Author Topic: Comp. Ratio (Read 4737 times)

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!!

Vladi · « **Reply #1 on:** October 18, 2007, 03:15:24 AM »

10:1.

EN318isPDX · « **Reply #2 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.

BrandC · « **Reply #3 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.

tjts1 · « **Reply #4 on:** October 19, 2007, 06:49:51 PM »

Waste of money.

gearheadE30 · « **Reply #5 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.

christophbmw · « **Reply #6 on:** October 20, 2007, 12:52:16 PM »

Quote from: tjts1;36120

Waste of money.

in the short run, untill you ruin your motor.

ChItalian1027 · « **Reply #7 on:** December 19, 2007, 12:14:36 AM »

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

tjts1 · « **Reply #8 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.

gearheadE30 · « **Reply #9 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.

strad · « **Reply #10 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.

sheepdog · « **Reply #11 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.

tjts1 · « **Reply #12 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.

318kid · « **Reply #13 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.

D. Clay · « **Reply #14 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.