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Removing Catalytic Converters Reduces Gas Mileage? #4672344
02/20/18 08:11 PM
02/20/18 08:11 PM
Joined: Aug 2014
Posts: 326
Florida
lawnguy Offline OP
lawnguy  Offline OP
Joined: Aug 2014
Posts: 326
Florida
I was talking to a mechanic/technician today regarding removing both my rear cats off of my 2000 F150 4.6 V8. He told me if I did that ,that I would get worse gas mileage. Is this an actual fact?

Last edited by wwillson; 02/21/18 08:40 AM. Reason: subject clarification
Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672354
02/20/18 08:24 PM
02/20/18 08:24 PM
Joined: Jun 2009
Posts: 3,694
Chicago, IL
tomcat27 Offline
tomcat27  Offline
Joined: Jun 2009
Posts: 3,694
Chicago, IL
I vote false; although your OBDII system is not gonna like your non-functioning cats. It should throw codes and illuminate the MIL for loss of efficiency from the cats


2019 Cherokee, 2015 Rogue, 2014 Sentra, 2001 Camry, 2005 Starcraft, 1998 Dodge B1500
1996 Sebring Jx, 1984 F150, 1970 Cougar, 2013 XT250, 2010 Vstar, 2014 CTX1300
Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672363
02/20/18 08:30 PM
02/20/18 08:30 PM
Joined: Sep 2004
Posts: 7,522
Marshfield , MA
andyd Online content
andyd  Online Content
Joined: Sep 2004
Posts: 7,522
Marshfield , MA
you wanna max out the cats. Pull the Vacuum line off the fuel pressure regulator and drive around moderately to load up the cats with a little richness. Reconnect and take the car for a 1/2-3/4 hr drive in a lower gear that will allow you to run the engine at high RPM and not get arrested. Its a hold over from carb days. AKA the Italian tuneup. The engine management wants to see those cats.New O2 sensors up front may help. Taking off the cats may probably throw the timing and fuel maps off is a wild guess.

Last edited by andyd; 02/20/18 08:33 PM.

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Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672366
02/20/18 08:39 PM
02/20/18 08:39 PM
Joined: Jul 2014
Posts: 913
Winnipeg MB CA
Number_35 Offline
Number_35  Offline
Joined: Jul 2014
Posts: 913
Winnipeg MB CA
Originally Posted By: lawnguy
I was talking to a mechanic/technician today regarding removing both my rear cats off of my 2000 F150 4.6 V8. He told me if I did that ,that I would get worse gas mileage. Is this an actual fact?
I would say 'possibly'. Your exhaust system is designed to provide an optimum amount of back pressure. Too little, and the engine will lose low-end torque (and thus use more fuel at low RPM). The cats provide some of the back pressure, so removing them may hurt idle quality and low-end torque. (The advantage would come at the top end, where the reduced back pressure would result in better flow and thus more HP at higher RPM.) It's a truck, not a race car - I'd leave them in place.

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672382
02/20/18 08:49 PM
02/20/18 08:49 PM
Joined: Apr 2008
Posts: 37,110
Ontario, Canada
OVERKILL Offline
OVERKILL  Offline
Joined: Apr 2008
Posts: 37,110
Ontario, Canada
Removing the secondary catalysts is going to provide nothing but a stronger exhaust note and will require a tuning device to disable the rear monitors or anti-foulers to trick the ECM.

What are you trying to accomplish?


2018 RAM 1500 Big Horn EcoDiesel
2016 Grand Cherokee SRT
Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: OVERKILL] #4672394
02/20/18 09:03 PM
02/20/18 09:03 PM
Joined: Jun 2003
Posts: 17,348
Deplorable in apple valley, ca
Chris142 Offline
Chris142  Offline
Joined: Jun 2003
Posts: 17,348
Deplorable in apple valley, ca
Originally Posted By: OVERKILL


What are you trying to accomplish?
looks like he wants to pay a huge fine for tampering with emissions equipment.


02 Wrangler durablend 10w40
87 F250 traveler 15w40
04 Tahoe super-s 10w30
Z400 castrol T 10w40
Can am maveric edge 5w40
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Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672414
02/20/18 09:12 PM
02/20/18 09:12 PM
Joined: Jan 2018
Posts: 93
Tennessee, United States
ZraHamilton Offline
ZraHamilton  Offline
Joined: Jan 2018
Posts: 93
Tennessee, United States
Exhaust systems do not need any backpressure at all to run right. Look at the video made by Engineering Explained. Exhaust VELOCITY is needed for low-end torque, not backpressure. However, to get higher exhaust velocity, the diameter of the pipe must be decreased, and a negative consequence is increased backpressure. When tuning an exhaust, you have to find a balance between sufficient exhaust velocity for low rpms, and the least backpressure possible for high rpms. If removing the cats doesn't decrease the velocity, then it wouldn't lose any low-end torque or mpgs.

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672427
02/20/18 09:21 PM
02/20/18 09:21 PM
Joined: Jan 2018
Posts: 93
Tennessee, United States
ZraHamilton Offline
ZraHamilton  Offline
Joined: Jan 2018
Posts: 93
Tennessee, United States
Also, where I live, there are no emissions requirements, so a lot of people drive around without cats lol. There are a lot of diesels blowing black smoke everywhere too.

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: OVERKILL] #4672450
02/20/18 09:40 PM
02/20/18 09:40 PM
Joined: Aug 2014
Posts: 326
Florida
lawnguy Offline OP
lawnguy  Offline OP
Joined: Aug 2014
Posts: 326
Florida
Better airflow equating to more HP.


Originally Posted By: OVERKILL
Removing the secondary catalysts is going to provide nothing but a stronger exhaust note and will require a tuning device to disable the rear monitors or anti-foulers to trick the ECM.

What are you trying to accomplish?

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: ZraHamilton] #4672452
02/20/18 09:41 PM
02/20/18 09:41 PM
Joined: Jun 2003
Posts: 17,348
Deplorable in apple valley, ca
Chris142 Offline
Chris142  Offline
Joined: Jun 2003
Posts: 17,348
Deplorable in apple valley, ca
Originally Posted By: ZraHamilton
Also, where I live, there are no emissions requirements, so a lot of people drive around without cats lol. There are a lot of diesels blowing black smoke everywhere too.
does not matter. It's still against federal law to tamper with emissions.


02 Wrangler durablend 10w40
87 F250 traveler 15w40
04 Tahoe super-s 10w30
Z400 castrol T 10w40
Can am maveric edge 5w40
57 case tractor 15w40
Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672467
02/20/18 09:51 PM
02/20/18 09:51 PM
Joined: Apr 2008
Posts: 37,110
Ontario, Canada
OVERKILL Offline
OVERKILL  Offline
Joined: Apr 2008
Posts: 37,110
Ontario, Canada
Originally Posted By: lawnguy
Better airflow equating to more HP.


Originally Posted By: OVERKILL
Removing the secondary catalysts is going to provide nothing but a stronger exhaust note and will require a tuning device to disable the rear monitors or anti-foulers to trick the ECM.

What are you trying to accomplish?


Yeah, that's gonna be another monumental waste of money.

We have talked about this how many times now? Buy a Lightning, you can gear this thing to the moon with 4.56's, spend 2K on a full custom tune with programmer, another 2K on a full exhaust system with shorty headers and you still won't make as much power as a stock PI 5.4L and a Lightning bone stock, would decimate you in every possible metric and have cost you less money than what'd you'll have into this rig.

This isn't a 302 where you can toss heads/cam/intake on it and make >300HP for a couple K. I've been there and done that. The 4.6L Modular is expensive to build and your best improvement is going to come from forced induction.


2018 RAM 1500 Big Horn EcoDiesel
2016 Grand Cherokee SRT
Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: OVERKILL] #4672474
02/20/18 09:57 PM
02/20/18 09:57 PM
Joined: Aug 2014
Posts: 326
Florida
lawnguy Offline OP
lawnguy  Offline OP
Joined: Aug 2014
Posts: 326
Florida
Got it. Thanks OverKill!

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: tomcat27] #4672478
02/20/18 10:01 PM
02/20/18 10:01 PM
Joined: Jul 2010
Posts: 18,669
PNW
ZeeOSix Offline
ZeeOSix  Offline
Joined: Jul 2010
Posts: 18,669
PNW
Originally Posted By: tomcat27
I vote false; although your OBDII system is not gonna like your non-functioning cats. It should throw codes and illuminate the MIL for loss of efficiency from the cats


If the ECU throws trouble-codes it may also go into a semi-limp mode which certainly could hurt engine efficiency and gas mileage.

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: ZraHamilton] #4672481
02/20/18 10:03 PM
02/20/18 10:03 PM
Joined: Jul 2014
Posts: 913
Winnipeg MB CA
Number_35 Offline
Number_35  Offline
Joined: Jul 2014
Posts: 913
Winnipeg MB CA
Originally Posted By: ZraHamilton
Exhaust systems do not need any backpressure at all to run right. Look at the video made by Engineering Explained. Exhaust VELOCITY is needed for low-end torque, not backpressure. However, to get higher exhaust velocity, the diameter of the pipe must be decreased, and a negative consequence is increased backpressure. When tuning an exhaust, you have to find a balance between sufficient exhaust velocity for low rpms, and the least backpressure possible for high rpms. If removing the cats doesn't decrease the velocity, then it wouldn't lose any low-end torque or mpgs.
Agreed, it's not the back pressure itself, but the velocity of the exhaust gas (which is, of course, dependent on the back pressure). Think of trying to blow through a straw that's too bid - the air velocity will be quite low. I believe that the exhaust system is designed around the cats such that exhaust gas velocity is maximized for idle and low RPM. Without the restriction of the cats, the exhaust gas velocity may be compromised. Make sense?

Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy] #4672482
02/20/18 10:04 PM
02/20/18 10:04 PM
Joined: Mar 2013
Posts: 581
Al
cashmoney Offline
cashmoney  Offline
Joined: Mar 2013
Posts: 581
Al
There are more than a few donkey holes out there these days illegally jacking with their exhaust systems to make their vehicle louder, supposedly faster (but mostly not really) and most impressively to make thick black smoke. The extra noise, pollution, smoke, and the clearly evident obnoxious stupidity and insecure selfishness that goes with it is very impressive.

Last edited by cashmoney; 02/20/18 10:05 PM.
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