How to determine Piston CR?

[evopwr]

Guys, possibly amateur question.

I'm looking at purchasing a set of CP Pistons from a private seller who claims they are 9.5:1, however there is no documentation to back this up (the piston spec sheet does not mention CP caralogue part number, CR or dome cc).

How can I confirm piston CR? Surely a high profile manufacturer like CP would state this somewhere on the product?

Alex

 

[4piknup]

How many different cr ratio do they make? If its only 2 then the dome will be a bit different. But not that hard to tell apart.
there may be a inked id stamped under the skirt on the small end boss area....

 

[4piknup]

Email a pic of the piston top to cp.
so the spec sheet is a generic sheet for multiple pistons and fitments. This does make sense, and reinforces my thoughts that the piston id is on the piston itself

 

[evopwr]

the spec sheet is specific to the job lot#, as far as I am aware.

There are several variations from 8.5 through to 10.5

 

[DOUGMO]

Ultimately comp ratio comes down to head volume, bore size, head gasket, deck height and piston dome/dish.

A piston will not give you a set comp ratio as all of those other things become a factor

 

[evopwr]

All those variables being equal (head, bore, gasket, deck), the piston dome/dish will determine CR.

 

[VIN18M]

Wouldn't the 10.5:1 pistons be flat top? That would cancel those out.

I have photos of 8.5:1 CP pistons but they are for a 4g93...not sure if that helps but might give you an idea of the dish.

Still a stab in the dark though...

 

[BYBY5L]

Use a c/r calculator like this

http://www.csgnetwork.com/compcalc.html

Edit: just saw a don't have dome cc, now I see ya dilemma, yea surely cp would have the dish listed on the spec sheet, post what pics ya do have up,

 

[ENGINR]

Are there any identifying numbers or letters on the piston castings that you could refer back to CP with?

 

[evopwr]

guys, see attached of the actual kit data sheet. I started asking questions when the seller told me they were 85.5mm when infact data sheet states they are 85mm.

 

[khubner1]

they look like they're flat pistons no dome or dish to speak of

only way to find out would be via a displacement test. i.e fill dish witha fluid to determine dis cc or do the reverse with dome. frome there your CR come down to what dougmo said

 

[DOUGMO]

If it's flat you can work it out.
Head gasket thickness, chamber size And deck height with a calculator

 

[khubner1]

well according to that spec sheet no dome height and a dish depth of .100 inch which i think translates up to 2.54 mm. which = a 14 cc dish now the dish isn't the complete top of the piston so i would hazard a guess at a 10cc dish on these pistons.

my working
dish in inch = .100
dish im mm = 2.54

dish volume
2.54 PI r^2

2.54 x 3.14 x 42.5^2
= 14413.23 mm cubed
move decimal 3 places for cc = 14.4 cc

subtract area for piston volume/piston integrity estimated 4 cc of space

leaves at my best guess a 10cc dish on the piston (which is a common number for dish)

from there you need to cc your head, get deck height and gasket thickness (after crush) to find final combustion chamber CC

deck height is center of main bearings to gasket surface on block

CR = swept volume / combustion chamber

 

[evopwr]

thanks guys! I've since contacted titan directly, these are 9:1 pistons (based on all other variables being standard).

 

[BYBY5L]

I done what kyle did and it came through at 9.5, add valve cuts in and yeah, guess would go to 9.1
are ya gunna get them?

 

[DOUGMO]

until you measure the bore an 85mm piston is a huge risk as you may end up with huge piston to bore clearance

 

[4piknup]

Ive bored it twice and the piston is still too small. ^