Reply To: 70/75hp HIgh Performance Cylinder Head Kit?

[Don]

OK, just looked at the 84 parts book on marinengine.com
The “low compression” kit is used on the 75hp models to “detune” them a bit, making them more tolerant of modern fuels….The 75hp models did NOT come with this kit/head…I’m not sure if this is the just the 70hp head, or a completely different head that could be used on both the 70 or 75hp models. These were the years that OMC started to detune/rerate engines in an effort to make them more tolerant of lower octane fuels. So, you will find similar kits used on V4/V6 engines.
The high performance cylinder head kit is another story that I am not totally sure of. This kit might just have the higher compression head used on the 75hp models, or it might have an even higher compression/squish head, which could be used on either the 70 or 75hp models…
The important thing to remember here is that the crankcase porting is different on the 70vs75hp models, so slapping a 75hp head on a 70hp won’t make it a 75hp….The carbs are different also…
The “kit part number” is not the cylinder head part number used in the kit, so I don’t have anyway of comparing all the cylinder head part numbers to see what heads were included in the kits.

The 52CI block is a completely different animal. And, it was designed to survive on lower octane fuels, so I’m not surprised it has lower compression readings…Keep in mind, the new block was introduced in 1986 on the 70hp models, but the 60hp models still used the small block up thru 1988. It is important to be able to recognize these different animals just by looking at them…