Re: how to improve a two stroke
OK! What you have is a VERY old designed block. If it were a Merc or an OMC with loop scavenging or if it even had a boost port, port work would be a lot more "iffy" and difficult. But your engine block is dirt simple, crossflow design, and will respond very well to porting.
The factory drills four ports on the bypass and four on the exhaust side. On the 90, the exhaust ports are elongated by milling but since it is a mass produced engine, there is still more you can do. I'm not sure on the '91 90 HP if they drilled the center inter-port web to join the two center ports and increase area.
First thing to do is to square and trumpet shape the bypass ports, streamlining the inter-port webs to an airfoil shape with about a 1/16 flat inside the cylinder for the rings to ride on. Bevel the bottom of the ports as far as possible to help direct the gas flow up into the combustion chamber inthe head. Even though the piston does not clear the bottom of the port and blinds about 1/3 of the port area, porting and beveling drastically increases the area that the engine "sees" and airflow is substantially increased.
Now: If your engine was a 4 cylinder engine, I would recommend leaving the exhaust side alone. The reason is this: since the port timing is about 120 degrees of crank and since the crankpins are 90 degrees apart, AND since all cylinders dump into a common exhaust chamber, there is about a 30 degree overlap. Porting the exhaust on a 4 cylinder Force will actually INCREASE exhaust blow-back into the cylinder and reduce performance to less than stock UNLESS the engine is set up for race purposes with zero backpressure.
That said, the three cylinder 90 is about 120 degrees port timing and 120 degrees crank angle so there is no overlap and very little exhaust blow-back.
Again, square the ports and trumpet shape the exhaust cavity side. Trumpet shaping is very important since in fluid dynamics, a trumpet or tapered shaped aperture will flow much more than a straight walled aperture of the same size. Since the exhaust side runs much hotter, leave a bit more meat on the inter-port webs
The work done with a Dremel and a straight carbide bit for the steel liners and a triangular bit for trumpet shaping the aluminum will take about 2 hours per cylinder. Run the dremel about 1/2 speed and constantly dip the bit in cutting oil otherwise it will gum up with aluminum.
You will see a few MPH increase in top speed and you will notice a slightly better fuel mileage at PART THROTTLE since the engine is working less hard to pump air. At full throttle you will notice a significant decrease in fuel economy since you are generating more horsepower.
Combine the port work with Boyeson reeds (which I have never used) and you will have a nice little package which should just about keep up with the 4 cylinder 90 HP Mercs.
Now, after all this comes the warning: Port work MAY increase the chances of ring catching. However, I have a ported 90 that now has probably thousands of hours on it and a 140 that I take onto big water without worry. Smatter of fact, the 90 is in the avatar above, running on the Chesapeake.
Now, the only photos I have of port work are over in The Chrysler Crew. If I try to import them here, the quality is very poor and almost unusable. SO: If you want to see them, Google The Chrysler Crew, join, scroll down to the questions forum, then on the left side of the screen, click on photos or photo albums. Look for Frank's Boats and scroll through until you get to the porting photos.
Oh, yeah! Force made two different types of reed blocks. If your engine has the plain 5 petal (per side) Vee blocks, improvement can be had by changing to the rubber coated 4 petal Vee blocks. They seal better and I think the reed petals are about 1 or 2 thousands thinner for more responsiveness. You need to get them off a junker though since new, the price would be more than your engine is worth.
