20:1 premix for aircooled 2-strokes?

[clmintie]

Just out of interest... I have some 2.5cc two stroke model motors that run on 4 to 1 methanol/castor. They make stupid hp for their capacity, nearly 1hp/cc at 38000 rpm and the oil is not just there to lubricate, but to cool as well. Most of it is unburnt and takes heat out with it... Messy, but sure works well.

[Reckless]

why would a air cooled engine need a different ratio to a liquid cooled one. Sorry if that's a stupid question but I don't know.

Also some of the guys on here would be running around on the old air cooled kdx's. What are they running at?

They are probably running or should be running a richer mix compared to the modern bike. Because they may/will have steel bores etc vs the more modern Nikasil bores and more modern metals in later engines. Also heat is far better controlled in a water cooled engine. That's not to say they can get away with running a leaner mix due to the better synthetic oils etc they have today. In their day we only had the tts Castrol R and the comparative Castor based 2smoke oils. Remember leaner usuallly means more heat, more revs = More wear. In the old days (air cooled) you had about 5 different piston sizes and you changed them as the steel bore whore out. Then resleaved the bore and started again. To reduce this wear and get good ring seal and compression you ran a richer mix.

[Reckless]

Just out of interest... I have some 2.5cc two stroke model motors that run on 4 to 1 methanol/castor. They make stupid hp for their capacity, nearly 1hp/cc at 38000 rpm and the oil is not just there to lubricate, but to cool as well. Most of it is unburnt and takes heat out with it... Messy, but sure works well.

Your sort of correct, the oil is only there to prevent friction which generates heat. Not really to dissipate heat.
Methanol is another kettle of fish again. Its claim to fame is it runs/burns coooool. So the advantage of running methanol is not so much the fuel giving more horse power. It is what you can do to the engine because you are running methanol. IE much higher compression ratios, advanced timing etc etc. That is why your little engine is producing silly HP not so much because of the methanol itself. This applies from your little 2.5cc engine to drag cars.

Do you get the theme, in all the posts, less heat, less heat.
Air cooled to water cooled =less heat. Synthetic oils =less friction = less heat. Nikasil bores= one ring=less heat. All of the above mean they can raise compression and revs etc to gain HP and performance in a 2smoker.

[Beeza]

Jeez! Looks like that ancient old 20:1 premix really IS the proper way to go on the PE175, then? Even with hi-tech modern fully-synthetic two-stroke oil in the petrol. Whoda thunk it?

Thanks for the advice!

[mc4aregreat]

mmmmmmmmmmmmmm!, ah those were the days.

[Beeza]

Well, I've been known to ride my 850cc Guzzi Le Mans 3 behind someone with Castrol R in his premix for his CZ175 2-stroke just because his blue-haze exhaust fumes smelt so wonderful. And this smell festival went on for 200km at 100km/h and my wrists ached from those Guzzi clip-ons travelling at such a low speed!

[Beeza]

If you're really flat-out racing that 2T motor as hard as it'll go whenever you can, would you perhaps require a 20:1, but if you're a very much meeker sort of trail rider, would you actually be better off with 32:1 or 40:1 of pure synthetic oil mixed in the petrol tank?

[Beeza]

Karters know what works and what doesn't when it comes to aircooled 2 strokes, so your input is very special! Looks like 20:1 premix really is the way to go if you want every last scrap of that low-end pulling power grunt which is always in pretty short supply on a light-flywheeled 175cc stroker! Thanks for the advice!