jhammons01

Just a quick question for the Exhaust Gurus out there.

Our Stock systems have Three Cats on it currently. Most ppl remove them all for freeing up the exhaust system flow capabilites. When Smog time comes around they then post in the for sale section "WTB Stock S5/S4 exhaust.....Desperate"

This situation has me asking, Could a person relocate the Cats?? Do they have to be in located in the down pipe to work? Is heat involved in the Catylizing?

What I would like to do is move the smaller cats to the Y-pipe and have an inter changeable system in-between a custom Header and the rear mufflers (stock or aftermarket)

Picture this, your stock Y-pipe hooks to the main Cat or presilencer. The Presilincer hooks to the Header or Downpipe. You could have the Main cat configured to hook directly to the Header. Then you have two "Post" Cats welded into another Y-pipe, returning you to a catylsist rate equal to the stock system.

after you pass smog you could then unbolt the cat systems and bolt back in your custom system.

You will be spnding a bit more to have both systems configured the same with all the same flanges and retrofitting your old system to be able to bolt in. But I think a bit of forethought coudl save you a ton of money later.

Right now I have my old system tucked under the work bench in one of those plastic containers.....waiting for the Smog notice.

macinslaw

Here's what I've read: (And I just went through some of this. I just purchased an 86, FC, NA, RX7 without cats. I went to emission test it, failed with an 18.6 instead of the 2.0 it's allowed!)

The catalytic converter is necessary for the car to perform correctly. The engine is designed with the back-pressure in mind. If you remove all the cats from the system, the ECU is no longer able to advance the spark ignition enough to compensate, thereby feeding the need for an aftermarket ECU to be able to do so.

That in mind, the 3 cats are placed on the car in locations for reasons. The one closest to the engine will heat up faster thereby doing its job earlier. The secondary, pre-converter I have no idea about. However the primary converter is where the majority of the 'stuff' happens to reduce emissions. The split air tube comes from the engine compartment and connects to the cat forcing fresh air from the intake into the cat. When the exhaust is hot, the unburned fuel is then burned off with the introduction of the air. Also, from what I've read, these cars, by design, pump some unused fuel through the engine into the exhaust.

My car was running poorly, that is, it screamed, but it wouldn't idle right, kept mis-firing and such. I just put on a new cat and re-connected the split air tube and now it idles perfectly, no more misses....it works great. So I have to believe most of what I've read about this issue.

-Mac

takahashiRyosukeFC3S

i just need to pass with numbers here in MA not system. the only regulations are that the system needs to be DOT stamped. (easily done if you have a set of lettered metal stamps, just a whack with a mallet and tadah it's "DOT")

TR

1SWEET7

iTrader: (1)

Sweet TR, I wish it was like that here in Cali.

sar

iTrader: (1)

The easiest way to switch cats in and out is with flanges. You have a straight pipe that flanges in the same location that the cat flanges. Go to track, put on test pipe, go to emissions, put on cat. If in Cali and visual is strict, keep hollowed precats.

Icemark

Yes, the way they work is by heat. An ideal tempature range. Exceed the range and they no longer work (or even worse melt down), too little heat and they don't work at all. The catalytic process doesn’t begin until temperatures inside a converter reach 500 to 600 degrees (F). If air/fuel ratio is on target, and the exhaust is free of contaminants, internal converter temperature stays at about 1200 degrees.

Nope wouldn't work. The main cat would be too close and work like a pre-cat being usless at normal operating temps, and the post cats would never get hot enough to scavange properly.

There are several high flow cats that work quite well. People are just too cheap to use them.

Icemark

That is radically incorrect. I won't even go into how many things are just old shade tree mechanic tales on that one.

ahh some major errors there as well.

#1 there are only two cats on most FC's. Only the 86-88 non turbo had three cats, and they worked based on temp. When the engine was cold, the first cat got hot faster working first. As the engine warms the first cat exceeded its operating temp and the 2nd cat came in play. As the engine hit normal operating temp, then the main cat came up to temp (and of course the first pre cat and 2nd pre cat (if equipped) exceeded operating temp and stopped working.

#2 the air pipe into the cat has nothing to do with unburned fuel being "burned off". It is simply to help the rear brick in a cat catylize the NO (oxides of nitrogen). Perhaps you are confusing the air pump operation with the air pipe. The ACV switches the air flow of the air pump into the exhaust manifold or the main cat (or both). When air is flowing into the exhaust manifold, then the unburned mixture is being addressed in the manifold, not in the cat.

So lets teach a little of the basics,
First there are two types of cat converters pellet type (common in the early days of cats) and monolith brick type. Our cars use Monolith Oxidation converters for the main cat.

From an emissions control standpoint, converter construction is of little consequence. However, pellet-type converters being very restrictive to exhaust flow, impose considerably greater power losses than monolith-type converters. With its honeycomb construction, a monolith substrate consists of a number of relatively small “tubes” or cells through which exhaust gasses pass. The size of the cells and their length determines the amount of restriction and to some degree, the extent of catalyzing action.

The most influential component of the catalytic reaction is the “loading” of the washcoat that’s applied to ceramic substrate. Heavier concentrations (loadings) of the precious metals that cause catalytic reactions increase the effectiveness of the process, with no increase in substrate surface area. Rhodium, platinum and palladium, which are used in various concentrations in the washcoat, aren’t bargain basement metals, so converter manufacturers (and of course cost being passed onto car manufactures) must tradeoff cost and effectiveness to produce converters that meet operational requirements, yet are affordable.

See, initially, catalytic converters addressed only hydrocarbons and carbon monoxide. But oxides of nitrogen constitute another compound that fouls the air we breathe and in the early 1980s, rhodium, another catalyzing agent, was incorporated with the resulting converters being known as “three-way” because they address three, rather than two pollutants. Rhodium functions as a reducing, rather than an oxidizing agent. In “chemistry speak” that means it separates oxygen from a compound instead of adding oxygen to it. Consequently, nitrogen oxides are broken down into nitrogen and oxygen. However, oxygen is as fickle as Lady Luck and tries to dance with any available partner. In the exhaust stream, that’s usually carbon monoxide, which the footloose oxygen atoms convert to carbon dioxide.

Most converters produced now contain two monolith "bricks" spaced several inches apart from each other. The washcoat on the forward brick typically contains rhodium which causes nitrogen oxides to break down into nitrogen and oxygen. After passing through the first brick, exhaust gasses pass through an air chamber before entering the second brick. This is known as “oxidation” types. A small tube passes through the chamber and injects air pumped in by an engine-driven “smog pump” (note in some (mostly domestic) vehicles, the "smog pump" incorporates an electric motor, which reduces accessory drive complexity and also allows for remote mounting.)

Injected air at the cat, simply brings additional oxygen into the exhaust stream to assist in the final oxidation process in that last part of the brick.

So you can see how that the cat has absolutly nothing to do with how an engine runs, other than if it is clogged/failed. If it is clogged or collapsed on itself because of internal overheating, (typically because of an excessively rich air/fuel ratio, oil or antifreeze in the exhaust system), that can cause poor idle and running conditions.

So in your case, you probably had a bad clogged cat and that is what cause the car was running poorly, wouldn't idle right, kept mis-firing and such.

inflatablepets

Zing'ed him real good Icemark. to add my own slice to this, When emissions tests are due, I install my Catco direct fit system. After the tests, I replace it with my downpipe. I can't give any H.P. numbers, the butt dyno says there is a significant increase though.

NISRCelica

Nice work, but he said that he bought the car without cats, so how could his problem have been a clogged cat? I'm not trying to nitpick; just curious.

Icemark

With so many things wrong with his post can you believe anything in it???



But anyway I suspect that he didn't do the work himself or that the smog shop tried to fix the high emissions by tuning up or down,

slpin

iTrader: (9)

jhammons01

So placement for heat purposes is detrimental.

I am trying to leave the RB header in place. I guess it won't work however. I just don't want to hav to pull the entire system out at smog time. Plus, being the opportunist that I am. I was also thinking of a rental situation.

But Ice, thank you for the post, that was definatly above and beyond.

sar

iTrader: (1)

Doesn't the visual test check for the location of the cats too? If you just need passing emissions numbers, a swappable main cat/test pipe is all you need as proved by the temps and activation that icemark posted. I mean, hopefully it's not that hard to warm your car up to operating temperature before going to get it tested.

jhammons01

^^No no. I'm not up against anything just yet. I just had my car smogged with the old stock exhaust. I am just thinking out loud so trying to pinpoint a problem is not needed. I am just trying to think ahead and possibly beat a system.

Richter12x2

iTrader: (1)

There are several that don't, too. My Eastern Catalytics high flow cat just disassembled itself on the way to my last RX7club meeting here in Dallas. Tessai was there when I dug the chunk of monolith out of the exhaust pipe (Thought someone had shoved a rock in there!) My exhaust system is about a year old, with less than 3000 miles on it.

Bigg2weed

If all I was faced with is switching out my downpipe with the cat system every other year just to pass smog, I'm totally ok with that...