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The reason lower octane ratings work at higher altitudes is pretty simple. The ambient air pressure is lower. That lower pressure results in lower chamber pressures (and possibly temperatures), thus less tendency to knock. That's all there is to it.
Predictions are very difficult, especially about the future.
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Starting Member
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Last Login: 7/16/2007 10:21:19 AM
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| Thanks, you guys, that's a great discussion. I learned a lot. Here's a question that verges on the hypothetical (or even science fiction). As we draw nearer to the end of the gasoline era, I suspect premium fuels will be harder and harder to come by. Even now, in the last little "fuel crisis" we had, premium was the first to go. Is there any way to modify a motorcycle (or any other) high compression engine to work properly on regular gas? There have to be hundreds of thousands of bikes and cars out there that are going to be left high and dry if premium becomes tough to get. Also, does gas become "stale" in the underground tank? Someone else told me that a good reason to use regular is that the premium goes stale because it's not sold as fast. True? CP
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Average Member
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Last Login: 11/14/2008 10:08:58 AM
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Just talk to anyone who is running a BMW airhead from the seventies. They were high compression engines that required a MINIMUM octane rating of 98. The first thing that was tried was advancing or retarding the timing slightly to allow for the difference in octane.
Then they came up with double plugging the cylinders which worked better than fooling with the timing and allowed an even lower octane to run in the bikes.
I just went out and bought a few cases of octane booster on sale and added it to my tank. Worked great with no hassles and was a lot cheaper than the other methods.
Today's engines are a lot more sophisticated so those simple fixes probably wouldn't work but I think most are designed to run on a wider range of octane.
A good site to read up on how fast today's gasoline starts to deteriorate is Randaak's Cycle Shak. The site is about maintaining old Goldwings but the section on keeping your gas fresh applies to most vehicles and his observations are kind of surprising.
Now, let me get this right - is it one down, four up?
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Starting Member
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| Hi All, Thanks again for sharing your knowledge. It boils down to this question for me: Is compression ratio the only determinant of required octane for a motor? My Bandit 1250 has 10.5:1 and calls for regular (87 or whatever here in Colorado) My BMW R1150R has 10.3:1 and call for premium unleaded (91) My F650GS has 11.5:1 or so, and certainly calls for premium. My old Kawasaki KLR650 had 9.5:1 and called for premium, as I recall. The new one has 9.8:1 and also calls for premium. I've never had the nerve to do the suggested test of running regular and seeing what happens. So what else is factoring in on fuel octane requirements for these motors? (I just want to go into the future with vehicles that use regular gas) CP
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Junior Member
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"Compression Ratio" is the ratio of the total volume of the cylinder (combustion chamber volume plus swept volume) to the combustion chamber volume. It is a mechanically determined ratio. However, there is another number, the "Effective Compression Ratio" which is more important. Compression cannot begin until the valves are closed. If the intake valve remains open some time after bottom dead center (usually done in the interest of high-speed air flow) then the effective compression will be lower. So an engine with a quoted ratio of 10:1 may have an effective ratio more like 9:1. The most common example of this was seen in the days of two-stroke engines, which often quoted compression ratios of about 6.3:1, measured after the exhaust port had closed. Effective compression is what determines the pressures in the combustion chamber. Combustion chamber shape, charge turbulence, engine speed, local hot spots such as the exhaust valve, and no doubt other factors actually determine the tendency of the engine toward knocking, and thus the octane requirement.
Predictions are very difficult, especially about the future.
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Senior Member
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| TORQUEY: Are you saying that the large station sign advertised one price (cheaper) but the sign at the pump was more (expensive)? They can do that only if the word "CASH" is on the large sign. If you're convinced that the signage at the gas station was wrong...at least here in beautiful downtown New Jersey, you can do something about it. (Law here states that the pump and station sign must match (except if "CASH" is used...and if the price goes down, the pump sign must go down first.) What was done...was this scenario occured. The customer did not pay. He simply sat there...called a cop. Cop verified . The state agency that oversees this type of stuff was called in...THEY verified it...and the station got a heavy fine. And was made to change their singage...RIGHT NOW. Of course, "no speakee English"...came into play, but to no avail. Thankfully. Customer got a 73 cent refund.
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Junior Member
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| On the older air head BMWs the timing was set at about 6 degrees BTDC at idle. Of course it advanced even more at higher engine speeds. The advance or "lead" was necessary to allow the flame front to go from the spark plug, where it starts, to the edge of the cylinder head at the proper time. In a hemispherical design that's a fairly long distance along the surface. Twin plugging a BMW allowed the timing to be set at 0 degrees BTDC at idle with a subsequent retardation effect at higher engine speeds as well. Starting the fire at two locations allowed the flame front to spread completely in less time, hence less advance required. Twin plugged BMWs usually ran cooler and smooher. They would run on regular easily with the modification though and with minimum pinging. The second plug was a smaller plug, 10 or 12 mm rather than the original's 14 mm size -- this to fit into the combustion chamber. New coils were needed too, coils with dual towers so as to fire two plugs from one source at the same time. Originally the BMW of that era used two coils, each a 6 volt unit, wired in series. The new coils were also wired in series, but were now required to be dual tower coils. Ray Nielsen, in MInneapolis
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