Quote:
Originally Posted by Red Hawk
Things I noticed after .040 was nocked off and the ports were opened up. It finally kick started easily. The starter motor has a workout now as it passed the compression stroke at TDC. The exhaust note at idle and at RPM was cleaner you can really hear it breathing now and no crack/pop on idle. It also feels like it added 1/4 of the bikes HP range. And you WILL be rejetting for you carb guys.
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Nail on the head. If you really focus on improving the exhaust port, which is by far the worst of the two, you will notice it at the muffler exit even at idle. The exhaust note is sharper - which is a good indication of increased exhaust gas pressure due to the increase in volume.
I still stand by what I say about where most of the improvement comes, which is the exhaust side.
The intake port is honestly not too terrible, mainly just needing some port matching and smoothing out with some material removed around the guide and a bit of blending here and there.
The increase in compression definitely can't hurt, but compression on it's own has never been a big factor in terms of power in my experience with engine building, and has often more been dictated by cam profile and maintaining a certain level of dynamic compression for proper ignition.
The exhaust side is what holds back these little motors the most. The bowl and the roof of the port around the guide is too low and forces the gas to make a very sharp turn right into a giant wall of excess material around said guide at a rather nasty choke point. if I had to guess, 70% of all of the material I removed came from those areas of the exhaust port.
My other suggestion, and one often left untouched by many, is to spend some time on the combustion chamber. At a minimum smooth out all of the sharp edges around the valves. I went a bit further and really blended all of the machinist ridges around the valves into the chamber itself to unshroud the valves as much as possible. This did increase my chamber volume a couple of CC's, but the benefit to flow past the valve into and out of the cylinder should not be under rated. This is how my chamber looked, almost all of this was done by hand except for the light polish job, which is kind of hard to see in the picture.
This is what I started with for a comparison.
