what size is my 390

[daddyjake707]

i have a 390 that has been rebuilt in the recent past the pistons say .030 so im assuming they bored them out 30 over when they built it. my questions is since im pretty sure it has the stock crank what size does that make the motor now i was told by some random guy at a bar one night who happened to be over hearing a conversation wit the friends i was with that a 390 bored 20 over is a 396. i know its a pretty technical question but im confidant you guys will have the answer

 

[LEB Ben]

.030 overbore adds about 6 cubes. I dunno what the actual displacement of a 390 is...so I dunno what you're total cubes are...ie a 400 is really a 402 plus a .030 overbore gives you a 408.


I believe a .020 overbore gives about 3.6 cubes, but I do KNOW a .030 gives you right around 6 cubes.


The equation to determine displacement is this...Displacement = bore x bore x stroke x 0.7854 x number of cylinder

 

[LEB Paul]

Depends on the bore, stroke, and number of cylinders..

For a 390 all I know is 8 cylinders.

 

[blacksnapon]

You remember that dreaded figure PI? First off find the area of the circle, then figure in the stroke, then multiply by 8 (number of cylinders)

 

[daddyjake707]

ok so im working with a 396 thats not bad hopefully it still runs strong so i can put that money somewhere else for the time being i tore it down to a short block and my friend says it looks good with no burns or build up on top of the pistons and everything rotates smoothly no i just have to wait unitl i do the front suspension before i can put it in and fire it up. is it easy to set up the motor to run on the stand?

 

[1985 Ford F-150]

Its around 396 I was lookin at some kind of a conversion thing and it said that mine came out to 396 its the same thing you got 390 with .030 over.

 

[SuperCab]

The stock bore of the 390 FE is 4.05 inches; the stroke is 3.78.

Bored .030 over, it would be a 4.08 bore, therefore a 395.359 actual cu. in. displacement, or 6.476 liters if you like metric.

 

[LEB Ben]

Thanks for clearing that up Matt...I knew it was about 6 cubes...Did you plug those numbers into a calculator...or just used a formula and the numbers you knew?

 

[SuperCab]

I knew the numbers and plugged them into a calculator...

IDK what the formula is exactly...

 

[SuperCab]

Oh, I found the formula.

Here, using Jake's as an example:

3.78 * 3.14 * ((4.08 / 2)^2) * 8 = 395.158902

Stroke times pi times the radius of the cylinder (half the bore) to the second power times the number of cylindrers is equal to the total displacement

 

[LEB Ben]

Ok...slightly different than my formula...and about .25 difference in CI...but oh well he get's the point...I just didn't know the exact bore and stroke, so I couldn't figure it out.

 

[Blade]

......... math, my head hurts now... thought i left that at school

 

[SuperCab]

Yeah, I know most of the numbers for th FEs so it was easy to plug em in...

 

[daddyjake707]

see i can always count on the ford gurus to give the most exact answer i was contendt with 396 haha thanks for the help fellas

 

[Moto Mel]

Thats a great formula for the guys that know and remember algebra, especially formulas. The formula that I've been using for a lot of years is;
bore (4.08) X bore (4.08) X stroke (3.78) X .7854 X number of cylinders (8)=
395.36 cid. I suffer from CRS and my formula works great for me, yours might be a bit more accurate but I like my bigger number.