Pontiac G8 2008-2009 - 2009.5 G8 GT X-Pipe mod (photos & dyno sheet added)

Special thanks to Mitch Mouton (aka Fatdaddy) for the idea. The stock 2009.5 G8 GT is blessed with 2 extra catalytic converters vs. 2008-2009 GTs so doing this mod is worth more power on our cars than earlier models. Mitch's car has identical mods to mine with the same tune, yet he dynoed 13rwhp more. When asked why his made more, he explained that he had swapped out the factory mid-pipes with 2.25" straight pipes and a Magnaflow X merge. Since it was worth some extra power on his ride, I thought it might be worth some power on mine. Here are the before and after photos. Unlike Mitch, I used 2.5" pipes. The factory pipes crush down to 1 7/8" between the mid muffler and the rear resonators. I will post dyno results tomorrow night. Also welded in an O2 sensor bung for better readings with the wideband sensor. Total cost of material was about $100. Labor depends on whether you do it yourself or use a shop. Surprisingly, the sound is unchanged from before. If you had not told me the mod was done, I would have never know.

http://guerragroup.com/2009_G8_10.JPG

http://guerragroup.com/2009_G8_11.JPG

http://guerragroup.com/2009_G8_12.JPG

http://guerragroup.com/2009_G8_13.JPG

http://guerragroup.com/2009_G8_14.JPG

http://guerragroup.com/2009_G8_15.JPG

http://guerragroup.com/2009_G8_16.JPG

That has to be the easiest exhaust modification you've done to date.

That has to be the easiest exhaust modification you've done to date.Especially since I wasn't the one welding it. :)

Turned out nice, huh? John and the boys did a great job. Let's see what she makes on the rollers tomorrow, OK?

I would love to see the gain, looks good

is it louder :) looks good i'm gonna have to look into this when i get home form offshore TOMORROW hell yea my 2 weeks is done ........

very nice though really

Surprisingly, the sound is unchanged from before. If you had not told me the mod was done, I would have never know.


that answers my question lol sorry i didn't read it all the way through till after i replyed i need sleep

posted before on grrrr8, all your observations are spot on.

take a look at the heimholtz resonator on g8board.

not alot of hp to be gained post-manifolds, but cheep to do!

posted before on grrrr8, all your observations are spot on.

take a look at the heimholtz resonator on g8board.I saw that. It was very impressive. Just the ticket for guys who want it loud, but without the drone. Excellent! :)

I have that same X pipe sitting in my shed next to my stock exhaust. Deff add some bass to the exhaust note.

Here are the dyno results. While you can see that I made a solid 6-8rwhp/rwtq gain under the curve, I had some knock retard slip in around 5400 rpm that didn't allow the "after" graph to stay consistently above the "before" dyno run. Without the KR, the peak difference would have shown a 8-10rwhp gain. Peak torque gain was a 10 lb ft increase. The before power/torque was 336/346 and the after power/torque was 342/356. Thanks to Kurt Decker from LSX Performance Power Tuning for use of his dyno tonight. :) For around $100 in parts, this is a worthwhile mod, especially since it's drone free.

Mitch (Fatdaddy), my graph was an exact overlay of yours in hp and tq until the knock retard at 5400. Our vehicles are extremely similar in mods and power. :)

http://guerragroup.com/G8_X-Pipe.JPG

I think I know the answer, but will this throw a code on an otherwise stock car? Did you change your tune post mod for the gains you've experience?

I wonder what shorties would net...

I think I know the answer, but will this throw a code on an otherwise stock car? Did you change your tune post mod for the gains you've experience?

I wonder what shorties would net...This mod will not throw a code because you still run the primary cats up front and your secondary O2 sensors are a way in front of the new piping and x pipe. I did need to lean out my tune a little after this mod, but my car still showed a nice gain even before the retune.

Right on! Good to know! Thanks.

good to hear
i did mine awhile back
http://www.g8board.com/forums/showthread.php?t=17858

http://www.youtube.com/watch?v=8z7TxV0XDtc

with gxp muffs...


i need to get onto the dyno and see if my car has loosen up at all with 7k on it now had i think 3k at my last dyno time.
plus i did a few little mods since then.