70/75hp HIgh Performance Cylinder Head Kit?

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  • July 30, 2019 at 5:19 pm #180505
    johnyrude200
    Participant

      Can someone shed some light on what these were all about…happened to notice this in the parts catalogue for ’84 70/75hp loopers.

      The parts book says the 70hp uses a higher performance kit, whereas the 75hp uses a low compression cylinder head?

      Always wondered what the difference was between these two motors other than the sticker on the cowel? I have noticed the older, smaller C.I. 60/70hp blocks (pre 1989) with the million cylinder head and thermostat cover screws always seem to have much higher compression readings then the ‘newer’ counterparts of post 1988-2001. I always just discounted it to a slightly larger C.I. block post 1988?

      July 30, 2019 at 9:16 pm #180521
      fleetwin
      Participant

        US Member - 2 Years

        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…

        July 30, 2019 at 10:11 pm #180536
        johnyrude200
        Participant

          Ah, so plot thickens.

          I’ll keep my eye out for some 75hp motors and try to learn more about the part number differences along with visual inspection of the head to see if there is a difference.

          Good to know there is in fact a difference between the 70 & 75hp. Wasnt sure if it was just another marketing ploy similar to the 28/30, 48/50, etc….

          July 31, 2019 at 7:00 am #180546
          fleetwin
          Participant

            US Member - 2 Years

            75hps are fairly rare, and they are not meant for the average boat. Usually, they have less mid range power than the 70hp models, and have to wind up higher RPMs to develop the extra horsepower…. So, generally speaking, the 75hp models are not a good choice for most rigs, and they often end up with piston damage due to over propping/lugging on average boats…

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