The 101 Association, Inc.
For the preservation and enjoyment of 1928 to 1931 Indian Scout Motocycles
"You can't wear out an Indian Scout"

compression test on 37ci

  • 10 Jan 2015 9:40 PM
    Message # 3187963
    Deleted user

    I'm hoping the shop manual I ordered will help with this but can anyone tell me what the compression should be on my 1926 Indian scout 

    Last modified: 10 Jan 2015 9:41 PM | Deleted user
  • 12 Jan 2015 5:08 AM
    Reply # 3189596 on 3187963

    About the same as a surprising burp from drinking a diet coke! :-) No actually I havent tested the pressure. A good, (sometimes better) test is the blow-by test where you apply a constant air pressure and measure the amount that leak past rings and valves.

    Last modified: 12 Jan 2015 5:13 AM | Carl-Erik Renquist
  • 31 Dec 2020 10:39 AM
    Reply # 9538837 on 3187963
    Tim Raindle (Administrator)

    Just for a frame of reference, if anyone else is asking this compression test question, I have been doing a little nosing around.

    Carl-Erik is of course correct, a leak down test is more relevant, but a compression test can be handy as well. The main issue it will tell you is if both cylinders are reasonably similar. If they are way different, you have an issue that needs checking. 

    Secondly, it is difficult to test under ideal conditions anyway, as kicking a bike over is not the same as running it, as intake flows will be quite different in the two situations. ( theoretically you should be testing with the throttle fully open, to allow unrestricted flow, but it may well not make much difference with a Schebler or linkert, would be interesting to know)

    Also, tests will be higher if the motor is fully warmed up.

    So, bearing all that in mind, we have tested a few 45 scout motors of the last couple of months just because, and they all seem to come in at somewhere around 62-65 psi cold. This would lead to a ballpark compression ratio of around 4.2 to 1, so possibly hot and under running conditions that would probably be a comp ration of 4.5 to 5 to one. Might get around to testing that hot. 

    I am not exactly sure what a Scout out of the box in 1928 should be, I have the swept volume and head volume etc written down on a scrap of paper from a few years back, but I never considered it particularly relevant. Bruce Artesinger at Enfield Racing quotes a stock Sport Scout  as 5.2 to one.

    I look forward to hearing from someone who approaches this a little more scientifically, I have a few personal theories about higher octane fuels and efficient combustion that I may well get around to testing out one day, or maybe not :)

    Happy New Year .


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