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Calculating Static Compression Ratio

Compression ratio put very simply is the ratio between the cylinder volume above the piston at BDC and the volume above the piston at TDC. This little calculation is for the serious engine builder. Normally in most standard engine rebuilds, calculating the compression ratio is not necessary as the ratio will be very similar to the manufacturers stated figures. So when is it necessary to do the maths? If you change the cylinder head to a different combustion chamber shape, fit a turbo or supercharger, maximum rebore, change the pistons to a different crown shape, stroke the crank or simply that you want to increase / decrease the compression ratio and need to know how much to add or remove from the Combustion Space. The method used is simple enough but needs to be done accurately, but the maths is a bit involved when all factors are taken into account. For a flat top piston with no valve reliefs the exercise is simple, but trying to work out a compression ratio with dome top pistons or eye brow type crowns is a fair bit more complicated. When I worked for Superspares in Rockhampton, I had to calculate compression ratios on a weekly basis as I was fitting turbo chargers and installing all sorts of high performance accessories that needed the compression ratios altered. Turbos and Superchargers required a decrease in ratio but most other modifications required higher ratios. There are two formulas (1 & 2) used to answer all your compression ratio questions. The formula 1b. is identical to the first, explained in the table of meanings and is not often required. It is used to derive the second formula.

1.  

1b.

2.

 

You will need a small piece of clear perspex with a hole drilled near to one edge to cover the combustion chamber and large enough to cover the bore, 100cc burette filled with a light grade fluid (not water) eg. 10grade oil, machine oil. My set-up is shown in the right picture above, working out the compression ratio for a blown 186 Holden engine. NOTE that the head is slightly inclined to allow for the air to escape through the drilled hole in the perspex sheet as the oil fills the chamber. The actual maths is shown in the bottom left diagram above. This one was easy, flat top piston and the HGV was supplied with the ACL Racing Series head gasket. If you haven't got a burette, then give the job to someone who has or borrow one. A small error in volume measurement of just a couple of cc's may render the whole exercise a waste of time. I would highly recommend that you work in metric units or at least have a converter calculator, otherwise you may get confused. Remember to work either in Imperial or Metric but not bits of both. If the measurements are in millimetres then convert them to centimetres, that way the answer given will be in cc's and most burettes measure in either ml or cc, same difference. One ml = one cc. You don't need a burette to calculate SV so that's the first calculation out of the way and obviously DHV will be next.

SV = swept volume

Calculate SV using this formula, Πr2h. Where r = half the diameter of the cylinder and h = the stroke of the crank. Pye is equal to 3.14159
CCV = combustion chamber volume The cylinder head should be assembled, spark plug installed and the head placed at a small inclination so the hole in the perspex is at the top of the incline. This will force any air from the chamber as it is being filled. Put some grease around the chamber and fit the piece of perspex over it and fill the chamber with fluid until the fluid is at the base of the filling hole. Note: ensure the valves don't leak otherwise it will give you inaccurate readings and your final CR figure will be lower than it actually is.
HGV = head gasket volume

You need to know the volume of the head gasket once compressed. A point to note here is that the bore hole in the head gasket may not be perfectly round or even the exact diameter of the bore. You will have to work out an ingenious method to measure the volume of this space exactly.  Read more....       A good head gasket company can supply this information.
DHV = deck height volume You use the same formula to calculate this as the SV except the height will be very small. Place the piston on exact TDC and measure the distance from the top of the piston to the top of the block. Take the measurement from the centre of the piston in line with the gudgeon pin. You need to take this measurement even if you have dome or dished pistons.

EDV = effective dome volume

 

CR= SV + CCV + HGV + DHV + 5 - 20

 

This calculation is only required if you have valve reliefs (eyebrows), dished or domed pistons. The best solution is to ask the piston manufacturer what this volume is. A dome will have a negative number where as a valve relief or dished crown will be a positive number. Flat top piston is zero. How do we measure the dome? A piston without rings fitted, has to be inserted in the bore and dropped down the bore exactly 20mm or whatever it takes to ensure the top of the dome is below the deck height. The top of the piston is coated with grease around the circumference so that it seals against the bore. Clean all excess grease from the piston and bore. From this we can calculate the exact volume of a 20mm stroke, if the piston was flat using the same formula as used to calculate SV. Now using the perspex sheet to cover the bore, fill the bore with fluid. Ensure the block is on a slight incline and fill from the top just the same as the cylinder head CCV is done. Take the burette measurement away from the calculated measurement and you have the EDV. Remember it is a negative number if domed. It won't matter if there are also valve reliefs as long as they are smaller in displacement than the dome. If the reliefs are bigger than the dome then you will have a higher burette figure than the actual calculated figure. This number will now be a positive in the formula. If you get this info from the manufacturer but modify the piston dome shape for whatever reason then you must measure it as previously described. An example of this may be that the valve reliefs need to be cut deeper because of an extreme lift cam.
CS = Combustion Space

Combustion space is CCV + HGV + DHV + EDV. This is all the space above the piston at TDC where the combustion will take place.

There you go, nothing to it really. Remember a burette is a must have tool to perform this job. As an added point - if you plan on building a good engine, then you should measure the CCV of each chamber of the cylinder head and have the chambers equalised. Also don't measure the CCV and then send the head off to be re-conditioned as the CCV will change and therefore change the CR. The best approach is to have the valves, seats and guides done but do not get the head shaved at this stage. Then measure the CCV and do the calculations. Then if changes need to happen, you can take it back to the machine shop for the shaving of the head if you need to pick up the CR or if chamber porting work is required to equalise the chambers.

There are several ways to modify compression ratio. So let me cover the second formula I have used to help determine what changes I need to make to get a desired CR. Here is where we now simplify the original formula. So the formula now looks like 1b and when transposed, we come up with formula as shown in #2. This new formula will be an invaluable aid in calculating new CS volumes to increase or decrease your current CR. If we know the current compression ratio, be it from a manufacturers specification or you have actually measured everything , we can calculate the CS required to get a desired CR. If you calculated everything then you already know your current CS and therefore know your exact CR. You can check that your calculations are correct with this formula. This formula is saying that if you divide the SV of the cylinder by the known CS, the answer will always be one less than the actual CR. Pretty tricky isn't it. Don't panic if you can't see it. You will once you transpose the CS with CR - 1. Try it out. Let's use the one I have already done on a blown engine I just did. I calculated the CR to be 8.43:1 rounded up to 2 decimal spaces. Now we divide the SV= 507.5783  by the CR - 1 which is 7.43. The answer is 68.31cc. If I had calculated to 4 decimal places then my answer would match almost exactly. Check out what my CS added to, in my original CR calculations, remember that CS= CCV, HGV, DHV and EDV all added together. Now I have proved my original calculation was good, so what if I now want to change the CR.

No big deal, I now need to calculate what my CS needs to be if I am to raise the CR to 9.5:1. SV = 507.58cc / 8.5 = 59.71cc is what I must reduce my CS to. So 8.6cc needs to be removed from my CS by reducing the CCV, HGV and DHV or any combination of these. The general methods used are to re-deck the block which reduces the DHV, use a thinner head gasket which reduces the HGV, shave the head thereby reducing the CCV and lastly fit dome top pistons which effectively reduces the CS. All have consequences for the engine builder. A standard engine like the example I have used is generally not a problem as there is usually plenty of clearance between the piston and cylinder head and valves, so any of the methods mentioned will be suitable. On high compression engines, the accepted method is to change piston dome shape. To gain compression a higher dome is used and to lower the reverse is required. If you have followed me this far and have got the grasp, then well done.

Practical Exercise:

Lets do one more exercise but this time the only thing we know about the engine is the bore size, stroke and the current CR. What is the CS volume? I will let you do the maths yourself. The bore is 91.1mm, the stroke is 98mm and the CR is 9.5:1. The first thing to find out is the SV. The formula to use is listed in the table above. Once you have the SV, the next formula shown can be used to calculate the CS because you also know the CR. To check your answers click on the links in the formula. CS = SV / (CR -1) . Now we know the CS of this engine in standard trim without even dismantling it. A word about factory specs though. The CR given by the manufacterer is always rounded to only one decimal place. This is adequate for general calculations and planning whatever changes you want to make to the CS. During the dismantling of the engine though you will of course want to individually measure and calculate the individual components that make up the CS volume. ie. CCV, HGV, DHV and EDV.

With the two formulas I have provided, you have the tools to work out any combination of changes you wish to make that will affect the CR of your engine.

Just to repeat them again.

To calculate Compression Ratio -     .      To calculate Combustion Space  -