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working out compression ratios
#21
I found another one today, Ian! Thanks to looking through Chris Gould's store now have evidence of 1A 38 with taps, 1A 38 without, then 1A 684 dated 1931-ish, and a 1935 dated 1A 7-something.
I was going to try filling them with fluid and seeing the relative differences, if the heads are of similar height and haven't been skimmed... but if you have already done it, yes, there is no hurry.

It does at least suggest that there is a more acceptible route for slightly enhanced tuning on an early car, just by selecting a later standard LC head?
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#22
(10-10-2018, 08:53 PM)JonE Wrote: I found another one today, Ian! Thanks to looking through Chris Gould's store now have evidence of 1A 38 with taps, 1A 38 without, then 1A 684 dated 1931-ish, and a 1935 dated 1A 7-something.
I was going to try filling them with fluid and seeing the relative differences, if the heads are of similar height and haven't been skimmed... but if you have already done it, yes, there is no hurry.

It does at least suggest that there is a more acceptible route for slightly enhanced tuning on an early car, just by selecting a later standard LC head?

The cards and parts books show-
 
Casting number 1A 670 was introduced under Part Number BC 82 in August 1933 with a slightly raised compression.

Casting number 1A 684 still under Part Number BC 82 appeared sometime before 1936.

So far I can't see a 1A 7** head number on the cards or parts books. 
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#23
Here are the variances I have found Jon, I am sure if I went through all the heads in my shed I could possibly find more, all these heads have not been skimmed with exception of the IA38 which has 10 thou off. What is interesting is the variation in the plug depths, very early having 1/2" reach then all mid production 5/8" reach until we get to IA684 then back to 1/2". There is quite a variation in chamber depths, I have marked these up in thousands if an inch and measured them all in exactly the same places, the IA684 plainly has the highest CR but it also has the greatest restriction in the all important transfer port.

   
   
   
   
   
Location: Auckland NZ
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#24
Great! I'll try and do the later head. What does 1/2 or 5/8 reach actually mean.. is that saying the former is less recessed and the measurement from spark plug tip to face of head?

And what is your thought for why the later 1935 head has more restriction in the transfer port? What does it likely correct from service feedback?
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#25
Be interesting to know the actual chamber volumes although I don't expect you to measure them all Ian! The difference in the spark plug holes are interesting. Did all the heads use the same type of plug?

Simon
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#26
It is the depth of the thread rather than the recess, the chamber shape appears to change subtly throughout the progression of years.

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.
Location: Auckland NZ
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#27
From Chris Gould:
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.


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#28
Neat, I've never seen it done that way before. Always just with a flat plate over the chamber with an inlet hole and a breather hole. Where do you put the scriber point? At the edge or the middle of the chamber? I did consider the space between the piston and cylinder down to the first ring gap but figured that it was likely far smaller than the errors in my other measurements so I ignored it!

Simon
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#29
Thanks Ian and Tony, I'm in the middle of a big clear out under our kitchen, 18' x 10' x 5' which has been a store for anything I might need. I'm a bit of a squirrel, a must when running a Seven. In it were a load of early low compression heads and whilst I'd worked out what most were this thread confirms my deductions. Whilst most have dates cast on there are about three 1A38 without dates.
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#30
Dave I concur most of the IA38 I have are undated whereas almost all others are.
Location: Auckland NZ
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