Chronology – all but 1A 38 tend to be dated
XL 506 (first 100 cars)
XL 845 (1923) with priming plugs – perhaps a short run prototype for…
XL 1253 (1924-6) with priming plugs; (above on a May 24 Scoop Scuttle car), which became
1A 38 with priming plugs up until at least 1929 (some marked OY or possibly CY 2)
1A 38 The primer holes were deleted, but other than the parts listings deleting the plugs there wasn’t a change in the part number or casting number shown on the cards for this later style of ‘low compression‘ head up to 1933. There is no mention of a higher compression ratio at the time of removal of the priming plugs.
1A 670 AF1T: (1934) Aug 1933 with raised compression c.5.2. Examples found up to 11.34
1A 684 AF2T (1935) one example 12.4.(193)5 . Can others’ collections narrow that down? It is listed for the three bearing engine, possibly.
1A 911 (1936)- ‘high compression‘ head with two bolt water outlet.
Standard Compression Ratios quoted
1923-1933: 4.8 (or 4.9) to 1 – was this stated variation perhaps down to the loss of priming holes?
1933: 5.2 to 1
1936-1939 6.2 to 1 noted in a 1936 Commercial Motor report on the new engine
If these are between 17 and 20 times the Compression Ratio, a good engine gives
4.8 to 1 = 80 to 96 psi
5.2 to 1 = 88 to 104
6.0 to 1 = 102 to 120
Basic cubic capacity comparisons for compression ratio
Basic water filling experiments on a combustion chamber with a D16 Champion plug give:
1A38 with taps 40cc (tap hole is approximately 1cc)
1A 670 about 37cc
1A 684 35cc
By comparison, a new Ricardo head (mildest compression version with space for high lift valves) 30cc, which is about par for sports heads.
Ian Williams reports there is quite a variation in chamber depths. His images show heads marked these up in thousands of an inch, measured in comparative locations.
What is interesting is the variation in the plug depths, the very early having 1/2″ reach then all mid production 5/8″ reach until we get to IA684 when back to 1/2″. This is the depth of the thread rather than the recess; the chamber shape appears to change subtly throughout the progression of years.
The IA684 plainly has the highest CR but it also has the greatest restriction in the all important transfer port. Looking at them all, with some small modifications to the transfer port (to raise the roof into the plug recess) and a skim the IA684 would probably give the best performance, good flow and an element of ‘squish’, It could be made into a reasonable head by releasing the transfer port restriction and wedge skimming to improve squish, but plug position will always be poor from a flame travel perspective. It will appear standard and thus likely be VSCC accepted.
The later 911 High Compression head is generally a good design, i.e. plug position is better for flame travel, but they can oil plugs if bores are not perfect. More power can be extracted with this head but it is not historically correct on an earlier car and not accepted for VSCC, unless tapped out to take 18mm plugs.
As there are so many variables, it is difficult to be definitive and much also depends on the condition of the rest of the engine.
First step: improve breathing and combustion; the transfer port and squish
Secondly: raise CR to what ever the bottom end is capable of coping with
Thirdly: perhaps plug position (and others may wish to add thoughts here)
Original ports and manifolding will have inefficiencies. Simply blending the sharp edges in the inlet and exhaust ports is well worth the trouble if you are making head improvements.
Chris Gould’s method for compression ratio measurement
I used to measure compression ratios at Ricardo. The clearance volume was measured by resting the cylinder head on V blocks on the level marking off table. This was a large cast iron table with a machined surface. I doubt that anyone reading this has one, but we can manage without it. Make up a board as shown with studs to fit into three of the stud holes in the cylinder head. Put it on a rigid bench and adjust the studs to get the cylinder head level in all directions using a spirit level.
Support a burette with a retort stand and fill it with fusus oil. I don’t know where to obtain this, but diesel oil is a good substitute. If you don’t have a retort stand you can make a substitute with a piece of pipe, a large close peg and a bit of ingenuity a shown.
Put a scribing block onto the head surface and adjust it so that the scriber just touches the head surface.
Fill the burette and let a little of the oil out to remove any air under the tap. Position the burette and the scriber over the combustion chamber and measure the amount of oil to fill it to just touch the scriber. When doing this adjust the tap so that it is just dripping to get the final level. It will cause a ripple that will rise up the scriber. Lift the scribing block and lower it carefully to see if the level is correct. If it doesn’t touch the oil put in another drop, and repeat this until it does.
Measure and calculate the volumes of the valve protrusion and around the piston down to the top piston ring. We used to measure the gasket area with a planimeter. Tracing the shape on graph paper and counting the squares and half squares etc. is a good alternative. Measure the thickness of a gasket that has been compressed. Add or subtract these volumes as necessary to calculate the clearance volume.
The compression ratio = (Swept volume + clearance volume) divided by the clearance volume or 1 + swept volume divided by clearance volume.
Thanks for notes taken from Austin 7 Friends forum contributions. Please comment if you have additions/corrections.