View Full Version : What determins engine size?

09-11-2005, 11:59 PM
I realize that its 5.8L means 5.8 Liters, but what determines the liter bit?

09-12-2005, 12:11 AM
It's a designation of engine displacement.
Displacement = Pi x r² x stroke x No. of cylinders.

The displacement normally doesn't include the head volume. If you use milimeters in the above formula, you get cubic mm which is converted to liters. If you use inches, you get cubic inches. (The old designation.)

Ops, for some reason the Pi symbol doesn't show up right.

[This message has been edited by CCWKen (edited 09-11-2005).]

09-13-2005, 08:18 AM
it is the amount of air displaced on one full revolution.

09-13-2005, 08:38 AM
What Bob says is true for a 2 cycle engine, but for a 4 cycle it would be 2 reveloutions. Ken say it best, it is the swept volume of all the cylinders in the engine.
no neat sig line

09-13-2005, 09:31 AM
<font face="Verdana, Arial" size="2">Originally posted by CCWKen:
The displacement normally doesn't include the head volume. .]</font>

That's new to me. I would define displacement with the head volume and piston dish/dome included. JRouche

09-13-2005, 10:44 AM
Displacement is the swept volume per one revolution. See here:

The shape of the combustion chamber in the cylinder head along with the shape of the top of the piston affect the compression ratio, but not the displacement. See here:


09-13-2005, 12:19 PM
It's been a long time but I seem to recall when rule testing the displacement of engines for racing they will fill the cylinder with oil and see how much it pumps out taking the average of several strokes. This obviously does not measure head space.

Paul Alciatore
09-13-2005, 12:39 PM
Actually it is an arbituary number selected by the sales department and designed for the sole purpose of helping sell more cars. I just can't figure why they switched from ccs to liters as the numbers are so much smaller. Perhaps they just couldn't count that high.

Paul A.

3 Phase Lightbulb
09-13-2005, 12:39 PM
<font face="Verdana, Arial" size="2">What determins engine size?</font>

The diameter of your brass determin the engine size. Larger diameter brass balls need larger engine size/displacements. http://bbssystem.com/images/smiles/bigups.gif

09-13-2005, 06:27 PM
CCWKen & winchman yer right.

It was early and I WAS thinking of compression determination. Volume won't change with piston or head geometry. Thanks, JRouche

09-13-2005, 07:30 PM

09-13-2005, 07:44 PM
Company I worked for built the first Ford Mountain motors.. Ford MOtorsport block came in with no bore holes.

My old boss made a engine out of a block of aluminum, reground a crank eccentric *Hank the crank?.. and had rods made. Seems it was about a six hundred cubic inch motor. It sounded like crap, you could not get a high enough lift cam to sound right. Some Nasa software fluid-dynamics recomended using a intake runner 42 inches long. (one air-gasoline pulse entering plenum while first went through vales into cylinder) He had already split the Holley dominator carburetors.

That first motor took him to the nationals, he was 3rd in the nation.. Too bad as he was racing and having fun his company took the bite with his kids running it into the ground. When they quit paying the bills I left, owing payroll was the next thing, people raced to the bank.

On the bright side? I bought most his good tools at auction about a year back.

On and that First motor? it broke clean in half, took crank, cam and heads off it and it fell in two pieces.. A pot smoking kid put a tarp over it, put a kerosene heater in there to preheat and tig welded it back together inside and out. It is still running last I heard. No bull. He Had Gliddens old ford Probe chassis-body.. Seems that was around 88 or 90? I can't remember.

I always calculated engine size by cylinder area x stroke.