This aint no Hemi

[Mark75H]

By the time Lon Stevens got done chopping the heads off of 6 cylinder Mercs for F hydros & runabouts and welding back on chunks of aluminum to be super padded heads, this is all that is left of the combustion chamber. This chunk of Play-Doh is about 10 cc's. A normal fishing Mk75 combustion chamber is about 26 cc's. Do all the math on the 10 cubic inch per cylinder motor at 10 cc's and you will come up with a full stroke compression ratio of about 18:1. Calculate compression from port closing and you will still be over 15:1, well beyond what you can run on even race gas.

Besides using alcohol, which is only a little help, there are 2 things about the motor that allow it to run with compression that high. First is the relatively small bore, practically no part of the combustion chamber is more than an inch from the spark plug. Second is the extreme amount of turbulence caused by completely filling the intake side of the deflector on the piston. When the piston comes up to the head, mixture shoots up the corners of the deflector pocket and violently swirls around.

[Roy Hodges]

By the time Lon Stevens got done chopping the heads off of 6 cylinder Mercs for F hydros & runabouts and welding back on chunks of aluminum to be super padded heads, this is all that is left of the combustion chamber. This chunk of Play-Doh is about 10 cc's. A normal fishing Mk75 combustion chamber is about 26 cc's. Do all the math on the 10 cubic inch per cylinder motor at 10 cc's and you will come up with a full stroke compression ratio of about 18:1. Calculate compression from port closing and you will still be over 15:1, well beyond what you can run on even race gas.

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Yeah, he showed me all that stuff in his shop , about 32 years ago , something I never thought about ,till now. Even with ALL that compression , those motors were all Hand rope started. 4 cylinders, 6 cylinders ,up to 66 cubes ! No thanks , not for me

[Mark75H]

Some welded in pads, maybe Quincy, but this block is standard bore and doesn't seem distorted. This block would not have the same very small combustion chamber. Probably 16 or 17 cc's. I have this motor together and just haven't set up the burette and checked it - I even test ran it on my Zorkan runabout

[Mark75H]

A rack of different pistons. The only one that significantly raises the compression by itself is the one on the far right.

[Dave S.]

Are those Blue things parts of some old Aztec Indian pottery?

[Roy Hodges]

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Yeah, he showed me all that stuff in his shop , about 32 years ago , something I never thought about ,till now. Even with ALL that compression , those motors were all Hand rope started. 4 cylinders, 6 cylinders ,up to 66 cubes ! No thanks , not for me

...........................He also told me they had very little spark advance with his electronic ignition, like .100" Before top center .

[Mark75H]

That's about where I have to run the 15cc/head gas motor, too