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A major seller of aftermarket cat-back exhausts says this on their advertising:

Performance Exhaust
Today, choosing performance exhaust can be confusing. A common misconception is that "bigger is better". Many enthusiasts, and manufacturers too, seem to feel that very large diameter pipes are the only way to go for muscle car performance, while we at XXX have found for street use, big pipes just take energy out of the exhaust, both by slowing down exhaust gas velocity and by cooling off the gases. While exhaust may leave the port at 1500 degrees or more, at the tailpipe it’s down to 150 degrees -- a sign energy has been lost. Velocity and uninterrupted flow are the real performance-making factors.
I'm a bit confused here. Once the exhaust exits the engine, what is it still imparting to the system? (Disregarding a turbo) How does heat 10 feet away in a pipe help the engine perform better? If the EGT at one header is 1,500 degrees, and the second header the same temp, what will the engine care about temp at the tailpipe? Less restriction I can understand. Less temp twelve feet away? Is this marketing hype or should I strap some propane heaters to my tailpipe?
 

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I don't know how it is important, but what they say about going bigger isn't always better is true. I know if you go too big you lose some of your lower end. I'm not sure how it all exactly works, but I know backpressure has a lot to do with it.
 

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I think they are saying that air at a higher temperature has a higher velocity, meaning it will exit the exhaust faster. The faster you can pump air through a motor, the more power you can make. You cant just throw a 5" exhaust on and expect it to work right. It wont have enough velocity. Bigger isnt always better. Smaller tubes will be better for low end torque, while larger tubes will allow the motor to flow enough to make more power at a higher RPM.
 

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Looking at that article they are talking about the primaries. Aren't those the pipes from each cylinder in a header? So I guess that since I'm doing a cat back system the only piece of information that article tells me is: There should be absolutely no back-pressure from the collector rearward. Which to me means Bigger is better!!

Their chart shows a 360" (which is the 5.7 liter) to have 1.5" primaries. So if I have 8 1.5" primaries then to just have the same flow through a common exhaust I would need a 4.5" exhaust. Now that would be sweet!
 

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JJSEBASTIAN said:
Looking at that article they are talking about the primaries. Aren't those the pipes from each cylinder in a header? So I guess that since I'm doing a cat back system the only piece of information that article tells me is: There should be absolutely no back-pressure from the collector rearward. Which to me means Bigger is better!!

Their chart shows a 360" (which is the 5.7 liter) to have 1.5" primaries. So if I have 8 1.5" primaries then to just have the same flow through a common exhaust I would need a 4.5" exhaust. Now that would be sweet!
Also 360 is not a 5.7, I believe 348cu is our 5.7.
If you did do a 4.5" exhaust you would lose power. A friend of my had a 2005 Hemi Ram and did a 4" daul exhaust system and lost back pressure and HP when he dynoed it. I had on my 04 HEMI 2.25" dual pipes with High Flow cats through a Super 40 Flowmaster and it made a big diff. Our Hemi Engines like some back pressure. Also they like a lot of air. Wish Volant would make a intake for our Jeep COmmanders because again on my Ram it also made a huge diff. The most Ive seen guys go is 3" with success. Any bigger I believe would HURT our performance.
 

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OK. I knew it wasn't exactly our 5.7 but it was the closest example in the chart.

I get that the fuel mixture can change depending on the backpressure, but if your engine controls correct for that, then I still would like someone to explain how adding back pressure can add horsepower...
 

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the only thing you don't need back pressure on is a turbo'd motor.

i made my supra NA motor have a 2.5" exhaust. i lost power over the 2" exhaust. i lost the power on the lower end. torque.
 

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sespe commander said:
A major seller of aftermarket cat-back exhausts says this on their advertising:



I'm a bit confused here. Once the exhaust exits the engine, what is it still imparting to the system? (Disregarding a turbo) How does heat 10 feet away in a pipe help the engine perform better? If the EGT at one header is 1,500 degrees, and the second header the same temp, what will the engine care about temp at the tailpipe? Less restriction I can understand. Less temp twelve feet away? Is this marketing hype or should I strap some propane heaters to my tailpipe?
I was watching a car show today and they mentioned that the hotter the exhaust the more efficient your exhaust is at removing underhood heat...and underhood heat is bad
 

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The largest exhaust i would ever go with is 2.50 inch unless i was running a turbo and that is where 3 inch exhaust is beneficial!
 
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