![]() ![]() Most of the gasses aren't lost because a compression pressure wave has reflected from the baffle cone of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the exhaust port. Now the loop of unburned mixture gasses have traveled into the exhaust pipe's header section. The more unburned gasses you can squeeze into the combustion chamber, the more the engine will produce. That is the key to making more power in a two-stroke engine. It is critical that the burnt gasses are scavenged from the combustion chamber, to make room for as much unburned gasses as possible. The gasses travel up the backside of the cylinder and loops around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. Now the transfer ports are uncovered at about 120 degrees after TDC.The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase. The blow-down phase has started and will end when the transfer ports open. A pressure wave of hot expanding gasses flows down the exhaust pipe. At about 90 degrees after TDC the exhaust port opens ending the power stroke. This pressurizes the crankcase causing the reed valve to close. Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. ![]() The following is an explanation of the basic operation of the two-stroke engine. If a two-stroke engine could retain the same percentage of air, they would be twice as powerful as a four-stroke engine because they produce twice as many power strokes in the same number of crankshaft revolutions. Some of the air is lost out the exhaust pipe. Two-stroke engines aren't as efficient as four-stroke engines, meaning that they don't retain as much air as they draw in through the intake. This is necessary because a two-stroke engine completes a power cycle in only 360 degrees of crankshaft rotation, compared to a four-stroke engine, which requires 720 degrees of crankshaft rotation to complete one power cycle. This article is an overview of how porting is performed and how it can benefit your performance demands.Īlthough a two-stroke engine has fewer moving parts than a four-stroke engine, a two-stroke is a complex engine with different phases taking place in the crankcase and in the cylinder bore at the same time. Yet the tooling is easily available and the design of the ports is actually quite straightforward with resources like computer design programs. So much myth and misinformation is associated with this complex machining and metal finishing process. The people in the market to buy it are looking for information and the people in the market of selling it are hiding information on porting. The process of cylinder porting is a funny paradox. ![]() In any case, I hope this expands your two stroke tuning minds as it did mines: ![]() There's some tips I used in the past as well as some new information I didn't know. It explains the two stroke process pretty well. I found a good one on a Singapore scooter site who got it from a UK two stroke racing site. I'm always on the prowl for a good read about two stroke porting. ![]()
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