Compression Ratio
There is no magic number here, the ratio needs to be selected with regard to the type of fuel used and also the cam timing. I'm not convinced that the petrol currently sold is as bad as some people make out, though there does seem to be some variation from batch to batch. Cam timing, and in particular the intake closing event has a huge effect on cylinder pressure and therefore compression ratio selection. At slower speeds, a delayed intake closing (such as is used with hi-po cams) will regurgitate lots of cylinder pressure back into the intake, making it possible - and desireable - to use a much higher ratio. As speeds increase and the engine gets "on the cam" the momentum of the intake charge causes this reversion to decrease or stop altogether, leading to much greater cylinder filling and pressure, and this in turn to a jump in torque and horsepower. At these higher speeds there is also a big increase in the amount of turbulence in the chamber, and this is why we can get away with the higher pressures at high speeds without detonation. It's also the reason that no more spark advance is required over a certain speed.
Despite all the other shortcomings of the Holden heads the chambers seem to support reasonably efficient combustion but even so I wouldn't get too greedy. As a very very rough rule of thumb, I'd suggest limiting CR on an engine with relatively short cam timing (similar to a stock cam) to around 9.5 to 1. Higher revs and longer cam timing will allow CRs of up to say 11:1 to be used with decent petrol, but if you are going to run high compression you'd better make sure you're right on top of the cooling, ignition timing and mixture distribution, otherwise it's easy to rattle the engine badly.
One good way to select the compression ratio of the engine is
to pick an appropriate dynamic compression ratio and
then work backwards from there to arrive at the corresponding static
ratio. What's the difference between the static and dynamic figures?
The dynamic ratio takes into account the distance the piston travels after BDC
before the intake valve closes, and therefore gives a better indication
of cylinder pressures when the engine is operating. Generally a dynamic
ratio of 9:1 will be in the ballpark for an engine running on PULP.
Adjust this figure up or down to allow for different fuels, air densities
due to altitude or boost etc. There are some good online calculators (see the
links section) or you can use the table below as a rough guide (the cam timing is
"advertised", not at 0.050"):
| -Intake Closing after BDC-- | --Static CR- |
| 60 | 9.6:1 |
| 65 | 9.8:1 |
| 70 | 10.1:1 |
| 75 | 10.4:1 |
| 80 | 10.8:1 |
| 85 | 11.2:1 |
Just remember these figures are only a guide; they may need some amount of adjustment to suit your engine and its operating conditions.
Builders of blown engines often have to reduce the ratio, and to be honest I'm less than impressed with some of the methods used. Things like pistons with deep circular dishes and decompression plates certainly reduce the pressure, but they kill any squish that may have existed in the chamber. This is just throwing horsepower away. If I was building a blower motor I'd be trying to keep the clearance between the piston and the flat face of the head to a minimum to promote turbulence (squish), just like in a normally aspirated engine. Then to reduce the CR, Id try to make the chamber deeper, possibly cutting a bowl in the piston that matches the head chamber but without touching the squish pad area. This approach should make significantly more power than simply spacing the head away from the deck.
For a naturally aspirated high-performance engine, you'll probably be using a small chamber head to get the desired ratio. Measure the chamber volume carefully to verify the ratio, the usual method being to cover the chamber with a piece of perspex with a small hole in it, through which you can pour light oil from an accurately graduated container or burette. You may need to machine material off the head to increase compression, and obviously if this is taken to extremes the stiffness of the head may be reduced to the point where it becomes difficult to keep the head gasket in. In these cases you may need to resort to o-ringing or domed pistons.
Here is the formula for calculating CR (the volumes for the
cylinder and chamber can be in cc or cu.in so long as you use the
same units for both. One cubic inch equals about 16.39cc):
CR=(D + V + DC + G + CC) / (V +DC + G + CC)
where:
CR = Compression Ratio
D = Displacement of one cylinder
V = Piston Volume (will be a negative value with a domed
piston)
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume
The formula to work out gasket or deck volume is:
V=0.7854 x d x d x g
Where:
V = volume in cc
d= Bore diameter in cm
g = gasket thickness (or deck height)
You can use the above formula with a slight modification to work
out how much a chambers volume will be reduced by if it's machined
by a certain amount:
V = 0.7854 x d x d x tm x ca
Where:
V = volume reduction in cc
d = bore diameter in cm
tm = the amount machined off the head in cm (10 thou is about
0.025cm)
ca = is the proportion of bore covered by the chamber bowl eg. if
the chamber area is about 60% of bore area use 0.6 for ca
Because there is so much possible variation in chamber volume from engine to engine it's important that the actual volume is measured so that the actual compression ratio can be calculated and adjusted if necessary. It's been my experience with the little Holden that the CR nearly always works out to be quite a bit less than Holdens original rating. So an engine that was originally rated at 9.4:1 might only measure 8.9:1 or even less. Piston manufacturers ratings are equally unreliable so whatever you do don't base your work on an assumed existing CR - you'll probably end up with less compression than you planned to have. As a very very rough rule of thumb though, on a street engine you could reasonably expect to use a small chambered head (as used on stock 149/161/173/179/186 engines) with flat-tops on engines with 186cu in. or less. The smaller of these engines will probably need some machining of the head to get the compression over 9:1. Engines of say 192 or 202cu in. or more with flat-tops might be better off with the bigger chambered 202 heads, depending on the cam timing.