On page 117 of "Two Stroke Tuners Handbook" Gordon Jennings wrote how one could grind transfer and exhaust port ramps into the piston crown to test porting ideas without grinding on the more expensive cylinder. He suggested it as a temporary trial for porting without mention of the aspect I would like to focus on here.
Flow Restiction in Ports
His focus was on rounding the edges of the port whereas mine is on rounding the edges of the piston top where it is adjacent to the transfer ports as a means to reduce flow restriction due to the near-90-degree edges. What I did was I ground down the piston edges (adjacent to the ports) .8mm at a 40 degree angle to horizontal. Actually I made the transition curved somewhat at the beginning and end of each "ramp". The more the edges are "in line" with the desired flow, the more efficient and less restricted that flow will be. In other words, for the same area of opening you can have more flow as if the piston had already moved farther down and out of the way.
To be safe this mod should only be done when there is at least 2mm between the crown edge and the top of the piston ring. (Mine has 2.8mm)
What I did was I ground
down the piston at a 40 degree angle to horizontal
that gave the equivalent of .8mm higher at each port. And I made the transition curved somewhat at the beginning and end of the
"ramp". For the same area of opening you can have more
flow as if the piston had already moved farther down and out of the way.
Jennings had wrote that differences and inconsistencies of the transfer ports tops mostly affect just the low and mid range
power because at top rpms the time that those
inconsistencies are significant is too short. So in regard to the
transfers intake flow being less restrictive with this
mod I would say Jennings has explained why the biggest effect is not in the high rpm range. The engine doesn't hardly recognize this mod at all above 7000 rpm. It really affects only the lower rpms which is how it broadens the powerband without changing top rpm porting. It also bleeds off the crankcase pressure at low rpms which allows the majority of the intake charge to enter the cylinder at a lower speed so that it doesn't loop around and exit the exhaust port as much. This allows more of the intake charge to stay in the cylinder for combustion which is how it increases power at lower rpms.
My final thoughts is that this .8mm ramp will have the greatest effects on engines w/o a high peak rpm. The higher rpm an engine is ported for, the less noticable will be this mod because the lower rpm range that this mod effects would be farther from the existing powerband. Someone with a 14,000 peak rpm engine tried this and reported that he got no benefit. My engine peaks at 7500 rpm. So any non-screamer engine with enough space on the piston to put a ramp on it can probably benefit from this mod. On my 55cc a .8mm piston ramp adds 6 degrees transfer port duration. For the same duration change a Honda 125 (with 54.5mm stroke) needs a 1.1mm ramp, and a Honda 250cc (with 72mm stroke) needs a 1.45mm ramp. Also, the longer the piston stroke (ie: larger engine), the faster the piston speed at the same rpm. I believe that the ramp effect is not realized at high rpms. So if my 55cc engine gets the most benefit at less than 3000rpm then a Honda 125 gets the same benefit below 2100rpm and a Honda 250 at below 1654rpm. These figures depend on the piston speed at transfer opening.
With four strokes taking over the motocross world I think the last remaining competing two strokes need every little trick in the book to be competitive again. 4 strokes main advantage is the wide powerband they have. This mod restores some width to the 2 strokes powerband.
Vito's superstock pistons have a bevel on the exhaust side that is 1.5mm down at the side and 2.5mm from the ring. One customer of theirs told me it increased his Blaster's "power everywhere" but Vito's don't make any claims other than to say it raises the powerband. [A Blaster has a 195cc single cylinder engine with 57mm stroke.] A person on www.bansheehq.com wrote: "I used them in my cousins Banshee a while ago. Put about 4-5 hard seasons on them. He claims he could notice a seat of the pants difference in power. He did pretty well in his heads up class. Only thing that beat him was a CRF450 dirt bike. Also used one in one of my Blasters. I could notice it being a lil more peppy. It also held up quite well. As far as I know it's still going strong and that was about 6-7yrs ago."
But I don't recommend a piston ramp for the exhaust since it would have the opposite effect in that it would benefit high rpm power and lessen low rpm power. At slow piston speeds the port (exhaust or transfer) would have a greater port duration. At the transfers that would benefit low rpm power by lessening the pressure at port opening. (The higher the transfers are, the less the crankcase compression ratio.) At the exhaust that would only benefit high rpm power at the expense of low rpm power.
The higher the crankcase compression ratio, the more of an effect this mod would have. To accurately determine your ratio click here.
Unfortunately the 48cc and 66cc have really good transfer port timing that would be thrown off too much by addition of transfer ramps. If you buy a taller piston or lathe off 2mm from the cylinder base then you can do the ramps. Somewhere in these pages I list where you can buy a 1mm taller piston for the 48cc. With it you can add 1.8mm ramps. The 55cc and 60cc cylinders have too low transfer ports that allow you to utilize 2mm transfer ramps. Also raise the 55cc transfers 1mm, and the 66cc transfers .5mm higher for a perfect port duration of 117 degrees. Then for perfect exhaust port duration of 157 degrees raise the 55cc to 25.45mm and the 60cc to 25.6mm from cylinder top.