Chris-PA
Where the Wild Things Are
The concept is pretty straightforward: The exhaust port opens first and cylinder pressure starts to drop as gas flows out. But the transfers open quickly after and there is still residual pressure in the cylinder. If this pressure exceeds the case pressure then exhaust will flow down the transfers, and the arrival of the fresh charge will be delayed until this exhaust gas is pushed out of the transfers.
Some time ago there was a bit of controversy about this topic in a 661 thread (of course), as Stihl had released a document claiming the 661 incorporated this technology. My thought at the time was it probably did to some extent, as all chainsaw engines will likely have some of that going on, but that it was a typical case of Stihl marketing putting a fancy name on normal stuff (i.e. "Intellicarb"). I also though that if you wanted to enhance this effect you could alter the port timing in fairly obvious ways - although it would likely not be optimum for power.
Recently I came across an engine that did exactly what I was thinking about - it's a garden variety 42cc Poulan engine from shortly before they went with the present strato design. This saw used a very restrictive cat muffler. Here is the timing:
The exhaust is fairly high for a stock saw, and it has only 14deg of blowdown. The intake duration is pretty short, which provides a lot of case (primary) compression. So what happens is that the exhaust opens into a restrictive muffler while the cylinder pressure is still high, and the transfers open very shortly after. Because of the restrictive muffler you'll get a high pressure pushing exhaust down the transfers, and I assume the big case compression angle is to make sure it will eventually be overcome.
If this were how it worked then you might expect to see a lot of exhaust deposits in the transfers - and indeed it has that. These are open transfers so the side of the piston is exposed as well. I had already started cleaning up the piston before I thought to take these pictures:
There are basically exhaust deposits all over the bottom end of the engine like it were a muffler. This is a non-plated engine and is in good shape other than the carbon, so I don't think it's a sign of abuse.
Fortunately they canned this turd in favor of the fresh air strato design. The good part for me is that the transfers are already high, so I can raise the exhaust and not have to bother with them. On the other hand, the combustion chamber is clearly larger than on older cylinders, and the transfer runners are narrower/lower volume - that last is probably not a bad thing.
Some time ago there was a bit of controversy about this topic in a 661 thread (of course), as Stihl had released a document claiming the 661 incorporated this technology. My thought at the time was it probably did to some extent, as all chainsaw engines will likely have some of that going on, but that it was a typical case of Stihl marketing putting a fancy name on normal stuff (i.e. "Intellicarb"). I also though that if you wanted to enhance this effect you could alter the port timing in fairly obvious ways - although it would likely not be optimum for power.
Recently I came across an engine that did exactly what I was thinking about - it's a garden variety 42cc Poulan engine from shortly before they went with the present strato design. This saw used a very restrictive cat muffler. Here is the timing:
The exhaust is fairly high for a stock saw, and it has only 14deg of blowdown. The intake duration is pretty short, which provides a lot of case (primary) compression. So what happens is that the exhaust opens into a restrictive muffler while the cylinder pressure is still high, and the transfers open very shortly after. Because of the restrictive muffler you'll get a high pressure pushing exhaust down the transfers, and I assume the big case compression angle is to make sure it will eventually be overcome.
If this were how it worked then you might expect to see a lot of exhaust deposits in the transfers - and indeed it has that. These are open transfers so the side of the piston is exposed as well. I had already started cleaning up the piston before I thought to take these pictures:
There are basically exhaust deposits all over the bottom end of the engine like it were a muffler. This is a non-plated engine and is in good shape other than the carbon, so I don't think it's a sign of abuse.
Fortunately they canned this turd in favor of the fresh air strato design. The good part for me is that the transfers are already high, so I can raise the exhaust and not have to bother with them. On the other hand, the combustion chamber is clearly larger than on older cylinders, and the transfer runners are narrower/lower volume - that last is probably not a bad thing.
