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This is one of the first things I learned when I got my M3 so read and learn.
Most automotive enthusiasts are familiar with the concept of compression ratio. We know that in general a high compression ratio is good for performance and efficiency, but that it can also lead to "knocking" or "detonation". Therefore high compression ratios are usually associated with the requirement for high octane fuel and careful engine management. But there is more to it than that. Compression ratio is also dependent on cam duration, as we shall see. Let's begin by reviewing the definition of compression ratio. We will also investigate how basic engine dimensions affect the static compression ratio.
The schematic above shows the parameters that go into calculating an engine's static compression ratio. Compression ratio if defined quite simply as the volume above the piston at bottom-dead-center (BDC), divided by the volume above the piston at top-dead-center (TDC).
C.R. = V1 / V2
The volumes involved are the volume of the combustion chamber (V2) and the "swept volume" of the cylinder. The swept volume is obtained by multiplying the cross-sectional area of the cylinder bore (Ac) by the stroke. Thus the compression ratio can be written as:
On my 2.5L S14 the total volume of the combustion chamber (including valve reliefs, piston pop-up and head gasket thickness) is V2 = 59.52 c.c. The stroke is 87.0 mm and the bore is 95.0 mm . Thus Ac = 70.9 cm² . If we plug all of this into the equation above we get:
Thus my compression ratio is 11.36:1
Now if we look carefully at the equation for compression ratio above we note some interesting trends. Even if the volume of the combustion chamber (V2) is kept constant, we can increase the compression ratio simply by increasing the stroke and/or the bore size.
So let's say you rebuild your engine, but all you do is bore it out slightly and install identically shaped, but bigger pistons. You don't shave the head or change the stroke, or decrease the volume of the combustion chamber. You will still increase your compression ratio.
Or let's say you rebuild your engine but all you do is add a stroker crank. And you also order new pistons with the pins mounted farther up so that the piston does not intrude farther into the combustion chamber at TDC. Thus your combustion chamber volume is unchanged - all that changes is the stroke. You will still increase your compression ratio, as the equation points out.
None of this is rocket science really. But it is interesting nonetheless, and might not be noticed without a careful examination of the compression ratio equation.
It is instructive to remember that the static compression ratio that your engine displays on paper does not translate directly to higher cylinder pressures. The cylinder pressure (prior to ignition) during engine operation is dependent on what can loosely be called "dynamic compression ratio". The pressure is greatly affected by the timing of your valve events - i.e. cam duration and timing. Specifically, the intake valve closing point is intimately related to an engine's dynamic, or "effective" compression ratio.
Above is depicted the situation during the compression stroke of a performance engine operating at high RPM. Most are probably familiar with the strokes in a 4-stroke engine:
(1) intake (2) compression (3) power (4) exhaust.
Obviously during the intake stroke the intake valve must be open in order to let the air/fuel mixture into the cylinder. Then at BDC the intake valve closes so that the piston can move up and compress the mixture right? Wrong! The intake valve on a modern performance engine stays open well into the compression stroke.
Notice above that the piston has moved past bottom-dead-center (BDC), and is on its way up the bore in an attempt to compress the air/fuel mixture prior to ignition. Yet the intake valve is still open. In fact, with a any kind of performance cam the intake valve will not close until 50° - 75° past BDC! That's 28% - 42% of the way into the compression stroke!
But we just learned before that static compression ratio is directly related to stroke. In principle the piston cannot compress the mixture until the intake valve closes. Thus if the intake valve closes when the piston has already moved quite some distance up the bore, then the amount that the intake charge will be compressed is reduced. The "effective compression stroke" has been reduced. Does this mean that when an engine is operating that the dynamic compression ratio is lower than the static compression ratio? Well yes and no.
An engine with a performance cam operating at low RPM will suffer a loss of torque due to the fact that the effective compression ratio is reduced by the late intake valve closing point. However, as the RPM increases "inertia supercharging" becomes important. At high RPM's the intake charge is is moving into the cylinder at high velocity. As such it has a lot of inertia and will continue moving into the cylinder past BDC, even though the piston has changed direction and is now moving up the bore (towards the incoming charge). Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. When this happens an engine is said to have "come on the cam". In order to ensure that the mixture is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio. This is why high performance engines with aggressive camshafts also tend to have high static compression ratios.
Bottom line: Static compression ratio and cam choice should be considered as a system.
A mild cam with an early intake valve closing point will work well at low RPM. But at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. As a result performance at high RPM will suffer. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed". This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure.
On the other hand, an aggressive cam with a late intake valve closing point will work well at high RPM. But at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur. As a result torque and performance will suffer. If a low static compression ratio is used with an aggressive cam (i.e. a late intake valve closing point) then the mixture may end up being "under-compressed". Thus a high performance cam with long duration should ideally be combined with a higher static compression ratio. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio.
Hope you enjoyed that.
Most automotive enthusiasts are familiar with the concept of compression ratio. We know that in general a high compression ratio is good for performance and efficiency, but that it can also lead to "knocking" or "detonation". Therefore high compression ratios are usually associated with the requirement for high octane fuel and careful engine management. But there is more to it than that. Compression ratio is also dependent on cam duration, as we shall see. Let's begin by reviewing the definition of compression ratio. We will also investigate how basic engine dimensions affect the static compression ratio.
The schematic above shows the parameters that go into calculating an engine's static compression ratio. Compression ratio if defined quite simply as the volume above the piston at bottom-dead-center (BDC), divided by the volume above the piston at top-dead-center (TDC).
C.R. = V1 / V2
The volumes involved are the volume of the combustion chamber (V2) and the "swept volume" of the cylinder. The swept volume is obtained by multiplying the cross-sectional area of the cylinder bore (Ac) by the stroke. Thus the compression ratio can be written as:
On my 2.5L S14 the total volume of the combustion chamber (including valve reliefs, piston pop-up and head gasket thickness) is V2 = 59.52 c.c. The stroke is 87.0 mm and the bore is 95.0 mm . Thus Ac = 70.9 cm² . If we plug all of this into the equation above we get:
Thus my compression ratio is 11.36:1
Now if we look carefully at the equation for compression ratio above we note some interesting trends. Even if the volume of the combustion chamber (V2) is kept constant, we can increase the compression ratio simply by increasing the stroke and/or the bore size.
So let's say you rebuild your engine, but all you do is bore it out slightly and install identically shaped, but bigger pistons. You don't shave the head or change the stroke, or decrease the volume of the combustion chamber. You will still increase your compression ratio.
Or let's say you rebuild your engine but all you do is add a stroker crank. And you also order new pistons with the pins mounted farther up so that the piston does not intrude farther into the combustion chamber at TDC. Thus your combustion chamber volume is unchanged - all that changes is the stroke. You will still increase your compression ratio, as the equation points out.
None of this is rocket science really. But it is interesting nonetheless, and might not be noticed without a careful examination of the compression ratio equation.
It is instructive to remember that the static compression ratio that your engine displays on paper does not translate directly to higher cylinder pressures. The cylinder pressure (prior to ignition) during engine operation is dependent on what can loosely be called "dynamic compression ratio". The pressure is greatly affected by the timing of your valve events - i.e. cam duration and timing. Specifically, the intake valve closing point is intimately related to an engine's dynamic, or "effective" compression ratio.
Above is depicted the situation during the compression stroke of a performance engine operating at high RPM. Most are probably familiar with the strokes in a 4-stroke engine:
(1) intake (2) compression (3) power (4) exhaust.
Obviously during the intake stroke the intake valve must be open in order to let the air/fuel mixture into the cylinder. Then at BDC the intake valve closes so that the piston can move up and compress the mixture right? Wrong! The intake valve on a modern performance engine stays open well into the compression stroke.
Notice above that the piston has moved past bottom-dead-center (BDC), and is on its way up the bore in an attempt to compress the air/fuel mixture prior to ignition. Yet the intake valve is still open. In fact, with a any kind of performance cam the intake valve will not close until 50° - 75° past BDC! That's 28% - 42% of the way into the compression stroke!
But we just learned before that static compression ratio is directly related to stroke. In principle the piston cannot compress the mixture until the intake valve closes. Thus if the intake valve closes when the piston has already moved quite some distance up the bore, then the amount that the intake charge will be compressed is reduced. The "effective compression stroke" has been reduced. Does this mean that when an engine is operating that the dynamic compression ratio is lower than the static compression ratio? Well yes and no.
An engine with a performance cam operating at low RPM will suffer a loss of torque due to the fact that the effective compression ratio is reduced by the late intake valve closing point. However, as the RPM increases "inertia supercharging" becomes important. At high RPM's the intake charge is is moving into the cylinder at high velocity. As such it has a lot of inertia and will continue moving into the cylinder past BDC, even though the piston has changed direction and is now moving up the bore (towards the incoming charge). Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. When this happens an engine is said to have "come on the cam". In order to ensure that the mixture is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio. This is why high performance engines with aggressive camshafts also tend to have high static compression ratios.
Bottom line: Static compression ratio and cam choice should be considered as a system.
A mild cam with an early intake valve closing point will work well at low RPM. But at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. As a result performance at high RPM will suffer. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed". This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure.
On the other hand, an aggressive cam with a late intake valve closing point will work well at high RPM. But at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur. As a result torque and performance will suffer. If a low static compression ratio is used with an aggressive cam (i.e. a late intake valve closing point) then the mixture may end up being "under-compressed". Thus a high performance cam with long duration should ideally be combined with a higher static compression ratio. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio.
Hope you enjoyed that.