Calculating Compression Ratio (CR)

[Cooperman1]

I said I would put on here the method for calculating your CR and how much to machine off a head in order to get the CR. It's a bit long, but I think everything is there so that anyone could do it.
First of all you need some measuring kit: A digital vernier, a 5cc syringe (from any chemists), a steel ruler and a spirit level. You'll also need a calculator and a paper & pencil. Keep a record of your calculations.
This is what you do:

1. Measure the average distance from piston top to block deck level with the pistons at TDC. Take the average for all 4 pistons.They should be virtually identical.

2. Calculate the capacity of this volume. Call it ‘A’.
For 0.010” piston top to block deck this capacity will be:
Standard bore at 1275 cc = 0.99 cc
At +0.020” overbore = 1.01 cc
At +0.030” overbore = 1.02 cc
At +0.0402 overbore = 1.025cc
At +0.060” overbore = 1.04 cc
From the measurement of your own engine you can work out the figure accurately. It's roughly 0.1 cc for each 0.001"

3. Measure the volume of the dish in the top of the piston using the 5-cc syringe with clean water. With Hepolite 21250, 21251 & 21253 pistons this is 8.3 cc.
Call this ‘B’

4. Set the head up to be absolutely level using a spirit level. Measure the combustion chamber volume in the head using the syringe and clean water. Do this for a couple of chambers and take the average. Call this ‘C’

5. There is a volume in the head gasket area and we’ll make an assumption here that with the average gasket, say a BK450, it’s 4 cc. Call this ‘D’

6. There is the ‘ring land volume’. That’s the volume down the side of the piston to the top of the top ring. Allow 0.75 cc for this. Call it ‘E’.

7. Now divide the actual swept volume of your engine by 4 to get the swept volume per cylinder. Call this the swept volume ‘SV’
For a standard 1275 cc = 318.75 cc
At +0.020” i.e. 1293 cc = 323.25 cc
At +0.040” i.e. 1310 cc = 327.5 cc
At +0.060” i.e. 1330 cc = 332.5 cc

8. Add all the unswept volumes together i.e. A+B+C+D+E.
Call this the Total Unswept Volume ‘UV’

9. The Comp Ratio (CR) = (SV+UV) divided by UV
So, if you have a standard bore 1275 cc engine with the SV = 318,75
And if the total of the other volumes, the total unswept vol. UV = say, 36 cc (which is the sort of figure you'll get)
The CR = (318.75 + 36) divided by 36
= 354.75 / 36 = 9.85 to 1

However, if that is not the CR you want, you have to work backwards to calculate what the combustion chamber volume in the head should be.
Say you have the figures we just used, but you want the CR to be 10.4 to 1.

The formula to work out what the UV should be is:

SV divided by (CR-1) = UV

So, in the case we are using:

UV = 318.75/(10.4 – 1) = 318.75 divided by 9.4. = 33.9 cc

Add A+B+D+E, as established earlier and subtract this from the new figure for UV to give the figure for 'C', in this case subtract it from 33.9. The resulting figure is the actual volume you need each combustion chamber to be.
Using the syringe again, after setting the head up to be absolutely level, put EXACTLY this amount of water into a combustion chamber.
The distance from the head face to the surface of the water is the amount to be skimmed from the head. To measure this I usually put a steel ruler across the head above the chamber and measure down to the water surface with a digital vernier, subtracting the thickness of he ruler afterwards. In practice, this is the most difficult dimension to measure as the water has a ‘meniscus’ which sort-of ‘reaches up’ to the bottom of the vernier calliper. You have to make a judgement on this, but don’t be put off, it is not that hard.

Once the head is skimmed as necessary you just fit it and your CR is correct. Remember if you are increasing the CR you will need to check and adjust the timing.

I hope this makes sense. Don't be put off doing it. it's really not that difficult.
Email me if you need any further info on this.

Peter