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Calculation of compression ratio
Posted: Mon Dec 05, 2011 7:58 pm
by TS1-200
Can somebody please post an idiots guide of calculating compression ratios, I have read about burettes and things, but what is the best way to do it, and what is a safe compression ratio these days.
Re: Calculation of compression ratio
Posted: Mon Dec 05, 2011 8:51 pm
by Supereibar
Another way to do it is with a Burette, measuring the head sitting flat facing up on a surface
Re: Calculation of compression ratio
Posted: Mon Dec 05, 2011 9:03 pm
by TS1-200
eden wrote:You can try different head volumes in the corrected and uncorrected compression ratios calculator here
http://www.lambretta-images.com/archive/porttiming.php
best way to measure volume is to do it with the engine built up with a little grease around the top ring and fill the head with 2 stroke oil to the bottom of the plug threads from a big hospital syringe and make a note of the volume it holds in CC's.
I assume the the piston should be at TDC
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 12:07 am
by TS1-200
Thanks that link is helpful, 2 questions, where is the height of the exhaust port measured from /to
and what would be a "safe" compression ratio, if there is such a term
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 4:25 pm
by TS1-200
Right By my calculations I`ve come up with 12:1 uncorrected & 6.68:1 corrected is this OK I`m not sure which is the favoured type of measured comp. ratio be it corrected or uncorrected,
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 4:46 pm
by nelson pk
I think you'll find thats a bit higher than what people tend to go for with todays modern fuel.
You'd probably get away with it if but make sure you retard your timing enough.
Thats what i personally think anyway.
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 7:58 pm
by TS1-200
Ah, looks like I`ve done it wrong I`ve measured from Bdc to top of exhaust port, I`ll do it again, my combustion chamber is 21cc 60mm stroke 70mm bore, the measurement from top of the exhaust to top of barrel is 30mm
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 8:02 pm
by sean brady scooters
actually its not true to say that compression can only start once the piston closes the exhaust port on the upward sroke ..because of the return wave /stuffing effect from an expansion chamber
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 8:14 pm
by TS1-200
So where do I stand, do I try to enlarge the combustion chamber? or leave as is
Re: Calculation of compression ratio
Posted: Tue Dec 06, 2011 8:19 pm
by sean brady scooters
i do agree eden as per corrected,but it is poss to be more corrected
