Exhaust bottleneck and power loss

[SheepDog]

Good evening everyone,

I was wondering what all of your thoughts were on power loss (if any) from the 2.5" bottleneck on the OEM connection point on aftermarket exhaust systems? I have Kooks headers with green cats, a Magnaflow Foose edition Cat back, but it still has the factory sized bottleneck connection. I am running a P1x Procharger at 13 PSI max, and e85 with Lund tune. I guess my question is, would it be beneficial to me to either change the catback for a full 3" design or have an exhaust shop make it 3" all the way through? How much power am I giving up with the bottleneck in place at this power level if any?

Has anyone done before and after dynos or have any other data (1/4 times) or just opinions on the subject? If the consensus is 5 HP gain, then not worth the trouble. If the consensus is 20+ HP, then obviously it is worth addressing. I'm not chasing quarter mile times or lap records. I do however like things to be as efficient as possible and give me all of what I paid for/put into it.

Apologies if this has been answered before, I couldn't really find any solid info on the subject

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[SheepDog]

No one else run into this issue?

 

[Burkey]

No one else run into this issue?

@Roh92cp IIRC, your issue was slightly different. Can you offer some insight?

 

[SheepDog]

@ProChargerTECH @beefcake

 

[80FoxCoupe]

There is def some power there. How much, I'm not sure. Modifications are a cumulative effort. Say you skip 5 small mods that don't seem worth it on their own. Together the gain may be substantial. You skip em, the competition doesn't. You get your ass dragged. Up to you...

 

[beefcake]

Until you're at 900-1000+ you should be ok on your current setup. We've ran 8's with the factory neck down connection.

 

[80FoxCoupe]

Until you're at 900-1000+ you should be ok on your current setup. We've ran 8's with the factory neck down connection.

Would have ran faster without the bottle neck!

 

[gimmie11s]

This problem is much more pronounced on turbo cars than blower cars, but definitely power to be had going to 3" dual or larger.

Ive seen 30+whp improvements moving from a factory catback to 3" on a ~650whp basic Paxton set up.

 

[SheepDog]

This problem is much more pronounced on turbo cars than blower cars, but definitely power to be had going to 3" dual or larger.

Ive seen 30+whp improvements moving from a factory catback to 3" on a ~650whp basic Paxton set up.

Thanks. Yeah that is the kind of difference that makes it worth doing. I wonder though, how much of a hindrance just the short section of 2.5" pipe is making, vs the entire length of the exhaust behind the cats being 2.5".(the rest of my exhaust is 3"). The exhaust gas, once it squeezes through the bottleneck, can then expand again and relieve some pressure. It may be that having high flowing headers, may exacerbate the bottleneck problem because the flow of exhaust starts out really good, then is hitting the smaller space even faster than it would be from the OEM manifolds.

 

[80FoxCoupe]

Thanks. Yeah that is the kind of difference that makes it worth doing. I wonder though, how much of a hindrance just the short section of 2.5" pipe is making, vs the entire length of the exhaust behind the cats being 2.5".(the rest of my exhaust is 3"). The exhaust gas, once it squeezes through the bottleneck, can then expand again and relieve some pressure. It may be that having high flowing headers, may exacerbate the bottleneck problem because the flow of exhaust starts out really good, then is hitting the smaller space even faster than it would be from the OEM manifolds.

Just fix it and know you did the right thing so you can sleep at night!!!!

 

[SheepDog]

Just fix it and know you did the right thing so you can sleep at night!!!!

Lol, Damnit! You know me too well. I'm not one to frankenstein stuff together. New cat back is on the way. At this point I'm just more interested in the science of the problem, and if others have any had any real world experience/data to support or disclaim this. And obviously, a like new Magnaflow Foose Edition Cat back will be up for sale in the classifieds soon for anyone interested.

This problem is much more pronounced on turbo cars than blower cars,

Wouldn't the same volume of exhaust be coming out of either a turbo or supercharged engine, as long as the same amount of air and fuel are being consumed?

 

[SheepDog]

Not sure how I did that.

Wouldn't the same volume of exhaust be coming out of either a turbo or supercharged engine, as long as the same amount of air and fuel are being consumed?

 

[engineermike]

At the same power level the turbo motor is probably actually flowing less exhaust, but turbo motors are running 30-50 psi of exhaust pressure vs supercharged more like 5-10 psi. Once back pressure is less than boost, it’s really diminishing returns. But going from 50 to 40 can net some major gains.

 

[SheepDog]

At the same power level the turbo motor is probably actually flowing less exhaust, but turbo motors are running 30-50 psi of exhaust pressure vs supercharged more like 5-10 psi. Once back pressure is less than boost, it’s really diminishing returns. But going from 50 to 40 can net some major gains.

As well as the fact that the supercharged engine still benefits from exhaust scavenging, whereas the turbo engine does not. Wouldn't you think though that at these kinds of power levels, squeezing all of that exhaust through a space that is considerably smaller, if even for a short distance, would cause a measurable amount of resistance, pressure, flow reduction?

 

[engineermike]

My advise is that if you really want to know, without changing it all, install a pressure gauge in the collector. I’ve actually thought about a method to use the o2 sensor hole to measure it. Lingenfelter (or maybe Vizard) stated decades ago to try to keep it under 5 psi. I’ve found that to be a good number, but supercharged is the least sensitive to it. An NA engine with a lot of cam overlap is more sensitive and turbo motors are as well due to the gross pressure imbalance across the cylinder. I’ve measured turbo exhaust manifold pressures from 34 to 48 psi on a 9 second single turbo setup running 17 to 22 psi boost, just for reference. Btw, you really don’t want scavenging in a supercharged engine because you already have boost tending to push charge through the chamber during valve overlap. More scavenging can just pull more charge out the exhaust. With supercharged, it’s like Goldilocks because too much or too little backpressure can hurt power. Again, cam overlap comes into play as well.

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