Cylinder Displacement Formula & Torque and HP Formula (Post your engine specs)

[Jawbreaker]

I thought some peeps would like to play around with the displacement formula.

I figured out that my motor is at 803cc displacement if oem stroke is 70mm. I was a little in doubt.
85.5mm pistons w/70mm stroke.

85.5mm x 85.5mm x .7854 x 70mm x 2 cylinders = 803805.849 / 1000 = 803.805849cc

Post your motor specs. Whacha got?



CYLINDER DISPLACEMENT
:: The engine displacement formula is the standard formula for computing the volume of a cylinder of any type with an added factor that represents the number of cylinders in the engine.

:: The cross-sectional area of the cylinders is determined and the volume of the individual cylinders is computed by multiplying the area by the stroke length. Multiplying the result by the number of cylinders gives the engine's total displacement. The formula is:
Bore diameter x bore diameter x .7854 x stroke length x number of cylinders = displacement
Dimensions in inches will give the displacement in cubic inches. Dimensions in millimeters will give displacement in cubic millimeters, which are then converted to the standard measure of cubic centimeters with the formula:
Cubic millimeters ÷ 1000 = centimeters
For example, compute the displacement of a single cylinder engine that has a bore of 54 mm and a stroke of 53 mm. The equation then is:
54 x 54 .7854 x 53 x1 = 123,672.42 cubic millimeters
We now reduce to cubic centimeters by diving by 1000
123,672.42 ÷ 1000 = 123.67 cubic centimeters (cc)



HORSEPOWER AND TORQUE
Horsepower and torque are measures of engine performance. They are related to the extent that one cannot exist without the other. Torque is the measure of the amount of work an engine can do and horsepower is the measure of the amount of work done in a given time. The time factor for horsepower computations is the engine's crankshaft speed, as measured in revolutions per minute.

Formula for computing horsepower:
:: Torque x rpm ÷ 5252 = horsepower
For computing torque when horsepower and rpm's are known:
:: Horsepower x 5252 ÷ rpm = torque

 

[keefer]

I hate to be the one to bust your bubble, but displacement is calculated on bore and stroke, not rod length. The stock rod length is 125mm, the stock Yamaha 650, 701, 760 stroke is 68mm.
So by your formula with 85.5mm bore and 68mm stroke you come out with a 780cc displacement. Thats what U got.

85.5 x 85.5 x .7854 x 68 x 2 /1000 = 780.8399676

 

[keefer]

All I can brag about is a puny lil 758cc brapper:biggrin:

84.25*84.25*.7854*68*2/1000 = 758.1752871

Here is an interesting chart I found somewhere.



Mo stroke is good:Eyecrazy:

 

[WaveDemon]

790cc on the cheap

 

[Freestylin-FX1]

790cc on the cheap

mfg:I got me one of those :friday::eek2:

 

[Speedfreak]

I was at a decent size but I don't know what I will have when I get it back this year... It might be much huger than before close to 890's for about the same money if it has to be built have it BUILT...

what I had was a 86mm bore X 86mm bore X .7854 X 73mm stroke X 2 cylinders /1000 = 848.08cc's

 

[Jawbreaker]

I hate to be the one to bust your bubble, but displacement is calculated on bore and stroke, not rod length. While the rod may be 70mm long, the stock Yamaha 650, 701, 760 stroke is 68mm.
So by your formula with 85.5mm bore and 68mm stroke you come out with a 780cc displacement. Thats what U got.

85.5 x 85.5 x .7854 x 68 x 2 /1000 = 780.8399676

View attachment 42333

No bubble to bust.

:doh: I was getting rod length and stroke confused.

 

[SUPERTUNE]

I think Jaws is confusing stroke with rod length...No watercraft would have such a short rod for the much larger displacements they have, now maybe a weed eater motor or some 50-100 cc dirt bike engines would have something that small.
He is right on the $$ with the displacement, 85.5mm bore X 70mm stroke (stock kaw 650 or +2mm yamaha stroker) = 803.8 cc's
Yamaha popular strokers are 72mm, 73mm, 74mm, 76mm, and the big one 78mm (+10 over the stock 68mm stroke of a 650/701/760 engine)

 

[Jawbreaker]

I think Jaws is confusing stroke with rod length...No watercraft would have such a short rod for the much larger displacements they have, now maybe a weed eater motor or some 50-100 cc dirt bike engines would have something that small.
He is right on the $$ with the displacement, 85.5mm bore X 70mm stroke (stock kaw 650 or +2mm yamaha stroker) = 803.8 cc's
Yamaha popular strokers are 72mm, 73mm, 74mm, 76mm, and the big one 78mm (+10 over the stock 68mm stroke of a 650/701/760 engine)

:doh: Post fixed. :bigok:

 

[Mark44]

w

Please step away from the calculations and leave them to the engineers.:biggthumpup:


Mark44

 

[njfl]

Here you go:

http://www.x-h2o.com/threads/5136&highlight=displacement

Note my comments/observations on the mm-for-mm increase on bore vs. stroke, but that's just the math side of things. Even though you get more ccs for a mm increase in bore, you have to factor in the physics leverage increase gained by increased stroke, but I don't have an easy way to quantify that for comparison at hand.

 

[keefer]

:doh: Post fixed. :bigok:

So do you have a +2mm stroker crank at 70mm?? I think you need to recalculate for the 68mm stock stroke which puts you at 780cc, right?? Check the second post. Chuckie was saying that your calculation was accurate with the numbers you plugged in, not that it was actually what you are running. And yes it appears that I am the displacement police:stups:

 

[Waternut]

Because I knew so little about my engine when I got it, I did a full inspection of it a week or so ago. Turns out my 750sx which really was 743cc engine is now a 762 thanks to the bore increase after the guy before me had it rebuilt. I measured the stroke with some digital calipers and the number was dead on with stock so I measured the bore and it was 0.040" over (81mm instead of 80 stock)...sweet!!

Use http://www.csgnetwork.com/cubicinchdispcalc.html for your calculations....waaay easier.

 

[justbrowsing]

correct displacement calculation

the web is a great place to find information and I stumbled upon this site researching 2 cycle engines. though not a jetski owner, it is unfortunate that there is occasionally mis-information that gets written and left to air for years and to be read by unsuspecting readers. the displacement formula needs correcting:

displacement is the area of the cylindrical shape swept by the piston. it is the area of a circle (pi * r squared, where r is the radius or half the diameter (aka: bore)). the volume(aka: displacement) is simply this area of the circle times the height of the cylinder (aka: stroke). to convert each mm figure to cm divide each by 10 (there should be 3 encountered as this is a volume) and so the point of "divide by 1000" is ok to give a final value of cm cubed ("cc").

here's a example clipping from elsewhere which contains correct figures:
Model aircraft glow engine, OS Max, 0.46 cu in (7.5 cc), acquired 2003. Bore = 22 mm; stroke = 20 mm.

 

[GIL]

Cool-that means I am now AM1 legal!!!

85mm bore x 73mm stroke=414cc!!!!!!! Hooray-I can't get busted now!!!

:bow28:














LOL

 

[BombThreat]

809cc's