How to calculate compression ratio
 

I built a test piston for the Holt 75. The clearance from top of piston to top edge of the bore is .201. The head button protrudes in the cylinder .030. Add a .010 thick head gasket & that leaves about .1.81 for the combustion space. The bore is 1.00 & the stroke is 1.250. How would I calculate the compression ratio? Thanks, C


http://www.rcuniverse.com/forum/atta...mentid=2068599

If the head button is flat on the combustion end, (no dome nor dish), and I understand your measurements correctly, I get 7.91:1 compression ratio.

AV8TOR

COMBUSTION SPACE = BORE/2 X BORE/2 X 3.14 X COMBUSTION AREA DISTANCE = xxx (cubic inches)
COMBUSTION SPACE = ((.5 X .5) X 3.14) X .181 = .142 (cubic inches)

SWEPT VOLUME = BORE/2 X BORE/2 X 3.14 X STROKE = xxx (cubic inches)
SWEPT VOLUME = .5 X .5 X 3.14 X 1.25 = .981 (cubic inches)

CR = SWEPT VOLUME + COMBUSTION SPACE/COMBUSTION SPACE
CR = (.981 + .142)/.142
CR = 7.908

Another easy way of figuring volume is ((Bore x bore) x stroke) x .7854 = Volume

(I have a spread sheet that I made up years ago that does all the math for me. It has formulas for lots of mechanical problems, and all I have to do is plug in the measurements and it spits out the answer.)

AV8TOR

av8tor, the data you gave me is really neat. I had a good idea that you could answer the question because I know you have had extenisive experience with engine building & racing cars. The head button does have a short passage to the sparkplug that comes with a 90 degree bend. Old style engine ! I will try to post a photo of the head...it may not work...it seems very hard to post any photos. Thanks, Your friend John

The photo did upload...but is on post number 1 I can see why not many photos are uploaded...it is a nite=mare to do!!!!!!!

No problem. Glad to help. Hope you finish that engine and get it running soon!

AV8TOR