Gases contract when they cool. Physics, don'tchaknow.

 

Ah, if it were only that simple. We'd both be rich.

The exhaust gases in the pipe are at higher than atmospheric pressure (>14.7 psi) but can "see" atmospheric pressure at the outlet, plus a whole world to mingle in, maybe find a mate with the right chemistry out there, form some long-chain bonds, settle down.

They'll stop expanding once they get there and get the party started. Before that, they're pretty pressured up.

Heat is not the only thing driving them to drink, distraction, and blood pressure meds. For one thing, some jerk keeps pushing them from behind, and every time somebody "goes for it" and jumps out towards the highway, there's a draft.

Bu, I can fix it if it makes you feel better:

" ... Exhaust gas expands, and slows down as it cools ..."

 

Yeah, operating in an open system does complicate matters some.... I think what we are both getting at is higher temps have a better scavenging effect on the exhaust, as the gases cool, they do indeed decrease in volume, which also gives a decrease in pressure, therefore, slower flow...... which is exactly what you DON'T want.

That's one of the reasons ceramic coated headers are nice. They retain the heat, so, scavenging is improved (over non-coated headers) and also have a side benefit of reducing underhood temps.... so, intake air is ALSO cooler. Which means: More Power. Which is something we ALL want.

 

doesnt the same principal appy in having a working cat which gets much hotter than the rest of the exhaust and aids in scavenging?... Seems like most here here think they will always get better performance without the cat...

 

But, the cat itself is a restriction. I could see how hollowing out the cat, as opposed to replacing it with straight pipe could cause turbulence, that would impede flow....... aside from that, I don't have any direct experience with JUST removing the cat......

On my Firebird..... I did a cat-back exhaust at the same time. The difference in performance was noticeable..... and it sounded SO good, I never turned the Radio on again. And I owned the car for eight more years.

 

The cat is supposed to run at a specific range of Exhaust Gas Temperatures (EGTs). The range is not difficult to achieve ... it will start working somewhere around 400~600F and normal is in the 1200~1600F range. Above about 2,000F it begins to degrade rather seriously.

I have a basic understanding of how catalytic reactions work, so they make sense to me but only in a general sense; I've never looked into the specific reactions in auto exhaust and what would be affected by, for example, using different materials or different quantities as the catalyst.

A good running engine should be spewing EGTs of around 1350F somewhere in the header; it will change depending on lean/rich condition.

I would expect there is some heat soak in the catalytic converter itself, so it might be pretty hot, maybe above the EGTs it's treating.

If you have an exhaust leak near the hot side of the cat, that could introduce oxygen into the pipes, which might burn and maybe is the reason you might get too hot and damage the cat.

I've never looked too deeply in the cat operation, partly because it's more-or-less mandatory to have one. For example I have no idea what a "high flow" cat really involves versus a stock one. But the catalyst materials are expensive, and you can't really get around the cost of metal raw materials, so it might just be pure economics.

If that turned out to be the case I would probably see if an OEM catalytic converter for another vehicle could be substituted for the one spec'd by a particular OEM. Assuming you could get the same emissions results from the substitute cat, I would expect a good candidate would be an European or Japanese luxury V-8, on the principle that those cars would tend to have the higher catalyst material count, since there is more "room' in the bill of materials for an expensive car. Perhaps such a cat would be less restrictive.

If the ECM and FI system can manage the parameters, you can delete it; if it can't you probably still can delete it but the engine might not run optimally. A similar thing would be happening if the cat was clogged; there has to be some point where it runs fine (as in the ECM can manage) and beyond that maybe not so much.

If I ever dig into it deeper I would probably want to explore why dual cats are so rare on OEM vehicles, and what, if anything, you could do to insure a dual cat system runs in the right envelope, and if there is any performance advantage in doing so.

I suspect it has something to do with that optimum heat range (keeping the cat hot enough to do it's job) and maybe going dual and getting that heat range means two cats would end up being more restrictive than one larger one. But that's just pure conjecture on my part.

Replacing the cat with a section of straight pipe the same ID as the exhaust system on either side is done with some import vehicles when performance tuning, especially if turbos are added. A hollowed-out cat won't be more restrictive per se, but you're introducing a low pressure area that will cause exhaust gases to cool faster than you might really want. It will have a bit of a quieting effect on the exhaust system; in essence you've just built a resonator.

David Vizard (who has done a lot of exhaust research for high performance over the years) had a system for racing engines where you dump the exhaust somewhere downstream of the collector into an empty container with 8x the volume of 1 cylinder. Apparently the 8x value is a minimum, and is the same if you're running fewer than 8 cylinders, but there's no advantage to going larger (or more precisely, the advantage is negligible and doesn't justify the room you would need to install it). You can then apparently add a muffler downstream, for noise control.

The resonant chamber is seen by the engine as atmospheric, so you tune your exhaust length as if it was an open header of a certain length and then hit the chamber instead of, well, an open header. The muffler downstream of this chamber can then dampen noise without the engine actually "seeing" the restriction of the muffler. To the motor, it's an open header; to a spectator at a race, it's a muffled system.

From that I would assume that a smaller resonant chamber (ie a hollowed cat) has no advantage over a straight piece of pipe, since it can't be made large enough to be a tuning aid, and is just "in the way" of good scavenging.

 

looks like the seller is changing his prices and raising them $80... there is only one auction left at $108....

I have the drives side installed and it bolted up and aligned with the exhaust perfectly...
I have a stripped out allen head on the passenger side so its going to have to wait until I get my unrelated misfire fixed I've been fighting for months now...

 

Ok so all I want to know is the cheapest easiest way to replace my exhaust system headers back only reusing the cat be sued my stock headers are shot.
Can anyone help with that? From this experimental deal seems like I can do it from eworld off eBay?

 

I cannot even answer that correctly since you have no vehicle information in your signature but I'll try... The exhaust changed in 98... older systems used a two into one output cat and the newer ones have a "y pipe" ... regardless these header bolt right up to the stock exhaust but there are short sections of pipes called down pipes... if yours are fine you can leave them... if you have a 98 or newer exhaust you may want to replace the y pipe with a bigger mandrel bent one since they are a restriction and hurt performance..

 

My 99 has a double inlet cat instead of a y pipe. Not sure what year the change was. But like Augie stated, without your info it is difficult for anyone to know what you have.