Tech: Whats the formula for finding engine displacement?

[AVTekk]

Ok I was in a heated discussion over which of these is right.

Heres the one that I learned:

bore x bore x .7854 x stroke x number of cylinders = ci

Heres the one that another person brought up:

(((bore x pi) stroke) numper of cylinders) = ci


Now my formula will pull up the same answer on any online engine displacement calculator. Heres one thats quite detailed:

http://www.syty.org/old/calculator.html

However with the other formula its like its only right half the time.
Lets do a few calculations..Heres a stock engine:

1. 4 x 4 x .7854 x 3.48 x 6 = 262.386

2. (((4 x pi) 3.48) 6) = 262.386

Engine with .030 overbore:

1. 4.03 x 4.03 x .7854 x 3.48 x 6 = 266.337

2. (((4.03 x pi) 3.48) 6) = 264.354

What gives?

 

[AUSyTyIN]

Volume of a cylinder= pi x 1/2 bore squared, times stroke. Multiply this by the number of cylinders to get volume (displacement).

 

[JSM]

SBC's formula is CORRECT. The ones you listed are not.

 

[E-Rue]

hello geometry!

pie x r squared x the length x the total number of cylinders.

e

 

[InvisiBill]

The second formula works out to the right answer on the first one, because (4/2)^2 is still 4.

Like they said, the volume of a cylinder is height x area. Area = pi x r^2. r = bore/2.

So (bore/2)^2 x pi x stroke x number of cylinders will give you total displacement.

 

[InvisiBill]

Actually, your formula is right. Rather than dividing the bore by 2, then squaring it, you just square the bore. The .7854 is pi/4. You divide by 4 to make up for not dividing the bore/2, then squaring that.

So yours is actually bore^2 x pi/(2^2) x stroke x cylinders, which is the same thing.

 

[Lynn D. Brown]

Engine Displacement

Engine Displacement

Just to be a nitpicky AHole (whaddaya expect from an Injunear), the volume displaced should also include the Combustion Chamber Volume, shouldn't it ? I think the displacement mentioned only includes the volume from the top of the block to the bottom of the stroke.

However, to negate my own anality, that volume is probably insignificant in the whole volume displaced.

 

[myclone]

Ummm.. pi r round not square. Yeeesh when are you city folk gonna learn that.









[sorry.. its been slow around here lately. back to your regularly scheduled thread]

 

[Don W.]

Actually you don't include the combustion chamber because 1) it's static volume, and 2) the reference is displacement which is how much air the engine displaces or moves as it runs. Now compression ratio -does- include combustion chamber volume. It's the ratio between cylinder volume and combustion chamber volume. Assuming flat top pistons.

Don

 

[AVTekk]

Volume of a cylinder= pi x 1/2 bore squared, times stroke. Multiply this by the number of cylinders to get volume (displacement).

Thank you invisiBill for strengenthing my formula. I forgot to point out what the .7854 stood for. As for sbc ans JS, just becuase my formula isnt the same as yours, doesnt mean its wrong...The other one that was pointed out to me was indeed wrong, and I agreed with you on that from the get-go.