WORKSHOP: TWO-STROKE HEADS

Steve Cooper gets the goss on how to sort them.

Even if there is no substitute for cubic capacity when it comes to horse power for any engine, of any size, it’s the cylinder head that determines the bulk of power that is generated. One of the best mechanisms with which to gauge engine output is ‘brake mean effective pressure’ or BMEP. This unit of measurement gives us a very effective benchmark for comparing the performance of an engine against another of the same type along. By swapping head designs on any engine you can ascertain the various impacts of profiles, angles, compression ratios etc. In a nutshell cylinder heads aren’t just lids on combustion chambers; they also have a vital and strategic impact upon how an engine runs and develops power. With four-strokes there are a lot of variables to take into account such as cam design, valve size, valve timing/ overlap and a host of other mission critical factors. With a two-stroke it’s generally only the positioning of the spark plug and the volume/profile of the head that influence power output allied to the size and geometry of what’s referred to as the ‘squish band’. This is the outer periphery of the cylinder head close to the inner edges of the head gasket. Its role is principally to direct the fresh fuel/air mixture towards the spark plug thereby maximising power and efficiency. Not so long ago most tuners were inclined to raise compression ratios whenever possible when they were tuning two-strokes simply because it was a well proven way of increasing the engine’s power. Within limits, the more compression an engine had the better the BMEP figures. Unfortunately with the advent of modern unleaded fuels aimed exclusively at fuel injected four-strokes a lot of what went before in terms of tuning knowledge is out the window. Many of those that build strokers are now actually looking at reducing compression ratios. This generally does away with pre-ignition/detonation and the dreaded knocking sounds that herald damaged pistons and even seizures. Our opening shot shows the typical profile of a pair of Yamaha RD400 heads with a fairly recessed central plug and a relatively narrow squish band. It’s a better design than some period stroker heads but not necessarily ideal. What follows is a series of shots showing original profiles that are giving cause for concern when used with modern fuel, some reworked profiles and the technology that goes into getting the best overall performance out of stroker heads.