Re: Fuel quality and octane level?

Posted by mattofvinings on March 05, 2009 at 16:56:41 from (67.142.161.28):

In Reply to: Fuel quality and octane level? posted by SDE on March 05, 2009 at 15:44:00:

In terms of IC engine theory, you want to avoid a knock event.

Knock is a point where the air/fuel mixture becomes unstable and explodes and releases a concentrated bit of heat. Normal combustion can be thought of as a smooth-flowing "ghost" that "sweeps" thru the chamber.

Main factors that affect knock are octane rating of fuel, how much oil is being consumed (oil has a very low octane rating...80 or below, tampering with the fuel's rating), the compression ratio, spark advance, and any "hot spots" in the combustion chamber ala a "hot" spark plug. Air density (for charging), air temperature, and relative humidity all make a difference.

You can think of knock conceptually as sandblasting your fingertip. It gets "hot" and one time doesn't cause any permanent damage. But if you did this 15 times per second (@ 1800 rpms for example, the crank rolls over at 30x per second and 15 of those are "power" strokes or combustion events), the heat would build up quickly...and a piston that can't get rid of that heat will eventually become aluminum gravel. Sharp edges on a piston for valve reliefs are an area it is difficult to remove heat from quickly and so are at greater risk.

My personal thoughts are that as long as you are not knocking audibly, then 87 octane is fine. Tetraethyl lead of days gone by did wonders for very inexpensively raising the octane rating of gasoline. The Farmall/IHC 4-cyl letter-series engines I've dealt with are all "low compression" in relative terms. The 6-cyl 221cid I just got has a 7.2:1 CR which is considerably more.

As TractorVet says, the duty cycle is important...if you are going to do your own Nebraska testing day in and day out you might want to think about premium fuel. All others I would personally recommend using 87 octane.

This post was edited by mattofvinings at 16:58:15 03/05/09.

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