Installed a set of 1 7/8" ARH headers and catted Y-pipe on my 2008 Seirra Denali this week, and here are the details . . .

First - I would have never guessed in a million years just how "rowdy" my exhaust would sound with headers! At certain RPMs and throttle openings, all he!! breaks loose (a raucous, bellowing roar is my best description) at the rear of my truck, LOL! I still have a completely stock cat-back, mind you! I'm very glad that I had not removed the resonator, as I had been considering!

That said, during normal cruise, it sounds very near stock, just a tick louder. VERY nice. Also interesting that during part throttle acceleration, say approx 1/4 to 1/3 throttle - at each upshift, when the ECM cuts fuel delivery during the upshift, there's a quite noticeable "back-rap" sound. Overall I must say that I'm pleased with the sound, though. Definitely NO cruise resonation or any other such nonsense.

As far as the performance difference, it's hard to say at this point. I suppose that the exhaust note difference makes the performance difference difficult to determine. Seems that it MAY have lost a bit of low-RPM torque, though at higher RPMs I believe there may be some improvement. To be expected, I suppose, with 1 7/8" primary tubes on a 378" engine. Admittedly, the traffic and weather yesterday afternoon and this morning have not yet allowed me to make any heavy or full throttle runs through the gears yet.

The installation was an absolute breeze. If ARH would have sent the correct Y-pipe to start with, we'd have been done in less than 3 hours. No fitment issues, excepting that the #1 and #3 tubes are EXTREMELY close to the steering shaft. Had to use a cutoff wheel to remove the protruding threads of the upper pinch bolt and part of the pinch bolt spring-steel flange nut. The battery cable to the starter solenoid had to be relocated forward. The oxygen sensor wiring all reached fine with no extensions necessary. My only fitment gripes have to do with the steering shaft intermediate shaft interference, and that the Y-pipe hangs down below the transmission crossmember just a touch.

One more BIG gripe though - on the way to the shop this morning my CEL came on. Before I even hooked up my Tech 2 to check codes, I knew what I'd find - sure enough, P0420 and P0430 catalyst efficiency codes for both banks! There's simply no way that those tiny "high-flow" cats have sufficient O2 storage capacity. Oh well - I'll just have the cat monitors "tuned out".

I have an appointment with Pat G on the 9th for a full dyno tune with wide-band. Hoping my truck is a beast after that!

 

Lloyd, I have the catted 1 3/4" ARH on my truck with stock exhaust. I love the sound and like yours, it is quiet on the road. It can draw too much attention at times, it is so loud at part throttle and low RPM's. I have had more than one person tell me they wish their car or truck sounded as good as mine. Most don't believe me when I tell them the exhaust is stock. You will be amazed after the tune. Your truck will have a new personality and sound different too. Let us know your HP before and after the tune. I could not find anyone with a 4 wheel dyno in my area, so all I got was the drive around tune with no dyno.

 

Well, I've got about 300 miles on my SD since the header install, and performance is definitely getting better and better! I've yet to check fuel trims at cruise or during acceleration, but at idle and fast idle they're going quite a bit positive, so I'm sure that the ECM is compensating the more I drive and therefore the performance increase.

Part throttle response and torque have improved, and a 25 MPH stomp on dry concrete yielded so much tire spin that the traction control AND stability control kicked in - it's NEVER done that before from 25 MPH!

Still cannot get over just how much the exhaust note changed (certainly due more to the "high-flow" cats than the headers themselves), but it's genuinely "rowdy". Not overly instrusive during cruise conditions, but hit certain RPMs at certain throttle openings and all hell breaks loose. I kinda like it, LOL!

Overall I'm very pleased with the headers, and will be even more pleased I'm sure once I get my Patrick G tune in a couple of weeks!

 

Ditto!

By the way, when I contacted Patrick G to schedule my dyno tune, he asked if my SD was AWD since he uses a 2WD dyno. When I told him that mine was 2WD he was relieved and commented that being 2WD would save some time in that he would not need to remove the front driveshaft before the dyno runs. So . . . I'm guessing that an AWD SD or YD can be dyno tuned on a single roller dyno by simply removing the front DS?

I'll definitely record the before and after dyno numbers and post them up.