Author Topic: Gas? (Read 3863 times)

-eur04lif3- · « **on:** November 22, 2011, 02:00:36 PM »

Which gas are you putting in your m42 and why?

I've been putting regular in my car. Last weekend a had a conversation with another BMW owner, and he was saying that he wont put anything less than premium.

So i did some digging, and found out that high octane contact only has a good effect if the engine is a high compresion one.

Can anyone shine some light on this theory?

-Vic

DesktopDave · « **Reply #1 on:** November 22, 2011, 05:45:05 PM »

He's right. Octane is a measure of gasoline's resistance to burning. It's possible for gasoline to ignite before the spark plug fires. It's called pinging (or pre-ignition). It sounds like a rattle or cackle under high load. Excessive pinging is really bad for the motor. It can blow out head gaskets, crack rings, melt pistons, that sort of thing.

To solve the problem, a lot of cars now have knock sensors fitted. If they note pinging, they retard timing and richen up the mixture to compensate. Later M42's do, but IMHO neither version of the motor is all that vulnerable to pinging.

I've used all grades of fuel, haven't felt much difference in my '91 M42. Several brands of 93 seem to provide the best fuel mileage. It's not much, maybe 1-2mpg highway, but definitely repeatable. I suspect some 93 octane is mixed with less alcohol, perhaps using superior additives?

deansweet · « **Reply #2 on:** November 22, 2011, 08:58:06 PM »

I swear minesays '91 Octane or better' on the inside of the gas door?
Mine seems to like Shell Premium 93 octane...
Oops, it says 'Premium Fuel Only'

-eur04lif3- · « **Reply #3 on:** November 26, 2011, 12:32:15 PM »

Quote from: DesktopDave;108255

He's right. Octane is a measure of gasoline's resistance to burning. It's possible for gasoline to ignite before the spark plug fires. It's called pinging (or pre-ignition). It sounds like a rattle or cackle under high load. Excessive pinging is really bad for the motor. It can blow out head gaskets, crack rings, melt pistons, that sort of thing.

To solve the problem, a lot of cars now have knock sensors fitted. If they note pinging, they retard timing and richen up the mixture to compensate. Later M42's do, but IMHO neither version of the motor is all that vulnerable to pinging.

I've used all grades of fuel, haven't felt much difference in my '91 M42. Several brands of 93 seem to provide the best fuel mileage. It's not much, maybe 1-2mpg highway, but definitely repeatable. I suspect some 93 octane is mixed with less alcohol, perhaps using superior additives?

I'm trying 93 right now, so we'll see if there's any improvement in MPG.
To me is not the MPG that's concerns me the most(m42's have good gas mileage), its the carbon build up.

Is our engine a high comp or low comp engine? (sry, im just a noob to cars)

Quote from: deansweet;108263

I swear minesays '91 Octane or better' on the inside of the gas door?
Mine seems to like Shell Premium 93 octane...
Oops, it says 'Premium Fuel Only'

thanks man

rstimpreza · « **Reply #4 on:** November 29, 2011, 01:48:00 PM »

I go with my owner's manual 91 or better since I live here in Texas we only have 93 so I use that instead. To me the better gas you use the the longer your motor will live and my opinion 87 octane are leftover dirty gasoline.......

jrobie79 · « **Reply #5 on:** December 02, 2011, 01:02:40 PM »

Ive read (no idea where) that it only matters with temperatures over 85 F. I have never put premium in my car unless its over that temp and have had zero problems with pinging

DesktopDave · « **Reply #6 on:** December 02, 2011, 02:29:42 PM »

Quote from: -eur04lif3-;108344

I'm trying 93 right now, so we'll see if there's any improvement in MPG.
To me is not the MPG that's concerns me the most(m42's have good gas mileage), its the carbon build up.

Is our engine a high comp or low comp engine? (sry, im just a noob to cars)

Our engine is 10:1 IIRC, middle of the road for gas motors. My M30 633CSi is only 8.8:1. I've heard of relatively streetable 11.25:1 motors, racing engines go upward from there. Turbo & supercharged motors are less, usually 8:1 or so to deal with very hot forced induction air.

That's nuthin' in the diesel world...22:1 isn't all that unusual.

bmwconnect · « **Reply #7 on:** December 02, 2011, 02:31:31 PM »

Unfortunately due to transportation of fuel in both the tanker and personal vehicle, there is a dilution factor due to the condensation build up as well as other variables.

BMW designed the engine management to run on a bare min of 87OCTANE or RON equivalent. Unfortunately what the pump says your getting in octane value is not always going to be correct and is most cases will be a lower octane value. This is why some people will ping on 87octane and some will not. Until they design some kind of live digital octane measuring device at the pumps, you will never know what your really getting, so 91 is the safe approach for BMW Engine and BMW's insurance policy to keep running car's on the road

My 2003 330ci MS43 engine management actuality tests the fuel grade in the tank by advancing the timing till knock and then transitioning to a different RON spark table - Just to give you a idea of advancement in the newer cars

-eur04lif3- · « **Reply #8 on:** January 04, 2012, 02:53:33 PM »

So, a little update...
Since ive posted this thread, ive been putting Premium Gas (93 octane) in my car. During Christmas i was at my parents and spoke with dad about it(this was his old car) and he said that i shouldnt put it in since the engine cant handle such explosions(keep in mind, my dad is not a mechanic by any sorts).

So i called him bluff on it.

Fast forward to new years eve. I get to my aunts house for the party and on the way there, i hear this noise....
http://www.youtube.com/watch?v=UybAQmbV3-4&feature=g-upl&context=G2ed1226AUAAAAAAABAA

The next morning, back to normal.
http://www.youtube.com/watch?v=-VQP6nHka04&feature=g-upl&context=G2f3f212AUAAAAAAAAAA

Now to today, i put Plus (87 octane) in this morning and i get this noise again. Same knocking noise.

What you guys think/know? Is it have anything to do with the fuel i put in? or is this a separate issue, like the chain tensioner... (my brother has a e30m42 and that was his guess).