Compression Ratio Calculations, Cosworth F'UP or mine?
 

Can't figure this one out. I've generated my own CR calculator and checked it against an online calculator. I've CC'ed the pa2688 pistons and have the dome as an advertised -2.83 cc (as per Cosworth documention) however when I work out the static CR I get a low 9.744:1 (with stock heads) or 9.42 with Cosworth heads. Whereas PA2688 are documented at 10.3:1. I wondered if someone could check my calcs and tell me where I'm going wrong.

What's also of interest is the Wiseco 11:1 CR pistons have an identacal deck height but a +7cc dome. My calculator works the Wiseco's to be 11.08:1 CR.

Heads Cosworth
Pistons Hi Comp

Bore 95.55
Stroke 81.4

Total Displacement cc 583.6825163

Head Gasket Bore 96
Head Gasket Thickness 0.5

Total Gasket 3.6191232

Piston Deck 0.6

Total Deck 4.30232813

Piston PA2688
Dome -2.83

Heads Cosworth
Combustion Chamber 58.5

Volume at TDC 69.25145133

Volume at BDC 652.9339676

Compression Ratio 9.428451752