Author Topic: fuel grade and comp ratio (Read 577 times)

SFRT - Frenchy · « **on:** December 26, 2004, 08:28:51 PM »

Is it the compression ratio in the engine that defines what fuel grade you should use?

Maverick · « **Reply #1 on:** December 26, 2004, 10:56:19 PM »

Short answer, yes for the most part. That is as far as wheeled vehicles go. If you are referring to aircraft it is a different situation and involves temperature among other things.

Staga · « **Reply #2 on:** December 27, 2004, 03:22:17 AM »

Mostly compression ratio and slightly ignition timing.

beet1e · « **Reply #3 on:** December 27, 2004, 04:34:55 AM »

Quote

Originally posted by Staga
Mostly compression ratio and slightly ignition timing.

...and also altitude. At 6000' a car that runs on 89 octane at sea level might run on 85 octane up in the mountains. When I was working in Denver (1997) the octane ratings were lower because of altitude - which effectively reduces compression because of rarefied air.

In Europe, we've always had higher octane fuels than the US. 95 is normal for unleaded, 98 for super unleaded.

Anyone know if there are octane ratings for diesel?

FUNKED1 · « **Reply #4 on:** December 27, 2004, 05:08:51 AM »

Yep, octane numbers are determined by testing on an engine with a variable compression ratio. Higher rating means it will work in a higher compression engine without detonation. Pressure altitude, RPM, ignition timing, intake air temp, etc., are all held constant for the testing.

Also Beetle the euro fuels are rated on the RON scale and US pumps show (MON+RON) / 2. RON gives higher numbers, so 93 octane at a US station is equivalent to 98 octane at a frog station. Different rating systems, not different fuel.

mora · « **Reply #5 on:** December 27, 2004, 05:17:09 AM »

Quote

Originally posted by beet1e
Anyone know if there are octane ratings for diesel?

There's a setane(sp?) rating which determines how easily it will ignite.

mora · « **Reply #6 on:** December 27, 2004, 05:21:55 AM »

Compression pressure is ultimately what causes detonation. Contributing factor is heat. Heat is affected by intake air temp, ignition timing, and fuel mixture(lean mixture causes more heat).

lazs2 · « **Reply #7 on:** December 27, 2004, 09:13:04 AM »

short answer is yes. ignition timing is criticle and aluminum heads will allow increase in compression because they cool faster. High lift and long duration cams will allow a little higher compression...sophisticated ignition controls have allowed higher compression than with the old carb and distributor. Run the best stuff you can find in your trans am.

an engine that has an overbore will have more compression than the stock bore also. With multiple spark distributors you can get away with a little more compression.

What is your initial and total ignition timing? Is the vacumn advance hoioked up?

lazs

VWE · « **Reply #8 on:** December 27, 2004, 11:15:04 AM »

#2 diesel, which is the highest grade, has an octane rating of 42 in the U.S.

mora · « **Reply #9 on:** December 27, 2004, 12:14:21 PM »

Diesel engine doesn't reguire a high octane rating because the fuel is injected to the combustion chamber at a precise moment, and it's much less prone to ignite explosively.

The cetane rating describes how easily diesel fuel will ignite. If it's too low the engine will not start and/or runs badly. If it's too high it will kill the engine.

Another important factor is fuel waxing which starts to happen just below freezing and will glog the fuel filter. There are different grades of diesel fuel for different temperature ranges.

Nash · « **Reply #10 on:** December 27, 2004, 12:16:38 PM »

Mee ho mah? Ding bok choi rangwa.

SFRT - Frenchy · « **Reply #11 on:** December 27, 2004, 12:34:34 PM »

How high of a compression ratio can I safely run with US 91?

Depending on the heads, I can end up at 440HP with 9.5 CR, or 490HP with 10.5 CR. I would still like to run on 91 pump gas.

indy007 · « **Reply #12 on:** December 27, 2004, 12:50:05 PM »

Toyota Celicas, with the 2ZZ engines, have iirc an 11.5:1 comp ratio. Runs like crap on 87 octane though, very noticeably smoother with 91 octane.

nuchpatrick · « **Reply #13 on:** December 27, 2004, 01:23:41 PM »

Yeah.. I'd say anything with 9.5 compression and higher really need the extra octaine..

I know my Mazda BP-ZE motor with its 10.0:1 needs 93 and anything really below 89 it runs like crap.. And my autocross/track car is also a Mazda with BP-ZE motor with running on 9.0:1 needs 105.. lol..but thats because it has a modified MAF and larger throttle body

lazs2 · « **Reply #14 on:** December 27, 2004, 01:31:08 PM »

I'm running 9.5 in my 468 for a very streetable engine... even so.. it will rattle a little if I gas it at 2000 rpm with full vacumn advance.

my Healey runs 11.5 to one but it has a nasty solid lifter cam and I run octane booster in it.

What are you planning to run for initial timing and total timing?

lazs

Author Topic: fuel grade and comp ratio (Read 577 times)

VWE