Engine assembly: 87 mm barrels with EMPI heads

[Malc]

I have a technical question about engine configuration, parts compatibility and compression ratio...
I've bought a set of 87 mm barrels and pistons from VW heritage, along with some EMPI GTV2 heads (40 mm inlet 35.5 exhaust). My crankcase is a stock rebuild I think, crank stroke is 69 mm. Deck height at TDC appears to be about 3 to 3.5 mm, and cylinder head combustion chamber is 55 cc. Based on that, I calculate my compression ratio to be 6.6 to 1. However, my original 85.5 mm pistons had a clearly bigger deck height, around 8 mm. The old heads have (I think) a combustion chamber volume of 47 cc (043 series heads) and I calculate this to have a compression ratio of about 5.5 to 1. Both of these seem really low, and the Haynes manual says the original compression ratio was 7.5 to 1. Any thoughts? Don't understand... Any wisdom greatly appreciated!

 

[gagvanman]

Hi. Your bore and crank size gives 1641 cc's.
With a deck height of 3mm the compression ratio is 6.6, the 3.5mm it is 6.4
To get a compression ratio of 7.5 you need to make the deck height 1.36mm

 

[Malc]

It probably won't come as a surprise to hear that I was the problem... Took no. 1+2 cylinder head off this afternoon, this time I measured the original deck height carefully : about 2.5 mm, so that works out at about 7
5 to 1, as expected. So now I hope that the problem I saw with the new pistons and cylinders was also user error, probably not taking care that I had the cylinder really properly pushed into the crankcase when I checked the clearance... Keeping my fingers crossed for when I fit the next new pair of cylinders and check the deck height again (but carefully this time)