are hotter and moving faster.
That being the case, when you get way to the back of the car the gasses are cooler and slower therefore smaller tubing won't be restrictive.
The real answer is to dyno test. You will put the finished car on the chassis dyno right?
Do we know the context of Vizard's statements? Unless we know otherwise I'll assume he was talking about flow at the front of the car. ie. in the headers where the gasses are hotter and moving faster.
That being the case, when you get way to the back of the car the gasses are cooler and slower therefore smaller tubing won't be restrictive.
The real answer is to dyno test. You will put the finished car on the chassis dyno right?
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sure do, I have to get a new clutch in though as due to extreme incompetency by absolute morons who refer to themselves by the name of "mechanics", I have a stock clutch in there now and it can't hold the power of the cam, let alone cam + spray.
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I've already modified the tips so that they're 3" inlet, but I think my first round of dyno runs is going to be Spintech/no Spintech. I've come to accept the fact that I'm going to loose some decent power with it in place, but I'm going to find out how much. If it's around or less than 10HP I'll leave it on, if not I'll just throw a 4" diesel muffler in there. Then I'll decide what to do with the tips becuase I can't have them hooked up in a functional configuration if I end up with the diesel muffler on there.
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Originally Posted by jyeager
Hotter gas will be flowing faster.
Do we know the context of Vizard's statements? Unless we know otherwise I'll assume he was talking about flow at the front of the car. ie. in the headers where the gasses are hotter and moving faster.
That being the case, when you get way to the back of the car the gasses are cooler and slower therefore smaller tubing won't be restrictive.
The real answer is to dyno test. You will put the finished car on the chassis dyno right?
Do we know the context of Vizard's statements? Unless we know otherwise I'll assume he was talking about flow at the front of the car. ie. in the headers where the gasses are hotter and moving faster.
That being the case, when you get way to the back of the car the gasses are cooler and slower therefore smaller tubing won't be restrictive.
The real answer is to dyno test. You will put the finished car on the chassis dyno right?
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I have measured the exhaust backpressure on several vehicles. I just drilled and tapped the exhaust pipe near the header for a 1/4" pipe fitting, added a foot or so of copper tubing to isolate the plastic aquarium tubing I used from the heat, and ran the tubing to a fuel pressure/vacuum gauge. A stock single exhaust V-8 can have 12 psi or more at peak RPM. A common pair of performance mufflers on a 400 HP V-8 might give ~ 5.0 psi. With a big cam (290 + plus seat duration on the exhaust) Vizard says we need less than two tenths psi to eliminate muffler-induced power losses!
Tips:
1. If you don't want to drill your pipe, you can get an O2 sensor plug, drill it for the fitting, and temporarily replace an O2 sensor with it.
2. Cats have more backpressure when hot. Be sure to fully warm up the engine for accurate results.
3. Bulletin: Big cams cause strongly pulsating exhaust! Clamp a small pair of vice grips on the hose and squeeze it almost shut to damp the signal, otherwise the gauge needle will beat itself to death in seconds (don't ask me how I know this...
)
Tips:
1. If you don't want to drill your pipe, you can get an O2 sensor plug, drill it for the fitting, and temporarily replace an O2 sensor with it.
2. Cats have more backpressure when hot. Be sure to fully warm up the engine for accurate results.
3. Bulletin: Big cams cause strongly pulsating exhaust! Clamp a small pair of vice grips on the hose and squeeze it almost shut to damp the signal, otherwise the gauge needle will beat itself to death in seconds (don't ask me how I know this...
)