Determining ideal catalytic converter placement?

Thinking ahead here, related to my exhaust question thread, the only mod I might do to my exhaust later if everything works out is to add a small performance cat to it, just to make the car more eco-friendly. No ECU I'll be using with this engine supports a cat so conditions for it won't be regulated in any way.

Furthermore this car will have a fully wrapped exhaust so I'll have to put it somewhere further back than stock to get it in the right temp range...so how can I determine ideal cat placement for cheap? Or is it really that important?

Pointing a temp gun at the exhaust won't work since the temp ranges will change when I add wrap...which would then make it impossible to measure the surface temperature of the exhaust itself.

As close as possible. It's easier to cool it down than heat it up.

A few inches behind the point where the runners all come together would be good.

Why no ECU? Without it, it's not likely that the cat will last long, which, even on the cheap, makes it a waste of money.

The only ECU I could get that would support a cat is the stock US market ECU, and I need something much better for the engine mods I'm doing...I'll be putting in an MS2x probably middle of next year. Running the stock JDM ECU for now.

The stock location for the cat (on US spec vehicles) is at the end of the downpipe, which is at least 5 feet in pipe length from the exhaust ports.

What does the ECU have to do with the presence of a catalytic converter?

The cat works on chemistry.

I used a 300 cell per inch Random Technology Metal Matrix core cat on my turbo RX7. Mounted in the stock location. Worked well at getting rid of some of the nasty exhaust stench compared to an open exhaust. Made no difference with regards to power output or sound abatement. If it can handle the abuse of a high EGT turbo rotary in track usage, it will probably work for you. :)

http://www.randomtechnology.com/products.html

I'm voting for "In the corner of the garage so it can be put back on later if you have a change of heart."

I think the rally america guys stick it on the end of the tailpipe, right by the bumper. That way they tech guys don't have to climb under the car.

What does a cat require for proper function and reasonable life?

I'm a long way from that part of the project, but I'd been working under the rough idea that I'd want the cat as close as possible to the exhaust ports (probably right after the header's collector), and that I'd want to tune the car to run reasonably close to stoich most of the time.

I assume a cat can put up with limited stints of running rich for high-load situations. Is there worry about cooking the cat with a lean cruise? What other considerations?

amg_rx7, thanks for the link! There's some detail there... While they do go into what kills cats to some extent, they don't really approach "what to do" as much as "what goes wrong". They don't mention lean as a particular problem, just rich and contamination with oil or antifreeze...

Usually if I place them with a local recycler I get anywhere from $50 - $120.

The ideal placement for any catalytic converter is in the scrap yard.

amg_rx7 wrote: What does the ECU have to do with the presence of a catalytic converter? The cat works on chemistry. I used a 300 cell per inch Random Technology Metal Matrix core cat on my turbo RX7. Mounted in the stock location. Worked well at getting rid of some of the nasty exhaust stench compared to an open exhaust. Made no difference with regards to power output or sound abatement. If it can handle the abuse of a high EGT turbo rotary in track usage, it will probably work for you. :) http://www.randomtechnology.com/products.html

An ECU can actively control the chemistry, a carb can not. More importantly, and ECU can make sure that the engine wont damage the catalyst, which makes the investment of putting one on worth it.

GODDAMNIT, GPS.

GameboyRMH wrote: The only ECU I could get that would support a cat is the stock US market ECU, and I need something much better for the engine mods I'm doing...I'll be putting in an MS2x probably middle of next year. Running the stock JDM ECU for now. The stock location for the cat (on US spec vehicles) is at the end of the downpipe, which is at least 5 feet in pipe length from the exhaust ports.

So you DO have an ECU. Does it use the O2 feedback, or not?

MS all have the ability to run with an O2 sensor, which makes a massive difference in how well the cat operates. Again, if you are going to put one on, might as well really use it.

If 5 feet is as close as you can get, it's as close as you can get. For that position, I would ask about the planned usage cycle, since if the cat is cold all of the time, I, again, would question why use one at all- it would be a waste of money. Still, I know that even 5 feet away, it can heat up, and work quite well- BTDT.

ransom wrote: What does a cat require for proper function and reasonable life? I'm a long way from that part of the project, but I'd been working under the rough idea that I'd want the cat as close as possible to the exhaust ports (probably right after the header's collector), and that I'd want to tune the car to run reasonably close to stoich most of the time. I assume a cat can put up with limited stints of running rich for high-load situations. Is there worry about cooking the cat with a lean cruise? What other considerations? amg_rx7, thanks for the link! There's some detail there... While they do go into what kills cats to some extent, they don't really approach "what to do" as much as "what goes wrong". They don't mention lean as a particular problem, just rich and contamination with oil or antifreeze...

Three reasons catalysts die-
1) overtemp- when the catalyst gets hot enough, the metal on the surface starts to sinter, which eventually will turn it off. It's also possible that the cat can get so hot that the substrate will melt.
2) poison- for modern fuel, that means phosphorus from oil. So as long as your car doesn not burn oil, you won't have a problem. If you are worried about burning anti-freeze, there's more wrong in the engine that should be addressed. 3) physical- don't crash it into things and you won't break it. And don't splash cold water on a hot cat. they crack.

Basic operation- cycle the air fuel around stoich. And cycling is important. That brings a different level of chemistry to the ballgame that makes the investment into the catalyst much more reasonable.

Rich ususally isn't a problem, since you can keep the catalyst cool easier by going rich. TOO rich is a problem- if you get to a point where the engine partially misfires, then you are feeding hot air-fuel that was to propel your car down the road straight into the catalyst- which is happy to make the chemistry happen. Thus over temping the catalyst.

Lean isnt a problem- the engine will hurt itself far before the catalyst will get damgage running lean. Peak temp INSIDE the catalyst is right at stoich, relative to the exhaust temp. Leaner will reduce the HC so that the exotherm is reduced to almost nothing.

The other high temp area is high speed, high load, high exhaust temp. At that point, most engines will run lean (down to 11:1 or so) which cools the temps off a lot, including the catalyst.

Yeah both ECUs use a single O2 sensor (ms2x can use two)...if that cat amg_rx7 linked to stands up to being right in the line of fire of a turbo rotary, I'd put it in as far forward as possible and not worry about it.

. . .

pres589 wrote: I really don't understand the negativity shown towards cats around here. But hey, whatever.

because they are dead weight that add nothing in the way of performance to an engine and they never would have been put on cars if not for government regulations.

the best place for one is at the scrapyard, where they are worth a good fraction of a tank of gas.

Last time I dealt with a cat- I put it in my storage unit. What GPS said is right on the money.

I'm stoked, sounds like it shouldn't be too awful to get a cat to live and function on the 2002. Though I'm not sure about the cycling near stoich...

I don't recall how closed-loop on MS works... Would it sort of cycle just out of imperfection in the table and correction? Does the rate of cycling matter? I'd imagine it would have too at some point... Too long isn't really "cycling" as much as varying which direction it's gone, and too short could effectively be steady...

novaderrik wrote:

pres589 wrote: I really don't understand the negativity shown towards cats around here. But hey, whatever.

because they are dead weight that add nothing in the way of performance to an engine and they never would have been put on cars if not for government regulations.

Well, outside of less emissions. We don't look like a town in central China anymore.

Unless you think having a lot of HC, NOx and CO it breathe is a good thing. I don't. Outside of stinking bad, it's not good for you.

Moreso that modern catalysts don't hurt performance nearly as much as people think. Like a GRM test showed.

ransom wrote: I'm stoked, sounds like it shouldn't be too awful to get a cat to live and function on the 2002. Though I'm not sure about the cycling near stoich... I don't recall how closed-loop on MS works... Would it sort of cycle just out of imperfection in the table and correction? Does the rate of cycling matter? I'd imagine it would have too at some point... Too long isn't really "cycling" as much as varying which direction it's gone, and too short could effectively be steady...

I don't know how the closed loop fuel works on the MS, but being that the core control uses an O2 sensor, part of the cycling is inherent in how the sensor works. But that is used to great advantage in how catalysts actually work.

Having it cycle about at 1-2 htz will work fine. And it will cycle somewhere near 0.46V- just the manner of the sensor.

If you're using a narrowband, you'll get the cycling you need.

Raw fuel will kill them pretty quickly. If you get misfiring, your cat will not be long for this world.

I'm going to chime in here...

Catalyst plus carb can be tricky. As has been said, a cat's life can be dramatically shortened without careful fuel control. I bought an S10 from a kid who put a 5.3L with a carb conversion in it. He was being eco-friendly and put two cats on it. I drove it for about 3000 miles before parting it out, and the cats were already starting to clog. The tuning on the carb was too rich and the cats were too far back for the shorter trips I took. They got a 1-2 punch of too much HC and not enough heat.

I also have no problem if someone wants to put a cat on to help emissions. For that I thank you. I would gladly cut my emissions tenfold or more and sacrifice 6 lbs that I can recoup somewhere else.

wae wrote: I think the rally america guys stick it on the end of the tailpipe, right by the bumper. That way they tech guys don't have to climb under the car.

They do this because the cats typically can't take heat generated by the anti-lag systems. Too close and the cats burn up.

What do I look for when buying a cat?

What makes one " high flow"? Does that even make a difference? Is ceramic or metallic better, or is it not even an issue of "better"? Is 300 cell good, bad, or mediocre? Do I still feed it meow mix?