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  #11  
Old 14/06/2017, 09:34 AM
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Phil Hyde Phil Hyde is offline
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Good advice indeed.

I am intrigued how you are measuring the compression ratio? Not something you can measure on a gauge!

Last edited by Phil Hyde; 14/06/2017 at 09:35 AM. Reason: spellinn
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  #12  
Old 14/06/2017, 04:14 PM
Mpb1p Mpb1p is offline
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Default Re errr

I have done
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  #13  
Old 14/06/2017, 04:53 PM
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cray cray is offline
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Like Phil has mentioned you do not measure comp ratio with a gauge.
Suggest you google it!

Colin
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  #14  
Old 14/06/2017, 05:17 PM
Mpb1p Mpb1p is offline
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Thanks , I am not a mechanic so am a bit confused with all of this stuff,
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  #15  
Old 15/06/2017, 07:06 AM
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Colin Sparrow Colin Sparrow is offline
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Quote:
Originally Posted by Mpb1p View Post
I have done
Thank you and well done. I had a feeling that might be the case. Contact Rob to get an upgrade so that you can access the rest of the forum. Good luck with the project.
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Colin Sparrow
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  #16  
Old 15/06/2017, 01:00 PM
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Phil Hyde Phil Hyde is offline
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Very basic CR measurement.

Spark plug out, piston at TDC. Using a suitable liquid (thin oil) measure the volume required to fill the chamber to the bottom of the spark plug thread.

Calculate the swept volume of the engine pi x radius squared x height.

Divide swept volume by chamber volume.
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  #17  
Old 15/06/2017, 08:19 PM
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dave higgins dave higgins is offline
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Not quite right phil, you must add the chamber volume to the swept volume, then divide by the chamber volume to get the correct figure.
As when the piston is at bdc, the total capacity of the cylinder area to be compressed must include the chamber volume to start your calculation with.
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  #18  
Old 16/06/2017, 09:15 AM
Brian Catt Brian Catt is offline
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YES BUT>>>> I had a pal who tried to measure his CR on a 2 stroke and "Filled the cylinder" and wondered why his engine took far more oil than the nominal 250 + combustion chamber....WE had great fun trying to get it all out again!
Just bring it to TDC, add the oil (x) to the combustion space and once you have that figure, add that to your nominal capacity, so x+ 250 (or whatever) divided by x should give you the CR. WELL no it doesn't!!! This is called the nominal CR, not the actual because the compression will be lower due to silly holes that the manufacturer insists in making in your liner! Some people will calculate CR from Exhaust close on the upstroke, so will get a much lower figure.
Compression pressure should be taken AFTER removing the oil you dripped in trying to get your CR!

Last edited by Brian Catt; 16/06/2017 at 09:23 AM.
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  #19  
Old 16/06/2017, 07:32 PM
Mpb1p Mpb1p is offline
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Default Compression ratio

Well thanks everyone for the advice but this is getting a bit too complicated for me... Can anyone suggest the correct spark plug ?
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