I realize there are "stealth" exhausts already available and/or coming out soon (any day now AMS?!?), but a co-worker suggested that I could pretty decent hp gains just by swapping out the downpipe and replacing the cat with a high-flow cat. By leaving everything cat back stock, the exhaust note would still sound completely stock.

Has anyone done this to their X? What were the hp gains? Does anyone have video of the exhaust with this set up?

 

I wanted the same thing you do....

there will definitely be an increase in noise levels if you swap out the downpipe/cat but nowhere near as loud as it would be with an aftermarket exhaust.

I have the UR downpipe and testpipe along with the Works stealth exhaust (a modified stock exhaust) and am happy with the gains and noise levels.

 

I saw that Works muffler just the other day. If you get a chance post some videos of your exhaust. Thanks.

 

Not quite apples to apples, but I have a Works downpipe, HFC, and stealth muffler on my VIII. It's a bit louder than stock at idle on the outside, but on the inside, it's barely louder at all compared to stock at any rpm or throttle position. I can hear the drivetrain over the exhaust!

In going with the AMS housing/downpipe combo with a HFC, I'd expect ~25whp gain with almost no noise gain. You'll also gain a significant amount of spoolup!

 

I have an AMS DP and HF cat mated to a Greddy spectrum elite exhaust and it's a little louder than stock but when you get on it the exhaust really comes alive. Nothing too loud though and perfect for daily driving.

 

I added a HFC to my otherwise stock exhaust and could not tell any difference in sound. Then I added the cast AMS down pipe and still could not tell any difference. Sounds just like stock, still.

(I also have a Cobb Access Port and although I haven't had it on a dyno yet, the power feels great).

 

do you feel like you picked up any power when you put the dp and hfc?

 

Can you record a video of the sound?

 

x2

 

I've seen a couple people put a DP and HFC with the stock exhaust and it sounds stock. Barely any difference. But idk how it sounds at full throttle.

 

My progression path was HFC, AP, DP. I felt power increases in each step.

 

By upgrading exhaust pieces before the catback, the sensation of a power increase is more than likely from the spooling of the turbo. With the free'er flowing exhaust through the turbo it should spool up quicker and feel like its got more power. However when you leave the stock exhaust in place with its smaller diameter tubes, you restrict it further down the pipe when the need for free flowing is higher. Low RPM means little exhaust flow and feels fine with stock catback, high RPM on the other hand is where you begin choking because it cant push the gas out fast enough. So as it spools, it spools fast till it hits the capacity of the exhaust and then begins losing efficiency and gaining backpressure which as I understand, could damage your turbo over time. Kind of the same result from when you upgrade your turbo before upgrading your exhaust.

 

^
I’m not disputing this, butt dynos are notoriously unreliable.

 

There is Greddy SE exhaust for the Evo? Did this replace the SP2? I looked at Greddy's site and didn't see an SE listed for the Evo.

 

+1
It's like smoking a cigerratte but exhailing the smoke out of your nose rather than you mouth. lol.