Catalytic converter size and MPG, Miles per gallon

OK, my question is this, I have an aftermarket Catalytic converter that was installed to replace my plugged, broken cat 2 years ago. It was installed days after I bought this jeep. It was about $100 installed, and seems to be the aftermarket typical replacement (meets somebody's minimum specs) size. Ratherbecamping looked at it yesterday at tells me it is about half the size of the OEM Cat. The Muffler shop today looked at it and checked thier cat catalog and said it is the standard after market size looking at the outside of it, but said there are larger, high flow cats available, but they are like $200, versus the $100 one I have. Now I know the material on the inside could have larger or smaller diameter passages in the cat support material, so outside size is not the whole story.

So my question is:
1) should I replace the Cat?:dunno:
2) What size and brand should get?:dunno:
3) How much should I pay for it?:dunno:
4) Is it likely to significantly improve my my high rpm power, and overall MPG's based on my history in this thread?:dunno:

Here is a detailed 2 year background on my Jeep :read:, 87 Wagoneer Ltd., ed., 4X4, 4.0 Liter, automatic trans., Renix, all stock except thermostat and radiator.

On the general topic of possible causes of poor fuel economy, as many of you know from reading some of my other preiodic posts I have been working on that issue with my 87 Renix system for nearly 2 years now.

Yesterday, I went to a second muffler shop for a second oppinion on my exhaust system which I have thought for some time might be part of my low power, low MPG problem, and the second muffler shop oppinion was that my system was OK. He shot the cat converter with a IR gun and it read 235 F on the inlet and 285 F on the exit which he said was OK, but I was not impressed. He said it was on the F range, but that makes no sense to me, I am thinking those numbers had to be degrees C for the exhaust, Right? As I recall I check my exhaust manifold last week and read something 485 F on it.

Long story short I had him replace everthing after the Cat (the Cat was only 2 years old) including the muffler. Before this I could barely feel exhaust gas flow (pressure) at 3" from the tail pipe at idle (which two muffler shops said was OK). Now I can feel the exhaust pressure at at least 12" to 14" from the end of the tail pipe. I will post MPG results later after this and the next tank of fuel to see if it affected MPG's, but it has already improved the power and performance some (maybe 15%).

Ratherbecamping looked at my Cat yesterday and said it is about 1/2 the size of the OEM cat, so I will be looking at replacing the Cat next. But I need experience based feedback and oppinions on which Cat I should get????:dunno:

Also, additonal background for my question below, I run a 165 F thermostat, and I checked the O2 sensor performance on mine yesterday with an analog multi meter and at 165 F water coolant temperature it oscilated at a about 2 second frequency (360 degrees, 0.8...0.2....0.8), from 0.2 to 0.8 volts and on occasion closed into a 0.33 to 0.67 V range indicating that my O2 sensor is working, and indicating that my Renix is in the closed loop mode at idle even at 165 F (I verified the 165 F with two different IR gauges). Also it stayed in the 0.2 to 0.8 range during accelaration and fell to a steady 0.2 volts for about 5 seconds during hard decceleration.

I need to check, but if anyone knows for sure I am thinking the drop to 0.2 volts on the O2 sensor for 5 seconds during deceleration is normal???? :dunno:

Lastly my 2.2 L diesel has twice the exhaust gas flow rate (but a smaller diameter pipe) as my 4.0 gas jeep with the new muffler, so I am thinking my cat is also part of the problem, specifically that it is too small.

Lastly, I have run repeated tests on and replaced ALL questionable Renix sensors except the Block Temperature sensor which tested OK. Fuel injectors are fairly new, Fiveomotorsports, premium, new, multi pin/needle (4) injectors. Distributor, rotor, cap, plugs, wires, ignition coil and electronic ignition controller are all new. Good grounds, less than 1 ohm, new alternator....etc......New fuel pressure regulator a year ago, new CPS 2 years ago, fuel filter (old fuel filter one was OK), replaced yesterday, new air filter 8 weeks ago.....Fuel pump and new fuel presure regulator passed the fuel pressure and flow tests 6 months ago. Engine vacuum in park (idle, accell, decell) are normal. Tires are new, lots of front end parts are new too.

I am basically down to the Engine and the exhuast system or one hell of drive train loss for the last 2 years (brakes are new 6 months ago). Transmission is not the problem, it is not slipping, if it was wheel bearings or the like I would have heard it getting worse the last 20,000 miles and 2 years of driving it, right?

I may be wrong, but I think a good exhaust shop can install a pressure gage in the pipe ahead of the cat and check for excess back pressure.....and the 160 degree t/stat may be a major factor too.

The 4.0s are made to run a much higher t-state. Pull that 165* out and replace with a 195*. If you search around everyone will tell you to do this. The 4.0 likes to run around 200-210. The injection systems are vary picky about this.

So your running 3" exhaust? This doesn't allow for any backpressure. The largerst I have seen recomended is 2.5". The cat, as long as it isn't plugged, should be fine. Go measure it and tell us how big it is.

That was holding my hand 3" from the end of the exhaust pipe exit, not pipe diameter, I never mentioned the diameter, but it is stock what ever that is (2"-?).

5-90 is quite happy with his 180 F thermostat, and mine gets up 190 F and 200 F on the highway, and it makes no difference in power or mileage at that temperature, so the 165 F thermostat is not the problem.

I was expecting the shops I went to, to cut a hole in both sides and run their hand near the hole to do a pressure test like I had seen done before, but they did not see the need in their opinions. I have not seen a pressure gauge for those temperatures and those low pressures before. I am sure they exist, but I have not seen a muffler shop use one before.

Ok, I have not measured it, but it looked about 6" wide max, 3.5 to 4" hgih and maybe 12" long.

I could probably fit 4 of them in my muffler!

why are you so against running the correct thermostat, 195? do you have a problem with the cooling system? if you have a properly working closed cooling system, there is absolutely no reason to run a 165 thermo. 180 is as low as you should go and that would be for reasons such as a performance chip, possibly a stroker, or very hot climate with alot of stop and go.

i have a direct fit aftermarket cat on my 89 and its about 14" IIRC. i got the direct fit because of the 4 bolt flange made it easier to attach to the downpipe. the length of the cat is not a problem. exhaust size should be 2.25 or 2.5" diameter. the cat converter on the 4.0 is not a big factor in terms of hp and mpg. its a low reving, low torque fuel injected engine. cat convertor and large diameter exhaust is a different story on a high reving, high horsepower, or turbo engine. it seems your problems are somewhere else.

Or just beacuse you think 215* is too hot for an engine to run (like me.) I've got emissions reports that show that it actually helps engine fuel efficiency (not economy!) to a measurable extent.

I've also got an 88 that doesn't like having a thermostat at all - it takes about a minute longer to get to temp (also 185-190*) and runs fine without it - but overheats everytime I put one in. I've replaced just about all of the cooling system, so it's not failed components. Just an oddity - I've owned two vehicles before that were that way (a Cavalier with the 2.8L V6, and a Honda Accord with the 1800cc engine.)

Shell size has nothing to do with catalytic converter flow rate - in fact, smaller converters can be a bit more efficient in flow than larger housings - less disruption due to cross-section changes. (No, I don't know why they make them so damn big OEM either.) I cut the bolt flange off of the OEM units, install an aftermarket (usually PFP) "universal" unit for a big block Chevvy, and call it good. BD sez it runs a little better - but I've not had a chance to do a "before and after" dyno test. Gotta build a dyno first...

PFP "universal fits" are a damn sight cheaper than OEM, about a third the size, and easier to find anyhow.

universal fit cats can be ordered thru any napa, at a very good price. ive had a napa universal on my 95 for 4 years. my smog just came up again this year and i got the same scores i got right after i put it on.

So when I was having cooling issues and pulled the 180* stant to replace it with a 195* it sounded stupid to me. Well it worked. Sure it will get 210-215 but then it goes back down. With the 180* in place it would just keep climbing. I don't like it running that hot either but functions a whole hell of alot better then with a lower t-stat in it.

I have a '92, with a gutted cat, and removing the ceramic honeycomb inside made no difference in MPG or performance. However, the 160 t-stat was causing the engine to stay in warm-up (rich) mode all the time. I got the proper t-stat from the dealer- it has a tiny hole in the flange to always allow coolant flow- and viola!- it jumped to 23 MPG with Interstate driving at sane speeds. Don't forget, that O2 sensors have a finite lifespan, and the XJ's, at least my '92, had the timer, at, I believe 87,500 miles, to 'service engine'. At today's prices, a new O2 sensor may pay for itself in a short time!

89xj said:

why are you so against running the correct thermostat, 195? do you have a problem with the cooling system? if you have a properly working closed cooling system, there is absolutely no reason to run a 165 thermo. 180 is as low as you should go and that would be for reasons such as a performance chip, possibly a stroker, or very hot climate with alot of stop and go.

i have a direct fit aftermarket cat on my 89 and its about 14" IIRC. i got the direct fit because of the 4 bolt flange made it easier to attach to the downpipe. the length of the cat is not a problem. exhaust size should be 2.25 or 2.5" diameter. the cat converter on the 4.0 is not a big factor in terms of hp and mpg. its a low reving, low torque fuel injected engine. cat convertor and large diameter exhaust is a different story on a high reving, high horsepower, or turbo engine. it seems your problems are somewhere else.

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I live in the great gulf coast swamp lands of Houston, Texas where it reaches 80 F on a cold day in February. I actually had to run the A/C here once in February this year when hit something like 82 F at 95% humidity. Talk about Global warming, Geesh! Actually I installed the 165 F thermo 2 years ago when it had serious overheating problems. Then a new radiator, new fan clutch, new cap and plastic bottle, and a few other items over time fixed that. My exhaust is either 2.0 or 2.25, I will check it tomorrow. This Cat was welded in at Muffler shop. Mine gets up to 210 F in the summer with the A/C anyway. I just have seen no performance differance between 160 F and 210 F, so I have not been in any rush to replace the thermostat. I am sure I have read a few others like myself that posted using 165 to 180 F thermos that were getting excellant mileage. I know 5-90 is running a 180 F thermo in his Renix with no problem, and mine is a Renix, 87, mostly stock.

Was that 14" long or wide?

Onkover said:

So when I was having cooling issues and pulled the 180* stant to replace it with a 195* it sounded stupid to me. Well it worked. Sure it will get 210-215 but then it goes back down. With the 180* in place it would just keep climbing. I don't like it running that hot either but functions a whole hell of alot better then with a lower t-stat in it.

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I would be willing to bet there was a flow rate problem with the lower temp stat. I suspect there is big difference in the throat opening area in many of these stats that never gets discussed or noticed!!!

heyhar said:

I have a '92, with a gutted cat, and removing the ceramic honeycomb inside made no difference in MPG or performance. However, the 160 t-stat was causing the engine to stay in warm-up (rich) mode all the time. I got the proper t-stat from the dealer- it has a tiny hole in the flange to always allow coolant flow- and viola!- it jumped to 23 MPG with Interstate driving at sane speeds. Don't forget, that O2 sensors have a finite lifespan, and the XJ's, at least my '92, had the timer, at, I believe 87,500 miles, to 'service engine'. At today's prices, a new O2 sensor may pay for itself in a short time!

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I already have a nearly new (maybe 6000 miles on it), working O2 sensor.

So what was the equivalent driving condition mileage with the 160 F stat? Did you change anything else beside the 160 F stat before the mileage changed.

RichP just made a post in a similar thread here where he noted that he changed the muffler and the exhaust pipe 3 weeks ago with a seemless SS system on an old oldsmobile (third muffler since they bought the car) and the mileage jumped up from 22 to 27 mpg and they had never gotten better than 22 mpg before. 5% increase in performance! So I guess it depends on the entire system.

Flowmonster cats are pretty inexpensive, of course you can get a universal Magnaflow cat for about the same price
http://www.flowmonster.com/mm5/merchant.mvc
http://stores.ebay.com/DEPENDONAUTO

I haven't played with my cat yet, but my cat back is 2.25" with a Flowmaster 40 series. I got a noticable gain in power and mileage. I run a 180 thermostat because of slow running on trails during the summer. The 195 was just to hot on those days. The only downside that I've seen is the longer warm up time during the winter. I live in Idaho where we have all 4 seasons. When I had the 195 in there during the summer it would go well above 230, which scared the piss out of me when I was out in the middle of nowhere. I've been looking into a new cat. I also need to put in my Borla header. Someone dropped it off at my house one day. I still have no idea who it was though. I would like to thank whomever it was though.

XJIDAHO said:

I haven't played with my cat yet, but my cat back is 2.25" with a Flowmaster 40 series. I got a noticable gain in power and mileage. I run a 180 thermostat because of slow running on trails during the summer. The 195 was just to hot on those days. The only downside that I've seen is the longer warm up time during the winter. I live in Idaho where we have all 4 seasons. When I had the 195 in there during the summer it would go well above 230, which scared the piss out of me when I was out in the middle of nowhere. I've been looking into a new cat. I also need to put in my Borla header. Someone dropped it off at my house one day. I still have no idea who it was though. I would like to thank whomever it was though.

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Can you quantify the noticable increase in power and mileage? Was it an OEM Cat that you replaced, or do you know?

Thanks for the incouraging feed back.

XJIDAHO said:

Someone dropped it off at my house one day. I still have no idea who it was though. I would like to thank whomever it was though.

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Maybe it was Mr. McFeely?

For future reference here is some useful testing information, and a rough idea of how much back pressure a cat puts on the engine:

http://www.warpspeedperformance.com/metalsubstrate.htm

I quote: "[FONT=Trebuchet MS, Arial, Helvetica]THE BACK PRESSURE TEST:
This test will require a back-pressure gauge. An inexpensive back-pressure gauge can be made from a small pressure gauge that reads from 0 to 30 PSI, a piece of vacuum tubing and a small 3 mm 0.D. piece of copper tubing with a 3 mm I.D. Viton 0-ring. This 0-ring will help to seal the hole that the copper tube is inserted into, or a back-pressure kit may be purchased from numerous sources pre-made. Drill a small hole in the exhaust pipe for the tube at the front of each converter. With the engine running, check the back pressure. The back pressure should be no more than 4 to 5 PSI. This reading is with the throttle partly open at 2500 rpm. When checking exhaust back pressure, be sure to check at the inlet and outlet of each converter or exhaust component tested. A reading of 1/2 to 2 PSI is normal when idling."

It also has some good info on what can kill an O2 sensor, specific warnings about avoiding silicones and antifreeze contact (which has silicates) with O2 sesnors as it can kill them!
[/FONT]

What do you do with these holes after you're done testing?

Not too long ago I had a local muffler guy check my exhaust --I suspected my cat might be plugged up and restricting flow. He drilled a small hole (1/8"?) downstream of the cat and then inserted a prssure gauge. Next he repeated the reading upstream of the cat, and compared the pressure readings--about 1 psi under high rpm difference. He then repeated downstream of the muffler and we saw enough psi difference there to swap mufflers. After testing was done he just zapped the holes closed with the welder.

So back to my original question with a twist. I see lots of ads for "we have the greatest Cats" but no where have I found any hard test data regarding operating back presure, or pressure losses at various exhaust gas flow rates (or test engine RPMs) for any of these guys.

Does anyone know where there is any preformance test data posted on any of these Cats?