true displacement for a 390 ci bored .30 over engine - Ford Mustang Forum
 
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post #1 of 6 (permalink) Old 10-24-2009 Thread Starter
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true displacement for a 390 ci bored .30 over engine

does anyone know what the "true" displacement is or the formula to calculate it for a 390 engine bored .30 over?
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post #2 of 6 (permalink) Old 10-24-2009
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I *think* you're looking for 0.030" over, not 0.30" (which is more than a quarter inch.)

The formula for calculating the volume of a cylinder is pi*r^2*h (pi times radius squared times height). You're just finding the area of the piston, and multiplying it by height.

And then, since it's a V8, multiple the result by 8.

From:

Ford FE engine - Wikipedia, the free encyclopedia

Quote:
The 390, with 389.6 cu in (6.4 L) true displacement, had a bore of 4.05 inches (102.87 mm) and stroke of 3.78 inches (96.01 mm).
To get the 389.6 number using this formula:

If the bore (diameter) is 4.05", the radius (half the diameter) is 2.025".

The area of the piston is pi*(2.025^2), or 12.88 square inches.

Multiply by the stroke (height) of 3.78", and you get 48.7 ci per cylinder.

Multiply again by the number of cylinders (8), and you get 389.6ci

So...

Boring 0.030 over increases the diameter of each cylinder to 4.08", and the radius to 2.04".

The area of the piston is now 13.07 square inches.

The stroke is still the same (3.78"), so the volume of each cylinder is 49.42 ci (an increase of 0.72 ci)

Multiply by 8 cylinders, for 395.4, an increase of 5.8ci.

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post #3 of 6 (permalink) Old 10-25-2009
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Quote:
Originally Posted by smokeshow View Post
I *think* you're looking for 0.030" over, not 0.30" (which is more than a quarter inch.)

The formula for calculating the volume of a cylinder is pi*r^2*h (pi times radius squared times height). You're just finding the area of the piston, and multiplying it by height.

And then, since it's a V8, multiple the result by 8.

From:

Ford FE engine - Wikipedia, the free encyclopedia

To get the 389.6 number using this formula:

If the bore (diameter) is 4.05", the radius (half the diameter) is 2.025".

The area of the piston is pi*(2.025^2), or 12.88 square inches.

Multiply by the stroke (height) of 3.78", and you get 48.7 ci per cylinder.

Multiply again by the number of cylinders (8), and you get 389.6ci

So...

Boring 0.030 over increases the diameter of each cylinder to 4.08", and the radius to 2.04".

The area of the piston is now 13.07 square inches.

The stroke is still the same (3.78"), so the volume of each cylinder is 49.42 ci (an increase of 0.72 ci)

Multiply by 8 cylinders, for 395.4, an increase of 5.8ci.
Amazing what one can figure out with basic arithmetic isn't it. I liked your solution. since the starting point was not given, and I didn't know it, I was about to offer a X more than it was solution. It paid to pay attention in math class.

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post #4 of 6 (permalink) Old 10-26-2009 Thread Starter
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thanks smokeshow - you sure are correct .030 not .30 - i lost a zero in that figure
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post #5 of 6 (permalink) Old 10-26-2009
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Man, you had that figured out before I even found my abacus!
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post #6 of 6 (permalink) Old 10-26-2009
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Quote:
Originally Posted by 68390gt View Post
does anyone know what the "true" displacement is or the formula to calculate it for a 390 engine bored .30 over?

In Cubic Inches the formula is:

Bore x Bore x Stroke x Number of Cylinders X .7854
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