Settle an argument-Is it better to leave good cat converter in?

On Sun, 10 Sep 2006 15:23:45 -0700, with neither quill nor qualm, Eric R Snow quickly quoth:

Both of you, but him mostly. With less backpressure, the engine will develop more horsies and torquies. With a reprogramming to offset the removal of the cat, the engine will run marginally better, too.

The guys on Trucks! (back when Stacey was on) and Xtreme 4x4 talk about the HyperTech (IIRC) units for gas engines.

Then again, removing cats is illegal unless the vehicle is used strictly offroad.

------------------------------------------- Stain and Poly are their own punishment

Comprehensive Website Design ======================================================

The cat does not do much if everything is working properly. It is there to burn off any fuel that escapes the combustion cycle.

The question of removal is moot as the law requires the cat to be there and functional.

You are correct. The back pressure is something that helps in most cases for a streetable motor anyway.

Well if it wasn't for OBDII and possibly EGR rates, pulling the cat would probably give some gain. Since the EPA required OBDII, (on board diagnostics phase 2), all cars are required to have built in diagnostics to determine if the cat is bad. This is typically a upstream and downstream O2 sensor is used for both AFR control and measuring catalyst health. The downstream sensor will switch at a lower rate than the upstream sensor, (with a functioning cat), due to the oxygen storage capacity of the catalyst moderating the lean rich swings. Replace the cat with a straight pipe and the system will eventually set a fault code and put on the MIL light on the dash.. Usually this is benign, but it will be picked up on the vehicle inspection, and the system could possibly enter a limp home mode, with reduced performance. On some cars the EGR rate is strongly effected by small backpressure changes, and the system may not be able to completly compensate, resulting in lower EGR rates. Most people think EGR kills mileage, but in fact when done correctly it can improve mileage by reducing pumping losses at part throttle. Lower EGR rate may also result in knocking, which in turn may cause the ECU to back off the spark timing due to feedback from the knock sensor... Even on a new car with the engine functioning perfectly, the engine out emissions are many times the catalsyt out emissions. People do not realise how low the emissions must be to pass the current EPA standards. Using a can of oil based paint will produce more unburned hydrocarbons than a new car will produce at the tailpipe in 20K miles. In any case in most naturally aspirated newer cars the catalyst really is not a major cork in the exhaust, and performance cats can be purchased. The code description on a modern engine calibration runs to many thousands of pages to describe and handle the many control interactions and self diagnostics.

My truck has the O2 sensors on the engine side of the cat. So what sensor determines fuel ratios and so forth?

Gunner

"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences."

- Proverbs 22:3

It does quite a bit even if everything is working properly because it can be much more efficient to clean the exhaust than to burn cleanly in the first place. In fact the first year cats were introduced nationwide, about 1975, GM decided to qualify all its cars with cats while Ford tried to avoid them, and Ford's gas mileage suffered so much in comparison because of this that they soon installed cats on those cars and had the EPA tests done again. They widely advertised this with the Pinto, with a version called the Pinto MPG.

The only problem I see in removing the cat is with the oxygen sensor located after it, found in all 1996 and later cars. It normally generates a feeble signal, but without the cat to clean up the exhaust the signal will be much stronger and identical to the one from the O2 sensor ahead of the cat. But it should be easy to create a fake signal (will a fixed frequency oscillator work?) or greatly weaken the signal from that sensor (maybe just a high resistance voltage divider will do, but it may be necessary to use a buffer amplifier).

Both of them. Their signals are either averaged (all cylinders will receive the same amount of fuel), handled individually (fuel is calibrated for each bank of cylinders), or the exhaust from each cylinder is read individually (fuel is calibrated for each cylinder individually). Any O2 sensors located behind the cat merely verify that the cat is working.

OK give me an explanation of this. Efficiency would be getting power out of the fuel you burn in the combustion chamber. Burning it in the tail pipe or letting it escape the combustion chamber unburned does nothing to move the machine down the road.

Indeed. So if I pull the cat...nothing bad happens unless I get nailed by the Smog Police. And a tuned exhaust may actually help things.

Gunner

"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences."

- Proverbs 22:3

But...

...if you remove the cat, the O2 sensor downstream from the cat will start to put out a much stronger signal, and the computer notices this and turns on the Check Engine light. And just removing that lightbulb may not let you pass smog inspection if your vehicle is 1996 or later (sometimes 1995), and I think it is because you rarely find a downstream O2 sensor in older ones, because the smog test consists of the inspectors plugging their computer into your vehicle's computer and reading out the actual state of the system. It may take time for your vehicle's computer to notice a missing cat, so try clearing all the trouble codes (AutoZone will do it for free, but there may be a jumper you can connect, or you can simply disconnect a battery terminal for a minute) and see how long it takes for the codes to return. If that time period is longer than the wait at the smog inspection station, you could have troubles passing.

The sure-fire way to pass inspection is to splice a buffer amplifier into the downstream O2 sensor's wiring so that the computer is fed a much weaker signal. The amplifier will protect the sensor against damage (they can't stand much of a load) and let you weaken the signal by the necessary 90-95% (I'm guessing at the amount). This isn't a difficult or expensive do-it-yourself project (see: single-voltage op-amp for DC signals), but you may want to ask at

or the sci.electronics.basics newsgroup. Try to get National Semiconductor or Linear Technology to send you a copy of their Linear Applications Handbook (completely different books, identical title) -- excellent, easy to understand, yet plenty of in-depth information. But for an automotive environment you want the device to be able to stand at least -32 to +160F temperatures, meaning power ratings may have to be doubled over conventional practice

-- that means rating parts for 4x the worst-case power rather than just

2x (try to install it inside the passenger compartment -- less heat, cold, and moisture), have plenty of surge protectoin (not just MOV and reverse polarity diode but also a choke-capacitor line filter), and be able to withstand at least 20V, preferrably 60V (about the worst case if the alternator regulator shorts). Use either Teflon-coated wire or cover wires with either Teflon or polysulfone heatshrink (vinyl heatshrink doesn't stand heat as well). Not that I would encourage anyone to cheat on an emissions test.

I wouldn't expect much improvement in mileage or power unless other changes are made as well.

That's what I would think, but the matter is more complex than tha, and I hope some expert will interject.

1. Why is maximum power obtained with a rich mixture rather than a stoichiometric one? Shouldn't the presense of unburned fuel that contributes nothing to the burn actually reduce energy output?
2. Similarly, why is fuel economy maximized with a lean mixture, where some oxygen contributes nothing to the burn?
3. And why do engines produce less HC and CO (before any cat) when tuned to burn more fuel rather than less fuel?
4. How much of a modern car's pollution output is reduced by the cat?

It's not like letting it down the tailpipe is intentional. There will always be some unburned HC molecules due to the fact that the HC will be quenched by the relatively colder cylinder walls and that some HC will be trapped above the top piston ring between the piston and the cylinder wall.

Maximum heat is at .95 lambda, which is 5% richer than stoichiometric.

Indeed it does, but that is not necessarily the same as above.

Maximum power occurs when there are no left over O2 molecules (all O2 is utilized), maximum economy occurs when there are no left over HC molecules (all fuel is utilized).

Chemical balance.

Lots of variables, so it depends. A 0.020 inch diameter evaporative leak can be a bigger HC loss than what is coming out of the exhaust.

I was trying to get a 1987 Jetta to work right last year. Never could get the O2 sensor into the normal range. Turned out the honeycomb cat was completely blown out. The O2 sensor was in an angled area in the front of the empty cat shell, where it was offset from the main exhaust flow. Without the slight turbulence and backpressure of the cat honeycomb, the sensor never warmed up to it's working range to enter closed-loop. The solution was to relocate the sensor to the downpipe, where it sticks into the hot exhaust flow. So at least on some cars, O2 sensor function is dependent on a good cat. By the way, there are O2 sensor cheaters that replace a missing sensor and send a signal to the CPU equivalent to a correct AF mixture. Often used to replace the downstream sensor after catalyst removal.

Rex, who remembers when "Test Tubes" were a hot item at my auto parts stores

My truck is a 94...no down stream sensor.

Gunner

"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences."

- Proverbs 22:3

Coincidentally. I'm having trouble with a catalytic converter right now... I got in a car accident a while ago, and the car I usually drive was totalled. So I bought an old junky-looking van for fairly cheap from the university I work at. It's a 1985 Dodge Van that is kinda beat up, but it was maintained well and has only 55,000 miles on it. It runs fine, but I'm having trouble getting it past the smog test. As near I can tell it just needs a new catalytic converter. I've asked at two different parts shops, but neither of them carries catalytic converters any more.

I'm sure I could go to the dealer, but I don't want to pay more than I need to... I don't really know anything about them, so I'm not sure how to choose a good one. I'm not even sure geting a good one is important... I don't plan on having this car for more than a couple of years. But if I can get a good cat for the same price as a cheap one (that will only last a year or so) then I'd rather get the good one. So, where would you go to get a catalytic converter?

(BTW, I'm in Orange County, Southern California)

Jeff P.

snipped-for-privacy@rgs.uci.edu wrote in news:1157995071.226831.48990 @q16g2000cwq.googlegroups.com:

An exhaust/muffler shop? 'especially one who does 'performance' exhausts custom.

You're a lucky man. ;)

You will not be able to buy a cat convertor in Kalifornia lest it be blessed by CARB. So anywhere you buy one will be good. Muffler shops install universal blanks, just like they used to do glasspacks. As long as it's big enough to not restrict flow, and small enough to maintain it's minimum temp, you are good.

And I can' imagine any decent parts store that can't get you a catalytic convertor, OE-duplicate or universal. Avoid AutoZone and the like, seek out an independent where the guys know what the heck they are doing. for that matter, a speed shop might be a worth a stop. Surely there are still speed shops in CA :)

Don't your smog checks also check the actual emissions? Over here in Blighty the test goes by what the exhaust gas analyzer at the testing station says, cross referenced to the vehicle and age.

Expiring minds need to know.

Mark Rand RTFM