I'm getting 1mm oversized pistons in engine build?? How much will this change cc. Will it be 2.1 or 2.15 L then or wat???

 

well come on?

 

Don't they teach you maths over there in Ireland?

 

no lad so wats answer????

 

If you don't give me an answer then your as stupid as i am

 

I dont think the capacity will increase much at all, you may get a slight increase but it wont be a 2.1 or 2.15.

banny

 

Ok so now i know!! thanx lad

 

Windows calculator and a memory of maths.

Bore x Stroke ripped off IWOC, so if it's wrong it's not only my fault

Volume of a cylinder is the area of the circle (in this case the square centimeters of the bore) multiplied by the height (in this case the stroke). Oh, times 4 'cos it's it's got four cylinders.

9.2cm=bore (Radius=4.6cm)
7.5cm=Stroke

PI * Radius^2 * Stroke * 4=1994.28cc

Assuming a 1mm overbore.
9.3cm=bore (Radius now 4.65cm)
7.5cm=Stroke

PI * Radius^2 * stroke * 4=2037.87cc


Alternatively...
The capacity (volume) is related to the square of the radius (the Radius^2 bit). So if I know the difference in the area, I can multiply the existing volume by it.

Find the difference ratio of the radii 4.65/4.6 = 1.0109
Square the ratio of the radii 1.0109 * 1.0109 = 1.0219

Multiply the old capacity by the ratio 1994.28*1.0219=2037.95cc

I know the numbers are a little different but I rounded the numbers in the second attempt.

J.

 

Vindaloo, you have just qualified as a scoobynet techie!!

 

And how will it effect the C/R


Mark.

 

 

depends what pistons are used

 

c/r um... right.. OK. Have to think about that one as they didn't cover it directly in O-level maths...

Given an STi (1999-2000) - static compression ratio = 8:1
Engine capacity = 1994.28cc
Cylinder swept volume = 1994.28/4 = 498.57cc

...at this point I cheated and googled/doodled until I understood the c/r.

Head volume = 498.5/7 (!) = 71.22cc (It was the divide by 7 bit that got me until I drew it out and puzzled for a while).

Assuming a 1mm overbore...
Engine capacity = 2037.87cc
Cylinder swept volume = 2037.87/4 = 509.47cc

c/r = (swept volume + head volume)/head volume

c/r = (509.47 + 71.22)/71.22 = 8.15

So the new compression ratio is 8.15:1 (ish)

J.

 

LOL.................. That's pretty good, it was really about how increasing the bore generally increases compression, if you retain the same piston dish volume.

It's impossible to actually work out the C/R unless you know the compression height, and piston dish volume, and then the head volume and gasket thickness.

After market piston spec's can vary between manufacturers, along with the thickness of gasket they recommend you use.


Mark.

 

Agree with Mark, if you are trying to work out an accurate CR with all the figures from a spec sheet then you are pissing in the wind. The only way is measure the volumes and thickneses yourself.

As also noted any increase in bore size (assuming everything else is the same including crown height and dish size of the new piston) will automatically increase the CR

Good maths though vindaloo

 

So assume increase in c/r is good thing yeah???

 

Increasing C/R can improve off boost performance, and throttle response, but the increase in C/R from just an over bore is very small.

That said, it would be worth checking what the spec of your pistons are, because they can vary depending on manufacturer, and part number.


Mark.

 

they are woosner 1mm o/s forged so would love to know? look forward to an answer?

 

The "Wossner" web site doesn't give the information, it just says the pistons are 8.35:1, but without knowing what model car you have, or the piston spec', it's not possible to say exactly what the compression is.

The first thing to do,, is make sure the piston part number you have, matches your year of car, but I'm sure your supplier will already have done this.



Mark.