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How To Raise The Compression Ratio, Safely And Effectively.

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Raising the Compression Ratio.

How to do so safely and effectively, by measurement and calculation.


Some people want to keep their Triumph exactly as it left the showroom, others like to modify it.       One of the most popular ways of doing the second is to improve performance, and a simple way to do so is to increase the "Compression Ratio" (CR) of the engine. But what is CR, why is it important and how can it be raised safely, without risking damage to the engine?             The basis of raising the CR is to have a thin skim of metal taken off the face of the cylinder head, to make the combustion chamber slightly smaller, so that the mixture in them reaches a higher pressure before it is ignited, giving greater power.   There are "recipes", recommended thicknesses to achieve a given CR, but they just don't work if someone has skimmed it before, or the combustion chambers have been modified.

I've raced six cylinder Triumph engines for twenty years and always built them and done any modifications myself, including "flowing" the cylinder heads and getting them skimmed to increase the CR.    I've learned how to do so by reading and practicing.   I may not be able to build you a 100% race-winning engine - I'd like to be able to do so for myself! - but I can show you how to measure your present CR and to calculate, not guess, how thick that skim should be, to achieve a useful and above all, safe, raised CR.


Warning: "Heere be Dragonnes"! 

I have included several equations, but if an equals sign - = - makes you nervous, please persevere.    They all represent simple calculations.    

When the equation says  This / That it means "This" must be divided by "That"

And R means R times R.



An internal combustion engine burns fuel to produce heat and energy.  Although the conventional four strokes are often labelled "Induction, Compression, EXPLOSION, Exhaust", it is not true that the third stroke includes an explosion.  The third stroke is really one of "Combustion", or burning.


A pool of petrol will burn.  Pour some into a plastic container and throw it in that fire.  It will heat up, vaporize, and raise the pressure in the container until the plastic burns through.  The confined petrol will burn very quickly, forming a fireball.  (Don’t try this at home, kids!)  If you put the same amount in a light metal container, the petrol vapour will heat and compress, but the container will not melt.  Eventually, if it gets hot enough and there is enough air in the container, the petrol vapour will get so hot and compressed that it will detonate, explode.  The spontaneous explosion will shatter the container, throw shrapnel about and make a loud noise.  More heat, more compression and the fuel/oxygen reaction will happen more quickly and with more power.


What happens when you increase the CR?




As shown in this graph, an internal combustion engine will run more efficiently if the fuel/air mixture can be compressed as much as possible, so that it gives more power for less fuel consumption, BUT too much compression and it will detonate, diesel style.  The shock wave can easily shatter a four stroke piston.

 A diesel engine is designed to compress a fuel/air mixture until the pressure and the increased heat produced by that compression cause the fuel to detonate.  As a result, diesels have the advantage of high efficiency in terms of fuel use, but the disadvantage of needing to be very strongly constructed.


If the hotter, more compressed mixture is more efficient, why are all production engines not high compression for more power?  There are several answers, including emissions as the extra heat produces more oxides of nitrogen (NOx), and less reliability as the engine is under greater strain.  Fuel additives, including lead, and high octane petrol are necessary to allow reliable high compression without detonation.  Fuel free of lead and benzene is all we have now and modern engines have electronic engine management that adjusts the ignition timing, so that they can run near the detonation point without being damaged.


But we want that power!  If we are willing to put up with the downside, and without sensors and computer chips, how can we safely make the engine compress and heat the fuel more?


What is the Compression ratio?

The CR is a number, that shows how much the fuel/air mixture in the cylinder is compressed before it is ignited.  The simplest way to calculate it is to take the stroke of the cylinder, the maximum volume of fuel/air, and divide it by the volume of the combustion chamber into which all that volume is compressed.


CR = Stroke Volume / Combustion chamber volume




But that's too simple.  When the piston is at the bottom of the stroke, the total volume in the cylinder includes the combustion chamber, so we must include that in the equation:


                                                                                                                                CR =    (Stroke volume + Combustion chamber volume) / Combustion chamber volume


Again this is too simple.  The head and block are separated by a gasket, which has a definite thickness, so that the hole in the gasket for the bore has a volume - Gasket Volume.

 And a smaller but still significant volume is in the bore, above the piston and below the top of the block, when the piston is at the top of its stroke, the "In Block Volume".


Thus the volume into which the stroke volume is compressed, the Compressed volume, is made up as follows:


                                                                                Compressed Volume = Combustion Chamber Volume + Gasket Volume + In Block Volume









Thus, an accurate measurement of CR is made by:


CR = Stroke volume + Compressed volume / Compressed volume


In fact, this calculation is entirely theoretical!  The actual compression ratio will be less, depending on how easily the fuel/air mix can get in and out of the cylinder.  Flow through the carburettor and inlet ports, the shape and lift of the valves, the relation between inlet and exhaust lift of the camshaft and the design of the exhaust system are all factors.  The less the resistance to flow and the better the exhaust gases leave and the new mixture enters, the nearer will practice be to theory.


In Practice.

The CR of a standard Triumph varied with different models and markets.  Early UK Spitfires were 9 to 1, while later versions for the USA emissions affected market were as low as 7 to 1.  How high you raise your CR depends on how you feel about the downside, emissions and reliability, and on the fuel you have available.

The "Octane number" of petrol tells how much compression it will stand without detonating.  100 octane petrol, meaning that it is as resistant to detonation as 100% octane, will be safe up to a CR of 11-12 to 1, but such high octane petrol is rarely available these days.  You can run your engine on methanol with a CR of 14 or 15, but that is a different story!  If you only have 95 octane, a CR of 9 should be your limit, but 98 octane (Shell V-Power) will be safe to 10.5 to 1.


The octane numbers above are from the UK, where the Research Octane Number, RON, is used, but  another method of measuring is the Motor Octane Number, MON, which is usually 8-12 units lower.    North and South America use the Anti-Knock Index, AKI, the average of RON and MON, so that the AKI for the same fuel as in the rest of the world my be sold as 4-6 units less than in the UK.    Subtract that from the above octane numbers if you live in those parts of the world! 


Calculating the CR

Our object is to raise the CR to a known, safe figure, and the method is to measure all the items listed in the equation:


CR = Stroke volume + Compressed volume / Compressed volume


Remembering that:  Compressed Volume = Combustion Chamber Volume + Gasket Volume + In Block Volume


Stroke volume we know, as long as you are sure of the engine type and can divide the total engine capacity by the number of cylinders.  But blocks can be bored out.  If you want to be sure to be accurate, measure the cylinder bore and crank stroke, and use the equation for the volume of a cylinder with radius R and height H, and using 'Pi' the ratio of diameter to circumference of a circle.   Pi =  3.1416 (approximately!)

Volume  = Pi R2H


Gasket volume can be measured in the same way, from a good but used cylinder head gasket of the same type as you intend to use.  Measure the hole for the cylinder, and the thickness of the gasket, and apply V = Pi R2H again.  Don't use a new gasket for measurement, as it will be a lot thinner after it is fitted.



In Block Volume.  Rotate the engine with the head off, and you will see that the pistons rise nearly to the top of the bore.  Find top dead centre for each piston, and measure how far the piston is from the top of the block.  Apply V = Pi R2H.

Ideally, the piston to block top distance should be identical for all the cylinders, but they may not be due to manufacturing tolerances.  If one is very different, suspect an odd piston or even a worn big end, and deal with it.


   A process known as "decking the block" can reduce the In Block Volume to zero. A machine shop skims the higher pistons until they all rise to the same height, and then skims the block so that they come up as far as desired, usually to level with the block top, so that  InBlock Volume is zero.  Then, the compressed volume is the same for all cylinders and all the cylinder CRs will be the same.  This is expensive, time consuming and really not necessary unless you are going for a full race engine.


Combustion Chamber Volume(CCV).  There is only one reliable way to measure the CCV, and that is by pouring a known volume of a liquid into the chamber until it is full!  To do this accurately you need a chemist’s burette, that you may remember using in school chemistry.  They are available on the Internet, go for a plastic, 50ml one. Buy, or make, a burette stand, to mount the burette vertically on the bench.


You also need a sheet of glass a bit bigger than the cylinder bore, with two holes in it. Two holes make getting bubbles out easier! Below is a diagram of the square I use.  Your local glaziers will have lots of pieces this big in their scrap bin, but ask for plate glass, ¼†(6mm) thick so that it is durable.  Get them to chamfer the edges and corners to protect your fingers.





Buretting the Combustion Chamber Volume

Put the cylinder head on your bench, chambers up.  Arrange some chocks to make it level, secure and high enough to allow the valves to seat.  Put the valves in place, with a little grease to seal them.  Don't forget the spark plug!  Place your glass square over the chamber, again with a little grease around it to seal it to the face of the head.  Both holes should be over the chamber.   Position your burette so that it will drip into the chamber via one of the holes, and fill the burette.




  Water is useless, as it will not "wet" the metal and leaves bubbles to confuse your measurements.  Some people use paraffin but it gets everywhere, and I hate the smell, so I use "Liquid paraffin".  You should be able to find this in 100 ml bottles at your local pharmacist who may think that you wish to use it to treat your constipation!  Liquid paraffin is a light mineral oil that wets metal surfaces but is water soluble, so it is easy to clean up and doesn't smell!

Make a note of the volume of liquid in the burette, and open the tap to run it into the chamber.  Go slowly, to avoid bubbles and splashing.  When the chamber is nearly full, add the liquid drop by drop.  You may have to tilt the head around a little to persuade the last bubbles to leave.  The very last bit of filling is into the thickness of the glass cover, so you may need to practise a bit, and to find an endpoint at which you decide that it is full.  Use the same endpoint for each chamber, or you will not be able to tell if they are same size.  Read the remaining volume in the burette and work out the volume you have run in.  That is the Combustion Chamber volume!

Measure each chamber.  Even more important than the actual volume is that all the chambers should be the same, within +/- 0.5mls.  There are about 15 drops to the millilitre, so you should be able to measure the volume to less than +/- 0.1mls.


Alternatives to the burette.

            Disposable medical syringes are an alternative.  Try and get a 20mls size, for the major filling, and a 1 or 2ml size for final topping up, so that you can measure down to 0.1mls.     Plastic syringes should thrown away after use, as the liquid paraffin will slowly damage the seal on the plunger.


How to use the measurements.

Now you can work out your present Compression Ratio, that you want to change. 

CR = Stroke volume + Compressed volume / Compressed volume


(Remember that Compressed Volume = Combustion Chamber Volume + Gasket Volume + In Block Volume)


While it may interesting to know exactly the present CR, you may think that we knew that already, from the model, year and possibly engine number of the car.  Sadly, not so!  The previous owner may have modified the engine, or have fitted an engine from a different model.  If you assume that the CR is the same as when it left the factory and that it will be safe to skim X thousandths of an inch off the head, you will risk ruining the cylinder head.  Now that you know the true CR, you can work out how much to skim to raise the CR precisely to the level that you want.


Calculating the skim to achieve a raised CR.

First, work the CR equation backwards from the CR that you want, to the smaller Combustion Chamber Volume (CCV) required to get it.  All the other quantities will stay the same.

Sparing you the algebra to turn the equation around:

NEW Compressed volume = Stroke Volume / New CR - 1


            The Gasket Volume and In Block volume will not change if you skim the head, unless you deck the block as well, so subtract them from the NEW Compressed volume:

New Combustion Chamber Volume =    NEW Compressed Volume - Gasket Volume - In Block Volume



Volume to be removed by skimming =  Measured Combustion Chamber Vol.  -  New Combustion Chamber Vol.


Now work the formula for the height of a cylinder V= PI R2H backwards, from volume V to height H.  Again, sparing you the algebra:


H= V/ PiR2



Height = Volume to be removed / Pi x radius squared


This is the height or thickness that must be skimmed from the head!


Yes, I know that this equation assumes that the combustion chamber is circular, when clearly it is not!  However, the difference from a circle is small enough not to matter.  For R use the average of the widest and narrowest width of the chamber, but remember to divide the width by 2, to get a radius.


If you want to be as accurate as possible, measure the area of the chamber directly.  Place a piece of graph paper over a chamber, and rub your dirty thumb, or a soft pencil across the edges.  This will imprint an outline of the chamber on the paper.  Count the squares of the graph paper, and multiply the number of squares by the area of each square.

Use this in the above equation, replacing the PI R2 with "Area", and calculate


                                                                     Height =  Volume to be removed / Area






You can use Imperial or Metric, but burettes are always calibrated in millilitres.  Be very certain that you use the same units all the way through, to get an answer in centimetres or decimal inches.  NASA crashed a Mars probe by making just this mistake!

One thou = one thousandth of an inch = 0.001 inch

10 thou =  0.254 mm, or one quarter of a millimetre

1 millilitre = 0.06 cubic inches







Now you can take your cylinder head to the machine shop and tell them exactly how much to skim off!   How good is that, when you are a mere garage hand to the gods of the milling machine and master of the lathe?


This may sound like a whole heap of trouble, when Joe Bloggs says, "Just skim it 30 thou and you'll be right, mate."   You may be alright, you may not, but the satisfaction of knowing that you are right, and how, is immense!



Edited by JohnD
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