When going for a bigger engine There is one thing
that is often overlooked and that is the compression ratio.
With a bigger engine, the SD1 head and just some standard pistons the compression ratio could
be raised too much. Just see the effect for your self with the calculator on the right by
increasing the bore and/or stroke and watch the compression ratio rise
The compression ratio is the ratio between the swept volume of one cylinder including the
combustion chamber area on one hand and the volume of the combustion chamber alone on the other hand.
In formula: CR = (V+C)/C
The swept volume of a cylinder can be calculated from its bore and stroke.
Getting the volume of the combustion chamber is a little more difficult.
It is not just the volume of the combustion chamber in the head but it can also be
for instance the volume of the combustion chamber in the piston. To sum it all up:
Combustion chamber volume
- Volume of the combustion chamber
- Volume of the combustion chamber in the piston
- Volume of the area above the piston but below block deck
- Volume in the cylinder head gasket
- Volume in the area between the piston/cylinder wall above the piston rings
One picture says more than thousand words so the figure below shows the various areas
which make up the total combustion chamber volume.
Volume in the combustion chamber
The standard SD1 volume of the combustion chamber is 36 cc. The Buick 300 head used to have
56 cc. Later bigger Rover engines have smaller combustion chambers with about 29 cc
Volume of the combustion chamber in the piston
The standard piston does have a small chamber area in the head. The compression ratio
is mainly altered by Rover by selecting different pistons with larger or smaller
dish volume in the piston. Range Rover engines for instance
have quite a large piston dish volume to reduce the compression to 8.35:1
Volume of the area above the piston but below block deck
The top of the standard piston is flat with the deck of the block. But if you have a shorter piston
you will have created extra volume above the piston, reducing the compression ratio.
Volume in the cylinder head gasket
The thickness of the head gasket also has an effect on the volume of the combustion chamber.
A thicker gasket will give a lower compression ratio. Notice that the thickness
of the gasket has to be calculated when the head is torqued down. Increase the thickness
in the calculator and you can see it has quite an effect actually.
Volume in the area between the piston/cylinder wall above the piston rings
The compression ratio as calculated on the right can be found in the
technical specifications of an engine. However in the real world there are more variables wich reduce the actual
compression ratio, factors such as:
This little area also plays a small role in determining the compression ratio
- Timing of the inlet valve
- Inlet resistance, including the throttle position
- Leakage along valves and piston
Timing of the inlet valve
To allow more fuel/air enter the engine the inlet valve closes after the BDC mark. At low revs the air/fuel mix is moving
relatively slow and there is time enough for the piston to push the mixture back into the inlet. The later the inlet closes
the stronger this effect will be. Only when the revs are high enough will the full amount enter the cylinder.
This is around the point where the engine delivers maximum torque.
When the inlet valve closes later the effective compression ratio will also be reduced. The effect can easily
be seen when entering different values for the closing angle in the calculator on the right. Until about 35 degr.
the effect on the CR is not very big. But after that it really brings the CR down fast!
This is how an economy cam works......it closes the inlet valve earlier resulting in a higher CR and thus
better thermal efficiency
Inlet resistance, including the throttle position
The compression ratio is based on the compression of air/fuel under atmospheric pressure. When the revs rise the
inlet resistance begins to play its role this means the pressure at the start of the compression stroke is lower than the
atmospheric pressure because of the inlet resistance. The ratio between the amount of air theoretically entering the cylinder and the
amount when the inlet resistance is taken into account is called the volumetric Efficiency (VE).
Normally the VE is around 70-80% on a very good designed inlet system around 90%. Some racing cars with pulse tuning can achieve
ratio's above 100% in a narrow power band.
The factors above reduce the compression ratio. When the throttle is closed the ratio gets especially bad. Sometimes
not even more than 3:1 ! That this reduces engine efficiency comes as no surprise. It is the main reason a Diesel engine
wins in efficiency over an Otto engine because the Diesel engine doesn't have a throttle valve as such which reduces flow and
thereby the compression ratio. In case of the Diesel engine the power is regulated by injecting more or less fuel instead of
regulating the amount of air.
For optimal efficiency the compression ratio should be as high as possible. A too high ratio can give premature ignition
resulting in engine damage. There are a lot of variables in an engine which determine the maximum compression ratio an engine
- Octane rating of the fuel used
- Form and size of the combustion chamber
- Cylinder bore diameter
- Location and type of spark plug
- Ignition timing
- Engine running temperature
- Fuel/air distribution in the combustion chamber
- Fuel/air ratio
The table on the right gives some compression ratio's for the V-8 in relation to
the octane rating and the bore diameter.
Thanks to: Adriaan Briene