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2004 CTS-V
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Discussion Starter #1
I've got a 2004 CTS-v with the following performance mods: magnacharger (older style) with a smaller 2.8 pulley; CIA; Kooks 1 7/8 lt headders; stock catback with no resonator.

I only put down about 420 whp before my headders. I was producing 8.4 lbs boost. I expected headders to net me a pretty sizeable gain and drop my lbs boost. My tuner told me that he was worried that the stock catback may be too restrictive. Without a reasonator, how restrictive is the stock catback with the stock mufflers?

What could I expect to net from a 2.5 magnaflow system? Would I get the same results from just removing the stock mufflers and welding in some magnaflows?

Would I net a meaningful gain by going with a full custom 3'' system?

Thanks for the advice!!!

BTW, it is not an option to buy a corsa system with these style headders, and I don't want to spend 1.1k on a BB system.
 

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Discussion Starter #2
Oh, and I don't have new numbers yet with the headders. I'll get those today.
 

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im pretty sure they are not restrictive at all. From what I know getting a cat back may give you a 1-3hp gain but thats probably the butt dyno speaking. manifolds, and cats are the only restrictions that I know of. Replace both or anyone of them and youll gain some powa back. lol only 420 w/o headers, I wish I was there. Your set up seems awesome, how is the sound with the maggie, 1 7/8 headers and a stock catback??
 

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Discussion Starter #4
I like to work my way down the foodchain of expensive mods, instead of up. I bought the car, got a maggie and 2.8 pulley, got an RPM Level IV Transmission, LS7 Clutch and Flywheel, Nitto NT-05's, CIA (was basically given to me during a tune), and now headders. Next on my list is to finish the cat back system without making my car sound like a damned dragster, and then a new shifter/bushings.

I just don't want to choke the motor on the top end. I was hoping to be around 440whp. I'm on a pretty conservative/realistic mustang dyno, so I'm not stressing about the numbers, just the gains. I have the utmost faith in my tuner - Scott from InTune Motorsports in Charleston, Tennessee - I just want to make sure I know my options regarding the exhaust.
 

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'05 CTS-V, '00 SLS, 98 SLS, 89 Eldo, '80 Eldo
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If anything, weld in some mufflers for better sound. For a HP gain, spend your money on something else.
What's your boost with the headers? And post up before/after torque numbers. Same dyno, right?
 

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2014 CTS Vsport, 2016 Chevy SS M6
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Long-timers may already know this, but the stock resonator actually has an x-pipe built in to it (at least on my 07). That with the 2.5" mandrel bent pipes, it's no wonder why a cat back yields little in terms of horsepower. I had a local shop weld in some magnaflows out back and a 2.5" magnaflow dual in, dual out resonator with x pipe. Nice deep bass tones without any resonance in the cabin.
 

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2006 CTS-V
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I'm a maggie idiot, but I do read all the FI threads I can find. so take my comments with a grain of salt.. I would think w/ ~8lbs of boost and your mods...you'd be in the 460-480 range ( pry not a mustang dyno )

Dynojet man found a lot of power by changing his maggie intake... but it sounds like you did something along those lines already... did you try a pull on the dyno with it off?
 

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Discussion Starter #9
The only thing I can figure that may be holding me back is my 78mm throttle body. I'm considering buying a 90mm and the appropriate Magnacharger TB plate for the 90mm. However, since I'm already making 8.4lbs boost (with stock manifolds - I expect it to drop with the headers), I don't think getting air into the engine is my problem. I also am dumbfounded as to why my car is only making 420 whp. Although, on this dyno, my baseline was under 300 whp. With just a tune and no cats, my stock V made 311 whp, if I remember correctly.
 

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Hmm that does seem a shade low for a baseline... REALLY low if it was tuned w/o cats.....

Maybe it's the ruler you are using to measure? ROFL (seriously I'm not laughing at your situation... your car is way more bad a$$ than mine)
 

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2005 CTS-V
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The only thing I can figure that may be holding me back is my 78mm throttle body. I'm considering buying a 90mm and the appropriate Magnacharger TB plate for the 90mm. However, since I'm already making 8.4lbs boost (with stock manifolds - I expect it to drop with the headers), I don't think getting air into the engine is my problem. I also am dumbfounded as to why my car is only making 420 whp. Although, on this dyno, my baseline was under 300 whp. With just a tune and no cats, my stock V made 311 whp, if I remember correctly.
If my memory is correct I think we have established in earlier discussions that your HP numbers are from a Mustang dyno which includes a factor for weight and aerodynamic drag that makes them lower than a Dynojet - I am thinking at least 10% in your case.

Finally, I think (but cannot prove) the stock catback is not an issue up to around stock +100 HP but starts becoming an issue after that. Maybe you can determine that with a dyno run wihout the catback in place before thinking about replacing it.
 

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IMO 110rwhp gain is pretty good. Three of us did Maggies at the same time last summer, and we got a lot less than that.
 

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You said that was your gain w/out the headers, so yes.
Like I said, this is my opinion, and I'm going by what I've seen since I got the Maggie.
 

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Discussion Starter #17
I just got word that with the catback attached, it made 444 and 431 at the wheels, and without the catback hooked up it made 455 and 440 (the numbers may be slightly off - I'll get the graph this weekend). I'm pretty happy with that, but I'm definitly going to swap mufflers.
 
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