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Hemi Head Design

The stock head many two stroke engines are equipped with are usually left with casting flash, dips and pockets which can be improved upon - that's easy to fix. What's not so easy to fix is a cooling system that suddenly becomes inadequate due to the increased power output after an engine build. Enter the after market head. But that's only the most obvious reason to make the change.

The combustion chamber shape impacts not only the way the engine delivers power but how efficiently it can get rid of excess heat, or better still how it can better put that heat to work. It is not uncommon to improve throttle response, overall power and peak power, thermal efficiency and fuel economy with a chamber shape designed for the specific riding style. An engine can be built to a higher output specification if its cooling ability increases as well. So, it's compression ratio can be higher and still allow it to run on the same octane gasoline.

The factories make wonderful engines and they generally top them with good heads. The products are delivered for a very well targeted specific riding style and minimum safe octane. In most cases if nothing were changed the engine would operate as intended. When we start changing things, like pipes and porting to make the engines the way we need them, the head shapes too must be changed.





In some cases the change from a safe pump gas head (usually a tub design of sorts) to a design that's more spherical is a change for the better and often yields more gain than the sum of its parts would seem to indicate - and it will still run well on pump gas as long as the compression ratio is kept within check. The spherical, or hemi spherical head design is not new technology but its principles are sound and advantages specific. They are possible because of the many things outlined in these other posts. Perhaps the most important of them is that it allows the spark plug to be placed directly in the center of the combustion chamber. Simply stated, it is the perfect place to start combustion.

Combustion chamber design may be the most misunderstood component within the modern two stroke engine. It's been said that the chamber shape can "make or break" an engine design. It is because of this that it can not be overlooked or ignored. In fact it's a great place to start looking when looking for more power.




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