M42club.com - Home of the BMW E30/E36 318i/iS

DISCUSSION => Engine + Driveline => Topic started by: ChItalian1027 on October 17, 2007, 11:50:15 PM

Title: Comp. Ratio
Post by: ChItalian1027 on October 17, 2007, 11:50:15 PM

hey guys does anybody know the compression ratio for the M42???

also would the two higher grades above Regular Unleaded Gas be ok for the engine??

thanks in advance!!

Title: Comp. Ratio
Post by: Vladi on October 18, 2007, 03:15:24 AM

10:1.

Title: Comp. Ratio
Post by: EN318isPDX on October 18, 2007, 10:55:09 AM

Quote from: kickass95318ti;35962

hey guys does anybody know the compression ratio for the M42???

also would the two higher grades above Regular Unleaded Gas be ok for the engine??

thanks in advance!!

I run a minimum of 91 octane.

Title: Comp. Ratio
Post by: BrandC on October 19, 2007, 06:13:17 PM

Quote from: kickass95318ti;35962

hey guys does anybody know the compression ratio for the M42???

also would the two higher grades above Regular Unleaded Gas be ok for the engine??

thanks in advance!!

Run at least 91 octane, do not put anything with less octane in an M42.

Title: Comp. Ratio
Post by: tjts1 on October 19, 2007, 06:49:51 PM

Waste of money.

Title: Comp. Ratio
Post by: gearheadE30 on October 19, 2007, 07:29:57 PM

Quote from: tjts1;36120

Waste of money.

Huh? If you run the M42 hard on 89, then it will ping badly and will destroy the engine. That is why the manual and gas cap both have stickers saying premium only.

Title: Comp. Ratio
Post by: christophbmw on October 20, 2007, 12:52:16 PM

Quote from: tjts1;36120

Waste of money.

in the short run, untill you ruin your motor.

Title: Comp. Ratio
Post by: ChItalian1027 on December 19, 2007, 12:14:36 AM

so medium and the higher grade of gas are bad??

Title: Comp. Ratio
Post by: tjts1 on December 19, 2007, 09:52:23 AM

Quote from: christophbmw;36165

in the short run, untill you ruin your motor.

147k miles on 87 and I still haven't ruined the engine. I'll let you know when I do. I use 91 when the temp gets above 85f or I plan to spend a lot of quality time at or near WOT.

Title: Comp. Ratio
Post by: gearheadE30 on December 19, 2007, 02:33:34 PM

Quote

so medium and the higher grade of gas are bad??

no, the higher grades are better, hence the cost. The higher octane number means that the mixture will explode at a more controlled point. Pinging and detonation occur when the mixture explodes due to pressure and compression heat before it is meant to.

Title: Comp. Ratio
Post by: strad on December 19, 2007, 03:37:13 PM

My refueling door sticker says, and I quote, "premium unleaded fuel only."  The owner's manual says the same thing.  

Whether that's really necessary, I don't know.  I'll let you guys do the experimenting though.  I'm going to run 91 or higher.

Title: Comp. Ratio
Post by: sheepdog on December 19, 2007, 04:59:38 PM

Quote from: tjts1;39736

147k miles on 87 and I still haven't ruined the engine. I'll let you know when I do. I use 91 when the temp gets above 85f or I plan to spend a lot of quality time at or near WOT.

I have heard from at least 2 guys who have ruined the engine this way, but they ran 87 and 89 all the time. It killed the main and rod bearings.

Title: Comp. Ratio
Post by: tjts1 on December 19, 2007, 05:08:27 PM

I don't see the connection between octane and bearing wear. If octane was the issues they would have burned through valves or pistons long before the bearings would show any wear. I suspect those engines saw other kinds of abuse like poor oil maintenance combined with high load, high rpm, high temps etc.
This engine's achilles heel is the lack of knock sensor. When the air temperature gets up above 80f and the engine is at high load I can hear it knocking occasionally. Thats my signal to up the octane to 91. But any temp below 80f  and it never knocks. If I ever kill the engine it'll be an opportunity for a fun rebuild. I'm not trying to push 87 octane on anyone. By all means, please follow your owner's manual to the letter.

Title: Comp. Ratio
Post by: 318kid on December 19, 2007, 08:58:42 PM

I run 87 and its been fine for me, My dads has 100,000 miles and thats all he's ran, and mine has 180,000 and I've ran about 5,000 with 87. I don't know what the PO put in it before I got it.

Title: Comp. Ratio
Post by: D. Clay on December 19, 2007, 11:10:48 PM

Gas has changed a lot since 1991. Especially the alcohol. They can put up to 15% alcohol in gasoline without any labeling. Avoid knocking or pinging (and any resulting damage) as the E30 does not have a knock sensor. Cars with knock sensors change the timing to prohibit this. However, performance suffers. My car runs fine without knocking or pinging on 89 octane.

Title: Comp. Ratio
Post by: nicknikolovski on December 20, 2007, 02:22:36 AM

Don't know how it is in the US, but down in Australia, we run minimum 95 octane for Bimmer's. I run 98 octane premium unleaded in my car. I noticed slight better fuel consumption with 98 over 95. When I first got my car I ran 91 octane on 1.5 tank refills and it ran like crap. It wasn't pinging just lacking power more than anything. They have even introduced 100 octane with 5%ethanol blend.

Title: Comp. Ratio
Post by: strad on December 20, 2007, 09:13:57 AM

^^just out of curiosity, do you rate by RON or AKI or something else down there?  We go by AKI up here, but RON is usually provided on the pump's label.