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Exhaust bottleneck and power loss

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SheepDog

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Anyone have any data on HP gains from Green cats to no Cats?
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Jay-rod427

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Your "high flow" cats are causing more restriction than that reduction. At BIG power levels it might cause a slight decrease in power, but sub 1,000+ it isn't really an issue. The first couple feet of the header are what's critical where the charge coming out of the combustion chamber is at it's hottest and highest volume. As the exhaust travels down the headers it cools and shrinks it's volume becoming less critical. Albeit a high rpm engine, it's still only 5 liters of displacement so the air volume is small relative to say a 540" big block chevy.

Along the lines of the old school header thinking. "I got 2.5" headers on my stock headed small block Chevy!". Cool but those heads have a 1.5" exhaust runner and port. so the giant header tubes offer no gain. All the while the actual exhaust valve opening is and always will be the smallest volume/biggest restriction. With that said if the headers and pipe are too small they could decrease power via further restriction.

2.5" exhaust is more than enough to support a coyote into 4 digit power.
 
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That being said, any idea what the exhaust port size is on the stock heads?
 
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So the 1 7/8" and now the 2" offerings are pointless above the 1 3/4" size? I guess before, I wasn't really considering that the Green cats that Kooks told me were good to 900hp were creating that much of a restriction, but after seeing so many posts where people have had them fold up/break apart, they are coming off.
 

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I made power gains going from Kooks long tube no cats and the mid pipe choke down. I measured 2 1/4” at the choke down and made 730 wheel with that. Took out the choke point and made 784. I had also went from 12gph meth nozzle down to 7 GPH so that may have also contributed to some of the gains.
 

Jay-rod427

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So the 1 7/8" and now the 2" offerings are pointless above the 1 3/4" size? I guess before, I wasn't really considering that the Green cats that Kooks told me were good to 900hp were creating that much of a restriction, but after seeing so many posts where people have had them fold up/break apart, they are coming off.
Header primary needs to be bigger than the port to some degree. The valve is the first restriction, next the runner and port, next the header flange, and primary. If the primary isn't big enough it adds another restriction. Any time you can limit the restriction at any of of those points it keeps from compounding restrictions. The exhaust coming out of the head is still quite hot trying to expand, and high volume so the primary is still bigger than the port somewhat to keep from adding to the piling up restrictions. And it's all dependent on air volume. IE sforced induction pumps more volume. But the general concensus is 1 3/4 again is fine up to the 1,000+ range.

As long as any point in the exhaust stream can flow equal to or more than any of the upstream points it doesn't hurt, but that's double edged sword. The further the gas gets the lower the volume becomes as the air charge cools.
 

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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
Don't have data for an S550, but I have good numbers for 4.6 Terminators w 2.3 Whipples. Bro in law and I both had em. Identical setup, identical tunes from JDM. I had factory cats and Magnaflow 2.5" catbacks. He had Kook's headers, cats taken out and some seriously loud 3 inch header back exhaust. Me 610 to the wheels. He did 615. Draw your own conclusions.
 

OniVita

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Since you have started thinking about it, I say go 3" all the way. Whenever you dyno your car, race someone with the same setup..etc you will be always thinking that it's the 2.5" exhaust fault.
I didnt dyno my Mustang before and after, I had only one choke point and that was the connection between the kooks headers and the corsa catback.
After that I ordered that 3" connection from kooks and install it, did i feel any improvement? No, but it felt really good having 3" exhaust lol
 
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Indeed, and in conjunction with removing the cats, I will be able to sleep at night. Is 700hp enough? Of course not. Is 720hp enough? Certainly not, but it's better than 700
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