Worse gas mileage after removing catalytic converter ? - S-10 Forum
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post #1 of 53 Old 08-12-2008, 03:39 PM Thread Starter
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Worse gas mileage after removing catalytic converter ?

I removed my catalytic converter because it was shot anyway and put a piece of straight pipe instead until I get around to buying a new converter. It seems like I am getting significantly worse gas mileage now , does anybody know why removing the cat would use more gas? I originally thought it would be the other way around.

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post #2 of 53 Old 08-13-2008, 01:41 AM
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Re: Worse gas mileage after removing catalytic converter ?

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I removed my catalytic converter because it was shot anyway and put a piece of straight pipe instead until I get around to buying a new converter. It seems like I am getting significantly worse gas mileage now , does anybody know why removing the cat would use more gas? I originally thought it would be the other way around.

well im not to seaure but i have to ask you 1 juestion when your driving do you accelorate al littler hard to just hear your exhaust? cus thats what i do!!!!
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post #3 of 53 Old 08-13-2008, 01:43 AM
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Re: Worse gas mileage after removing catalytic converter ?

do you do a lot of in-town driving? some vehicles lose a lot of low-end torque...leads to better mpgs in town with a light footed driver

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post #4 of 53 Old 08-13-2008, 02:00 AM
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Re: Worse gas mileage after removing catalytic converter ?

i took mine out, replaced it with a straight piece of pipe and now it gets way worse gas mileage and runs like complete shit. half throttle bog down, full throttle bog then it kicks in but not full potential, same problem every is having on the forum.... i think i need to put the cat back in.. even if it isn't loud anymore

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post #5 of 53 Old 08-13-2008, 02:03 AM
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Re: Worse gas mileage after removing catalytic converter ?

I ran a pipe through my cat, still getting worse milage I think.

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post #6 of 53 Old 08-13-2008, 09:05 AM Thread Starter
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Re: Worse gas mileage after removing catalytic converter ?

I do almost nothing but in town driving , a lot of stop and go , 5 speed trans. but I usually drive very conservatively. Its been louder since I removed the cat especially with the exhaust leak somewhere. I think it really needs the cat to work right and get more MPG's.
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post #7 of 53 Old 08-13-2008, 10:17 AM
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Re: Worse gas mileage after removing catalytic converter ?

Seems like something else is causing the bad gas mileage, I ran no cat for years and if anything I picked up maybe .5-1mpg.

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post #8 of 53 Old 08-17-2008, 02:38 PM Thread Starter
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Re: Worse gas mileage after removing catalytic converter ?

I dont know what else it could be since the drop in mpgs happened in the one day I removed the cat. Its hard to believe I was getting better gas mileage before when the cat was clogged. If it was not instant change then I probably wouldnt have noticed but since it was, I can only guess that for some reason my particular truck ( '98 with 2.2 and manual trans. ) was more efficient with the extra backpressure ????
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post #9 of 53 Old 08-20-2008, 02:28 PM
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Re: Worse gas mileage after removing catalytic converter ?

Does it feel like more power? Maybe if it couldn't get much exhaust past the clogged converter, it couldn't get much fuel/air in.

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post #10 of 53 Old 08-20-2008, 03:51 PM
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Re: Worse gas mileage after removing catalytic converter ?

Put a new aftermarket cat converter in there and do a routine tune-up. The cat is in there for a reason and it shouldn't be much more than $200 to purchase one and have it welded in at a muffler shop.


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post #11 of 53 Old 08-20-2008, 04:21 PM
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Re: Worse gas mileage after removing catalytic converter ?

I took my cat off and started getting shitty gas milage.. replaced my o2 sensor and it runs great now.

All i can tell you

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post #12 of 53 Old 08-20-2008, 04:23 PM
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Re: Worse gas mileage after removing catalytic converter ?

I have no cat on my 97' 4.3 and I am getting normal gas mileage. On the freeway I'm getting like 21mpg...

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post #13 of 53 Old 08-27-2008, 10:38 AM Thread Starter
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Re: Worse gas mileage after removing catalytic converter ?

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Does it feel like more power? Maybe if it couldn't get much exhaust past the clogged converter, it couldn't get much fuel/air in.
It does have more power now. So you could be right, its just able to comsume more fuel now to do what it used to do with less or something. I am wondering if those who get better mileage or the same have an automatic trans. because even with the increased power mine has now you still have to wind out the lower gears before shifting so that it dosent lug a little in a higher gear , and that would effect the mileage. I think these little underpowered engines ( 2.2 ) do better with an automatic than a 5 speed.
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post #14 of 53 Old 08-27-2008, 11:15 AM
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Re: Worse gas mileage after removing catalytic converter ?

Although you don't indicate the year or engine type of your truck and whether or not it's otherwise modified......I'd say......

If you had a scan tool, you'd probably see that the computer is adding fuel because the o2 sensor is sensing a "leaner" condition due to the lowered exhaust restriction. Just not enough to set the MIL

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post #15 of 53 Old 08-27-2008, 11:27 AM
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Re: Worse gas mileage after removing catalytic converter ?

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I think these little underpowered engines ( 2.2 ) do better with an automatic than a 5 speed.

actually for MPG, a 5 speed does way better than an automatic.

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post #16 of 53 Old 09-01-2008, 11:14 PM
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Re: Worse gas mileage after removing catalytic converter ?

your burning more gas becuase your O2 sensors arent reading correctly therefore dumping more fuel in. also your loosing alot of backpressure... though loosing some backpressure is good for an exhuast system, you still need some backpressure... do you have a performance muffler on the truck? or none at all?... anyway its mostly becuase your 02's are reading that there is no cat or a significant change in the exhuast system and cuasing you to burn more fuel
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post #17 of 53 Old 09-03-2008, 01:45 PM
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Re: Worse gas mileage after removing catalytic converter ?

Don't know if you've tried yet but maybe unplug the battery for 1/2 an hour and then hook it back up to try resetting the ECU. It's free and can't hurt.
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post #18 of 53 Old 09-05-2008, 03:45 AM
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Re: Worse gas mileage after removing catalytic converter ?

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Don't know if you've tried yet but maybe unplug the battery for 1/2 an hour and then hook it back up to try resetting the ECU. It's free and can't hurt.
hes right it cant hurt!!!! or sence you want your mpg back you should just get a free flow cat. there only $100... you you would get some back preasure back and the cat comes with an 02 sensor so you computer should just fix it self, and youll get your mpg back!!!!!

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post #19 of 53 Old 09-05-2008, 02:25 PM Thread Starter
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Re: Worse gas mileage after removing catalytic converter ?

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actually for MPG, a 5 speed does way better than an automatic.
Hmmm, .. normally with a bigger engine I can see that because it has more power and you can actually shift when you want but an engine that lacks power there is such a small margin of time between getting the speed up and ready for the next gear and giving too many rpms for best fuel economy. If you change gears too soon you lugg a little , extra strain on the drive train burns more fuel...too late and you have more rpms than you need doing the same thing. In town driving exaggerates the effect because the constant shifting. Im not saying its hard to shift at the right time but the different road conditions sometimes change how you drive. A bigger engine has the power to shift earlier without lugging. With an automatic its all computer controlled making it shift at the right time all the time for best fuel economy. You may be right , I dont know but seems like computer controlled shifting would be more economic. I have the stock muffler and no modification of the system/engine. I will try disconnecting the battery for a while and see if that helps. Eventually I will get a new cat , but need to go with the cheap fix for a little while. Thanks for the info guys.
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post #20 of 53 Old 09-05-2008, 02:54 PM
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Re: Worse gas mileage after removing catalytic converter ?

I picked up about 5MPG in my lumina not running a cat or muffler... Cops didn't like it all too much though


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post #21 of 53 Old 09-11-2008, 06:17 PM
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Post Re: Worse gas mileage after removing catalytic converter ?

i just recently took out my cat bc it was shot and making rattleing noises.
i replaced it with some strait pipe and welded it.
i get the same mpg if not a little better!!!
as for the power diff.. i accelerate a lil faster and it sounds alot better
until i get my new exaust in that i am developing myself
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post #22 of 53 Old 03-09-2012, 05:15 PM
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Re: Worse gas mileage after removing catalytic converter ?

Trying to save a buck I went without the converter for about 1000 miles and the gas mileage was terrible. Put it back in the truck ,2000 4 cyl auto and the gas mileage is back to normal and the truck sounds much better. It is worth it to not see that check engine light on all the time.
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post #23 of 53 Old 03-11-2012, 03:59 PM
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Re: Worse gas mileage after removing catalytic converter ?

bump of a 4 yr old thread. glad you searched.


my 01 sonoma got 27mph on the hwy with a gutted cat along with 250+ miles.

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post #24 of 53 Old 03-11-2012, 04:59 PM
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Re: Worse gas mileage after removing catalytic converter ?

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I do almost nothing but in town driving , a lot of stop and go , 5 speed trans. but I usually drive very conservatively. Its been louder since I removed the cat especially with the exhaust leak somewhere. I think it really needs the cat to work right and get more MPG's.
Fix the exhaust leak and see if your mileage returns to normal.

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post #25 of 53 Old 03-15-2012, 12:54 AM
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Re: Worse gas mileage after removing catalytic converter ?

Removing and or gutting cat will not affect your mileage in anyway shape or form. You guys saying otherwise have other issues like you jacked your O2 sensor up or it just needs replacing. Done this too many times. I know what I know because I have done this many times and various types of vehicles and have my own experience to rely not something I read on the interwebz............lol
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post #26 of 53 Old 03-15-2012, 12:56 AM
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Re: Worse gas mileage after removing catalytic converter ?

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Trying to save a buck I went without the converter for about 1000 miles and the gas mileage was terrible. Put it back in the truck ,2000 4 cyl auto and the gas mileage is back to normal and the truck sounds much better. It is worth it to not see that check engine light on all the time.
Check engine light can be fixed wit one of these..................

http://www.afterthoughtsauto.com/o2sim.html
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post #27 of 53 Old 03-31-2012, 04:50 PM
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Re: Worse gas mileage after removing catalytic converter ?

have you tried putting a riser on the post cat sensor? with the cat gone it will give a "dirty" exhaust reading if you use a riser, provided the sensor isnt shot, is should send the right signal. the sensor will still get hot but it will read "cleaner" do to the fact that its not in the direct path of the exhaust. also should get rid of the c/e light or just reset it as mentioned above and it shouldnt come back.
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post #28 of 53 Old 04-05-2012, 01:11 PM
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Re: Worse gas mileage after removing catalytic converter ?

Well looks like you're getting a new cat in the future. I still will post this for you.
http://www.afterthoughtsauto.com/o2sim.html
My CEL light was on with my straight pipe and I had clearance issue for the anti-fouler trick on the 02 sensor.
The simulator got my millage back to normal...spendy though.


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post #29 of 53 Old 04-05-2012, 01:58 PM
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Re: Worse gas mileage after removing catalytic converter ?

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your burning more gas becuase your O2 sensors arent reading correctly therefore dumping more fuel in. also your loosing alot of backpressure... though loosing some backpressure is good for an exhuast system, you still need some backpressure... do you have a performance muffler on the truck? or none at all?... anyway its mostly becuase your 02's are reading that there is no cat or a significant change in the exhuast system and cuasing you to burn more fuel

Thats def NOT true.


Only your upstream sensors control a/f mixture. Downstream only monitors cataylist efficiency and do not controll a/f mixture AT ALL.

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post #30 of 53 Old 04-06-2012, 01:28 AM
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Re: Worse gas mileage after removing catalytic converter ?

ok so for me i have a problem. cat is about 80% plugged. its derivable but super hard on fuel. i need to get it to town to get it fix would cutting a hole before the cat make it run better to get it to town? sound is not a problem.

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post #31 of 53 Old 04-06-2012, 01:30 PM
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Re: Worse gas mileage after removing catalytic converter ?

Hell yeah, just take the sawzall and cut it off right before the cat.


Id say unbolt it, but these are old enough you will spend more time trying to remove rusted bolts then its worth.

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post #32 of 53 Old 04-06-2012, 04:15 PM
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Re: Worse gas mileage after removing catalytic converter ?

Ok thanks i have a magnaflow cat system coming in for it. Should see mpg gain from the old one. Im going to do cat back later no money for it lol. Till i start working my summer job.

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post #33 of 53 Old 04-23-2012, 10:32 AM
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Re: Worse gas mileage after removing catalytic converter ?

With the 4.3l you need back pressure from the exhaust for low end torque and power.
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post #34 of 53 Old 04-23-2012, 10:47 AM
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Re: Worse gas mileage after removing catalytic converter ?

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With the 4.3l you need back pressure from the exhaust for low end torque and power.
INCORRECT.

You need proper exhaust scavenging.

http://en.wikipedia.org/wiki/Scaveng...8automotive%29


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post #35 of 53 Old 04-23-2012, 12:48 PM
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Re: Worse gas mileage after removing catalytic converter ?

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Check engine light can be fixed wit one of these..................

http://www.afterthoughtsauto.com/o2sim.html
I just got rid of both my cats and did the non foulers but think I could have damaged the o2 when I used a impact gun to get it out. What o2 simulator is the one I need the the type5 or type 6? I would rather buy one of those then buy another o2 and still have the cel.
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post #36 of 53 Old 04-23-2012, 03:11 PM
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Re: Worse gas mileage after removing catalytic converter ?

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With the 4.3l you need back pressure from the exhaust for low end torque and power.
Its a wonder vehicles ran at all BEFORE CATS.........................lol
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post #37 of 53 Old 04-23-2012, 09:35 PM
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Re: Worse gas mileage after removing catalytic converter ?

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INCORRECT.

You need proper exhaust scavenging.

http://en.wikipedia.org/wiki/Scaveng...8automotive%29
which means some back pressure thru one instrument or another. no? what would be the best solution? what cat and muffler set do you suggest for the first gen 4.3l vortec with medium mods?
thanx
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post #38 of 53 Old 04-23-2012, 10:51 PM
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Re: Worse gas mileage after removing catalytic converter ?

i dunno, my 95 4 banger obx header no cat 2 1/4" straight pipe to thrush turbo muffler out factory tubing to stock location getting 30/31 mixed mpg shifting less than 2K rpm...id like to see an autotragic s10 get that.

also more exhaust leaks than you can shake a stick at...AND SES light on constantly due to no EGR lol
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post #39 of 53 Old 05-21-2012, 10:15 AM
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Re: Worse gas mileage after removing catalytic converter ?

Thanks for posting your problem. I read this about a few hours before i was going to take mine out. I'll leave it in.

About your leak.

Can exhaust leaks be filled with duraglas fiberglass? I want to try tonight
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post #40 of 53 Old 06-01-2012, 10:36 AM
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Re: Worse gas mileage after removing catalytic converter ?

you want the least backpressure, the more air and fuel you cram in the more power you can make, the reason the mileage went down is I bet the check engine light is on and the computer is in limp home mode and is adding more fuel to protect the engine,,,either replace the cat, or get a antifouler for the rear O2.
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post #41 of 53 Old 06-01-2012, 10:43 PM
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Re: Worse gas mileage after removing catalytic converter ?

Catalyist efficiency code will not throw the ecm into limp home mode.

Rear o2 has no effect on drivability. It doesnt add more fuel.

89' S10 blazer 4wd: 355, 650 dp, rpm intake, comp 282 roller cam, modified vortecs, T56 trans, 8.8 ls rear, headers, x-pipe, spintech pro street mufflers, and......AC

94 sunbird-"turbochicken"Turbo 2.0 5spd getrag swap
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post #42 of 53 Old 06-05-2012, 07:31 AM
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Re: Worse gas mileage after removing catalytic converter ?

ok.....
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post #43 of 53 Old 06-05-2012, 02:10 PM
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Re: Worse gas mileage after removing catalytic converter ?


89' S10 blazer 4wd: 355, 650 dp, rpm intake, comp 282 roller cam, modified vortecs, T56 trans, 8.8 ls rear, headers, x-pipe, spintech pro street mufflers, and......AC

94 sunbird-"turbochicken"Turbo 2.0 5spd getrag swap
Tbi heads, intake,timing set, dist for sale
Blazer Video!
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post #44 of 53 Old 06-05-2012, 10:43 PM
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Re: Worse gas mileage after removing catalytic converter ?

My converter is roted. I have an 86 so i don't take emissions tests. My mechanic said i can take it off and pass inspections. I kept it on there even roted and planned to paint it just to pass visual inspections if i come across a dick cop. Then i found this thread and learned with out it i may have drivability problems.

Whats the verdict. Is the first gen cat an irrelevant piece of equipment for drivability?
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post #45 of 53 Old 06-06-2012, 01:12 PM
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Re: Worse gas mileage after removing catalytic converter ?

no for christs sakes, its ok bro's!

89' S10 blazer 4wd: 355, 650 dp, rpm intake, comp 282 roller cam, modified vortecs, T56 trans, 8.8 ls rear, headers, x-pipe, spintech pro street mufflers, and......AC

94 sunbird-"turbochicken"Turbo 2.0 5spd getrag swap
Tbi heads, intake,timing set, dist for sale
Blazer Video!
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post #46 of 53 Old 06-06-2012, 01:15 PM
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Re: Worse gas mileage after removing catalytic converter ?

I know your position on the topic.
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post #47 of 53 Old 06-06-2012, 01:16 PM
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Re: Worse gas mileage after removing catalytic converter ?

LOL, that is all you need.

89' S10 blazer 4wd: 355, 650 dp, rpm intake, comp 282 roller cam, modified vortecs, T56 trans, 8.8 ls rear, headers, x-pipe, spintech pro street mufflers, and......AC

94 sunbird-"turbochicken"Turbo 2.0 5spd getrag swap
Tbi heads, intake,timing set, dist for sale
Blazer Video!
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post #48 of 53 Old 06-08-2012, 09:22 AM
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Re: Worse gas mileage after removing catalytic converter ?

Quote:
Originally Posted by Styleten View Post
My converter is roted. I have an 86 so i don't take emissions tests. My mechanic said i can take it off and pass inspections. I kept it on there even roted and planned to paint it just to pass visual inspections if i come across a dick cop. Then i found this thread and learned with out it i may have drivability problems.

Whats the verdict. Is the first gen cat an irrelevant piece of equipment for drivability?
For fock's sakes. This idiot could use some of the money he swindled from the insurance company and replace his rotted cat for a new one, but instead will rather drive around with faulty parts so he can invest his swindled money into more online pyramid scams.
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post #49 of 53 Old 06-08-2012, 02:08 PM
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Re: Worse gas mileage after removing catalytic converter ?

Lol

89' S10 blazer 4wd: 355, 650 dp, rpm intake, comp 282 roller cam, modified vortecs, T56 trans, 8.8 ls rear, headers, x-pipe, spintech pro street mufflers, and......AC

94 sunbird-"turbochicken"Turbo 2.0 5spd getrag swap
Tbi heads, intake,timing set, dist for sale
Blazer Video!
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post #50 of 53 Old 06-27-2012, 01:54 AM
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Re: Worse gas mileage after removing catalytic converter ?

with no cat theres less back pressure, which makes for more power needed to actually move the truck but will help with power at top end, unless you turbo or maybe supercharge not 100% on the S.C but in those cases you actually want less back pressure to a point which you would would want maybe a 3.5 inch straight at tops for the 4.3 liter. if you put the cat back on you wont notice largely but it will give you less power at top end but better gas mileage, the same thing would happen if you were to take off the muffler
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