The V8 Owners Forum -

hi few questions, why did land rover or rover itself decide to make engines with different compression ratios? what's the benefit of a low com engine? and how do you make and engine either low compression or high? cheers

At the time the RV8 was introduced in the UK, there were different grades of petrol, according to octane ratio. Commonly 2-5 star. 92-100 octane. 2 star was used for trucks etc - 5 star for performance cars. The majority of cars needed 3-4 star.
The first Rover V8 as fitted to the P5&6 was designed for 5 star - 100 octane petrol. Within a few years that was phased out and 98 4 star became the best available, so the CR was dropped. Better head design with the SD1 allowed it to be raised slightly. The low compression units as fitted to Land Rover etc are designed to run on the lowest octane petrol.
SD1s are marginal on 95 octane and run best on Super Unleaded which is 97-98 octane. Most later engines are fine on 95 octane.

Just to add,
Rover achieved different comp ratios by changing the volume of the dish in the top of the pistons.

Denis

Different compression ratios also affected maximum power output.

Chris.

DaveEFI wrote:At the time the RV8 was introduced in the UK, there were different grades of petrol, according to octane ratio. Commonly 2-5 star. 92-100 octane. 2 star was used for trucks etc - 5 star for performance cars. The majority of cars needed 3-4 star.
The first Rover V8 as fitted to the P5&6 was designed for 5 star - 100 octane petrol. Within a few years that was phased out and 98 4 star became the best available, so the CR was dropped. Better head design with the SD1 allowed it to be raised slightly. The low compression units as fitted to Land Rover etc are designed to run on the lowest octane petrol.
SD1s are marginal on 95 octane and run best on Super Unleaded which is 97-98 octane. Most later engines are fine on 95 octane.

Further to this Dave , what about LPG ... is it better to have a high or Low CR for best MPG etc ?

You know the answer to this one Keith!!, high comp for LPG.

Chris.

Thankyou Chris

I believe low compression commercial engines use a very mild camshaft profile which increases dynamic compression ratio and low speed torque.

Static compression ratio is therefore decreased to prevent detonation on low octane fuel.

As the camshaft gets more radical and valve overlap increases, more compression is needed. A full race cam needs up to 15:1 to work properly as the piston is halfway up the bore before the valves are shut.