Preferred Fuel

Optimax for my M3 and nothing else.Read other posts about people trying the BP stuff and making their car run cr4p,so they swapped back.

I had 200litres of 102 octane race fuel in my possesion a couple of weeks ago but resisted the urge to put any in the M3, thought it might do more harm than good - probably needed to adjust the ignition timing as well or something?? Optimax all the way otherwise..

Jonners wrote: am confused on this some say no point in running higher octane if the car is properly set up for 95 octane but no matter how the engine is tuned surely a higher calorific value fuel will be better or perhaps if the 98 octane burns differently then it won't suit the ignition timing set up for 95 octane can anyone explain?? the theory seems to conflict with people's experiences - or maybe there's a psychological factor at work...

As far as I understand it it goes like this. A higher octane fuel allows for a more advanced ignition or a higher compression ratio before knock occurs. This alone gives the increase in power, not the fuel. So if your car doesn't have knock sensors (allowing it to alter ignition timing depending on the fuel) and you haven't raised the comp ratio then it won't make any difference what fuel you use (as long as its not diesel!).

I have also heard that using higher octane fuel alone can actually slightly reduce power as it has a lower calorific value?!

just to clarify something.
The benefit of having a knock sensor is the ability to run fuel WITH LESSER Octane, not the other way round.
If your ignition is set to max XY the knock sensor will not exeed that value. It will only retard to prevent/stop knocking.
Higher Octane fuel to an otherwise not changed engine will have NO benefit in terms of power. And not all "racing fuels" are manufactured to the same norm as "road fuels".
Once the barrel is opened and the stuff is not used the "Octanes" are making a runner anyway and it's not much better than any other fuel
As mentioned already from Mike, it can have negative effect as well.

But that's all in a nutshell only. There's more to it.

BMG M3 wrote: Reading this months Evo mag's Q&A page, it says..... " most modern engine management systems can sense the fuel RON and advance the ignition timing to make the most of it"

yes but only to a given range. Is there anywhere in Europe fuel available from petrol stations with more than 98 Octane? (don't say Shell V-power now, that's only available in Germany and only since last year).

We sell proper leaded 4 star where I work, but it will kill your cats!

When I was working for a Rover dealer back in 1990, I went on a course for engine management and ignition systems. Thier Mems systems all had knock sensors. BUT, thier systems constantly wound the ignition advanced untill it detected pinking, then back off a few degress, then after a few seconds, advance it again. Mind you, it lead to plenty of burnt out valves and head gasket failures on the old A series (Maestro) and O' series (Meastro/Montego/boxy 216) and M' series (820,220,420) Funily enough, none of the Honda engined Rovers ever suffered from that!

Before mapping cars, I make sure the customer fills the car up with the fuel they are going to be using everyday, especially when they want the "max" out of the engine. Agressively tuning the engine with Optimax then the driver putting in 95 octane fuel may do damage to the engine, even with an knock sensor, as they only retard to a limit too.

But, I agree with Uwe, a higher octane fuel won't really make your knock sensor tell the ECU to advance up the ignition, its only used for "safety" incase the quality of fuel is poor for some reason.

 

Edited by SHEPSM3

Horsetan wrote: My car's so old, it doesn't have any cats!

 You are lucky!

95 E36 M3.0

I have a factory sticker in the corner of the windscreen stating 98 RON or over. Optimax is 97.....!?

I know that the octane rating differs over the world and has something to do with combustion temperatures (i.e. US gas is rated something like 88 but is equivalent to uk unleaded, 93-5)

 

///Mister_G wrote: 95 E36 M3.0 I have a factory sticker in the corner of the windscreen stating 98 RON or over. Optimax is 97.....!? I know that the octane rating differs over the world and has something to do with combustion temperatures (i.e. US gas is rated something like 88 but is equivalent to uk unleaded, 93-5)

Optimax is 98. When it was first introduced in Germany they said 99.....
Now they have Shell V-Power with 100 (and it cost's a fortune)

Optimax is 98 RON. The handbook for the E46 M3 also states to use 98Ron where possible although the car can be run on 95 if need be.

Going back to the orginal question, When I had my old car (mitsi FTO) It actually seemed to prefer the 97 BP Super, seemed smoother somehow. I haven't noticed any difference between BP and optimax in the M3 so I stick to opti as its at the recommeded RON as is usually cheaper than BP where I live too.

Just for info, the Japanese have 100Ron fuel which the FTO was set up for and that worked fine on any grade/make of petrol. Even Jet!

///Mister_G wrote: I have a factory sticker in the corner of the windscreen stating 98 RON or over. Optimax is 97.....!?