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Discussion Starter #1
So it seems like most are in agreement that a cat back on an otherwise stock G8 should add approximately 10 hp give or take. I want to know how much hp gains can be had from any full cat back on my engine over the stock exhaust system.

I’m doing a dod delete on my 09 g8 gt. Cam specs are 218/226 .595 lift on 114+3 lsa. Already have a specter cai. I will also be adding kooks mid lengths with high flow cats. And a mail order tune from one our vendors.

Now with the motor set up like this, how many hp will a full cat back give me over the stock setup ? This is my dd car. Any info is much appreciated. Thanks Patrick
 

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Every one of these G8 GT are different. Catback, intake, cam swap, tune, headers, cats or not, etc etc..... You can do the same mods to 2 identical cars and get different results.
 

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every village has one
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If specific numbers in regards to gain are your goal, you need to dyno the car and get a baseline. If my memory serves me correctly, most stock G8’s are 280-290 rwhp. If you dig deep enough here on the forum, there’s probably a thread on this.
You mention using Kooks mods and high flow cats. Whose cats? What catback are you considering, and what diameter? Whose tune?.
Been my experience to strap it down and spin it to get a baseline. Do your mods, and get it dyno tuned with the same dyno, then you’ll get your number.
 

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F1rehawk
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Do it right the first time. Get Kooks long tube headers, a better CAI and a street tune customised to your car and driving conditions. I think canned tunes of any kind suck, but that's my opinion. Get a tuner to drive your actual car and adjust everything on the go.

Like the others said all our cars are unique. It depends on a lot of factors, including the parts you use. I recommend the Solo exhaust system.
 

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Discussion Starter #5
I get what you all are saying but I think you’re missing my point. I’m just wondering if anyone can give an approximate hp increase from any cat back ( Solo, Corsa, Magnaflow ) say 2.5” diameter when hooked up to our 6.0 L engine that already has good exhaust flow out of the heads (Kooks mids with their high flow cats) has a relatively small cam, but much larger than stock and a good tune ( Pat G or RCR) Gotta be more than 10 hp I would think.
 

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Discussion Starter #6
Let me put another way. With the above mods, how much hp am I leaving on the table with a stock cat back ? Specifically the mid pipes and 3 resonators.
 

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Discussion Starter #7
Let me put another way. With the above mods, how much hp am I leaving on the table with a stock cat back ? Specifically the mid pipes and 3 resonators.
 

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F1rehawk
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Not much HP, but why do headers and leave the rest stock? Maybe something like 15-20HP?
 

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Discussion Starter #8
Not much HP, but why do headers and leave the rest stock? Maybe something like 15-20HP?
I live in a complex and have yet to hear a cat back that's quite enough on cold start. I leave for work pretty early every day so trying not to be that ***hole neighbor. I have Magnaflow axel backs on now just for a bit of sound. I like it the way it is and I think it will be a bit louder with the Kooks mids and HFC's. Adding them to get rid of the biggest exhaust restriction.
 

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Like others have said. Everyone of these cars reacts differently. I have however done very similar mods to what you are looking for. I don’t remember exact cam specs but it has very good street manners. I have K&N CAI, Borla cat back, kooks mids with high flow cats and custom tune. Dyno read 386hp to the wheels. Again as others have said. Get a custom tune, remember all of them react different and good luck!!
 

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I live in a complex and have yet to hear a cat back that's quite enough on cold start. I leave for work pretty early every day so trying not to be that ***hole neighbor. I have Magnaflow axel backs on now just for a bit of sound. I like it the way it is and I think it will be a bit louder with the Kooks mids and HFC's. Adding them to get rid of the biggest exhaust restriction.
Regarding this. I previously had magnaflow axlebacks and added in Kooks mids and HFCs (the rest stayed stock). It was obnoxiously loud IMO. It wasn't solo J pipes loud, but still very loud. I ended up swapping back on the stock G8 GT axlebacks and I really like how it sounds. On cold start and just cruising it is quieter than my previous stock setup with Mangaflow axlebacks only, but sounds about as loud as the Manaflows when really getting on it. For me, this is perfect and I'm very happy with the setup.

To me, it sounds like this is exactly what you'd want. One thing to keep in mind is that with the thinner walls of the Kooks mids, you may hear more valvetrain noise which can be bothersome. You have a pretty low lift cam picked out so the valvetrain noise might not be too bad, but just be aware of this.
 

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I live in a complex and have yet to hear a cat back that's quite enough on cold start. I leave for work pretty early every day so trying not to be that ***hole neighbor. I have Magnaflow axel backs on now just for a bit of sound. I like it the way it is and I think it will be a bit louder with the Kooks mids and HFC's. Adding them to get rid of the biggest exhaust restriction.
Buy an XForce catback and be done with it. Full 3 inch system, and dual mode mufflers. Mine with cam, long tubes, and no cats is quiet enough to not really piss anyone off when cold starting. Once you're out on the road, open up the mufflers and let it rip. Oh, and no drone. Yes, it's a fairly expensive catback, but the quality is top notch, and it fits the car perfectly, even being 3 inch all the way.

Ask @executor999 about his recent switch to XForce. He hooked them to Kooks mid-length headers.
 

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Buy an XForce catback and be done with it. Full 3 inch system, and dual mode mufflers. Mine with cam, long tubes, and no cats is quiet enough to not really piss anyone off when cold starting. Once you're out on the road, open up the mufflers and let it rip. Oh, and no drone. Yes, it's a fairly expensive catback, but the quality is top notch, and it fits the car perfectly, even being 3 inch all the way.

Ask @executor999 about his recent switch to XForce. He hooked them to Kooks mid-length headers.
This would be a great option for him as well if his budgets supports it. I looked into the XForce mufflers when I did my DOD delete and they looked really solid. They unfortunately weren't in the budget for me though.
 

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This would be a great option for him as well if his budgets supports it. I looked into the XForce mufflers when I did my DOD delete and they looked really solid. They unfortunately weren't in the budget for me though.
Unfortunately you can't get just the mufflers, you gotta get the whole catback. I finally pulled the trigger on it when Summit had a sale. I started with the Kooks mid-pipe and didn't like it much at all. The absence of mid-muffler or resonantors makes it very tin-can-like. Once I priced out a custom setup and doing something like cutouts, the XForce quickly makes sense and I was pleasantly surprised when I saw the quality of the whole system.
 

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Discussion Starter #14
Regarding this. I previously had magnaflow axlebacks and added in Kooks mids and HFCs (the rest stayed stock). It was obnoxiously loud IMO. It wasn't solo J pipes loud, but still very loud. I ended up swapping back on the stock G8 GT axlebacks and I really like how it sounds. On cold start and just cruising it is quieter than my previous stock setup with Mangaflow axlebacks only, but sounds about as loud as the Manaflows when really getting on it. For me, this is perfect and I'm very happy with the setup.

To me, it sounds like this is exactly what you'd want. One thing to keep in mind is that with the thinner walls of the Kooks mids, you may hear more valvetrain noise which can be bothersome. You have a pretty low lift cam picked out so the valvetrain noise might not be too bad, but just be aware of this.
Appreciate your feedback. Thanks
 
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