2007CalSpec

Hey everyone! I used to run an off-road prochamber for 6 years when my car was brand new until I had to put my stock catted mid-pipe back on for smog purposes a few years back. I'm currently running a Pypes x-pipe with Borla Stingers and this setup sounds good but I want it louder. I want to put my off-road prochamber back on and I have this idea of adding high flow cats to the flat area of the down-pipes (see picture). This should increase the exhaust volume and also eliminate the exhaust smell which I didn't care for. I plan to use my 91 Street tune again. It was made for the off-road prochamber and it turns off the rear O2 sensors. I figured I would use this tune instead of my 91 Smog tune because the high flow cats don't have O2 sensor bungs anyway. Would there be any problems with this setup? I'm basically going to run an off-road tune and the HF cats are just there to eliminate the exhaust smell.

m05fastbackGT

I used to run a Mac high-flow catted prochamber, but had to put the stock cats back on when one of the high-flow cats bit the dust.. Unfortunately Mac discontinued it's catted Prochamber series years ago..

m05fastbackGT

Here's a couple of images of my Mac catted prochamber..

2007CalSpec

I remember seeing the catted prochamber but never got a chance to buy one. I was bummed when I learned they were discontinued.

m05fastbackGT

Same here.. As I still have both the Prochamber and catted legs sitting in my basement, but don't know if I'd be better off hollowing them out or just look into purchasing off road legs from MAC..

As for your particular application.. In theory you should be able to either bolt in or weld aftermarket cats in the flat portion of your off road legs..

However If your not 100% sure, I would seek the advice from an exhaust shop just to make certain for your own piece of mind..

In regards to having your rear 02 sensors turned off along with your high-flow cats not having rear 02 sensor bungs, this may present a problem with passing emissions.. Therefore I really don't know what your state emission requirements are in Calif, but here in PA unless you meet the emission exemption requirements of 5000 miles per year or less, your vehicle is then required to pass the full emissions test..

Anyhow just something more you may need to look into

2007CalSpec

California no longer requires a sniffer test. They only check your OBD2 system to make sure there's no problems and do a visual. I know I won't pass with aftermarket cats and rear O2's off so I plan to switch back to my stock system when it's time to smog again. Switching out the mid-pipe is easy enough.

m05fastbackGT

I don't think too many states use sniffer tests anymore to begin with anyhow as most cars on the road today have OBDII ports, with the exception of older cars prior to 1996..

However just as you mentioned, they will most definitely do a visual, so unless your state has an emission exemption requirement, you won't pass the visual inspection with aftermarket cats nor pass the OBDII system check with your rear 02 sensors turned off..

At any rate, as long as it's easy enough for you to switch back and forth from the stock mid-pipe to off road, you'll be good to go

07 Boss

If you're looking for sound quality why not weld in a couple of resonators. I have a couple of old school Cherry Bombs welded in just before the axle. Then make some MIL eliminators to get by smog. I used to swap my mid-pipe every year but now I just have to swap tunes to pass the OBDII testing.

m05fastbackGT

If I'm understanding his post accurately.. I think the reason for switching the mid-pipes is he's concerned that his car can still fail the visual inspection for not having his OEM catted mid-pipe installed..

So I can understand exactly where he's coming from, as here in PA they also have the same requirements.. How I'm able to get around the visual inspection is by meeting the emission exemption requirements for having 5000 miles per year or less on my vehicle..

2007CalSpec

I love the way my car sounds now with Borla Stingers but I just want it louder and the only way to do that is to put high flow cats on. I've thought about just going full off-road and eliminate cats all together but I can't deal with the exhaust fumes now like I did in the past. I know I won't pass smog when they do a visual and see the high flow cats or no cats at all.

07 Boss

Well then to answer your original question, I don't see any issue with putting cats where indicated. I have an O/R pipe that I welded cats in right about there. The only issue I had was that it seemed to hang down low and would scrape them occasionally. That is why when I went with resonators I welded them in just before the axle. No scraping there.

m05fastbackGT

There are certain states that also perform a visual inspection.. What you are not understanding is, if an inspection station happens to notice that the vehicle's OEM catalyst equipment has been removed in favor of either an aftermarket catted or non-catted exhaust, they can fail the vehicle for removal of the original OEM catalyst equipment, as doing so is in violation of federal and state law..

07 Boss

What you are not understanding is that he swaps in his stock mid-pipe for inspections. That is not the issue. He wants the hi-flows for sound and smell when not running his stock set up. He knows what he has to do to get inspected and pass the visual, he wants to know how that particular location will work for cats while he is running his non-inspection exhaust.

m05fastbackGT

I understand where your both coming from.. Apparently I misinterpreted part of your post and thought you meant that his car would pass the OBDII test with a different tune running aftermarket cats and therefore wouldn't have to switch between mid-pipes in order to pass the visual inspection requirements for emissions which is not accurate at all..

At any rate, my mistake for the confusion, as all I stated was as long as he re-installs his OEM mid-pipe prior to inspection, that he won't have any problems for passing both the visual inspection and OBDII emissions test..

As for installing high-flow cats to work with his MAC prochamber setup, I addressed his question by stating that I didn't see any reason why he couldn't either bolt in a pair or have them welded onto his prochamber pipes..

2007CalSpec

Here's some pictures of the stainless steel high flow spun metallic cats I'll be installing. They are 2.5" inside diameter so they'll slip right over my exhaust pipes. Took a bit of searching to find a dealer that would ship aftermarket cats to me and I'm so glad I found one. I'll be running them with my off road tune (rear O2's off). Come smog time, I'll just swap out the downpipe with HF cats for my OEM catted downpipe and reload my smog tune. Easy enough.

Measurements:
Inlet Diameter: 2.5in.
Outlet Diameter: 2.5in.
Body Length: 4.5in.
Overall Length: 10in.
Width: 4in.

07 Boss

Why did you have problems finding someone to ship cats to you? I got mine from JEGS I think. Ordered them online.

m05fastbackGT

Are these clamp bolts on's.. Or do they require welding ?

m05fastbackGT

The cats you ordered from Jegs, do they have 02 bung sensors for plugging in your rear 02 sensors and can they also be bolted on with clamps or do they require welding ?

I'm looking for high-flow cats to install on my MAC procchamber, but don't want to cut any portion of the off-road pipes just in order to install them

2007CalSpec

As of January 1st, 2009 California doesn't allow buying, selling, installing catalytic converters that don't have CARB EO numbers. Just about every store I checked won't ship them to this commie state.

I plan to use lap-joint clamps since these will slip over my 2.5" exhaust pipes.

Unfortunately, you'll have to cut the off-road pipes or legs or downpipes as they call them. I don't see any other way to install the high flow cats unless you stick them on the end of the "chamber" and your over-axle pipes but that might be too far from the motor for cats to be effective. I don't know.

m05fastbackGT

Here's another option.. I can just use my existing ProChamber with my stock mid-pipe and cats.. All I'll need to do, is just cut the H section off and clamp the ProChamber in it's place..

Problem solved