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Discussion Starter #1
Hey guys,

searched around and i have yet to come across any actual info about how much worse MPG gets after going catless.

i’m currently getting 10-12mpg on the highway and 6-9mpg in city, So im hoping MPG might improve a bit after replacing my front set of o2 sensors and running some CRC intake valve spray into the Air intake manifold... along with a few bottles of B-12 Chemtool (gas tank). Later on, ill replace oil with 5w-50... not sure if that will help but troy mentions up to 20mpg is possible with it

i was actually getting 6mpg (9mpg on highway) before i replaced sparkplugs and so im glad mpg has improved a bit since then.

can’t imagine the 6mpg getting any worse than it is if i remove the cats though.
 

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Hey guys,

searched around and i have yet to come across any actual info about how much worse MPG gets after going catless.

i’m currently getting 10-12mpg on the highway and 6-9mpg in city, So im hoping MPG might improve a bit after replacing my front set of o2 sensors and running some CRC intake valve spray into the Air intake manifold... along with a few bottles of B-12 Chemtool (gas tank). Later on, ill replace oil with 5w-50... not sure if that will help but troy mentions up to 20mpg is possible with it

i was actually getting 6mpg (9mpg on highway) before i replaced sparkplugs and so im glad mpg has improved a bit since then.

can’t imagine the 6mpg getting any worse than it is if i remove the cats though.
Is something wrong with your car? On a flat highway (with 10w60), I can get up to 22mpg in cruise control at 70mph (non-M mode).


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Discussion Starter #3
Yea, probably. Thats exactly why im trying to figure out what could be causing it to run so lean but everyone so far keeps saying “dont expect any better than 10mpg” — yea when flooring it but not when driving calm lol. I guess i’ll find out once i replace the o2 sensors as i’ve gotten “increased emissions” warnings on and off for some time now

anything else i should look at besides o2 sensors?

Is something wrong with your car? On a flat highway (with 10w60), I can get up to 22mpg in cruise control at 70mph (non-M mode).


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Yea, probably. Thats exactly why im trying to figure out what could be causing it to run so lean but everyone so far keeps saying “dont expect any better than 10mpg” — yea when flooring it but not when driving calm lol. I guess i’ll find out once i replace the o2 sensors as i’ve gotten “increased emissions” warnings on and off for some time now

anything else i should look at besides o2 sensors?
I’ll defer to someone who has more technical experience regarding this platform. But, as mentioned a healthy S85 should be getting around 21mpg or so in cruise on the highway. I’ve had it for two years without major issues or concerns.

Speaking in general car logic, something is wrong with either fuel, spark, or air. If you’re not feeling a loss of power, maybe you have a fuel leak somewhere. Something is off, that’s for sure.


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Have you checked the codes which caused "increased emission" and what is your coolant temp when cruising on highway? I had bad thermostat and it affected to fuel consumption. I have now made couple of business trips in this month with the M5 and one trip is approx. 500km (310miles) Average consumption with the cruise control set to 112km/h ( 70mph) was 11.3l/100km (25mpg).
 

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I am able to get 10.9L/100 at 120km/hr in p400 all day long.
in the city however i will get 17 to 22L/100km
 

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Flatland cruising 75-80 mph at 3k rpm I get 19-20 US mpg in P500S. I really haven't found much of a mileage difference if at all between P400 and P500.

Performance wise my car is bone stock with P4S tires. I think with a smg having an extra gear compared to a manual you could get very low 20's cruising.
 

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Discussion Starter #10
Have you checked the codes which caused "increased emission" and what is your coolant temp when cruising on highway? I had bad thermostat and it affected to fuel consumption. I have now made couple of business trips in this month with the M5 and one trip is approx. 500km (310miles) Average consumption with the cruise control set to 112km/h ( 70mph) was 11.3l/100km (25mpg).
Which thermostat? The small $10 temp sensor on the passenger side, the big one on the driver side, or the engine oil temp sensor?

or are you talking about the thermostat that opens/closes coolant automatically?
 

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Which thermostat? The small $10 temp sensor on the passenger side, the big one on the driver side, or the engine oil temp sensor?

or are you talking about the thermostat that opens/closes coolant automatically?
Yes, coolant thermostat. You can check temperature from instrument clusters secret menu.
 

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what values did you see that made you think it needed replacement?
On highway i got 75C which is too low. 78C and over is ok. You can see higher values in the city even your thermostat is not working ok. Also when the climate went below 0C i got fault code also...
 

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It think the issue is something else and not cat removal. I have my cats removed and I get 12-13 mpg in city driving in M mode and driving spiritedly.


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Discussion Starter #15
It think the issue is something else and not cat removal. I have my cats removed and I get 12-13 mpg in city driving in M mode and driving spiritedly.


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i havent went catless... yet. I just wanted to find out from you guys just how much worse it is for MPG... before and after going catless. I’m assuming its not a noticeable amount?
 

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You could see an improvement if you go catless and install a tune since the tuned software will allow for a leaner mix since there is no longer the risk of burning the cats. That is also assuming you drive moderately most of the time.


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Discussion Starter #17
You could see an improvement if you go catless and install a tune since the tuned software will allow for a leaner mix since there is no longer the risk of burning the cats. That is also assuming you drive moderately most of the time.


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So its best to go with a tune after going catless? I was actually thinking to have a muffler shop drill out the cats instead of welding a new pipe in its place or installing catless headers... since i wont need to pass strict emissions in the place where i live
 

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A tune is definitely the best and cleanest route but if you have to pass emissions it might be best just to install a catback exhaust or do stock muffler delete and call it a day. In the end going catless might not yield more than a 1 mpg benefit.


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Discussion Starter #20
Ah ok, seemed like everyone kept mentioning how much worse their MPG gets after going catless. Sounds like from what you’re saying it doesn’t negatively effect gas mileage and if it does... theres a small positive gain with a tune

any cheap tunes you guys would recommend now since FAMS seems no longer an option anymore... the closest thing in terms of similar vanos and smg tuning along with price that i have found is a Bren tune
 
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