thanks wendell and pninto, u actually get how new motors are tuned from the factory

 

The entire fuel system is NOT tuned around the catalytic converter. The rear O2 sensors only measure catalyst efficiency. The FRONT sensors are what the ECU uses to control the fuel trims.

 

yes but the restrictions in air flow are part of the tuning affecting the valve timing wich if you have less restriction the valve will be open to long to just get eauxaust gasses out of the cylinder. wich is what scavanging is and takeing out cats cause over scavaging wich takes of so much that some fual air mix gets sucked out to

 

There are other reasons that running without cats can have negative effect too. It alters the way air flows through the exahust, and the power will tend to be more focused into a narrower powerband with lots of intermittent power spikes that occur when a lot of coplex things are happening. The peak output will tend to increase, but honestly, unless you spend a lot of time on a track, you spend most of your driving time between 1500 - 2500 rpm. At these ranges, the cats aren't restricitive at all. The factory exahust tuning is also meant for these ranges, so messing with it too much can cause adverse effect to fuel economy and low range power.

BTW, the factory systems are tuned around emissions. You can think whatever you want, but this was the design and intent of OBD-II. When the rear sensor detects an converter fault, most systems will try to minimize emissions without a working cat. This will often harm economy and performance.

There just aren't enough good reasons to remove converters, whether you are OBD-II or not. Now before you judge my motives in this matter, let me explain my experiences. I sell a lot of replacement converters for vehicles where people have taken them off and found that bad things happened to the way the vehicle runs. Now take them off if you want, I make more money when people do, but its not legal to remove them, and it won't help fuel economy. And my second strong reason for wanting you to keep it on is that my brother almost died from carbon monoxide poisoning from a vehicle.

 

I think that you are somewhat confused as to how the scavenging in an engine works. Read this link: http://www.superchevy.com/technical/...exh/index.html

Bottom line is that removing the catalytic converters will not cause this over-scavaging that you claim.

 

I did not say that he should or should not take the cats out. All else equal (rear O2 sensors bypassed, etc) an engine without catalytic converters should make more power and get better mileage simply due to the fact that it does not have as much restriction in the exhaust. It sure does alter the way the exhaust flows as it doesn't have the restriction in there. I would like to see some dyno charts or a study or something that supports your claim that removing the cats (in essence removing restriction in the exhaust) will hurt your mileage and power.

Whether or not to remove them is a choice left to the individual. Legal or not, they will do it if they want to.

 

i've had experince to differ... though it was not drastic it still accurd droping my the mpg about 1-2 though this was with a differnt vehicle and i know there are differnt ways to cause scavaging

 

and also i am not removing the cats at this point, may replace down the line though

 

My '89 Ranger came with a stock tach, 2.9l. V-6. It'll spin the tires on hard accelleration from a dead stop and turns 3,100 rpm at 60mph. I measured the height of the stock tires, and since I needed a new set, I bought a new pair for the rear about 2" taller than the original tires. The taller tires dropped the rpm down to 2700 rpm at 60mph(the speedometer is slow 5mph now--when the speedo. shows 55mph, I'm actually doing 60 mph--had a friend follow along side to verify it). (Saving 400 rpm every minute going down the highway doesn't seem like much, but that's 30 or 40 extra miles on a tank of gas)
good news is---I'm getting 2 mpg better with the taller tires
bad news is---It wont spin the back tires now
I don't think I'll go with 3" taller tires--that might lug the little v-6 down too much, and I'd end up with worse gas mileage!

 

Engines are designed to run with back pressure from the exhaust. Thats why if ya put too big of an exhaust pipe on or take the cats off a car the performance drops

 

Ok then explain to me why the (both) O2 sensors switch from rich to lean many times a second?


Catalyst performance is based off of both rich and lean pulses of air. Its the whole key to how it works. If the O2 sensors gave straight 450 milli volts the cat would not operate correctly.

Yes the 02 sensor ziconea elements can help aid the fuel and ignition system correct itself, but the basis for their correction still has to suite the needs of the catalytic converter hense the switching. If you ever see straight voltage either the O2 isn't warmed up or its trashed.

So in a sense when 02 sensors are working correctly, there really is no set rich or set lean in closed loop. It constantly switches lean rich lean rich etc.

 

Oh and as far as exhaust performance....

cranking...you want vaccum
at idle... you want vaccum.
wide open throttle...you do not want vaccum
decel(engine revving throttle closed) ......you will get the most vaccum ever.

So out of the above situations think of it this way. Taking away restrictions is going to hurt vaccum

taking away restrictions has no negative effects at wide open throttle since no vaccum is needed. As much air as the Maf can sense coming in the better, so can you theoretically hurt the top end power of the engine? No.

A. at WOT (wide open throttle) your system usually switches open loop rich and takes away 02 feedback anyway. So at this point catalyst removal has no effect.

B. You are risking a negative effect on your engines vaccum readings at low throttle.. Probably won't effect it much.

Basically if you do it right, running without cats for high end performance can do some good power wise. You can argue me all you want but if you truly understand how throttle opening and vaccum affect engine performance you'll see how this works.

 

Who-da thought--I had a '84 Firebird with a v-6/4 speed manual xmsn. It would barely spin the tires in 1st gear. Engine lite started acting up-took the exhaust off behind the cat. It was starting to disintergrate and plug up the muffler. I revved up the engine till the exhaust blew enough junk out of the cat to the point that I could see thru into the front opening. Cleaned out the muffler and bolted everything back up with new gaskets. Took it for a run down the interstate--by the time I got back home, the v-6 would spin the tires in 1st, 2nd, AND 3rd!!! GO FIGURE!! (For entertainment purposes only--I'd never tell anyone to gut their cat.-meow)

doorgunner
(an old steel Coke can fit perfectly on the M-60 machine gun in Nam--the ammo belt fed over it perfectly eliminating jam-ups! American "engine-uity"-have a Coke & a Smile. This also for entertainment purposes only!)