All the power is in the head. Part 4 - Compression Ratio

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Guy Croft
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All the power is in the head. Part 4 - Compression Ratio

Post by Guy Croft »

In part 3 we saw the influence of burn rate and examined some of the factors relating to it. Here we discuss one of the most important and misunderstood issues:

Compression ratio

Static CR

The 'static' compression ratio is defined as:

CR = (swept volume + clearance volume) / Clearance volume

Or = 1 + Vs/Vc

The clearance volume is assessed by a combination of measurement and calculation:

Vc = volume of combustion chamber + volume of head gasket +/- volume from piston crown to block deck + valve reliefs + clearance between piston crown and top ring + volume of piston bowl (or - volume of piston dome). In practice the top ring clearance is usually fairly insignificant and ignored.

Vs is the literally the volume 'swept' by the piston crown from bottom centre to top centre, and is calculated from the bore and stroke according to Vs = Stroke x Pi x diameter squared/4. If for example the swept volume is 500 cc and the clearance volume is 50cc, the CR is 11/1.

However when Vs is calculated not from the commencement of the compression stroke (at BDC) but via the ‹Å“effective stroke¢ž¢ measured from the closure point of the inlet valve (say 70-90 crank degrees after BDC) - the static CR can appear quite low. Some say that as a ‹Å“rule of thumb¢ž¢ for a race engine the CR assessed by this method should not fall below 9.5/1 and thus the piston dome and head need to be tailored to this end.

Normally aspirated race engines need a high static CR and boosted (turbo or supercharged) ones need a low static CR. The limit of the CR that an engine will tolerate before the onset of knock (detonation) is the octane rating of the fuel, though it is certainly true to say that the onset of knock is affected by other factors such as air density, humidity and temperature. Run a boosted engine with too high a static CR and detonation is sure to occur.
The optimum value of 'static CR' can unfortunately only by dyno and track test experience. Static CR of 12.7/1 or higher are not untypical on F1 engines, where the pistons are junked after each Grand Prix, but on a historic rally or endurance unit 10-10.5/1 might be a wiser choice.

There is certainly a trade off - discussed in Part 3 between high CR and good burn, a threshold for many engines that is not worth exceeding. Additionally the stress and extra heat that comes from running a very high static CR ¢‚¬Å“ it affects the oil and cooling system as well as the mechanical components - must not be overlooked. The pressure and wear on the rod bearings especially and piston pins, rod bushes can be quite severe, while piston alloys - even the best forged ones, have a finite fatigue life especially at the elevated temperatures that a very high CR can create.

There are many ways to raise the static CR - bigger bore or longer stroke, domed pistons, milling down the block and head, thinner head gaskets, etc. To lower it dished pistons and all sorts of other methods are used, including decompression plates under the head, shorter rods, fitting the valves deeper into the head, opening out the combustion chamber, milling the pistons, bigger valve reliefs.

The static CR suggests that a volume of air equivalent to Vs (ignoring the gasoline here because it occupies an insignificant percentage of the swept volume) with the cylinder fully filled to capacity at atmospheric pressure (but excluding the clearance volume) - will be compressed to a much smaller volume Vc at top centre (TDC), and that this is good for thermal efficiency. In practice - as a determination of an engine's real behaviour static compression ratio does not tell the whole story. We need to look at it a bit closer.


The 'effective' CR is the real arbiter of how much torque the engine generates and it depends on the truevolume Vs of air in the cylinder and the extent to which that volume is compressed. We are assuming here, that a race engine is capable of superior cylinder filling than its production counterpart, and this is generally true.

With all gasoline engines - boosted or normally aspirated the higher the 'effective compression ratio' the higher the thermal efficiency, and the higher the torque output - everywhere in the rpm band. High effective CR shows up as a powerful, driveable torque characteristic with a high peak torque figure.

Effective CR is very much tied in with Vr - volumetric efficiency. The extent of the filling throughout the speed/load range of the engine depends on the air's mass flowrate into the cylinder from the moment of inlet valve open and the momentum it has to continue filling the cylinder after bottom centre (BDC) even as the piston is going up. This determines whether the cylinder will fill at atmospheric pressure, or over it ¢‚¬Å“ a condition that it is certainly possible to achieve; and if not thru the whole rpm band then certainly in parts of it.

One reason why F1 engines are capable of power outputs of 800+ bhp from 3 liter engines ¢‚¬Å“ apart from the fact that they have 8-10 cylinders (more firing cycles) ¢‚¬Å“ is that they can generate volumetric efficiencies well over 100%.

The cylinder filling depends on engine speed, throttle position, the flow characteristics of and pressure wave effects in the head as a whole, camshaft timing (particularly on the overlap phase), contamination of the intake volume by exhaust residuals left over when the intake stroke commences - and many other factors.

A boosted engine may well require a low static CR to prevent detonation on a certain octane grade of fuel, but the high Vr and thus effective CR of even mildly boosted engines like the Lancia Volumex - running static CR 7.5/1 and blowing at maximum 5-6psi boost - will create enough torque to match the most powerful normally aspirated 8v equivalent.

On any given n/a engine that is 'running quite well already', raising the static CR alone will only yield a modest increase in torque. In very general terms the more lift at overlap and duration the cams have - the higher the static CR needs to be. This a rather indeterminate thing. The lowest limit for a scratch-built competition engine needs to be say 9.6/1 or higher is better. A low CR n/a engine will never give of its best if fitted with really high performance cams.

With turbocharged engines CR is a bit of a non-issue. It is certainly most unwise to build a turbocharged motor with a high CR to try and 'improve' the off-boost' performance. With boosted engines one should nearly always reduce the static CR compared with the OE unit if the boost is to be run higher, unless using very octane fuel - and then treat the unit, in tuning terms much as a normally aspirated one. That is to say try to gain performance by increasing the 'effective CR'.

With turbo engines this comes from a combination of high mass flow from the compressor and high flow thru the inlet tract of the engine. Naturally none of this will work unless the turbine end is getting the energy from the exhaust phase. It does not come from merely building up huge boost in the manifold - this often means the unit is 'overboosted' and may be running borderline surge in the compressor. A good turbocharged engine run back-to-back with the same turbo, will give more power for lower boost, after good head prep to improve flow and maybe cam change too - showing that the air really is getting into the cylinder at the right time.

We want - in summary - to build up high effective CR by means of high volumetric efficiency and 'ramp up' the cylinder filling massively, then evacuate it far more effectively. All this comes from the head. .

The 'bottom end' of the engine merely serves to transmit the energy to the drivetrain, and all we can do there is try to make it strong enough to do the job - and minimise the losses. How we do this practically and how different heads and setups respond in different ways, comes next.
GC race LC (low compression) piston alongside OE one, this Uno Turbo motor was prodigiously powerful due to high effective CR & fantastic Vr - even with 7.2/1 and only 12psi boost, OE cam..
GC race LC (low compression) piston alongside OE one, this Uno Turbo motor was prodigiously powerful due to high effective CR & fantastic Vr - even with 7.2/1 and only 12psi boost, OE cam..
NC Uno T piston comparison.JPG (55.07 KiB) Viewed 50755 times
This style of dome on a high compression forged piston gives 10.8/1 in a normally aspirated 2 liter Fiat TC engine.
This style of dome on a high compression forged piston gives 10.8/1 in a normally aspirated 2 liter Fiat TC engine.
GC pistons 8v HC.JPG (10.34 KiB) Viewed 49434 times

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