Before / After full exhaust mileage B7 S4

[chris4521]

Been a lurker for a while now. Recently bought a 2008 B7S4 6sp.

Car has 120km
Maintenance:
-OEM coils
-iridium plugs
-k&n air filter
-winter/summer tires
-lubrimoly oil / filter each 6000km

Bone stock when I bought it - i sometimes use it for long road trips from Toronto to Quebec.

Was getting about 12L per 100km driving just about 120.

Stock tune. Recently installed catless downpipes (stock exhaust). After that i Noticed a slight increase in mileage, maybe 11.5L per 100km.

A few days ago I installed the FI 2.5" catback non res. Still have the stock tune. Since I cheaped out and got the XS power downpipes, naturally they didn't fit well to the FI exhaust. Shop had to cut them back and install an offset so they removed the bungs for the aft O2 sensor. Not a big deal because I plan on getting a JHM tune shortly.

One issue is my mileage has taken a shit! On my last road trip - same roads, "same" gas, same driving and roughly same weather conditions - I was seeing about 13-13.5L per 100km!

Can anyone explain why? Maybe this is a stupid question but I figured mileage would improve from opening up the exhaust.

The car sounds sweet and is noticeably quicker...but I'm stumped. Think it has anything to do with the o2 sensor?

Any thoughts ?

Thanks guys

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[bmwpower603]

Been a lurker for a while now. Recently bought a 2008 B7S4 6sp.

Car has 120km
Maintenance:
-OEM coils
-iridium plugs
-k&n air filter
-winter/summer tires
-lubrimoly oil / filter each 6000km

Bone stock when I bought it - i sometimes use it for long road trips from Toronto to Quebec.

Was getting about 12L per 100km driving just about 120.

Stock tune. Recently installed catless downpipes (stock exhaust). After that i Noticed a slight increase in mileage, maybe 11.5L per 100km.

A few days ago I installed the FI 2.5" catback non res. Still have the stock tune. Since I cheaped out and got the XS power downpipes, naturally they didn't fit well to the FI exhaust. Shop had to cut them back and install an offset so they removed the bungs for the aft O2 sensor. Not a big deal because I plan on getting a JHM tune shortly.

One issue is my mileage has taken a shit! On my last road trip - same roads, "same" gas, same driving and roughly same weather conditions - I was seeing about 13-13.5L per 100km!

Can anyone explain why? Maybe this is a stupid question but I figured mileage would improve from opening up the exhaust.

The car sounds sweet and is noticeably quicker...but I'm stumped. Think it has anything to do with the o2 sensor?

Any thoughts ?

Thanks guys

Sent from my SM-G900W8 using Tapatalk

You could have a faulty coolant temperature sensor.

Thats would cause your mpg to dip

[Lucca M]

Or simply your old cats were messed up and now engine works better :)

[gaben2racin]

Stop worrying about the MPG's and enjoy your V8 AWD.
And get your JHM tune soon.


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[ZimbutheMonkey]

See here: I did a writeup on how catless downpipes affect intake resonance http://www.audizine.com/forum/showth...-loose-power-)

[chris4521]

I do not have the rear o2 sensors installed and I've read a few threads that say these are still used for "fuel trim" even though they may be coded out. I now see a CEL on my car. Ive messaged JHM over their fbook page and haven't had a response yet. The shop I had the tune done at said they would be doing the rear o2 delete. Anyone know more about what the rear o2 sensors are for? Am I at risk not having them installed, should I get the bungs welded back in?

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[eslatts11]

I do not have the rear o2 sensors installed and I've read a few threads that say these are still used for "fuel trim" even though they may be coded out. I now see a CEL on my car. Ive messaged JHM over their fbook page and haven't had a response yet. The shop I had the tune done at said they would be doing the rear o2 delete. Anyone know more about what the rear o2 sensors are for? Am I at risk not having them installed, should I get the bungs welded back in?

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Let me know what you hear from them, because I had the jhm 93 tune done and F.I. dp's + cat back installed on my b7 s4 and then a few months later I'm getting all these o2 sensor fault codes. They started on bank 2 sensor 2, now I have those fault 's plus bank 2 sensor 1 fault as well. I payed my shop to replace both sensors and it didn't do a damn thing and the fault code came right back a few days later.

[VinnysS4]

The front O2 sensor is for trimming the fuel. The rear is to make sure the catalytic converter is doing its job.

[chris4521]

Thank you I haven't been able to find much info so far. so really, is there any problem with having the rear o2 not installed?

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[CdnaudiS4]

I have a code right now from installing DP and cat back exhaust I believe its a bank 2 oxygen no activity. I've read that getting a JHM tune will fix this code and I've been debating for a year now if I should get the tune or not lol

(Is there a way I know that the shop actually tuned it?) Will there be that big of a power difference where I will notice it? I plan to get the 91 tune but fill 94 once in a while because it will learn anyways right? For me its about 1100 Canadian to get it done...