compression ratio

[revi]

I understand the formula to calculate compression ratio is (v1+v2)/v2 with v1 being volume @ exhaust port closing and v2 being volume @ tdc. I can do the math fine, but how do I acurately measure the those figures. I have read about using a graduated cylinder and a 30cc syringe and I see how that might work. Could someone describe the actual steps in that proccess?? What type of oil to use and where to get the materials?? Please??
After spending years of tinkering with my bikes trying to make them go faster and handle better, I am on a quest to actually understand what I am doing. First post to this form, but I plan on attending alot!!

 

[EricGorr]

I use a graduated cylinder with water, then dab grease around the piston rings and set the piston at the top of the exhaust port and measure the total volume. Its too hard to measure the closed valve compression ratio. But programs like TSR's Compress help you calculate both compression ratios with just simple measurements. For more info check out Tom Turner's web site www.tsrsoftware.com

 

[revi]

Can anyone explain the actual procedure step by step?? I would love to know, please!!

 

[EricGorr]

Its probably more accurate to calculate the swept volume of the bore before the exhaust port opens using the highest point of the exhaust port opening to TDC, using the simple geometric formula for the volume of a cylinder. Then position the piston at TDC and volume check the trapped volume with a graduated cylinder or the syringe. In the TSR software they recommend filling the head to the top of the spark plug hole them subtracting 1.75cc. This method allows for trapped air bubbles to roll up through the plug threads. Of course the cylinder bore must be straight up and down. when I perform a volume check on a head, I set the cylinder on a flat bench surface. If I'm checking it on the engine I'll block the engine so the bore is perpendicular with the bench. If the head has an off-set sprark plug, you'll be geometricaly and gravitationally challenged :yeehaw:

 

[bigred455]

Or you can use the static compression (BDC TDC),instead of the dynamic.

BORE X BORE X STROKE X.0031416 DIV BY 4000.

Example for a stock Kx250, mine to be exact.

66.4mm x 66.4mm x72mm x.0031416 /4000 =249cc this is the cv(cylinder volume).. After this, locate the piston at TDC seal the edges of around the piston like MR GORR suggested. Dont forget to use the head gasket when you measure the ccv(combustion chamber volume)put the head on, bolt it down,and start filling it up TO THE BASE OF THE PLUG THREADS .Take note of how many cc's it took. My 00 kx 250 CCV WAS 17.8CC CV 249CC (CV+CCV/CCV ) 249+17.8=266.8/17.8=14.9:1

Now of course the dynamic is going to be lower than this, because you are using the effective stroke volume(exhaust port closing).I have read graham bells book(very good) From what he was saying about the 2 different ratios he prefers the uncorrected(static) compared to the (dynamic) corrected or japanese way.using the dynamic let's say the volume of the cylinder (effective stroke) off the top of my head is 30mm instead of 72mm which gives you the volume somewhere around 120cc 6.1 cr or what ever,I know it is somewhere in the 8.1 range. so the cylinder only has this amount using the dynamic.He was explaining this cannot be accurate,because of the return charge of the pipe replacing the cc's of mixture back into the cylinder that was temporarily lost.Plus the high rpms of the piston will out run the fuel charge before all of it escapes out exhaust. uncorrected way for me

So pick your CR.