More reason to run higher oil to fuel ratio?

[Chris-PA]

Animation of functionality of the STIHL 2-MIX Engine - Vus TV

Thanks - I couldn't get that link to work, but I searched and found some other animations. I think I get it - they only pull air down the transfer port in reverse and then push it back into the cylinder first, followed by the real air/fuel charge.

As long as it does not pull too much air into the crankcase, then only the normal fuel mix goes into the crankcase. Even if some of the air does get pulled down into the case, you compensate by enriching the mixture. So strato or non-strato should make no difference as far as fuel/oil mixture ratio.

 

[David (saltas)]

sorry about the link with all the spam that you have to kill to get to the 15 second animation

 

[Terry Syd]

I played around with my strato when I first got it and tried blocking off the strato port. It ran excessively rich without the strato butterfly hooked up. What I came to realise was that there was a fair amount of air left in the cylinder along with the fuel mixture from the carb. Once the exhaust port closes, the turbulence of the airflows and the squish band turbulence creates a homogenous fuel/air mixture within the combustion chamber by the time of combustion.

Because you can compensate with the carb mixture, getting the strato port flowing is as important as getting the intake port flowing. If you port the strato ports for more flow, all you have to do is tweak the High speed needle to provide the extra fuel needed to maintain a proper air/fuel mixture for combustion.

 

[Chris-PA]

I played around with my strato when I first got it and tried blocking off the strato port. It ran excessively rich without the strato butterfly hooked up. What I came to realise was that there was a fair amount of air left in the cylinder along with the fuel mixture from the carb. Once the exhaust port closes, the turbulence of the airflows and the squish band turbulence creates a homogenous fuel/air mixture within the combustion chamber by the time of combustion.

Because you can compensate with the carb mixture, getting the strato port flowing is as important as getting the intake port flowing. If you port the strato ports for more flow, all you have to do is tweak the High speed needle to provide the extra fuel needed to maintain a proper air/fuel mixture for combustion.

I can believe that, but in this case the issue is about what's happening in the crankcase in regard to lubrication. If I understand it right, a quantity of clear air is pulled back down the transfer port in reverse, so that when the cylinder begins to fill that clear air is the first thing out. So that's mostly what gets blown out with the exhaust gases.

As far as the crankcase goes, it only gets conventional air/fuel/oil mixture - with two exceptions:

1. Any excess air that is drawn back down the transfers beyond what is needed to fill the runner volume

2. Any extra fuel/oil added to compensate for clear air that stays in the combustion chamber

I figure these will pretty much cancel each other out, which means the crankcase sees a normal air/fuel/oil mix and no increases in the amount of oil should be needed.

Pretty cool - I'd like to have a strato saw.