I would have put my Alpha onmy lift to get some better shots but it's too cold to be shuffling cars on and off the lift today so I just settled for crawling under and getting the following shots.

The driver's side "pre-cat" or "pup-cats" as they are also known. There is one of these coming off each of the exhaust manifolds, driver and passenger, just after the O2 sensor. The pup-cats light off faster than the main cats to keep cold start emissions low. I'm not familiar enough with the H3 to know if it also uses an Air Injection Reation (AIR) system to pump fresh air into the exhaust stream on cold starts to reduce emissions until the cats warm up enough. That air is injected for only the first few minutes of a cold start.



Here's the passenger-side O2 sensor there are two, one on each exhaust manifold collector. These are essential to the operation of the engine as they monitor air/fuel ratio and allow the PCM to constantly adjust fuel injector pulse width and try and maintain as close to stoichiometric (14.7:1) air/fuel ratio as possible except on wide-open-throttle (WOT) where Power Enrichment (PE) mode is enabled which is essentially just a rich mixture for safety. GM, as far as I know, uses only narrow-band sensors so they cannot be used for tuning purposes. Wide bands have to be added (as I do on my vettes) in order to be able to tune the car using one of the sw/hw packages available.



Here you can see the relationship of the driver's sensor to the pup-cat (shot taken pointing towards rear of rig).



Both sides continue back to a main cat, one of whichcan be seen in the pic below. Surprisingly, there is no secondary, colder O2 sensor behind each of these cats. It's surprising because the rear O2's in other GM setups like my vettesmonitor the operation of the rear cats and report an emissions DTC if one or both fail due to clogging or other problem. The tuning software allows these sensors (their trouble code generators, really) to be shut off so that they can be removed or replaced with a high-flow aftermarket version. This means none of the 4 catsis monitored by the PCM.



The exhaust then continues rearward where the two sides meet up and merge into a single pipe. The driver's-side flattens out for a short distance before the Yto traverse and clear one of the crossmembers where there's not enough room for the round pipe to pass.



The single pipe is interrupted at this point, just after the Y by the resonator . . .



. . . and then continues over the passenger-side rear axle to the muffler.



The exhaust gasses exit the muffler out the driver-side-only tail pipe.



It seems that running a true dual exhaust will be a little bit of a challenge due to the fuel tank that blocks access to the driver's side. Dual is still possible but would be circuitous as the driver's side makes a detour around the tank. An x-pipe section could be placed further back, possibly in the area where the resonator currently resides. Clearance for heade

 

So a true cat back would remove the resinator and rear mounted muffler...that would open up the sound a good amount I would think. Having no downstream 02 sensors also helps cause you can replace the back cats to a highflow cat if you want to really open it up....good stuff...thanks for the pics!!!

 

Patches,
I just signed on to post some pictures but alas you beat me to it! Thank you very much for the great shots as I know how difficult it was to get a decent shot without lifting the truck. Now we all can be enlightened.

 

mr. importkiller,
I have been around enough forums to see "little arguments" like that turn into nasty full blown wars and not knowing the relationship between youand TheGunny I did not know what direction that was going so I simply did not want to be involved. I'm at a point in my life that I don't need to prove anything to anyone and don't need any stress or BS from anyone either. Everyone here has been a great help to me but I certainly don’t expect anyone to get their shorts in a knot over something I’ve asked about. Thanks for all your help.

Gar

 

not to worry bro...we are all cool here. Even when arguments get really heated (and they have before) we always seem to find a happy zone and calm down. Normally it takes Bob posting something funny to bring us around but we hardly ever leave with hurt feelings. I have much respect for the gunny and his knowledge and my intent was not to hurt his feelings...I don't think he feels that way but if you do Gunny, I am sorry. I admin two boards currently and mod two others and this is the only place where almost everyone is man (or woman) enough to be able to have a productive argument where name calling and personal attacks never come into play. Hell, 75% of the arguments here that get really heated are by long time members that are just messing with each other. Some of the old arguments I had with shorty you would of thought we wanted to kill each other...but in the end Shorty is probably my best friend on the board and someone I respect a great deal. It's all good brudda

 

the flattening out of the driver side pipe to cross under the x-member is what I was [:-] at when I was under there today?? Don't you think that interrupts flow making for an unblanced exhaust pluse?

All the cats and res's/goofy bends, Nissan Z esque muffler......we have to figure out something better guys!

 

the I-5 has the same flattened out section of pipe. I don't like the design either. There was a member here awhile ago that redid that part of the exhaust after he did a cat back and said that he noticed no improvements. Coming out with something better definatelycouldn't hurt any though.

 

Man, what a mess. Everything after those cats is just sound reducing resonators or whatever you want to call them.

Here is what I think, I would see if the MAC GM 5.3 Headers fit, look at Dr Gas for an X-pipe that has a 90 degree bend off one legthen run that to a Spintech dual in and out and split them out the back.

The other thing you could do is volunteer your Alpha toone of the major Exhaust Makers if you live close to them.

 

I agree with TheGunny, what a mess! When I have the extra coin I'm going to replace the mid-truck resonator andinstall a hi-flow muffler in it's place, then some how with dual tailpipes exit out the back. There's a muffler shop in my hood that does custom work (bending etc.) and might be up to the task. I'llgo by and see what the muffler shopsays and let everyone know what the verdict is.

 

[align=left]Regardless the alpha has 4 cats. How many time does a man have to say it. I talked to a g.m. tech and he told me their monitored before the first cat and after the second cat. And removing or altering any of them could cause you to throw a o2 code. My only option with removing the pre-cats it to park my alpha,core my ecu.and get a new comp. tune to accept the loss of backpressure and possibly advance spark,time.,air,fuel, but you hafta run 91oct. Or get o2 simulators--- they make them for late model muscle cars.[/align]