I haven't been overly thrilled with the exhaust options available for my car, so I haven't done anything with it yet.

Here's what I've got. I have an 84 Z28, L69---T-5 tranny, 3.73 gears.

I've added a 650 Holley 4011 series, a Weiand Action Plus Intake, a Jacob's ignition system, a Comp Cam's 262 XE (Lift .462 .469
Duration 262 intake 270 Exhaust), roller tip rockers and a few other misc. things here and there... The only exhaust mods that have been done are the Flowmaster 80 series muffler, and I removed the AIR junk.

So how much do you think that the stock exhaust is holding me back?

 

headers are usually good for a about 15 hp

 

That depends on application. 15 hp is what someone could expect on a stock set up, at least that's what I understand.

Mine isn't a wild setup, but it's certainly not stock either. I would think that I'd get more than 15 ponies from a good exhaust upgrade.

 

With that cam in thier I think the number is way more than 15hp.

 

That's what I figured.

 

I just installed headers on my 87. I had stock lg4 manifolds. When I got the manifolds off I looked at the right side exit port. All 4 right side cylinders were trying to exit through a 1 1/4" hole. If I try to stick 3 fingers in there they stop at the second knuckle. I couldnt beleive how restrictive these things are!

I did gain some performance, but I am still breathing through the stock 2 1/4" pipe and muffler. Thats the next thing to go.

 

The stock exhaust is a huge restriction. It's usually the best place to start with your mods so that the rest of them will show up. If you replace the whole exhaust system (headers and a catback) you could gain up to 30-40hp on that alone. Maybe even more considering the mods you already have.

 

What ever way you go, headers and a catback are # 1!

 

Check my sig for my improvements. The first run was with a stock system with a Flowmaster 40-series muffler, the second was gutted cats, the third was 1-3/4" SLP headers, 2 new Catco hi-flow cats, and 3" Flowmaster cat back. The only other changes I made during this time was porting the plenum and taking out my AC.

So however much HP it takes to make a 3580 lb car run 1/2 second faster, is how much you're holding back.

 

Just curious ... what is the size of the stock Catback on an IROC?

 

If I remember correctly, it's 2-1/2".