Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

CID (Cubic Inch displacement)

What is the CID of an 8 cylinder engine with a 4inch bore and 4 INCH stroke?

Volume of a cylinder is πr²*h.

π = 3.14…

r is half of 4 inch bore

h is 4 inch stroke

3.14*(2)2*4=50.24 each x 8 = 401.

Boy it has been a long time since I used this type of math, hope I got this right

1 Like

402.12 According to the available online calculators.

The Ford 400M had a 4 inch bore and 4 inch stroke. It wasn’t a particularly good engine. The idea was that it would have the output a big block and fuel consumption of a small block. The reality of it was that it had the output of a small block and fuel consumption of a big block. Add in some questionable metallurgy for good measure as well.

1 Like

Just curious, I read references to the phrase “square engine” sometimes. Is this what it means, the stroke is the same as the diameter? If so, would a “square engine” perform differently from a non-square engine, presuming the displacement was similar?

Very generally speaking , and over square engine (larger bore than stroke) will give more HP and and Under square engine will give more torque. I do remember that Chrysler engineers at one time determined that a Bore/stroke ratio .90 or .91 was ideal but I no longer remember if the stroke or bore were larger.


And the answer is…wait for it…stay in school, they teach this basic math.


Interesting. I would have guessed just the opposite, more torque with a large bore and short stroke. I notice people w/short legs tend to be able to push larger weights on the leg machines at the gym. I guess in engines the longer stroke on an “under-square” – b/c the crankpin sticks out further – this gives the piston more leverage to produce a turning force on the crank.

Thanks for including the def’n of the terms over-square and under-square , I’ve wondered about those too.

The way I think of it-larger bores allow larger valves, resulting in better high rpm breathing and higher power at high rpms. Long strike engines can’t do that, so what power they make is at lower rpms.

1 Like

And as George rightly divined the longer stroke usually means a longer rod and crank throw and more leverage at mid stroke, so more torque.


Within the same family of engines you can look at HP/torque curves and see the torque advantages of long stroke versions. Granted the CID increase with the longer stroke. Chevy 327 vs 350. Studebaker 259 vs 289.
But turbos have changed all that, look at the torque curve of the Ford turbocharged 3.5L vs the non-turbo 5.0.

Actually the highest torque/displacement comes from a short stroke and a long rod. The long rod decreases the angle the rod makes with the power stroke direction, increasing the amount of the stroke in which power is applied.

Look at the crankshaft and pedals of a bicycle. If the pedal is straight up and you push down, very little force goes into the rotation of the crankshaft. Now as the pedal goes off vertical, more of the applied force goes into the rotation. The peak transfer of power occurs when the pedal is straight forward, 90* from vertical, then it goes back to zero as the pedal approaches the bottom of the stroke. At 30* past the top, 50% of the force is going into the rotation and at 30* before the bottom, 50% is going into the rotation. Most of the power is applied from 30* after the top to 30* before the bottom.

In an engine, since the top of the connecting rod is at the center of the piston, it makes an angle to the crankshaft journal. So the half power points shift a little. You get a small shift forward at the top but a larger shaft away from the bottom so that the half power points are less than 120* as you would get from a bicycle where the operator can move his knee so he is pushing straight down. The shorter the connecting rod, the shorter the power stroke. The power stroke is between the half power points.

A shorter stroke allows for longer connecting rod for any given configuration. Also since the crank journals are on a smaller radius, the angle the rod makes during the power stroke is less so the power stroke is even longer. There are other ways that racers use to increase the length of the rod, such as using pistons with the piston pin as high up as possible.

I will not argue that a longer stroke yield more torque overall, but it is less torque/displacement.

Figure 7 of this document is an interesting graph relating fuel economy to bore, stroke, and con rod length:

It sounds like there are three parameters to consider, not just two, when trying to optimize torque for a given displacement

  • bore diameter
  • stroke
  • distance from piston to crankshaft

In a given deck height engine block the rods can be longer. If you choose a shorter deck height, the engine can be more compact, the rod stays the same as the long stroke engine. The small bore, long stroke engine also makes it more compact in overall length.

Piston speed matters, too. 7000 rpm with a long stroke has greater speed than the same engine with a short stroke.

But stresses go up with the square of engine speed!

Lots of design considerations!

Many more than three parameters that affect torque, George. Compression ratio, intake, exhaust, cam timing and lift, …