Compession ratio advice required.

mgbv8 · Post by **mgbv8** » Sun Jul 20, 2008 5:20 pm

I've not quite got my head around the different compression ratio's on the earlier engines.
I've got a pre SD1 3500 @ 8.25:1
I've got a later stiff block @ 9.35:1

But apart from the fuel injetor coutouts, they appear to be the same. ie, the compression chambers look the same. And the piston tops look the same?
So how does the CR go up on the later block? , or have I got a wrong pair of heads on the one with the higher CR stamped on it.

Sorry for the numpty question, I looked back in the forum but couldnt really find owt.

And I've lent all my V8 reference books to a mate who's doing his first build.

Post by **katanaman** » Sun Jul 20, 2008 5:42 pm

Compression is set by the bowl on the pistons. You also need to remember that a larger engine needs a larger piston bowl for the same compression because it has a larger swept volume. On top of that the later heads (4.0 4.6) had had smaller chambers to compensate for the thicker head gaskets.
None of that explains why you have two 3.5 engines with different stamped compression ratios but have the same piston bowl. My guess is one of them has had the pistons changed to alter the CR at some time in its life.

mgbv8 · Post by **mgbv8** » Sun Jul 20, 2008 5:47 pm

Thanks Marki
So the next question is, how do I check which pistons I have?
Is there a table of piston bowl depths I can refer to?

Post by **katanaman** » Sun Jul 20, 2008 5:56 pm

A picture is worth a thousand words so might be worth trying that. They are usually quit noticeable between the ratios. Failing that measure the volume and someone is bound to recognise it.

bugster · Post by **bugster** » Tue Sep 02, 2008 10:21 pm

The two may have pistons with different pin heights which means that 1 piston may sit higher in the block than the other.

Post by **katanaman** » Thu Sep 04, 2008 4:44 pm

bugster wrote:The two may have pistons with different pin heights which means that 1 piston may sit higher in the block than the other.

Not on a standard 3.5 Rover they wont.