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Exhaust Sizing

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Old 11-08-2017, 03:58 PM
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I knew this info still existed somewhere. Below is one of the reasons why I ditched a 3” single setup in favor of true duals on my heads/cam car many moons ago and never looked back. Some good food for thought:

“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…

Now these numbers are assuming that everything else is set up perfectly. The muffler must flow as much as the open pipe or more to get zero loss at the listed hp levels. He also states that using a muffler with a larger inlet/outlet diameter than your exhaust pipe is a great way to get more out of a smaller diameter system since the muffler flow will be able to match the straight pipe flow. Now there are many other things to consider when designing an exhaust system but I figured this would give a great foundation to build on.

Just a side note that he stated that I thought was neat... “Just as fish don’t feel the weight of water, we don’t readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons!”

https://ls1tech.com/forums/generatio...ml#post2987587
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Old 11-09-2017, 08:40 AM
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Originally Posted by StealthFormula View Post
I knew this info still existed somewhere. Below is one of the reasons why I ditched a 3” single setup in favor of true duals on my heads/cam car many moons ago and never looked back. Some good food for thought:

“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…

Now these numbers are assuming that everything else is set up perfectly. The muffler must flow as much as the open pipe or more to get zero loss at the listed hp levels. He also states that using a muffler with a larger inlet/outlet diameter than your exhaust pipe is a great way to get more out of a smaller diameter system since the muffler flow will be able to match the straight pipe flow. Now there are many other things to consider when designing an exhaust system but I figured this would give a great foundation to build on.

Just a side note that he stated that I thought was neat... “Just as fish don’t feel the weight of water, we don’t readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons!”

https://ls1tech.com/forums/generatio...ml#post2987587
Post 19
Thanks for this - this is some of the general information I was seeing online when searching before. Everyone's calculations are slightly different, but in this case they are saying a 3" single pipe is good for 370 HP with no losses. By these types of charts, ideally I could run a dual setup, or a 3.5" single.

But I guess the other piece of the puzzle that's missing for me is the "with no losses" part - how much of a loss would we be talking? Obviously it must go up exponentially the more power over that supposed max number you get, but if you're running with a motor that makes 450 HP, I wonder how much is lost on a 3" single vs 3.5"? If we're talking 2 or 3 HP that's pretty negligible, if it's more then it's maybe something to think about.
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Old 11-09-2017, 08:49 AM
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Originally Posted by handyandy View Post
Thanks for this - this is some of the general information I was seeing online when searching before. Everyone's calculations are slightly different, but in this case they are saying a 3" single pipe is good for 370 HP with no losses. By these types of charts, ideally I could run a dual setup, or a 3.5" single.

But I guess the other piece of the puzzle that's missing for me is the "with no losses" part - how much of a loss would we be talking? Obviously it must go up exponentially the more power over that supposed max number you get, but if you're running with a motor that makes 450 HP, I wonder how much is lost on a 3" single vs 3.5"? If we're talking 2 or 3 HP that's pretty negligible, if it's more then it's maybe something to think about.
I agree, that would be very beneficial information that would really further tell the story. I suppose it’s not out there given all of the potential variables. I think the best that we can all do is search for and examine dyno results for folks at similar power levels that have gone from the traditional 3” single setups to larger single setups or duals. It’s definitely out there, I’ve seen many before and after results on LS cars over the years.

I wish I still had my 3” ORY and Magnaflow catback with cutout. If I did I’d take it to the dyno and get numbers with cutout closed, cutout open, and finally with true duals and overlay the graphs. Would be great info to have out there.

Last edited by StealthFormula; 11-09-2017 at 09:00 AM.
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