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Small Air Cooled Gasoline Engines

Engine design and octane.

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Old 12-23-2009, 11:57:09 AM
Jim M. Jim M. is offline
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Default Engine design and octane.

All other things being equal (displacement, HP) can an older flathead engine burn a lower octane gasoline than a newer OHV engine? My Wisconsin AKN manual states to use a gasoline with at least a 74 octane (research method) rating. It would seem to me that older engines might be better able to burn older stale fuel that might adversely affect todays OHV or even an OHC engines. Sound reasonable? Jim
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Old 12-23-2009, 12:10:20 PM
makoman1860 makoman1860 is offline
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Default Re: Engine design and octane.

Jim, yes and no. Flatheads by nature usually have lower compression ratio. However the combustion chambers arent very stable. So its a trade off. Cant really dictate octane requirements by overall engine design alone.
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Old 12-23-2009, 12:16:20 PM
Kevin Pate Kevin Pate is offline
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Default Re: Engine design and octane.

IIRC I think the fuels back then weren't as well refined as what we have today. If you wanted the good stuff you either had to get the ethyl or av gas at the airport....
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Old 12-23-2009, 02:32:35 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Engine design and octane.

Compression ratio, timing, and speed of engine operation all affect the octane needs. Most of todays autos use smaller faster running engines which need at least 87 octane to avoid pre-ignition and detonation. Your Wisconson would be hard pressed to hit 3500 RPM, being much more comfortable pulling at 1800 to 2000 RPM instead of the 5 to 7 thousand that the auto needs. There are reasons though why you would want to use a higher test fuel now days though. First and foremost, the E-10 crap fuel they call gasoline is anything but gas. It is a mixture if impressed volatile componants like Iso-propane, iso-butane, Esthers and etherated materials and a minimum of 10% ethyl alcohol, as well as liquids. All of these volatile materials evaporate from standing fuel, especially in unsealed containers like your machines fuel tank and the carb. Todays 'gas' also degrades due to the fact that it is Hydroscopic - that is, it absorbs water from the humid atmosphere, into the solution. The absoebtion hastens furl degradation to the point that after 30 days, the fuel has lost 50% of its burning capability, and after 60 days, it has seperated into 2 liquids that are both corrosive, and will not burn. Not only that, but the matreial will cause galvanic action between dissimilar metals like white metal (zinc), aluminum, brass, steel, and iron. This battery action will destroy brass floats, and eat zinc and aluminum like it had been dipped in sulfuric acid! Even fuel stabilizers will not stop the water absorbtion or most of the evaporation problems, although it will slow but not stop degradation. your engine may run on this panther pi$$ fuel longer than an auto engine would like to, but the harm done to the carb abd fuel system parts negates any savings. most small engine shops are now asking that you use a high test fuel, as it does not degrade as fast as regular. Sthil and other chain saw manufacturers are now demanding that hi test now be used, as the E-10 crap fuel has undependable alky levels. In testing different brands, alky content has been measured at up to 27% In a 2 cycle engine, this alcohol content is deadly to the internal componants, due to oil breakdown. Auto mileage will suffer as well, as alcohol does not deliver the same power that gas does.

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Old 03-05-2010, 03:59:39 PM
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OTTO-Sawyer OTTO-Sawyer is offline
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Default Re: Engine design and octane.

All things being equal, Displacement, RPM, & Compression Ratio, running the lowest octane (cheapest) fuel you can without getting detination (knock) and you will make the most power available from a given engine (at its rated RPM).

BUT, even though a low compression, low speed engine can run just about anything that burns, it will run smoother and have more low end torque with a higher octane rating which is less explosive and burns slower (or some old stale gas which is also less explosive and burns slower).

Low octane fuel has a more violent explosion than slower burning high octane fuel, and in a low speed (hit & miss) engine is what causes an engine to jump every time it fires and suddenly fling the flywheel around coasting the rest of the way, and hammers on the rod bearings. High octane fuel that was designed for high compression engines has a slower flame front to control detination (the sudden violent explosion), and in a low speed low compression engine will ignite without the violent explosion and continue pushing the piston all the way through the power stroke instead of hammering on the piston and rod at the top of the stroke.

A high compression motor needs high octane to control detination. A High Speed engine would actually make more power if it was set up for dual/fuel where it could run high octane at the lower RPM and switch over to a lower octane at the higher RPMs as the slower burning high octane fuel is still burning while it's going out the exhaust pipe at high speeds.

If it's limited use and you don't mind spending the extra money, go for the higher octane in an older engine, and the engine will live a longer happier life. If it's a daily use engine, and ecconomy is a bigger concern, then go with the lowest octane rating (cheapest) fuel that it will run on without knocking and/or jumping around, and your pocket book will be happier. If it's occasional usage and you want to prolong its lifespan, find the lowest octane it will run on and then bump up the octane rating a couple points to smooth it out without totally breaking the bank.
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