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2 1/2" or 3" exhaust?

I'll reveal my bottleneck secret when you can explain the catback gain.

Everyone knows how a cutout works.

It gains plenty of power on most ANY car/platform.

Don't mislead the kiddies, Sleeper.
 


The cutout circumvents the catback. If it gains power then there's an obvious restriction.
Restrictions that would be alleviated with a better flowing catback...

This is how cars work 101 for dummies edition stuff here…
Now why are two 2.5" pipes a restriction when fed by a 3" pipe?
 


To clarify, what I have now is junk. The previous owner had some pretty bad work done to the stock exhaust. The welding is extremely poorly done. He had a glasspack put on in place of a rotted resonator because it was cheaper. Not only that but the flange was butchered along with the pipe before the Y. Both mufflers have rust and need replacing. I'd like to go stainless to complete my exhaust as my ZZP headers are stainless and I live in Michigan where they think salt is the best way to get rid of a foot of snow. Right now it drones really badly and is pretty loud so I'd like a low restriction but near stock sound if possible. I'm hoping that's not a pipe dream.
 
I went from a 2.5" SLP cat back on the Regal to a custom 3" with a walker muffler. No crushed bends or anything. It was all pieced together from mandrel bends from summit.

With no other mods in done, I lost 25 hp on the dyno going to a 3". However I did lose a solid 2 PSI (11-12 down to 9-10). The only other thing I changed were the valve springs.

The graphs below don't show the boost. I don't think his gauge was working on the first one and the 2nd one it was a dyno day and he didn't have it connected.

Dynopull03regalSeptember12_zpsbe690326.jpg


20140810_144824_zps0c54115d.jpg


I plan on going back this spring to see what it'll do on a gen v with a 3.2 (currently seeing 10-11 psi) or my 3.0 which should be a few lbs more.

And before anyone complains the 2.5" was the SLP cat back and sounded horrible on the car. It came with the car when I bought it. Just didn't like the sound of it. I sold the cat back for $200 and pieced together my 3" complete for about $50 more and it's much better/quieter.

not to make any more excuses (lol) I just realized the first graph was on stock chrome wheels and the 2nd was on my heavy ass 17" lucerne wheels.
 
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Gained TWO HORSEPOWER WITH CUTOUT.

LOST HORSEPOWER WITH CATBACK.

Well then.

I knew I didn't know anything.
 


That was my point though. I've never seen much for gains with exhaust. The only reason I drop my exhaust for racing is to save weight. And so I can hear my car next to the one next to me with open headers. But what do I know....I built an xpz car with stock manifolds.
 
GTPpower said:
http://www.streetfire.net/video/ncgp-dyno-day-2009_645263.htm

Watch from 1:15 to 2:50

He had a fairly free flowing exhaust. Dyno showed 2hp gain with a cutout.
Click to expand...

Gained 2hp over having a 3" exhaust on a 277whp car. Sounds about right… when the exhaust flows well a cutout isn't going to do much because the restriction was alleviated.
As for losing 25hp going from 2.5" to 3" exhaust… something else went wrong there.
Per the post, you lost boost pressure going from 2.5" to 3" which means you had an increase in VE which will always equate to a gain in power.

It doesn't take a genius to tell you there was something else going on.
 
GTPpower said:
That was my point though. I've never seen much for gains with exhaust. The only reason I drop my exhaust for racing is to save weight. And so I can hear my car next to the one next to me with open headers. But what do I know....I built an xpz car with stock manifolds.
Click to expand...

Glad you've never done anything either, new guy to the scene.
 
SnowFlake said:
Gained 2hp over having a 3" exhaust on a 277whp car. Sounds about right… when the exhaust flows well a cutout isn't going to do much because the restriction was alleviated.
As for losing 25hp going from 2.5" to 3" exhaust… something else went wrong there.
Per the post, you lost boost pressure going from 2.5" to 3" which means you had an increase in VE which will always equate to a gain in power.

It doesn't take a genius to tell you there was something else going on.
Click to expand...

Everything was going through one flowmaster 50 series muffler though. So, I have to imagine there was some restriction.
 


Still a lot more volume in that 3" pipe than the stock stuff.
Woulda been interesting if he had a stock catback so you could corroborate my findings.
 
blueguy said:
boost stacking.
Click to expand...

Best day on GPF ever!

In the interest of helping:

My boost pressure went down making one change only: Switching from Pacesetters to Speed Daddies. Note that was a stock cam, pump gas, top swap car.
 
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