Maximum efficient power from a 2 1/2" exhaust
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Joined: Dec 2010
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Maximum efficient power from a 2 1/2" exhaust
My car is a stock block Z06 with full exhaust, blower cam, and an A&A v-3 blower kit. I have made up to 658whp in great weather, and as low as 608whp on a different dyno in hot humid weather. Currently I am running 1 3/4" ARH with the 3" x pipe that tapers to 2 1/2", and a 3" catback. I'm having some rubbing issues that is really annoying. I tried making some adjustments which made it better, but didn't solve the problem. I am considering changing exhaust, and thought about moving back to a 2 1/2" catback. Effectively my exhaust is only 2 1/2" now since it tapers down at the x pipe. My question is at what power level is 2 1/2" piping no longer efficient? I don't ever plan on upgrading the blower so I figured the maximum power I will ever see is about 700-725whp. Based off of a few charts I have seen, 2 1/2" is restrictive at that level. What your opinions on this?
Burning Brakes
Joined: Jun 2010
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From: Metairie Louisiana
So it's the catback that's rubbing? Are you sure a "smaller" catback will solve the problem or could it be alignment or another exhaust positioning issue? If you use a known high quality 2 1/2" catback I don't think you would lose hardly anything hp wise.
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I'm almost positive it is the catback that is rubbing. The only other thing that it could possibly be is the flange on the x pipe. I havent looked at it too closely to be honest. I took it to a reputable shop in Houston, but the problem was not completely solved. It is doesn't rattle as bad now. I will have to ge the car high enough off the ground to check it out. I doubt I would loose any power with a different catback being that the x pipe already reduces down to 2 1/2 inch. I am wondering if I am leaving power on the table by having it taper down, and if not how much more power can I efficiently make with 2 1/2 inch pipe.
Burning Brakes
Joined: Jun 2010
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From: Metairie Louisiana
I'm not a tuner or exhaust guru, if you would really want an exhaust to keep up with a boosted engine you might want ARH 1 7/8" headers feeding a 3" x-pipe and 3" connecting pipes to your present catback. That would cost a good deal of money. See if you can get the car on a lift and find the rattle problem, if the only way to fix it is to use a smaller catback you might have to do it. Something is not installed right, I don't remember alot of comments about the 3" catbacks rubbing or rattling. If the 2 1/2" catback is a must, I don't think your car will lose much of anything or be slower. Maybe it's a lose heat shield or something less complicated that's causing the noise.
Burning Brakes
Joined: May 2007
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From: San Francisco CA
As far back as the mufflers are on C5, 2-1/2" catback like Borla Stingers are about as free flowing as they come and not really going to cost power compared to 3". If you go 3" it needs to be 3" all the way from X to catback. There is no benefit to a 3" catback once the X goes to 2-1/2". Remember that the exhaust cools and condenses as it travels down the pipes requiring less area the further it gets downstream, especially if you have cats.
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I realize the 1 3/4 primaries are not optimal, but they were just right for my previous setup. I also know that I'm doing no good with the 3" CB with the tapered x pipe. Would 2 1/2 in exhaust be considered too small for lets say a 700 whp car if for some reason I go with something smaller? Would I gain much by chopping off the x pipe flanges and welding up 3" flanges to mate up with my current exhaust?
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I'm not a tuner or exhaust guru, if you would really want an exhaust to keep up with a boosted engine you might want ARH 1 7/8" headers feeding a 3" x-pipe and 3" connecting pipes to your present catback. That would cost a good deal of money. See if you can get the car on a lift and find the rattle problem, if the only way to fix it is to use a smaller catback you might have to do it. Something is not installed right, I don't remember alot of comments about the 3" catbacks rubbing or rattling. If the 2 1/2" catback is a must, I don't think your car will lose much of anything or be slower. Maybe it's a lose heat shield or something less complicated that's causing the noise.
Pro
Joined: Dec 2003
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From: chandler az
If it's only rubbing in a small area, get a sharpie and mark the area rubbing. Pull down the exhaust, put a bat on the area you want massaged and whack the bat with 5lb sledge, while rolling the bat into the area until you get the clearance you need. It sounds crude, but it works. The bat keep the exhaust from getting scratched up. Small dings will not choke off your exhaust.
Once upon a time you had to do this to headers to get spark plug clearance.
Once upon a time you had to do this to headers to get spark plug clearance.
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Safety Car
Joined: Aug 2011
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From: Portland OR
At the power level you are at, I believe your car would like a dual 3.5" system to the cat back which should stay 3" minimum. Especially if you strive for 700+ at the wheels.
The Nova in my .sig picked up 2 mph in the 1/4 when going from a full 3" system to a full 3.5" system. That is approx 30hp. It's a brick.
The engine made 700 at the flywheel on the dyno uncapped.
General rule of thumb for no loss from the exh is 2.5" thru 500hp, 3" thru 600hp and 3.5" through and beyond 700hp.
Because the exhaust system grows in length when hot, extra clearance needs to be given at cold temp. It's a pain to figure out where, but it will reveal itself with marks on the pipes.
Ron
The Nova in my .sig picked up 2 mph in the 1/4 when going from a full 3" system to a full 3.5" system. That is approx 30hp. It's a brick.
The engine made 700 at the flywheel on the dyno uncapped.
General rule of thumb for no loss from the exh is 2.5" thru 500hp, 3" thru 600hp and 3.5" through and beyond 700hp.
Because the exhaust system grows in length when hot, extra clearance needs to be given at cold temp. It's a pain to figure out where, but it will reveal itself with marks on the pipes.
Ron
Burning Brakes
Joined: May 2007
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From: San Francisco CA
Where does the X go down to 2-1/2"? Is it the catback flange or at the back of the cats/X outlet? I'd say running a full 3" X all the way back to the catback would be worth more power even if you connected it to a free flowing 2-1/2" system. I don't think 3.5" will fit. Overall full 3" X into 3" catback > full 3" X into 2-1/2" catback > 3"-2-1/2" X 3" catback. The X is your major choke point.
Safety Car
Joined: Aug 2011
Posts: 4,449
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From: Portland OR
I think 3.5" will fit. But it would have to be custom. And the X if used would have to be narrow. And it would be loud!
I measured the width of the exh tunnel to be 9".
My car had a couple of 4" x 18" round mufflers in the Magnaflow X that was on the car. fit fine.
Ron
I measured the width of the exh tunnel to be 9".
My car had a couple of 4" x 18" round mufflers in the Magnaflow X that was on the car. fit fine.
Ron
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Where does the X go down to 2-1/2"? Is it the catback flange or at the back of the cats/X outlet? I'd say running a full 3" X all the way back to the catback would be worth more power even if you connected it to a free flowing 2-1/2" system. I don't think 3.5" will fit. Overall full 3" X into 3" catback > full 3" X into 2-1/2" catback > 3"-2-1/2" X 3" catback. The X is your major choke point.
Le Mans Master
Joined: Sep 2000
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From: Clouds Over California
As far back as the mufflers are on C5, 2-1/2" catback like Borla Stingers are about as free flowing as they come and not really going to cost power compared to 3". If you go 3" it needs to be 3" all the way from X to catback. There is no benefit to a 3" catback once the X goes to 2-1/2". Remember that the exhaust cools and condenses as it travels down the pipes requiring less area the further it gets downstream, especially if you have cats.
If you had a 427cid + Naturally Aspirated (NA) and looking for every possible improvement to better efficiency to NA, then a full 3" system would be the way to go from headers collectors to tail pipe.
There is a reason FI setups don't need cams with high lift and lots of overlap like NA...FI has 100% Volumetric Efficiency, whereas NA is more dependenat because it is having to suck in air and scavenge exhaust better to make more power...lucky to have 85% VE.
Don't get me wrong FI setups benefit from the same priciples of removing restrictions and back pressure, I just don't know that you would benefit from simply changin one piece of your exhast system to 3" when you had other pieces that were 2.5".
Last edited by Shinobi'sZ; Dec 19, 2011 at 05:55 PM.
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I agree with this. I have made anywhere between 600-1K hp and I have always used the stock mid-pipe and either Borla Stingers, or straight exhaust (Cat back) exhaust..but it has always been 2.5". This was with the first A&A D1 Twin IC kit, first TTi kit, and first PTK kit.
If you had a 427cid + Naturally Aspirated (NA) and looking for every possible improvement to better efficiency to NA, then a full 3" system would be the way to go from headers collectors to tail pipe.
There is a reason FI setups don't need cams with high lift and lots of overlap like NA...FI has 100% Volumetric Efficiency, whereas NA is more dependenat because it is having to suck in air and scavenge exhaust better to make more power...lucky to have 85% VE.
Don't get me wrong FI setups benefit from the same priciples of removing restrictions and back pressure, I just don't know that you would benefit from simply changin one piece of your exhast system to 3" when you had other pieces that were 2.5".
If you had a 427cid + Naturally Aspirated (NA) and looking for every possible improvement to better efficiency to NA, then a full 3" system would be the way to go from headers collectors to tail pipe.
There is a reason FI setups don't need cams with high lift and lots of overlap like NA...FI has 100% Volumetric Efficiency, whereas NA is more dependenat because it is having to suck in air and scavenge exhaust better to make more power...lucky to have 85% VE.
Don't get me wrong FI setups benefit from the same priciples of removing restrictions and back pressure, I just don't know that you would benefit from simply changin one piece of your exhast system to 3" when you had other pieces that were 2.5".