Dhadley or Joe (OMC heads)

[bmurray]

Dhadley or Joe, just got my OMC crossflow heads in. What is the difference from these 79 Johnson 235hp heads and my 85 Johnson 150hp heads. They are supposed to be a higher compression head, but what makes them higher? Is it the dome? These are the ones that I will be machining down and polishing the domes. Thanks

 

[Hooty]

Re: Dhadley or Joe (OMC heads)

There's a technique where you plug the sparkplug hole and measure the amount of water the combustion chamber holds and compare it to the other. The one with the least volume is the has the highest compression ratio.<br />You measure the water in c/c's and you can use a large, graduated pipette or syringe.<br /><br />c/6<br /><br />Hooty

 

[Greg Lawrence]

Re: Dhadley or Joe (OMC heads)

any machine shop can cc a head for you... combined with bore/stroke/displacement/and cc you can figure any compression ratio and where to go from there. there is a program called comp calc that can be searched for on the net