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« on: January 25, 2014, 08:15:56 PM »
I get a lot of calls from people who want to mismatch a cylinder designed to work with the stock stroke to a stroker crank. The first problem with that is cylinder length. The cylinder is too short for the increased stroke resulting in the piston coming out the top of the cylinder. Of course these engine building geniuses have an answer for that problem. The obvious answer is to put a spacer plate under the cylinder. This creates the next problem, port timing. Increasing the stroke increases the port timing and because of rod angularly it increases transfer timing more than the exhaust timing. Even if you can space the cylinder to get a reasonable exhaust port timing, the transfer timing will be way too radical for good performance. The third problem is the port floors need to be dropped so the floor matches the top of the piston at bottom of its stroke. That is not usually a problem for the exhaust port. It is a problem for transfer ports. In most cases the port window too tall in proportion to the area of the rest of the port. This causes scavenging problems in the cylinder. It is important that the transfer streams attach to the top of the piston and travel to the rear cylinder wall in a coherent column. When the window is out of proportion to the duct, the streams become disorganized and turbulent. This causes charge impurity and charge loss out of the exhaust port. This will cause the engine to perform poorly in the low end and midrange part of the power band. So you have a peaky engine that is down on power.
The fourth problem is the combustion chamber shape. Any engine that employs a bore able cast iron liner needs the combustion chamber machined larger than the bore. This is so the piston does not hit the head after being bored to the last over size. If the piston is coming out the top of the cylinder the chamber must be machined so the piston does not touch the sides of the step in the chamber as well as not hitting the squish band. This means a lot of volume around the outside edge of the chamber at top dead center. This destroys the important function of the squish band. The squish band not only imports motion to the fuel charge, it also dissipates heat out of the end gases of the combustion process. This is very important because if the end gasses over heat, they explode rather than burn. That is what we refer to as detonation, and it destroys engines.
If you wish to build a stroker engine use cylinders that are designed for stroker crankshafts. Sphinx and Puma cylinders a available to work perfectly with stroker cranks. If you insist on using the stock stroke cylinder, the cylinder can be lengthened with a longer sleeve and a spacer plate for the top of the cylinder. This allows me to correct the port timings and shapes. It also allows for proper combustion chamber shape.
I hope this clears up any misunderstanding about 2 stroke stroker engine combinations.
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