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Was wondering what the recomended fuel for the M535 is. The PO of my car was using 91 however the MPG was horrid. I made the switch to reg and the car's MPG has gone up significantly, well over 5. However I have noticed the car runs a little rougher and is running rich. I am not really to sure if they go hand and hand or not. I have always ran reg in my M30's except when I had my E34 and that was ran on premium due to the chip. However with this motor I am unsure. I know the compression is higher at 10:1 so not sure if this would call for the change in fuel.

Thanks

Josh

10:1 compression and L-Jet, I'm saying 91 will almost certainly be necessary for good performance.

I can get away with 87 on a stock 528i but like the way it pulls without complaint at higher rpms with 89. Given the age, normal wear in the distributor mechanicals, etc., I think this is reasonable, even if it would have done fine on 87 out of the factory brand new.

YMMV,

KenB

If it's a real M535i, it's 9.3:1 compression, not 10:1. Regardless, you should be running premium, whatever the number is where you live.

Except when timing is changed to allow the use of better or worse fuel, the car should not run any differently with a different octane rating. But if you change the timing 5 or 10 degrees it will run much differently.

Mike W. wrote:If it's a real M535i, it's 9.3:1 compression, not 10:1. Regardless, you should be running premium, whatever the number is where you live.

Except when timing is changed to allow the use of better or worse fuel, the car should not run any differently with a different octane rating. But if you change the timing 5 or 10 degrees it will run much differently.

Then I guess I will be making the switch back to 91. I made the switch to 87 after a few tanks of running 91. My MPG was horrid at around 13. I switched to 87 and it shot up to just under 18. Why this is I do not know. I know that 91 does not burn as fast as 87 and if your car does not require 91 the extra fuel gets "wasted". At least that is my understanding in a nutshell after a few google searches.

No extra fuel should be wasted, just that the additional octane isn't needed. Octane does slow the burning process to a degree, and can make it run slightly cooler and with lower Nox emissions, but is not generally noticed in driving, except in reduction of knock or preigniton, which is a bad and destructive thing. Fill it with premium and check the ignition timing.

Mike W. wrote:No extra fuel should be wasted, just that the additional octane isn't needed. Octane does slow the burning process to a degree, and can make it run slightly cooler and with lower Nox emissions, but is not generally noticed in driving, except in reduction of knock or preigniton, which is a bad and destructive thing. Fill it with premium and check the ignition timing.

Thanks much appreciated