What does cubic inch displacement really mean?

The volume "displaced" by the pistons in all the cylinders.

OK, let's not "give him a fish" - Let's "teach him to fish" right from the git,-go, huh? :)

Pistons go up and down inside a cylinder. One end of the cylinder, the head, is fixed. The piston is the other end of the cylinder. In between the piston and the head is some volume.

When the piston is at the bottom of its stroke, the volume is X cubic "Units" - Units might be inches, centimeters, cubits, football fields - Whatever...

When the piston is at the top of its stroke, the volume is much smaller

- Let's call it Y cubic Units.

The motion of the piston from bottom of stroke to top of stroke is said to "displace" a certain number of cubic units by making the cylinder's volume get smaller. The exact number depends on cylinder/piston diameter and length of stroke.

Therefore, displacement of one cylinder, D, is figured as:

D = (X - Y)

or the difference between the cylinder's volume when the piston is at the bottom of its stroke, and when it's at the top.

Assuming you're using inches for your units, do that math, then multiply by the number of cylinders the engine has, and you get the engine's total "Cubic Inch Displacement" number. Use centimeters, and you get the engine's displacement in CCs. Likewise for whatever other unit you care to convert to/work with.

Good start, Don.

We can also tell him that large displacement numbers, whether in cubic inches or in litres, give you the potential for more powerful engines since this is the amount of fuel/gas mixture you can burn in the engine in a complete revolution.

So, to the OP, a 7 litre (or 7000 cc) engine has the rough potential to put out twice the power as a 3.5 litre engine (3500 cc) ALL OTHER FACTORS BEING EQUAL.

There are many ways to make the factors unequal, and that is what we hotrodders, benchracers, mechanics and enthusiasts are often interested in. For example, a highly developed 2 litre engine can easily put out a 750-1000 horsepower, while a detuned 4 litre engine might not even make a hundred.

That is one of the things we enthusiasts love to play with.

Guess that means my 2 liter isn't all that "highly developed" - 74 HP @

2750 RPM is its peak according to "The Book". (Big surprise, as it was sold to power a "sporty economy car", not a tire-eating monster :) )

================= For a four cycle gasoline engine wouldn't you need two full revolutions to burn the full displacement?

That clears that up! Thanks!

Jeff

Yes, you would, but the displacement by the pistons is achieved in one single revolution. In a four stroke cycle, the valving, spark, etc allow each cylinder to fire every other revolution, but the cylinder is still swept once per revolution.

I realize that this could have been misinterpreted, and perhaps I should have explained it better in the beginning.

Im glad we dont have to support the expenses that would go along with a 'highly developed' 2 litre engine, Don, aren't you?

I was thinking about the Formula I engines when I made that statement, which are-to me- marvels of engineering. That one can put together a small displacement engine which can turn 20,000 rpm for hours and put out astronomical power ratings is pretty spectacular.

The total volume swept out by all the cylinders (not including combustion chamber volume).

IOW, the surface area of each piston (pi*(bore/2)^2) multiplied by the stroke, multiplied by the number of pistons.

When it comes to calculating volume flow, that is correct. Of course that assumes 100% volumetric efficiency (the engine moves an amount of air/fuel mix EXACTLY equal to its displacement on every full power cycle, or 2 rotations in the case of a 4-stroke). But volumetric efficiency is all over the map. An idling 440 cubic inch engine, for example, only moves a tiny fraction of 440 cubic inches of air every 2 revs. On the other hand, at its peak power RPM the same engine (even without supercharging) might move MORE than 440 cubic inches every 2 turns (more than 100% VE) because of ram-induction effects in the intake manifold. Turbo-charged or super-charged engine consistentely operate at more than 100% VE whenever there is boost.

Yes.....the 4-stroke cycle does take two revolutions to complete one combustion cycle.