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) whereas they are documented at 10.3:1. I wondered if someone could check my calcs and tell me where I'm going wrong.

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

 

The dome is taken as a negative value in the calculator?

 

the Cosworth heads have a large cc vs stock = lower compression

 

There is a negitive value for the piston cc. Although the dome is high the valve cutouts are deep. I measured at -1.29cc Cosworth state -2.83
The Cosworth heads are taken into consideration.
Stock combustion chamber is 56cc CR is 9.74:1
Cosworth heads 58.5cc CR is 9.42:1

Even using my lower cc dish measurement I'm still getting a maximum of 9.9:1 CR. Whereas Cosworth state they are a 10.3:1 cr piston.

Wiseco 11:1 CR pistons have a 7cc dome. Which works out to be 11:1 CR in my calculations, as Wiseco advertised.

NOTE that the deck height in my engine is 20 thou less than that of a stock engine. I'm running Cosworth rods, which are slightly longer than stock. As such anyone running these pistons (sometimes sold online as high comp 11:1 CR) appear to have a CR considerably lower than stock!

 

http://www.z1auto.com/prodmore.asp?m...ne&prodid=4158

Cosworth makes both 10.3:1 and 11:1 pistons (as well as 8.8:1) - none of them are the same

 

Fully aware that there are multiple pistons by Cosworth. I own two sets, one PA2687 which I'm currently calculating at 8.9:1 CR (with my longer rods , hence the slightly higher CR) and a set of PA2688 which is currently calculating at 9.7:1 again with my longer rods. As such, unless my calculations are incorrect, with high comp 10.3:1 CR Cosworth pistons you will achieve significantly less than 9.7:1CR (I haven't got the stock headgasket compressed height or stock rod length)
Using stock approximation I'm getting around the 9.2:1 mark, a far cry from the advertised 10.3:1.