Re: 1994 Mercury Sport Jet 90HP - Porting for performance?
I port older Forces all the time. It's easy and you will gain 5-10 hp per cylinder. This is on 3 cyl engines. On 4 cyl engines, do NOT port the exhaust side.
Buy a dremel with a flex shaft. Buy a 1/8 inch CARBIDE straight bit and a 1/4 inch High Speed Steel triangular bit with 1/8 inch shafts.
Start with the bypass ports. Use the triangular bit to remove aluminum and the Carbide to remove steel liner. First, square the ports, then streamline them by removing aluminum. Shape the inter-port webs like a streamlined airplane wing with the piston side about 1/16 inch thick and the cover side sharp. Remove aluminum from the bottom of the port by angling the bit towards the front of the block. this does not enlarge the port any more, but it eases the airflow path and the piston "thinks" it "sees" a bigger port. Don't be afraid to remove a lot of aluminum. Run the Dremel at medium speed and use cutting oil. If you run at too high a speed, the aluminum will clog the bits.
Work slowly and carefully. I usually do just one set of ports a night. It should take somewhere around 2 hours to port each side--4 hours per cylinder.
Next move on to the exhaust side. do the same, except shape them like square trumpets. After you are finished, slightly chamfer the inside of the ports to help avoid ring catching.
If you are a gambling man, you can try raising the ports by about .060. I have never done it but I understand that it will produce more power. But this power comes at a higher RPM with a loss in torque. In essence, you are changing the engine from an all purpose engine to a racing engine with best performance over 6,000. Even with the ports not raised, in order to gain the most power, you must let the engine rev up. this may involve a racing impeller that will unload the engine a little--kind of like a racing torque converter with a high stall speed. Again, I have never tried them but I have heard that Boyeson reeds will also give a little more power. If you have the rubber coated reed blocks though, I think new reeds will probably not be worth the money.
But after all this, remember that a jet drive only provides about 80% of the thrust of a prop drive. All that work will yeild only about 5 MPH.
Have fun if you decide to do it. Go to Chrysler Crew, join, and go to photos. Go to Frank's boats album and you will see a 3 cyl I ported. You can clearly see the trumpet shaping and squared ports. This was one of my first and my technique has improved a little since then. BTW: After all this, The above assume that your '94 has an old style Force crossflow engine block. If Merc modified it and it has boost ports, or if it is loop charged, then you will not be able to port for more power.
Nice work on what you have done by the way Frank.