newb fuel question

Gordo said:

As to "castor in your 2 strokes and synth in the 4 strokes," That's pretty much 100% opposite what the standard thought is. If you look at the 4 stroke fuel it all has castor, which makes sense since a 4 stroke runns hotter. I figure I run my 4 strokes so rich I don't worry about it.

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Nope...I disagree :lol:
Virtually all 4 stroke fuels are 100% synth. Take YS's recommendation of 20/20 thats 20% Nitro and 20% all Synth oil. 4 strokes do not run hotter than 2 strokes either.....why you think that? They combust 1/2 the # of times as 2 strokes and they turn 20% less revolutions. Combustion is where heat is generated.... Why would you think a motor that runs slower and half as long would be hoter? Rather, 4-strokes (that burn alcohol) must "conserve" heat between strokes to stay stable...that was the problem with the first designed 4 strokes where they had to make them big and heavy to even be a little bit reliable.

The issue with heat conservation in a 4 stroke, goes to the point that Spastic is making. The oil must stay at temperatore for longer time.... In a 2 stroke, the oil is hot and is then expelled. In a 4 stroke, the castor sits there and "cokes". The result is a big mess of coked castor which gathers on the exhaust values and in the value cover and builds up rather quickly to reduce the power of the motor. The synth goes not polymerize to the same extent.

Read this

I am a strong advocate of castor....but not in 4 strokes.

3d-aholic said:

Virtually all 4 stroke fuels are 100% synth. Take YS's recommendation of 20/20 thats 20% Nitro and 20% all Synth oil. 4 strokes do not run hotter than 2 strokes either.....why you think that? They combust 1/2 the # of times as 2 strokes and they turn 20% less revolutions. Combustion is where heat is generated.... Why would you think a motor that runs slower and half as long would be hoter? Rather, 4-strokes (that burn alcohol) must "conserve" heat between strokes to stay stable...that was the problem with the first designed 4 strokes where they had to make them big and heavy to even be a little bit reliable.

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But, given that the thermal efficiency of the intenal combustion engine doesn't vary much from one to another, that the amount of heat produced for a given amount of shaft horsepower would be roughly the same, regardless of whether you're firing once per revolution or twice.

But then again, perhaps the two-stroke is vastly less (thermally) efficient than the 4-stroke (hence the greater fuel consumption) and this is why they run hotter.

But -- another thing to consider in the 2S vs 4S lube equation -- a 2-stroke is lubricated with a fresh fuel-oil charge once per revolution -- a fourstroke is only lubed with a fresh fuel-oil charge once every two revolutions.

Also remember that on most two-strokes, a small (but significant) portion of the new fuel-charge is lost out the exhaust port during the overlap of the transfer/exhaust phase. That fuel isn't burnt but it will absorb some of the heat from the engine as it passes through -- thus promoting greater cooling.

I truth is that we could probably speculate all day about what *might* happen inside these engines but the safest route is usually just to follow the manufacturer's recommendations as to oil types and percentages.

xjetBut said:

-- another thing to consider in the 2S vs 4S lube equation -- a 2-stroke is lubricated with a fresh fuel-oil charge once per revolution -- a fourstroke is only lubed with a fresh fuel-oil charge once every two revolutions.

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Yep, that is the crux of the issue.

Castor polymerizes when it sits at that high heat for such an extended "time". In a 4 stroke, oil can sit up inside of the engine not just for additional strokes but for a very extended time. Oil lubing the values, rocker arms, and tubes is for all intensive purposes trapped.

The castor gets up in to the top of the values and sits there for very, very extended times, turning from castor into vanish and then vanish into cooked "coked" black carboned castor. Its not the "amount" of heat...its the "extended" time of heat exposure. In a 2 stroke, the castor is there for only a short amount of time and out it goes. Fresh castor comes in. Due to the short period of time the castor is in there it only polymerized in cases of extreme heat due to poor running conditions. In all other conditions, it goes out only slightly thicker.

Run your 2 stoke at 100% castor no problem. Try running your 4 stroke at 100% castor and it will need to be cleaned within 3 gallons...and it will be all but a mess. I've done it.