Ok so I was wondering In addition to all the normal mods (intake, exhaust, pcm) what about a better flowing CAT? on my last car (Isuzu Vehicross) One of my CATs failed so I replaced them with Magnaflow CATs. I also had a K&N filter and Borla muffler on it, but when I changed the CATs I did notice a difference a little more power and better throttle response also I felt like it just ran smoother. I never really paid attention to mpg so I can't comment on that. Last I looked under my H3 the CAT and tubing around it looked cramped and I always felt like it needed to be rerouted to flow better. Maybe anew CAT in a different location with a smother exhaust route. Does anyone have any experience with this? or comments?

Also here is a cool web page I came across, it's just a write up on the H3 with all the stats. http://www.rockcrawler.com/features/.../hummer_h3.asp

 

that makes complete sense,a straight line is always a better flow

 

"Also here is a cool web page I came across, it's just a write up on the H3 with all the stats."

Hey, I like good publicity for the H3 as much as the next guy, but this is obviously a press release written by Hummer.

 

Thats what I was thinking hilljob26. So what the deal with everyone else? What do you think?

 

DOC4H the H3 already comes with a high flow catalytic. Re-routing to make a smoother flow may be an option but the problem lies in the second catalytic located up at the manifold. The ideal solution would be to install headers which would eliminate that second catalytic and give you the much needed increase in exhaust flow. The problem is till now is that no one makes headers for the H3 only the H2. So I would not bother with the re-routing until this becomes available.

 

Headers would be worth some small gain only, the H3 I'm referring to. The stock pipe actually had some flow engineering to help in the mpg department, believe it or not.

Messing with the CAT would not be a wise move foremission sensor/PCM reasons, again there are only a few HP there and the cost/time would render it a low return. I can't speak for all States or local shops, but tampering with the CAT presents a legal issue most places would not want to deal with.Some reading from the CAT mfgrs may helpthe forum get a better understanding of why I say this. JP349 has a handle on most of the issue. The H3 has a close-coupled CAT system, the O2 sensor, the close CAT, back CAT etc.. are located where they are for a reason, andthose locations (not physical, but for data stream purposes) are programmed intothe PCM for proper emmissionscatalyst temp control and efficiency. IF you mess with this you open up a can of worms on check engine light and O2 sensor reading errors. These could be moved and you might find someone with HP Tune to custom tune set these variables in your PCM, but they would have to do it on your truck to read what specifically your mod(s) did to notjive with fixed data tables. IMO, not gonna be worth it for 5-10 HP???

2.1 Close-Coupled Converters

Achieving high conversion efficiencies for both HC and NOx emissions during normal

vehicle operation represented by the FTP driving cycle, for example, has focused attention on

cold-start performance of catalytic converters for both Tier 2 and LEV II light-duty applications.

LEV I hydrocarbon emission requirements introduced by California in 1994 provided the first

regulatory driver that placed importance on cold-start emissions. Numerous studies published in

the late 1980s and 1990s have discussed the high percentage of FTP driving cycle emissions

associated with the early stages of vehicle operation following a cold engine start situation

(references include: 2,3,12, 22-24, 29-31, 38, 39, 41, 45, 52, 54-56, 66, 69, 75, 78, 80, 83, 88-

90). This is especially true for 1990s vintage vehicles sold in the U.S. designed to comply with

less severe Tier 1 emissions standards. Hydrocarbon tailpipe emission profiles during FTP

testing of Tier 1 vehicles are generally dominated by emissions emitted during the first one to

two minutes of operation after the cold-start. This large fraction of cold-start emissions in Tier 1

vehicles stemmed from significant fuel enrichment used by auto manufacturers to facilitate

engine start under cold conditions and significant delays in converter warm-up to catalyst

operating temperatures required for high conversion efficiencies (e.g., 350

[left]engine exhaust valves (e.g., at the exit of the exhaust manifold) are

 

Yeah its a boring pile of techy stuff, but you get the idea, the PCM is doin and measuring all kinds a stuff to maximize emmissions efficiency, ie.cold-start engine calibration stratigies. You'll throw it out of whack if ya mess with it, and to make your customized CAT system work reliably and properly will cost ya plenty of time an $$$$.

The data in this MECA manual is also couple years old, so it is more sophisticated now. [:@]

 

Yeah... What Doc Olds said.

In short... Don't expect headers for the H3 anytime soon and there's no need to change the Cat's. There won't be a noteable change for the cost, agravation, legal and/or warranty issue's you'll adopt.

 

WOW, why could I have said it that way? Crud, new guy showin me up already. [sm=sigh.gif]

Nice

 

I said it with the upmost respect and admiration.

But seriously thanks for the detailed explanation.