How to measure your compression rato

contributed by Ian Simpson

And what difference a skim makes.


To take into account:
Piston swept volume.
Gasket thickness volume.
Chamber volume.
Piston top protrusion or rebate volume.
Piston protrusion, if any, into gasket area.
Volume around the sides of the piston to liner, down to the upper piston ring.

For dished pistons compared to standard: 6÷2=3. 3x3=9. Pi x 9=29.27cm² x 0.15cm=4.24cm³ to add to chamber volume.

Standard engine:

A standard head is about 73.5mm thick.

I guestimated the base diameter of the raised centre area of the piston top to be 60mm.

I approximate the gasket area to be 9.07cc for a 2.2mm squashed to 2.0mm.

Originally the piston top raised area takes away about 2.12cc of that gasket area, leaving 6.99cc gasket area to add to the chamber area.

44cc + 6.95cc = 50.99cc + 349.25cc = 400.2cc / 50.95cc = 7.85:1 CR


My current engine

My cylinder head has 1.5mm skimmed off.
My pistons have the 30 thou' raised area removed and a 30 thou' rebate into the piston top.
Maybe 2cc or 4cc trimmed out of the the chamber in the head, either way, the head has been buretted at 39cc.

Height of centre piston top area is 0.75mm or 0.075cm. And depth of mine is 0.075cm.
Height + depth is 0.15cm or metal removed from combustion chamber volume.
With the dished piston the area below gasket area / piston top is also about 2.12cc.

39cc + 9.07cc + 2.12cc = 50.19cc + 349.25cc = 399.44cc / 50.25cc = 7.96:1 CR

So this is how I arrive at 7.96:1 for mine and, by the same method, 7.85:1 for the standard head and pistons.


Errr...

There is a great usage of the equation: Cross sectional area = Pi r².

This is used to calculate the gasket area.
So diameter of 7.6cm across, divided by 2 = radius of 3.8cm.
Squared (Multiplied by itself) 3.8 x 3.8 = 14.44.
Then multiplied by Pi (3.14159) = 45.36mm².
Then multiply this by the gasket thickness which is about 2mm or 0.2cm: 45.36 x 0.2 = 9.07cm³.

The pistons swept area is 349.25cc.

Each chamber in the standard engine is 44cc.

The area of the piston raised area or rebate area is worked out the same way as for the gasket area.
Diameter is about 6cm on mine.
Height is 0.75mm or 0.075cm.
So: 6÷2=3. 3x3=9. Pi x 9=29.27cm² x 0.075cm=2.195cm³ to minus from gasket volume or total chamber volume.

The the total area of swept area and chamber area are added = 400.2cc.
This is then divided by the compressed area of 50.95cc.
The product is the Compression ratio: 7.85:1.


To accurately measure the cc of the head:

You get a small square, maybe 10cm x 10cm, of flat rigid transparent plastic sheet. Install a 7mm, or so, hole centrally in it. Put a thin grease layer on the head around the chamber to seal the plastic which you then settle onto the head. Sit the head upside down on the bench and drip something like SafeClene or Paraffin from a burette through the hole. The air comes out. Once the chamber is full re-read the burette.

If you don't have a burette, they cost maybe £50, you might have a best estimate on the kitchen table using a 30cc medicine cup. By this method I got 40cc for my 39cc head so not far out. I had a smaller cup too that I could reasonable estimate 1cc amounts with.


See this for more details and a handy automatic calculating thing.

Pistons, how and what to measure.

Obvious to experience people but for first timers:

Measure the depth of the area rebated into the top of the piston and measure the width of that area.

To measure the depth you could lay a rigid bar across the whole top and use a dial or digital calliper to measure from the top of the bar to the bottom of the rebate. Then measure the bar thickness and subtract that from the overall, and that leaves the rebate depth only.

The width is a bit of a guess as the edges will be radiused a bit. So pick a place maybe half way up. Also it would be wide to measure the top of the rebate and the bottom of the radius, so as to be sure the pistons are all the same.

Also you could measure the overall diameter of the pistons across the width of the top, and the depth from the top to the upper piston ring. Should be the same but if the pistons are a different make from different years, even though they're all from the same 'tuning company' or parts seller, they could be different. 'Tuners' had some machine out the rebate in lowered compression versions, they don't come from Tarabusi or Perfect Circle like that. If they are forged, then from another manufacturer, but not dished unless specially ordered that way, and maybe not even then. From year to year 'tuners' may have use different machining companies and they may have take different sizes of metal out. It might be possible to get them matched if they are slightly different. They best be the same make though.

From those measurements you can calculate the volume of metal that's been removed and then the compression ratio.


Ian S RTOC Profile

My dished pistons