Single 4" Exhaust
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Single 4" Exhaust For a naturally aspirated engine, when does a single 4" exhaust become a restriction?
I've got a 12.5:1 solid roller LS7 setup with a Holley Hi-Ram intake. I'm thinking it should be around 630 to 670 at the wheels? Current exhaust is 1-7/8" ARH headers with 3.5" collectors, Custom 3.5" Y-Pipe into a custom single 4" catback with a Dyantech 4" muffler.
Just curious if anyone had a similar scenario where a 4" catback became a restriction for a naturally aspirated setup. I know forced induction setups can support crazy hp numbers.
I've got a 12.5:1 solid roller LS7 setup with a Holley Hi-Ram intake. I'm thinking it should be around 630 to 670 at the wheels? Current exhaust is 1-7/8" ARH headers with 3.5" collectors, Custom 3.5" Y-Pipe into a custom single 4" catback with a Dyantech 4" muffler.
Just curious if anyone had a similar scenario where a 4" catback became a restriction for a naturally aspirated setup. I know forced induction setups can support crazy hp numbers.
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Since it's not being forced out by a blower or turbo,,, yes, absolutely! If you're strictly relying on the pumping forces of the pistons (NA), the exhaust size becomes a restriction much lower on the HP scale.
Here's an example of comparison:
The factory cast exhaust manifolds with a turbo could possibly support 1200hp on a turbo application. How much hp will those factory cast exhaust manifolds support on a naturally aspirated combination? 400? 425?
Here's an example of comparison:
The factory cast exhaust manifolds with a turbo could possibly support 1200hp on a turbo application. How much hp will those factory cast exhaust manifolds support on a naturally aspirated combination? 400? 425?
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A turbo doesn't force out the exhaust. Its actually a huge exhaust restriction. That is why its not uncommon to see 40 PSI of pressure between the valves and the turbo, and the pistons have to work much harder to force out the exhaust which can cause pumping losses. Think of putting a multi-chambered, baffled, and packing filled muffler right on the exhaust of your 1200 HP car.
The factory cast manifolds will support tons of power N/A as well. I think you're confusing how much something can actually make, and the point at which is starts costing you horsepower, as they are 2 different things.
The factory cast manifolds will support tons of power N/A as well. I think you're confusing how much something can actually make, and the point at which is starts costing you horsepower, as they are 2 different things.
From the Sticky and the old David Vizard article...
Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
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Real information, holy ****! That's gettin it done.My math comes out about the same. Calculating area of a circle (x2 for dual exhaust) you can see that a 4" single lands you between a 2.5 and 3.0 dual exhaust. So about as good as you can get without going true 3" duals.
2.5 duals area = 9.81
3.0 duals area = 14.13
3.0 single = 7.07
4.0 single = 12.46
Also good evidence that a single 3" system is fairly restrictive. Making no claims on power like jake did though. But it seems pretty clear to me.
Last edited by AnotherWs6; Nov 28, 2016 at 03:02 PM.
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From: Vancouver, WA
A turbo doesn't force out the exhaust. Its actually a huge exhaust restriction. That is why its not uncommon to see 40 PSI of pressure between the valves and the turbo, and the pistons have to work much harder to force out the exhaust which can cause pumping losses. Think of putting a multi-chambered, baffled, and packing filled muffler right on the exhaust of your 1200 HP car.
The factory cast manifolds will support tons of power N/A as well. I think you're confusing how much something can actually make, and the point at which is starts costing you horsepower, as they are 2 different things.
The factory cast manifolds will support tons of power N/A as well. I think you're confusing how much something can actually make, and the point at which is starts costing you horsepower, as they are 2 different things.
Please read the very first question in my original post #1. Talking about when the exhaust size becomes a "restriction" for an NA engine. This is where you got off topic, not me.
Not talking about overall capability or even a forced induction engine at all. And you're correct, there's huge difference and very different comparisons. Of course you could pipe a pro-stock engine through a single 4" exhaust pipe, but it would be severely restricted (but my original question is,,,,, when, and what size for NA?). The gains vs size will not impact a forced induction car on the same scale as a naturally aspirated engine. If you don't recognize this fact then we can just agree to dis-agree. 
Thread Starter
11 Second Club
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From: Vancouver, WA
From the Sticky and the old David Vizard article...
Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
Thread Starter
11 Second Club
iTrader: (37)
Joined: Mar 2004
Posts: 2,046
Likes: 3
From: Vancouver, WA
From the Sticky and the old David Vizard article...
Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
Hey guys I just got done reading a good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.). From reading the article I think that a lot of people that see gains when going from a 2.5” exhaust system to a 3” dual system see them because the muffler on the 2.5” system didn’t flow enough for their application. David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
I was curious why the math was not computing with the numbers on that link above, so I calc'd it out by subtracting the tubing wall thickness and then it calc'd out correctly. Most tubing sizes uses outside diameters for sizing references ie, 3" = OD. After subtracting the wall thickness of 1/16th (1/8th overall), it has a significant impact on the numbers. I made a spreadsheet that came up with these:
Single 3" = 339hp
Single 3.5" = 467hp
Single 4" = 616hp
Single 4.5" = 785hp
Single 5" = 975hp
Dual 3" = 678hp
Dual 3.5" = 934hp
Dual 4" = 1232hp
Dual 4.5" = 1570hp
As you can see, a single 4" starts becoming a restriction after 616hp. I'm hoping mine will be closer to 750-775hp (at crankshaft). I'm now leaning toward some dual 3.5" pipes/mufflers straight off the 3.5" collectors and dumping before the rear axle. Not much else I can do. I'd love to keep a full catback but not at the expense of giving up potential hp losses.