I need help deciding what to do to my stock NA exhaust for best HP gains.
 

At first I was simply going to replace everything from the engine to the mufflers on my 88 NA FC3C. New headers, and pretty much straight pipe out to the dual mufflers. I thought I could simply take out the oem system and replace the whole thing. only putting the oem stuff back in come smog time.:) But then after some research I learned that the header might not really gain more hp than the stock manifold. And some headers do not have the o2 bung hole. That it's best to just replace the cat sections. This then raises the question of the ports not getting enough back pressure. and so on. I'm more confused than ever. :Wconfused
what's the best combination? any advice from the wise mod gods would be great!!

the big restriction is the catylitic converters, getting rid of those on a stock car will get you the biggest gain.

the headers help, but the gains are small becuase of a couple of things. the exhaust manifold isnt too bad, the rest of the system (intake manifold) doesnt flow more, and on an s5 the air/fuel is too rich

its up to you, but since its a vert, and without cats it smells, a header to high flow cat might be the best setup.

without the cat it smells?? seriously? I did not even consider that. suppose I go the header to high flow cat. would I need to buy one with an o2 bung hole? and I assume I could simply weld the pipe (for the ports) to the high flow cat. If I forgo the high flow cat for a straight pipe could I also weld the pipe to it?
in any case, I like your suggestion thus far.

The exhaust smell is barely noticeable except when your sitting at idle for awhile. I am running the stock exhaust manifold, racing beat downpipe, corksport racepipe / presilencer & racing beat catback.

I had to add about 3 inches of pipe onto the end of the downpipe because racing beat can't measure worth a damn, but now I can simply drop out the racepipe and put in a cat for smogging. It's 17.5" long, which is exactly stock cat length.

I used to run a straight pipe, but it leaves a lot of annoying high frequency noise that the mufflers can't dampen. The corksport pipe has no restriction at all, but cleans up the sound very nicely (and surprisingly quiet at low throttle). You can also go with racing beat's silencer if you think you need it at stock noise levels, but what I'm running really isn't much louder than most cars on the road. Consider the pipe length if you go that route though. Racing beat's silencer is 20" long, so you will need to extend your cat's length and modify the hanger bracket to have it bolt it easily during smog time.

Other than that, you will need to make sure your split pipe is welded in (or bought that way w/ rb's presilencer) so your 6PI system works.

that's exactly what I needed to know. It's also nice to know it came from a fellow vert owner. Did you notice a significant hp gain. How does it feel, know what I mean? could you tell me how much the set up cost you? BTW: is it dual?
This is great advice guys keep it coming!

Removing the cat's will disable your aux ports above 3800 rpm and it's not really worth it to remove them (aux ports) as you lose hp below 4k rpm and gas mileage.

I've had all the cats removed, cone filter, single straight pipe into a 4in single and the power loss after putting the cats back in was very small. I'd say it was worth it to get that 4-7 mpg in the city back to loose maybe 1-5hp in the top end.

Then again, if u have an S5 it will not be an issue at all, since the ports are mechanically activated.

Gutting the car made more of a difference in speed as opposed to power adders imho.

so you are arguing that there is no significant hp gain by removing the stock cats, that's a first. not talking smack, I'm just really surprised. I always thought removing the restrictive cats provided significant gains.
anyone?

you can put a turbo somewhere near the begginning for the exhaust

Well, i've had them on and off and there was not a major difference. Nothing like when my friend took them off his 87 turbo supra. This is on a rebuild with the throttle body mod and a K&N cone filter.

I read a magazine article awhile back regarding the removal of the catalytic converters and they did dyno charts comparing hi flows, test pipes, and the stocker on an integra gsr or a civic si. The dyno chart showed about a 5hp difference between the test pipe and the stock cat. I'm not saying it won't make a significant difference on a turbo charged car, only a n/a car (with 1.3 liter rotary). There are other exceptions like a supercharger or a big V8.

p.s. On my friends supra the boost went up about 4 psi after taking the cats. That's where most of his hp came from.