Exhaust question
 

Okay,so I've been looking into a different exhaust setup because Pettit Racing told me that I'm not making full boost due to a restrictive exhaust...So,I went to a custom exhaust shop today to see what I could have made,and the guy recommended a Hi-Flow cat with 2.5" piping all the way back..However,everyone I have seen on here seems to run atleast 3" back,and some people such as Goodfella even run wider than that..So that's what I told the guy,and he was convinced that I would lose a lot of torque if I ran a 3" back..Does this guy not know what he's talking about,or am I wrong??

Ah the old "you need backpressure or you'll lose torque" discussion. This gets some people really fired up. The bottom line is that you're not going to notice a loss of torque with the old butt dyno. Even if there is a loss at the bottom end, the high end (plus the mental effect of the increased volume) will stop you from noticing. A 2.5" will probably get along fine depending on your goals for the future (eg. if you're looking for 500 rwhp then a 2.5" will be too restrictive). For my dime its 3" all the way, then at least its done no matter what the future brings.

What's your current exhaust setup? Why do you need custom? standard 3" downpipe, midpipe, and cat-back exhaust is what HUNDREDS of owners run. Yeah, you lose a little bit of torque at 2k rpm. Who cares? You'll get on boost quicker and once on, will make more power than a narrower exhaust. 3" has been the standard for over a decade.

You can get a high flow metalic cat up to 3.5". If you want to get rid of the restriction, then just the metalic cat may do it unless you have the stock exhaust to. Some shops can do mandrel bends or have plety of stock on hand but only 2.5" which is most common for the small 4cyl (Honda, Doge, Subie, whatever).

You have a street ported motor and 3" DP. You would be going backwards to go to 2.5" and is the opposite of what Pettit Racing was telling you.

Going smaller for torque doesn't work on a turbo engine. Restriction will cause backpressure which in turn steals horsepower.

Stay far away from the shop that made that recommendation. It sounds like they aren't familiar with turbo cars or rotaries.

No need for a custom exhaust either. Pettit sells a good cat back and so does Racing Beat, and a dozen other companies.

Pettit definitely knows what they are talking about. I think the context is very unclear though, and that may be part of the problem here.

Authentikdit, how much boost are you trying to obtain? If you're having trouble making 10-12psi then it's an issue of a clogging exhaust component. If you have a stock downpipe or main cat then I'll bet that's the issue. If you're going for 14-16psi or more, then yes maybe opening the restriction is the best next step.

3" all the way through with a high-flow cat is pretty damn free-flowing. It's good for over 400hp. I would get set up with that first. You can buy/sell these parts cheaply and easily, to boot.

Dave

My setup is just a Pettit downpipe,stock cat,and a Magnaflow catback..Basically,i'm having trouble boosting the stock 10-8-10 pattern..I boost 10 pounds in 1st and 2nd gear on my primary turbo,but then when the secondary kicks in,i hold 5 pounds..So after 4500 RPMs,I can't boost more than 5 pounds in any gear..Pettit Racing told me it's due to a restrictive exhaust..But the shop I took it to apparently didn't think that I should run 3" piping..All i'm trying to do is open up the exhaust enough to get my car back to the stock boost pattern.

On a NA piston motor (and possibly rotary) you will lose torque if you go too big exhaust wise. However on a turbo car.......different story.

Go as big as you can, 3 inch at least. My set up id 3 inch dp to 4 inch downpipe back (once I get the HKS exhaust I bought from another forum member). Currently I have a 3 inch downpipe/midpipe/cat-back.

If the shop told you to go with 2.5 inch, on a 3rd gen (or even a 2nd gen TII), run.

All you need to do is replace the OEM cat. Get on the group buy with SMB.

I've got an M2 downpipe, factory cat, with a 3" RB catback. Runs so much better and can feel significant power increase with my :buttslay: dyno. 3" is where it's at!

Well have them pull the cat and inspect it for clogging. Or pick up a lower mileage stock cat and replace it for under $100.

However, when boost drops from 10psi to 5psi at 4500rpm, I don't think it's as likely to be the cat and more likely to be solenoid or rats nest related. Lots of solenoids and vacuum tubes are doing stuff at 4500, and most minor malfunctions in that system result in lower boost above 4500.

So while it's wise to consider replacing the cat, changing the rest of the exhaust is wasted money if your goal is to restore a 10-8-10. The next step would be to change a couple solenoids and look for system leaks.

Dave

I agree with DGeesman. Sounds like a turbo control issue not a clogged cat. I clogged cat would normally start out at the regular boost level on the secondary and decrease as the air flow increased through the rev range.

You can verify if the cat is clogged by removing the front 2 bolts and shining a flash light in there to check the condition. If it looks like mesh, honeycomb type of structure, its good. If it looks like a hair clogged shower drain, its melted and bad.

The FAQ sticky has a few good links to the troubleshooting info site for stock turbos.

I appreciate everyone's responses..The only reason that I believe that it's my exhaust is because Cam at Pettit Racing spent about 10 hours trying to figure out why I wasn't boosting properly,and finally came to the conclusion that my exhaust is restrictive..He didn't even say it was the stock cat,I believe he said it was the catback..I measured today and the catback piping is only 2"..

^Ah, that might do it. 2" is definitely pretty frickin small and Cam is a pretty sharp guy.

Check out the Racing beat exhaust. Pettit sells it.

My racing beat dual tip cat back (and my midpipe 3") will be for sale in just over a week. Once I get and install my new exhaust.

I was actually hoping to stick with something a little louder..I like the sound of my Magnaflow catback..Should I just switch out my catback and hope that that's what was restrictive?Or should I also go ahead and change out my stock cat to a hi-flow cat?Cam was telling me that he thinks the stock cat is best over a hi-flow cat or midpipe..Will a hi-flow make a much bigger difference?

If you have a 2-inch exhaust, that's certainly a problem...but I would be surprised if it is solely responsible.

Yes, your worst restriction right now is the 2" piping. You can still do a Magnaflow, in fact I did another thread showing some pics with one that comes with a polished tip. You will need some 3" pipe and mandrel bends and a 3" flange and gasket. The pipes the muffler shop may have and also the mandrel bends they can weld together (don't crimp bend) but the flange and gasket you may have to order from Racing Beat unless Pettit has them. Pettit may also still have a cat back exhaust but it may not be as quiet or as cheap.

Oh okay..Thanks so much for all of the help guys..Originally,Pettit Racing thought it was a faulty knock sensor causing the boost issue because it was actually bad,however it didn't turn out to be the problem,and they assured me that it's my exhaust..