Stroke	2.377 in. (60,37 mm)
	Bore	2.6770 in. (68,00 mm)
	Capacity	875 cc (53.4 cubic inches)
	Valve pockets    normal compression      low compression	yes 10 : 1 8 : 1 (?)
	Length	2.054 in.
	Compression height	standard Imp: 1.050 / 1.055 in. Imp Sport: 1.055 / 1.060 in.

Compression ratio

Imps have an unusually high compression ratio of 10 to 1, possible because of an advanced combustion chamber shape. (8 : 1 was available for certain overseas territories). The Rally Imp, built only to order, had a compression ratio of 10.5 to 1. The van had ... The 930 cc engine had ...

Octane rate
The octane ratings of petrols tells you how much the fuel can be compressed before it spontaneously ignites. Lower-octane petrol can handle the least amount of compression before igniting. When petrol ignites by compression rather than because of a spark, it causes 'pinking' in the engine, which should be avoided at all times.
Most standard cars run at a compression ratio of between 8 and 9 to 1. These engine will work quite happily on 3 or 4 star fuel. In the case of a CR approaching 9.5 to 1, they will run reasonably well on fuel with an octane rating of around 98. With a CR of 10 : 1   a rating of 100 octane becomes more or less essential. If petrol of an octane rating of 95 is used, most Imp engines will start to 'pink' easily and tend to 'run on' after the ignition is switched off.
(Modern fuels' octane rates have less to do with having strings of 8 carbon atoms, but it still measures the compressability. Hence you can have an octane rate of more than 100, where originally it was a percentage...)

Usable CR
A higher CR only means more power if the combustion chamber is modified in shape, so as to increase both volumetric and combustion efficiency, then a real increase and overal engine performance is possible. When CR is increased to well beyond 11 : 1, some engines start to get rough and will not develop any more power. A frequent phenomenon is 'combustion rumble' as well as other forms of indigestion symptoms, like eating head gaskets. This means that this particular combustion chamber design has been enlarged beyond its limit. It is probable that bore size, as well as 'squish' area, have an effect on maximum usable CR.

In competition, where power is the main concern, higher CR is wanted. Competition set-ups usually raise the CR. Get a cylinder head and diminish the combustion chamber volume with no sacrifice to breathing.
Hartwell would reshape combustion chambers to increase the compression ratio 10.5 : 1. (Hartwell folder; Hartwell parts list). And their competition 998 cc blocks would have 12 : 1 (the RP1 as well as the RP2-set ups). The Emery GTI in stage 4 tuning had 11.5 : 1

What is the compression ratio?

The engine compresses a cylinder-full of air and petrol into a much smaller volume and then ignites it with a spark plug. The amount of compression is called the compression ratio

Compression ratio: how the swept volume relates to the clearance volume at Top Dead Centre.
CR = (swept volume + clearance volume) / clearance volume

Compression Ratio Calculation By Bowling

If combustion chambers and pistons 'carbon up', this increases the CR noticeably.

Increasing the compression ratio

10.0 : 1 is high already - don't strive to blow up the engine. Paul Emery used tot say that "One of the main reasons for Imp engines giving trouble is when the compression ratio has been increased too far".

The head can be skimmed to increase the compression ratio, but this is not a good idea if the head has previously been skimmed, and should always be done with caution anyway - if reliability is not to suffer.

Paul Emery:

I often find Imp engines where machining of the head has been carried out not in an attempt to increase the engine's power output - but merely in an attempt at stopping the head gasket from blowing. The top face of the block is often machined for the same reason.

As an absolute maximum, I reckon, 2-3 thou should be all you should skim off an Imp head. And yet I find road-going Imp engines where some garage has hacked off as much as 15 thou... Wildly increasing the compression ratio in this way will nearly always accentuate any problems.

Having skimmed the head to clean up the gasket face, you may need to reduce resultant raised compression ratio by removing metal from around the inlet valves in the combustion chambers. Keep off the gasket mating area!

Compression ratio graph

As standard the Imp is high for a road engine at 10.0 : 1, but on (even mildly tuned) road engines it is possible to go up to 11.0 : 1 without the need for 5 star fuel. For rally engines I use between 11.5 and 12.0 : 1, and on race engines I aim for 12.5 : 1, although this is sometimes difficult to achieve with a flared combustion chamber.

To calculate your own compression ratio it is necessary to build up the volume from the dimensions below and then check the ratio on the graph.

Reinz 875 gasket	3.3 cc
Three layer gasket	4.0 cc
Wills Rings	1.9 cc
Std piston cut outs	0.7 cc
998 piston cut outs	1.6 cc
Large 998 piston cut outs	2.1 cc
Pistons down bore per thou	0.1 cc

Make sure that you add in one type of head gasket, one of the piston cut out sizes and the distance the pistons are down the bore.

A vernier sprocket can be used to increase compression ratio without the losses of inaccurate valve timing. At Imp 09 they were for sale (Shrigley Engineering) and I photographed the instructions.

Supercharging is not compatible with high compression ratio ... generally C.R. is lowered from the original specifications.