7:1 compression ratio

[Bronze Green]

Quick question for the more knowledgeable. Was the 7:1 compression engine only offered for foreign markets or could you get it in the U.K. also?

[geoff]

UK as well

[Wittsend]

When Series 2s 2¼ litre petrols came out "pool" petrol had a low octane value. And in some export markets pool petrol was all that was available at the time.

Soon after the higher comp heads, 8:1 CR came along.
I'm not sure if the high comp heads were an optional extra from the "off".

[Craig T]

What makes the difference, is it piston crowns or the head thickness?

I have a 7:1 engine but then mine was an export model. Engine number is 25289485J. I think it's 7:1 anyway, not sure how I found that out?

Craig.

[Wittsend]

I think the pistons are the same - it's head thickness. In the old forum archives, somewhere I have the head heights for 7:1, 8:1 and 9:1 heads.

Whilst the stamping (or not) on the head tab can be a good indicator of what you have - you can never be certain what a PO has had machined. Measurement of the head is the only sure way to know.

Note: you can not machine a 7:1 low comp head to 9:1 CR 

[Adam1958]

Add a turbo to increase cylinder pressure. 😉🥴

[Wittsend]

Before turbos were "invented" people had great success with supercharging blowers for their Series 1 engines 

IIRC I think Allard had a kit.

[Richard]

In the old forum archives, somewhere I have the head heights for 7:1, 8:1 and 9:1 heads.

I didn't have a look, but I found this on Expedition Land Rover about the difference between a 7:1, 8:1 and 9:1 head... (And here's a compression ratio calculator – with explanation and all.)
Richard

[Craig T]

Maybe I found the number 7 stamping in my head. It is the original engine and head and it had never been apart before, all original size bearings and pistons still in it with 78k miles on it now. Not sure what power difference it makes going to 8:1 from 7:1 but it feels lively enough for me.

I'll take a closer look again at the weekend if I get her out this time round.

Craig.

[diffwhine]

The best document I have found which explains all this is from ACR. Its in the downloads section of their website and is called "Cylinder Head Identification Notes".

[Bronze Green]

The best document I have found which explains all this is from ACR. Its in the downloads section of their website and is called "Cylinder Head Identification Notes".

Roland at ACR knows his stuff and a nice chap to deal with.