OldSchoolSS

Hey guys I just read in a magazine that putting the catalytic converters before an x-pipe takes away any benifit of using a x-pipe(other than a reduction in sound). They suggested placing the cats behind the x-pipe if possible. Would the exhaust even be hot enough to light of the cats if they were that far back? Also they mentioned if you sized your cats properly that they can act as a resonator chamber. How true is this?

waylow

I don't know for sure about the cats; but I live in Texas, and my county adopted emissions recently. On my 95 LT1 I had been running Hooker Long Tube headders, no cat's, and a flowmaster with a 3" QTEC to dump the exhaust right before the muffler. I wanted to be able to pass the inspection tests... so I got a Highflow dynomax cat from Summit. Not even sure I got the right one... it's a 3" highflow cat but it has Wagner stamped in it, not dynomax... oh well that's besides the point. The only place that I could get the cat to fit under the car was way after my Y-pipe, in the back, like just before the pipe gets bent up to go over the axle.

So after I got that welded up, I drove it around, got it good and hot a few days. I went and got it inspected. FAILED! too much nox at low rpm I think is what it said.
So next week I came back and this time I just had the QTEC opened, they stick the exhaust sniffer in the tailpipe and lookie there, Passed with flying colors. So... if you want it to be legal to the spotcheck... put the cat's in. If you want them to actually do anything.. I would stick em up front.. but If it were me, they would be behind the X-pipe and get a pair of electronic cutouts, and put them just after each headder, so when you open them, you're running straight headders.

Just a thought,
-Tony

Zero_to_69

My theory says the X/H pipe should be placed before any restrictions to
make use of the cross-over tuning.

Once the exhaust pulse hits the catalytic internals, the pressure wave will reflect back.

I would imagine any pulses getting past the catalytic will be attenuated
enough to negate the potential of the X/H cross-over tuning.

If anything, make sure you have high flow catalytics to get the exhaust gas
flowing if emissions are a concern.

nateh

I have seen testing that would seem to contradict the orginal premise here. The test was on a vehicle with dual exhausts, cats just aft of the manifold outlet, h-pipe/x-pipe, and dual mufflers to pipes routed out the back. The h-pipe helps acoustically, but the x-pipe was even better acoustically and had significantly higher power output. This vehicle was something with very good power.

Putting cats after the x-pipe might be better powerwise, might not, but it sure makes it tough for the cat to do its job. Those things need heat.

Also, cats do help quiet things down. It's not the same thing as a tuned resonator, but they do help. Sometimes, they can help sound a lot.

OldSchoolSS

I don't have emissions testing in my county. I just know how much of a difference that cats make on emissions output, so I wanted to put them on. I also like that they get rid of some rasp and drone. I was going to fab up a dual over the axel exhaust for my car with an x-pipe, but if the cats hurt the benifits of the x-pipe i might just leave them off. I don't think that they will get hot enough or even fit for that matter behind the x-pipe.

waylow - Dynomax is a division of Walker Exhaust. Walker was really big back in the day. I think they were the ones that made the chamber pipe exhaust systems.