N5 RTC3571

DETERMINING COMPRESSION RATIO OF YOUR 2.25L PETROL ENGINE:

There are multiple head castings used on the engine over the years. The two standard compression ratios used by Rover are 7:1 and 8:1.

The easiest way to determine your head's compression ratio is to look at the top surface between the valve cover and the carburetor. You will see a head bolt located there. On some heads there is a flat rectangular boss under the head bolt that extends towards the carburetor to the edge of the head.

If your head does not have this boss it is an early 7:1 compression head. If your head has the boss it is a later casting that came in both 7:1 and 8:1 versions, depending upon the country the car was originally sold in. If the boss has no stamping on the top or a big 7 it is a 7:1 head and was probably originally sold outside the United States. If the head has a large 8 stamped on the top (could look like two '0's) then it is an 8:1 head.