Andy:
Measure the actual bore diameter and divide it by two. Square
the result and multiply by pi, 3.14156, and multiply that result
by the stroke of 90 mm. Add the capacity of the cylinder head
chamber plus the gasket capacity of 4.5cc. Divide this quantity
by the sum of the chamber capacity and 4.5 cc for the gasket.
Or, use this information from another letter I sent out.
Blake
********************************
<<<snip>>>
I was browsing the mgcars.org BBS and found your post there. As
your engine is +0.040 overbore the compression ratios will be
different. So I've got some new data for you.
Bore Size Capacity Compression ratio with standard head.
Std. 1250cc 7.25
0.010 1260cc 7.3
0.020 1270cc 7.35
0.030 1280cc 7.4
0.040 1290cc 7.45
For the Laystall head with the 0.040 overbore on the 1250 engine,
the CR is in the neigborhood of 9.6 to one. (1290+150)/150 If
you cc the head because it was surfaced sometime ago, you can
figure the CR for yourself. Suppose you CC the head and all of
the chambers add up to 148 cc. The CR would be found by taking
the 1290 cc capacity of the plus 40 overbore engine, adding the
148 to it, and dividing by the 148. (1290+148)/148=9.7162 If
you don't know how to CC a head, I can send some information.
You will need a graduated burette to complete the task.
I also noticed that my copy of the email I sent has messed
up the format of the charts. I will add a straight text file
copy so it will format correctly. If you save it and bring it
directly into a text editor it should format just fine.
Blake
**********************
Perhaps you may be able to construct a chart for yourself giving
the data you
desire. I don't have a Laystall head, and the only one I've seen
was
installed on an engine. So I am going to give you some
information on the
standard head and perhaps it will help you with your question. I
found the
following data in a booklet written by WKF Wood in 1968. The
chamber size
is the sum of all four combustion chambers.
Head Chamber Ratio
Remove Finished Depth Size 1&1/4 Litre 1&1/2
Litre
Std. 76.75 mm (3.022 in.) 200 cc 7.25
8.33
1/16 in. 75.16 mm (2.959 in.) 175 cc 8.1 9.
3/32 74.37 mm (2.928 in.) 165 cc 8.6
9.9
1/8 in. 73.58 mm (2.898 in.) 150 cc 9.3
10.7
I checked this against an original factory tunning booklet
"Special Tuning of
the MG Midget type XPAG (As fitted to Series TB and TC Cars)"
Issue 1, June 1949.
It shows several stages of tune also. They list the chamber size
as 45.5cc
and the head gasket C.C. as being 4.5 c.c compressed. Adding
these together
and multiplying by 4 you get the total head cc as 200 which is
the same as in
the chart above. Using the data what can be gleamed from the
booklet, the chart
would be like this. The chamber size in this chart is for one
cylinder.
Head Chamber Size Ratio
Remove Finished Depth With Gasket 1&1/4
Litre
Std. 76.75 mm 50.0 cc 7.25
3/32 74.37 mm 8.6
1/8 73.575 mm 9.3
Checking the data in the first chart by recalculating it appears
that data is correct.
For instance, multiplying 76.75 mm by the metric conversion
factor of 0.03937 you get
3.0216475 in. for the original head thickness. The head
thickness of 73.575 converts to
2.897 in. using the same process. The cylinder capacity can be
calculated by taking 66.5
dividing it by two, squaring, multiplying by pi (3.14156) and
mulitply by the stroke of
90 mm which gives an individual cylinder capacity of 312.59 cc
for one cylinder and
1250.36 for all four. Using the head size of 50 cc and the
cylinder capacity of 312.59,
the Compression Ratio can be found by adding the 50 to 312.59 and
dividing by 50.
Doing that gives a CR of 7.2518. Using 1250cc for the engine
capacity and 200cc for the
total head capacity the result is exactly 7.25. We can check the
9.3 CR by adding 150 to
1250 and dividing by 150. (1250+150)/150=9.333... So The CC
capacities listed for the
head in the first chart must be very accurate.
Using that data, I found that every 1/64 inch (0.0625 in.)
removed from the head's thickness
reduces the chamber's capacity by 6.25 cc. This assumes that the
chambers walls remain more
or less vertical to the heads deck. This data could be used to
get an approximate CR of a
milled Laystall head's IF you know the original thickness.
Attached is a poor copy of a picture of a standard head and the
Laystall. The shape of the
chamber is slightly different. It appears that removing the same
amount of thickness from it
will leave the finished chamber size slightly larger than that
for a standard iron head. If
you can cc an original Laystall head and measure the head's
thickness you could complete
this partial chart for Laystall head which would probably be very
close. The article on the
Laystall head I sent yesterday was done on a TD (1250 cc). If
you happen to have the head
on a 1500 cc XPEG the results would be about what's in the chart
below. The minus sign
indicates the decrease in the heads thickness or the decrease in
the chambers capacity.
The ~ (tilde) sign is the best I can do on the keyboard for the
mathematical symbol "is
approximately equal too."
Head Chamber Size
Ratio
Remove Finished Depth With Gasket 1&1/4
Litre 1&1/2 Litre
Std. ? ?
9.3 ~10.7
1/64 in. -0.015625 in. -6.25 cc
~9.7 ~11.4
1/32 in. -0.03125 in. -12.5 cc
~10.1 ~11.9
3/64 in. -0.046875 in. -18.75 cc
~10.5 ~12.4
1/16 in. -0.0625 in. -25.0 cc
~11.0 ~13.0
This is the best that I can do with the books I have.
Sincerely,
Blake J. Urban
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