Hi there I'm wondering what's the optimal exhaust size for a cat back system on this car for best performance? Will a 2 1/2" exhaust result in poorer performance because of lack of back pressure? I have the 2 1/2" flex pipe and cat from ZZP.

 

Super generalizing here: higher back pressure is supposed to create higher velocities to help scavenge exhaust out of the cylinder, so in theory this is good for low end torque but it depends on many factors.

 

Unless you are going to tune your exhaust lengths for scavenging like Steelmesh said, back pressure really doesnt matter. 2.5 inch exhaust is plenty for an m62 car. If you have a turbo, go 3".

 

My turbo LE5 made 444 whp with a 2.5" single exhaust

 

I have no plans for forced air so an intake system is the extent of my plans for modifications. I'd go to 2.5" but I'm suspecting I would lose the low end power. Is that true? Would 2" or so be the sweet spot?

 

The idea of back pressure is one of those mythical characteristics, don't get too caught up here. Heck, you may even gain torque on the low end. ZZP will respond to your email, give them 24 hours during the week, ask about if their 2.5" and 3.0" system will lose torque at low rpms in your 2.4 car.

In your application, with an N/A 2.4 LE5, the main benefit of going with an aftermarket exhaust is sound (which is cool still, if you like it do it!), with all the bolt-on's you'll probably get a small side effect of HP. The best exhaust system you can possibly put on your car is a turbocharger, which actually muffles sound pretty well and gives you 1000% more horsepower than an intake+exhaust It's not like the 80's or 90's when N/A engines out of the factory were horribly designed (GM!). When I first tore down the LE5 I noticed how much effort they put into making it as efficient as possible out of the gate just looking at the head design. What I am getting at is that GM has optimized the engine pretty well, so the easy bolt-on mods don't gain you much.

When you go on youtube and watch videos of the big horsepower N/A 4-cylinders, you have to understand they invested $10s of thousands! There was one that was mid-300 hp, he said it was about $60,000. A $4,000 turbocharger kit could get you 400 hp.

 

2.5" or just use stock.

 

Pretty much any exhaust will work fine for NA. As steelmesh said, its mostly just for sound. Make sure you use mufflers or your ears will bleed on an NA Car.

 

Isn't the stock DP 2.25"? That would be the one restriction I would go aftermarket.

 

The exhaust manifold is the same between the SS/NA and the SS/SC according to GM ( PN : 12581284). The Converters are apparently different between the SS/SC and SS/NA (different part numbers) however they appear to replace the same previous part ( PN :15247471) . Since the parts are apparently the same between the SS/SC and the SS/NA I think the factory stuff is more than up to the task on a factory/ lightly modified LE5. The old school rules for intakes and exhaust are pretty much the same. Longer, smaller runners/primaries will tend to push low end torque at the cost of HP further up the rev range. Shorter, larger runners/primaries tend to support high end HP but at the cost of lower end torque. The further away from the engine a restriction is, the less effect it will have on performance.

Also, you are not looking to keep back pressure. The idea is to minimize the restrictions but keep the intake air/exhaust velocities up with the proper size piping.

If I recall correctly the cat/down pipe and catback are 2.25" from the factory but I don't have one handy to measure.