| [GMCnet] Fwd: GMC: Octane Rating & Detonation [message #134194] |
Mon, 11 July 2011 16:04 |
emerystora
Messages: 4442
Registered: January 2004
Karma:
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Senior Member |
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Here is something that I posted on this subject 7 years ago.
Emery
Begin forwarded message:
> From: Emery Stora <emerystora@mac.com>
> Date: February 27, 2004 10:57:15 AM MST
> To: GMCNET <gmcmotorhome@mailinglists.org>
> Subject: GMC: Octane Rating & Detonation
> Reply-To: gmcmotorhome@mailinglists.org
>
> Someone has pointed out that the GMC motorhome manual calls for an Octane rating of 91 while others say that the Octane rating of 86 or 87, which is Regular gas today, works fine.
>
> The thing that is being overlooked by most people is that the way that Octane is being measured at the pump is different today from what it was when our GMCs were built. So our manuals refer to the old way of measuring Octane.
>
> If you look closely at a pump you might see a sticker that says the octane number and also shows "(RO + MO)/2". This is the Octane measurement of today. It means Research Octane plus Motor Octane with the sum divided by two or in other words, the average of the two methods.
>
> I agree that regular gas works fine in our GMCs and that it is a large waste of money to purchase Preminum -- however, the timing of the engine MUST be set so as not to have the engine knock or ping when using Regular gas.
>
> Octane is a rating of a fuels resistance to ignite. The higher the octane, the harder it is to get it to ignite. A higher octane may be necessary to prevent pre-ignition and detonation in a high performance engine but the engine timing can normally be adjusted for whatever Octane you use. This is especially true in the lower RPMs that we encounter with our GMC engines. Higher octane fuel will generally burn slightly slower than a lower octane fuel which could also require a change in ignition timing. Interestingly, using more octane than you need will not help power, the slower burn rate will actually cause you to lose some power.
>
> Normal combustion will take place at a pretty steady rate (for a given rpm and load). When a large amount of the charge burns extremely fast and uncontrollably, it is know as detonation (or pre-detonation). Detonation can destroy an engine in a matter of seconds. It is common knowledge that excessive heat causes detonation, but heat it not the only cause. Pressure plays a big role in it also. You can get detonation by running with too much ignition advance. If heat was the only cause, signs of detonation would show up around the exhaust valve, but this is not the case. Usually when detonation takes place from too much advance, the detonation occurs on the intake side of the chamber, which is the coolest side of the chamber. This happens because detonation did not occur until the pressure got excessive, which was after the spark occurred. By the time that happened, the charge near the exhaust valve has already been burned. Most of the time, detonation will occur aft
er normal combustion has started. Apart from destroying pistons and spark plugs, light detonation can cause all sorts of other problems, like fatiguing cranks and rods quickly and pounding bearings to death, so avoid detonation at all costs.
> --
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
> --
> GMCnet Guidelines and Information at:
> http://www.gmcmotorhome.com/gmcnetguidelines.html
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>
>
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