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Discussion Starter · #1 ·
Background: I installed a Magnaflow catback exhaust a couple of years ago when I first got my Ecoboost Mustang. It's a 2.5" diameter setup that necks down to 2.25" where it connected to the stock downpipe. Just recently, I installed a cp-e downpipe that's 3" in diameter. The downpipe came with two adapters - one allows for it to connect to a stock (2.25") exhaust, and the other is for a 3" exhaust.

I installed the downpipe with the 2.25" adapter, so it mated up with the necked down catback. My plan was to take it to an exhaust shop near me, and have them remove the narrow, stock-diameter section (or possibly the entire section from there to the split) and fabricate a section of pipe that would connect to the downpipe with the 3" adapter.

So, the shop owner sent his exhaust expert out to assess what I wanted to do. I explained what I had, and he immediately commented that a 3" downpipe was too big. (?) He said it probably cost me horsepower, and removing the 2.25" section would probably cost me more, because I'd be robbing my engine of needed back pressure. He suggested leaving it as is, although he'd be happy to do whatever I wanted.

It wasn't what I expected. He spent a lot of time chatting with me, explaining how he would have built me a custom exhaust, which to be honest, sounded a lot like the Moxman exhaust I've read about here. He alternately sounded very knowledgeable about exhaust systems, but not necessarily too familiar with this particular 4-cylinder turbo (he actually didn't realize it was a turbo until I told him).

So, is he off base? Is the remaining stock-diameter section behind my downpipe actually helpful, or is it a hindrance? I thought I knew a little, but now I think I know less...
 

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Background: I installed a Magnaflow catback exhaust a couple of years ago when I first got my Ecoboost Mustang. It's a 2.5" diameter setup that necks down to 2.25" where it connected to the stock downpipe. Just recently, I installed a cp-e downpipe that's 3" in diameter. The downpipe came with two adapters - one allows for it to connect to a stock (2.25") exhaust, and the other is for a 3" exhaust.

I installed the downpipe with the 2.25" adapter, so it mated up with the necked down catback. My plan was to take it to an exhaust shop near me, and have them remove the narrow, stock-diameter section (or possibly the entire section from there to the split) and fabricate a section of pipe that would connect to the downpipe with the 3" adapter.

So, the shop owner sent his exhaust expert out to assess what I wanted to do. I explained what I had, and he immediately commented that a 3" downpipe was too big. (?) He said it probably cost me horsepower, and removing the 2.25" section would probably cost me more, because I'd be robbing my engine of needed back pressure. He suggested leaving it as is, although he'd be happy to do whatever I wanted.

It wasn't what I expected. He spent a lot of time chatting with me, explaining how he would have built me a custom exhaust, which to be honest, sounded a lot like the Moxman exhaust I've read about here. He alternately sounded very knowledgeable about exhaust systems, but not necessarily too familiar with this particular 4-cylinder turbo (he actually didn't realize it was a turbo until I told him).

So, is he off base? Is the remaining stock-diameter section behind my downpipe actually helpful, or is it a hindrance? I thought I knew a little, but now I think I know less...
With most turbo cars you want the least amount of back pressure to get the most HP out of the car. The more back pressure you have the harder you engine has to work to spin up the turbo. Theoretically a 3" straight pipe catless exhaust would give you the most flow and the most HP gains.
 

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I have always been told, never reduce your exhaust size/flow...
but a lot has changed since my school days!
I have recently made some changes to my exhaust and will be testing at the Summit Dragway in the near future.
I suggest running as-is and collect some data, then make your changes and collect more data. You can always change back, if you don't like the results...
 

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You are confusing "back pressure" with scavenging effect. You don't want back pressure in the exhaust.

What the tech is saying is that the bigger tube is causing you to lose the speed of the flow and making the exhaust less efficient.

Think of it as taking a garden hose at full water pressure, then attaching a fire hose to it. Your flow will drop considerably as the little garden hose tries to fill the bigger hose.

Sent from my SM-G991U using Tapatalk
 

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Discussion Starter · #6 ·

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Discussion Starter · #7 ·
You are confusing "back pressure" with scavenging effect. You don't want back pressure in the exhaust.

What the tech is saying is that the bigger tube is causing you to lose the speed of the flow and making the exhaust less efficient.

Think of it as taking a garden hose at full water pressure, then attaching a fire hose to it. Your flow will drop considerably as the little garden hose tries to fill the bigger hose.

Sent from my SM-G991U using Tapatalk
He made basically the same analogy, which I understand. However, what should that mean to me? Is the only real answer to measure, change, measure, change, etc... in order to find the optimal configuration?
 

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You have a catalytic converter. That will give your turbo something to push against. I'd go 3" all the way to your 3"/2.5"/2.5" Y-pipe.
You may also benefit from wrapping your downpipe.
 

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The bottleneck in the exhaust system of our Ecoboosts is the catalytic converter itself.
What you install after the cat, like a bigger catback exhaust, is not going to make any noticable difference to performance, back pressure or the scavenging effect.
All you will get is more sound. :)
 

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You don't need back pressure. You're exhaust guy is just plain wrong. A 3 inch downpipe has proven, many times, to make more HP on a dyno. After the 3" downpipe, you don't want it to neck down anywhere. After a 3" downpipe, you can utilize the factory 2.25" pipes, as two 2.25" pipes will flow more than one 3" pipe. Most people upgrade to two 2.5" pipes because that's what most aftermarket exhaust companies sell, and people buy their product out of ignorance. The 2.5" pipes add cost and weigh more. You'll probably want to get rid of the resonator and add a better flowing muffler. I think the best, and most cost effective, setup is a 3" downpipe to a 3" inlet muffler, with two 2.25" outlets to the factory 2.25" pipes, with the factory mufflers removed and straight piped. This can all be done at an exhaust shop, usually for way less than the cost of an aftermarket exhaust, and you don't have to get your hands dirty. Getting rid of the resonator, utilizing the factory 2.25" pipes, and using one muffler, instead of two, all saves weight. It's also very similar to the Moxman Exhaust, except that he used two 2.5" pipes. The 2.5" pipes are unnecessary, from a performance standpoint, but he works at an exhaust shop, and his guys did a beautiful job on the 2.5" pipes.
 

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You don't need back pressure. You're exhaust guy is just plain wrong. A 3 inch downpipe has proven, many times, to make more HP on a dyno. After the 3" downpipe, you don't want it to neck down anywhere. After a 3" downpipe, you can utilize the factory 2.25" pipes, as two 2.25" pipes will flow more than one 3" pipe. Most people upgrade to two 2.5" pipes because that's what most aftermarket exhaust companies sell, and people buy their product out of ignorance. The 2.5" pipes add cost and weigh more. You'll probably want to get rid of the resonator and add a better flowing muffler. I think the best, and most cost effective, setup is a 3" downpipe to a 3" inlet muffler, with two 2.25" outlets to the factory 2.25" pipes, with the factory mufflers removed and straight piped. This can all be done at an exhaust shop, usually for way less than the cost of an aftermarket exhaust, and you don't have to get your hands dirty. Getting rid of the resonator, utilizing the factory 2.25" pipes, and using one muffler, instead of two, all saves weight. It's also very similar to the Moxman Exhaust, except that he used two 2.5" pipes. The 2.5" pipes are unnecessary, from a performance standpoint, but he works at an exhaust shop, and his guys did a beautiful job on the 2.5" pipes.
I agree with this. From the little homework I’ve done backpressure and scavenging for turbo motors arevery different from NA motors. Turbo motors benefit more from the least possible resistance because it allows the turbo to spool faster. Quite some time ago, I did the math on exhaust volume.The area of a 3” circle is 7.07 and for 2.25” 3.98. Dual 2.25” is 7.95 (there is some rounding in the math where Pi is involved).

My car has factory active exhaust,so it has no resonator. However, the factory downpipe still tapered to 2.25” before the Y. I went catless 3” to a magnaflow 3”->dual 2.5” Y which the exhaust shop welded to the 2.25” and filled the slight gap with the weld. I also added a Vibrant race resonator (straight through resonator) before the Y to tune out some nasty rasp. I’m pretty happy with the result.

40307
 
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