static compression help
10-12-2009 | 03:08 PM
#1
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From: warminsterwiltshire, UNITED KINGDOM
static compression help
need help working out static compression for my 21, i got 13mm for stroke and 16mm for bore. i measured squish band to be 19 thou, cant measure combustion camber but not big, can anyone recommend a close chanmber size for a 21 standard plug not turbo. if anyone can work out or show me that would be fantastic
cheers
cheers
10-12-2009 | 09:09 PM
#2
RE: static compression help
Measure the stroke again because it should be closer to 16mm. With the 13mm stroke the engine size comes to 2.6cc (.16 cub inch) but with a 16mm stroke it's 3.22cc (.195 cub inch). Most engines have a very similar bore and stroke.
Assuming your engine really is 16mm (bore and stroke) then it's 3.22cc, and I'm going to work in metrics here. The .019" (.48mm) squish clearance gives a volume of near enough 0.1cc which is part of the combustion chamber volume for figuring out the compression ratio. The combustion chamber bowl in the head button will be somewhere fairly close to 0.3cc but this has to be measured quite accurately and that's the hard part
. Now, assuming it comes to 0.3cc you add that squish volume of 0.1cc for a total combustion chamber volume of 0.4cc. To find the compression, add this 0.4cc to the engines swept volume of 3.22cc to get 3.62cc. Now divide that by the combustion chamber volume (3.62/.4) = 9.1 which is the compression ratio.
As I said before though, the hard part is measuring the volume of the combustion chamber in the head acurately. If you miss by just .01cc then it'll change the calculated compression by around +- .2 so instead of finding 9.1 it could be anywhere from 8.9 to 9.3 which isn't really drastic but shows how you need to be quite accurate. The only way to do it is by using a syringe graduated to about that .01cc accuracy and carefully filling the combustion chamber until it's dead level with the outer edge of the squish band. Obviously you must have a plug fitted and this will also fill the plug cavity. Some do this by placing a plastic sheet over the head with a small hole in the centre for the syringe needle to go through and with a thin layer of grease to seal the plastic against the edge of the squish band.
I made my own "syringe" by using clear plastic tubing with about a 1/16" internal size and calibrating it so I knew exactly how much liquid was in a particular length as I dripped it into the head. With my tubing there's .0256cc for every centimetre it drips out and I watch a reflection off the surface to see when it's full.
Assuming your engine really is 16mm (bore and stroke) then it's 3.22cc, and I'm going to work in metrics here. The .019" (.48mm) squish clearance gives a volume of near enough 0.1cc which is part of the combustion chamber volume for figuring out the compression ratio. The combustion chamber bowl in the head button will be somewhere fairly close to 0.3cc but this has to be measured quite accurately and that's the hard part
. Now, assuming it comes to 0.3cc you add that squish volume of 0.1cc for a total combustion chamber volume of 0.4cc. To find the compression, add this 0.4cc to the engines swept volume of 3.22cc to get 3.62cc. Now divide that by the combustion chamber volume (3.62/.4) = 9.1 which is the compression ratio. As I said before though, the hard part is measuring the volume of the combustion chamber in the head acurately. If you miss by just .01cc then it'll change the calculated compression by around +- .2 so instead of finding 9.1 it could be anywhere from 8.9 to 9.3 which isn't really drastic but shows how you need to be quite accurate. The only way to do it is by using a syringe graduated to about that .01cc accuracy and carefully filling the combustion chamber until it's dead level with the outer edge of the squish band. Obviously you must have a plug fitted and this will also fill the plug cavity. Some do this by placing a plastic sheet over the head with a small hole in the centre for the syringe needle to go through and with a thin layer of grease to seal the plastic against the edge of the squish band.
I made my own "syringe" by using clear plastic tubing with about a 1/16" internal size and calibrating it so I knew exactly how much liquid was in a particular length as I dripped it into the head. With my tubing there's .0256cc for every centimetre it drips out and I watch a reflection off the surface to see when it's full.
