Rover V8 Compression Ratio - expert advice needed

[Ian C]

Hi,

I am on the same road as jonny m (see thread “Rover V8 Cylinder Head: Minimum Thickness) but I thought I would start a new thread as I have slightly different issues.

My engine is a 1992 3.9 RV8 from a Range Rover Classic, the block is stamped 9.35:1 and the head thickness (dimension A) is 0.93”. The Rover manual states this should be 0.94” – new so it would indicate that the heads have been skimmed by 10 thou !? However the face of the cylinder heads does not support this as it appears to have original casting marks still evident (see picture 04).

I have measured all the volumes above each piston (when at TDC) and have calculated the volume of the composite gasket (thickness 1.2mm before compression). I am currently measuring the volumes of the combustion chambers prior to establishing the current CR. The first combustion chamber volume (CCV) came out at 33.7cc giving a CR of 9.17:1. The engine has been rebored +20 thou so, in theory, the CR should have increased by approx 0.1:1.

My questions are –

Has the cylinder head been skimmed or were there variations in the “A” dimension when new?

Is the measured combustion chamber volume of33.7cc “normal”? – I was expecting either 28cc or 36cc.

Is the quoted CR of 9.35:1 accurate or does it vary from engine to engine? and, if so, by how much?

Any and all input appreciated.
Ian

[kdavies3]

The accuracy of the 9.35:1 should be within 0.01 cr.
You need to work with the compressed thickness of the gasket.
If the heads have been skimmed it will have been on the bottom face.

[Ian C]

. . You need to work with the compressed thickness of the gasket.
If the heads have been skimmed it will have been on the bottom face.

Kev,

Thanks for the response. I have assumed (always dangerous) that the composite gasket will crush from its relaxed thickness of 1.2mm (0.12cm) to 1mm (0.1cm) when in place and the cylinder head torqued to the correct setting – is this reasonable?

You can see the bottom face of the cylinder head in pictures 3 & 4 – does it look like it has been skimmed?

Ian

[Sidecarbod]

Personally I would not worry too much about any figures that Rover state, their engines came off a production line and therefore all sorts of things could varry.

You are doing the right thing by measuring the volumes, 0.2mm compression of the gaskets seems like a reasonable figure, you could go for 9.75:1 if you want to stick to plain unleaded or 10:1 if you don't mind using Super unleaded. Of course other things affect the actual CR, mainly the camshaft.

AJMHO!