Nice! glad to hear someone likes them. That might help me decide a little easier

Alright thank you. I will start looking around

OBX headers are $499 or best offer on ebay. I threw them an offer for giggles to see what they respond back with.

 

Thanks for this Will, this is some good info right here. Maybe I'll just hold out and do an engine/exhaust rebuild at some time in the future and do everything at the same time.

Maybe I missed it, but can you please address the whole theory of low-end torque loss and back pressure when running headers with a cat-back system; in most of our cases the MagnaFlow #16727? Is that realistic in our situation, in certain setups, and would a professional (exhaust) installer know what to do if tweaks were needed to achieve the correct specs for the exact engine/exhaust setup?

 

You can run headers into the Magnaflow 2.5" cat-back and still have noticeable horsepower gains, but the more power you try and make the more restrictive the 2.5" exhaust becomes, and it limits the power benefit subsequent modifications will make.

It's just a matter of capacity. Exhaust velocity is what we're really after, get from the engine to out the tailpipe as fast as possible. When an engine makes more power, it breathes more air in and out, and as it breathes more air into an exhaust system that can't handle the capacity, velocity stagnates, pressure starts to build back to the engine ("backpressure").

You'll often read that "some backpressure is good for torque" and that's true, if you oversize the exhaust you'll lose exhaust velocity until the engine rpms are such that the engine exhausts enough gases to fill the pipe, but that's not a concern with a 3" exhaust on a 300+ horsepower V8, you're not going to see significant torque losses like that, the LS4s that I've driven with 3" exhaust systems have felt very strong.

Also, regarding low end torque loss, the torque converter on the LS4 flash stalls to around 3500rpm when you hit the gas, so there's no particular worry about what the engine makes at 2000 rpm at full throttle launch, for example.

Best think of it this way, you should never lose power because of your air intake or your exhaust system. If you're at the point in your modification sequence when you're ready for headers, I'd run a 3" exhaust, get the proper benefit from a full exhaust setup rather than the headers dumping into a smaller cat-back. I quote the same on 3800 Supercharged engines. It's a balancing act but I'm confident that's the right formula.

An exhaust shop should be able to build a 3" system if they're competent. The dual split can be 2.25", the single 3" has slightly less capacity than dual 2.25" so you want to match the size but not restrict. If you're having a system built, I'd purchase and supply them with a Magnaflow Y pipe section, that can often be a restriction point if they build a poor flowing Y pipe on site. However, again, the ZZP system for an 04+ Grand Prix will fit on a 7th gen Monte minus the mufflers, but pick up some center in and out 2.25" mufflers and some ebay tips, it's an easy install for any exhaust shop.

 

Valuable information. Thank you Will!

 

Im very confused, what is a catless downpipe?

isn't the downpipe straight off the exhaust manifold which leads into our catalytic converters?

If I do a header install can't I get a downpipe plus keep my catalytic converter? I need to pass emissions in Ontario.


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I think I understand now actually. If I get a catted downpipe, it will simply replace the stock catalytic converter? Or is there 2 cats/mufflers on this car? (One on the downpipe itself and the catalytic converter which could be replaced with a high flow cat)

Im confused. Help lol

 

My bro's monte has a 3" highflow downpipe with a cat. Though his is a 6th gen, not a 7th. So I assume you could do it. Don't hold me to that though haha

 

With headers the easiest thingg would be to but a performance cat.. From like magnaflow or whoever and have the exhaust shop ddoing the work weld it in. No need to spend money on a downpipe to just have the shop cut it. There is only one cat on our cars then it Ys off into the 2 mufflers. Thanks will for all the good info it's nice having you on the forums

 

Is the catted version of the downpipe actually considered a high flow cat? This would replace my stock cat?

 

Yes. I think on most all the downpipes you can by they will put a high flow cat on them. I was going to go that route but I figured since i have no inspection why not go with out and make all the noise i possibly can

 

Thanks man