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#4672344 - 02/20/18 07:11 PM Removing Catalytic Converters Reduces Gas Mileage?
lawnguy Offline


Registered: 08/17/14
Posts: 326
Loc: 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?


Edited by wwillson (02/21/18 07:40 AM)
Edit Reason: subject clarification

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#4672354 - 02/20/18 07:24 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
tomcat27 Offline


Registered: 06/25/09
Posts: 3564
Loc: 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
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#4672363 - 02/20/18 07:30 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
andyd Offline


Registered: 09/25/04
Posts: 7125
Loc: 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.


Edited by andyd (02/20/18 07:33 PM)
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#4672366 - 02/20/18 07:39 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
Number_35 Offline


Registered: 07/07/14
Posts: 818
Loc: 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.

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#4672382 - 02/20/18 07:49 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
OVERKILL Offline


Registered: 04/28/08
Posts: 36416
Loc: 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?
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#4672394 - 02/20/18 08:03 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: OVERKILL]
Chris142 Offline


Registered: 06/05/03
Posts: 16811
Loc: 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.
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#4672414 - 02/20/18 08:12 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
ZraHamilton Offline


Registered: 01/15/18
Posts: 92
Loc: 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.

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#4672427 - 02/20/18 08:21 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
ZraHamilton Offline


Registered: 01/15/18
Posts: 92
Loc: 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.

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#4672450 - 02/20/18 08:40 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: OVERKILL]
lawnguy Offline


Registered: 08/17/14
Posts: 326
Loc: 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?

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#4672452 - 02/20/18 08:41 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: ZraHamilton]
Chris142 Offline


Registered: 06/05/03
Posts: 16811
Loc: 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.
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#4672467 - 02/20/18 08:51 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
OVERKILL Offline


Registered: 04/28/08
Posts: 36416
Loc: 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.
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#4672474 - 02/20/18 08:57 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: OVERKILL]
lawnguy Offline


Registered: 08/17/14
Posts: 326
Loc: Florida
Got it. Thanks OverKill!

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#4672478 - 02/20/18 09:01 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: tomcat27]
ZeeOSix Offline


Registered: 07/22/10
Posts: 17762
Loc: 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.

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#4672481 - 02/20/18 09:03 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: ZraHamilton]
Number_35 Offline


Registered: 07/07/14
Posts: 818
Loc: 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?

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#4672482 - 02/20/18 09:04 PM Re: Removing Catalytic Converters Reduces Gas Mileage? [Re: lawnguy]
cashmoney Offline


Registered: 03/23/13
Posts: 576
Loc: 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.


Edited by cashmoney (02/20/18 09:05 PM)

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