How does a 535i respond to different fuel octane ratings?

[pundit]

Is there much of an advantage in running a 535i on fuel with any higher octane rating than say 91? In Australia we have fuel with the following octane ratings...

Regular Unleaded - 91 Octane
Premium Unleaded - 95 Octane
High Octane Premium Unleaded - 98 Octane

While the Vanos version of the 525i would undoubtably benefit because of it's engine management system (knock sensing etc) would there be any gains with a 535i? For example 95 octane costs around 10% more than 91 octane so switching to 95 octane one would need at least a 10% improvement in economy/performance to offset this. I assume this would require the 535i to be retuned ie. have it's ignition timing advanced to make use of the higher octane. I have tried 95 octane in my '89 Toyota van (fuel injected - no knock sensing) and the result was less power and a reduction of about 10% in fuel economy compared with regular 91 octane! While I didn't alter the ignition timing, (it had already been pinging very slightly under load whilst on 91 octane), the pinging stopped on 95 octane but everything else went backwards! Does the EAT chip require the use of higher octane fuel?- Cheers.

[JonE]

EAT chip will require at least 91 octane. Mine seems to run fine with anywhere from 90 to 92 octane, no pinging noticed. Prior to the chip it would run with anything, but no signficant advantage with higher octane gas. I think the ratings I mention are the non RON octane ratings, your may be RON ratings.

[DanH]

I ran 89/90 (US ratings) octane in my 535i for a couple years and for the past few months I've used 87 octane and I am getting the same MPG either way. From my measurements, the difference is from 0% to 3% better and the cost of the fuel is more than 3% higher.
Also, there is no pinging problem and its not chipped.

For 8 month period May to December:

Year--Octane--Avg.MPG--Miles Driven
2003-- 89/90 ---20.17 ---- 13394
2004-- 87 ------20.09 ---- 8237

Therefore, it is not worth the extra expense to use higher octance than the minimum required for US spec cars without perf chips.

[Derek A.]

Higher octance fuels tend to have a higher detergent level. On the m30 motor the detergent is helpful in keeping the intake system clean. I have seen a number of motors come apart after running cheap fuel and its amazing the crud that is built up.