full credit to sicivicdude on bhq
you hae to have the engine perfectly level for this so put the front of you sled on blocks or remove the engine and use a level to make sure its perfectly level!!
i had a really informative talk with a buddy on another forum and i think this info would be very useful for some of you to know when choosing the right fuel.
as alot of you know compression is not a completely valid way to tell whether or no to run 87oct or 110oct another thing to take into consideration is your compression ratio.
a simple way to measure compression ratio when putting an engine together or bolting on a nice ported big bore is before installing the head put a nice thin coat of grease around the piston to completely seal the ring and role the engine to top dead center. after install the head, you do not have to torque it down.
next take some sort of measuring device like a big syringe of somthing you can measure in CC's with. fill the syringe or what ever you are using to measure cc with a thin oil yamalube may work but it would be best to use a cheap oil that way your not wasting good mix oil. fill the cylind to the bottom thread of the sparkplug hole, remember engine is at top dead center.
keep very close track to the exact amount of fluid you are putting in and this is your trapped volume for example my banshee has 22cc of trapped volume.
next take a calculator do some calculating
example i have a 421 cc yamaha banshee with 22cc trapped the bore is 68 by 58 stroke and it is a twin. so the math equation i am going to do is
bore X bore X stroke X 3.1416/4000 and this will give you the exact displacement per cylinder whether it be a single cylinder or a 4 cylinder this is the equation you would use because you are only measuring 1 cyl.
next you add the amount of volume to the equation then you divide the volume +22cc/22cc
so back to the banshee here is how i would do it
68x68x58x3.1416=842551.9872
842551.9872/4000=210.637 which is my cc in one cylinder.
210.637+22cc=232.637
232.637/22cc=10.5744544 or 10.5:1 comp ratio. which should not run pump gas
so now i have my trapped or uncorrected compression ratio.
7.5:1 and under ~170 psi would be my max for fresh 93oct anything over should really run a 50/50 of 93 and 110 or straight 110
now there is a little bit more confusing way to do your Corrected comp ratio it is really long to type out an explanation on how to do so but this should be a nice guideline to follow on your next build.
you hae to have the engine perfectly level for this so put the front of you sled on blocks or remove the engine and use a level to make sure its perfectly level!!
i had a really informative talk with a buddy on another forum and i think this info would be very useful for some of you to know when choosing the right fuel.
as alot of you know compression is not a completely valid way to tell whether or no to run 87oct or 110oct another thing to take into consideration is your compression ratio.
a simple way to measure compression ratio when putting an engine together or bolting on a nice ported big bore is before installing the head put a nice thin coat of grease around the piston to completely seal the ring and role the engine to top dead center. after install the head, you do not have to torque it down.
next take some sort of measuring device like a big syringe of somthing you can measure in CC's with. fill the syringe or what ever you are using to measure cc with a thin oil yamalube may work but it would be best to use a cheap oil that way your not wasting good mix oil. fill the cylind to the bottom thread of the sparkplug hole, remember engine is at top dead center.
keep very close track to the exact amount of fluid you are putting in and this is your trapped volume for example my banshee has 22cc of trapped volume.
next take a calculator do some calculating
example i have a 421 cc yamaha banshee with 22cc trapped the bore is 68 by 58 stroke and it is a twin. so the math equation i am going to do is
bore X bore X stroke X 3.1416/4000 and this will give you the exact displacement per cylinder whether it be a single cylinder or a 4 cylinder this is the equation you would use because you are only measuring 1 cyl.
next you add the amount of volume to the equation then you divide the volume +22cc/22cc
so back to the banshee here is how i would do it
68x68x58x3.1416=842551.9872
842551.9872/4000=210.637 which is my cc in one cylinder.
210.637+22cc=232.637
232.637/22cc=10.5744544 or 10.5:1 comp ratio. which should not run pump gas
so now i have my trapped or uncorrected compression ratio.
7.5:1 and under ~170 psi would be my max for fresh 93oct anything over should really run a 50/50 of 93 and 110 or straight 110
now there is a little bit more confusing way to do your Corrected comp ratio it is really long to type out an explanation on how to do so but this should be a nice guideline to follow on your next build.
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i know i didnt go into detail to tell you if you have to run 110 or safe to run 87 but once you get this number you can contact a builder and give him the info and he should set you on track.
another thing i should add if you dont know your stroke you can use a micrometer that has a depth feeler and roll the engine to tdc and measure through spark hole and then role to bdc and measure again and see what the difference is to get your stroke.
example say the depth gauge reads 10mm through the hole at tdc and 68at bdc your stroke would be 58mm
another thing i should add if you dont know your stroke you can use a micrometer that has a depth feeler and roll the engine to tdc and measure through spark hole and then role to bdc and measure again and see what the difference is to get your stroke.
example say the depth gauge reads 10mm through the hole at tdc and 68at bdc your stroke would be 58mm
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