Compression ratios in the toy shop

vibrac

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I have been asked recently how my play dough measuring works. Since I had to do some work on my next Egli Comet today I took some pictures:
Dont read from the pictures the measurement was 69cc
which on a 572 motor is 9.2

pdcr5s.jpg
pdcr2s.jpg
pdcr4s.jpg
pdcr6s.jpg
pdcr3.jpg
 

greg brillus

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Push the piston crown into some plastercine or play dough and measure the depression left in it. Tim's method looks good, as it is the total clearance volume you need to know, to work out the CR accurately.
 

davidd

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Push the piston crown into some plastercine or play dough and measure the depression left in it. Tim's method looks good, as it is the total clearance volume you need to know, to work out the CR accurately.

If you are using fluid to measure the volume, you still have to use a Plexiglas plate or the volume will be greater due to the meniscus.

Typically, you use a little white grease to seal the as John is doing. You can also measure the piston in the cylinder to get the number Eddy mentioned. A little white grease around the top ring and push the piston in the cylinder. With a straight edge across the top to the cylinder, push the piston down a measured amount, say one inch. Put the Plexi on the cylinder and fill with fluid from the burette. Now comes the hard part. Calculate the volume of the cylinder for the distance it was pushed down, so for one inch. Subtract that volume from the burette reading and you will have the remainder.

David
 

davidd

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I should have said "With a straight edge across the top of the cylinder" above. Also, John has made his Plexi plate round to fit into the counter bore of the head. A large flat plate across the face of the head will include counter bore volume as part of the combustion chamber volume. You can measure the depth of the counter bore and subtract it out, but it is easier to make a round plate and measure directly for good repeatability. Tim's method does this all automatically, but I suspect he is a major investor in Playdough.
 

vibrac

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I should have said "With a straight edge across the top of the cylinder" above. Also, John has made his Plexi plate round to fit into the counter bore of the head. A large flat plate across the face of the head will include counter bore volume as part of the combustion chamber volume. You can measure the depth of the counter bore and subtract it out, but it is easier to make a round plate and measure directly for good repeatability. Tim's method does this all automatically, but I suspect he is a major investor in Playdough.
Yes I am a Major player:cool:
 

davidd

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It certainly is simple, but I think you would have trouble with the SCTA inspectors who might find your combustion chamber to be 1000.1 cc and disqualify you from getting a record when you measured it at 998 back home. I would guess that John is using an expensive or Class A burette and a thick piece of Plexi.

David
 
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