TRunner

Anyone know how to figure it? The rough estimate that the machine shop gave me was about 4-5 extra cubic inches so not even 0.1L. Anyone know for sure?

bamachem

looking at the equations, and the diameter/radius is the only thing that changed (same rods and cam), then the difference is proportional to the increase in surface area of the piston head.

For the old pistions, Surface Area = SA(old) = (pi) x radius(old)^(squared)

For the new pistions, Surface Area = SA(new) = (pi) x radius(old)^(squared)

SA new divided by SA old is the ratio you want...

SAnew / SAold = [ radius(new) / radius(old) ] ^(squared)

Now all you need is the radius of the old piston and the radius of the new piston and you can calculate it...

Take the ratio you get with the above equation and multiply it by your old displacement to get your new displacement.

TRunner

Actually I already new all this. And you need to factor in the stroke. The information I don't have is the current diameter and the current stroke. The stroke will stay the same, but the diameter will increase by the amount that my new pistons are. I have 30 over pistins being installed, but is that 30/1000 from teh surface or 30/1000 of the diameter? Being that this is a common size in the rebuilds I figured someone would have done this by now...

bamachem

it's either 30/1000's in the diameter or 30/1000's in the radius...

i'm thinking diameter. can't do it w/o the piston diameter. look up what the stock pistons are on a parts webpage or call an auto parts store for the info. if the stroke stayed the same, then the displacement will change relative to the diameter/radius change only.

TRunner

You got a link?

Flamedx4

Here's how I did it. .030 over increases the cylinder volume by .02
(I computed the volume of a cylinder, then increased the diam by .030 and recomputed it, then divided to get the percent change.)
So, a 3.0 - 2958cc x 1.02 = 3017cc
a 3.4 - 3378 x 1.02 = 3445cc

Praufet

Bore: 87.5 mm
Stroke: 82 mm

Flamedx4

m'kay, I did the math with actual bore and stroke and the multiplier for .030 over is 1.019 (still pretty darn close to 2%...)

So, a 3.0 - 2958cc x 1.019 = 3014cc
a 3.4 - 3378 x 1.019 = 3442cc

Or alternatively, the formula for volume of a cylinder is PI x r^2 x h
3.14159 x bore-radius squared x height
87.5mm = 3.445" 82mm = 3.228"
Stock piston:
3.14159 x 1.72^2 x 3.228 = 30.001 cu in. x 6 = 180cu in. or 2949cc (spec says it's 2958cc)
.030 over:
3.14159 x 1.737^2 x 3.228 = 30.59 cu in. x 6 = 183.54cu in. or 3007cc

well, that double check revealed the errors of rounding off stuff and OH WAIT - the cylinder bore is bigger than the piston size - that would probably account for the discrepancy. (and I'm not looking up ring clearance and figuring that out...)

edit: to double check I looked up some displacement figures on engine builders websites and the 2% number is about right for a v6. .030 over ran about 2.6% more displacement for an eight cylinder engine.