Hi Mark,
I just finished doing that job. I also ported and polished and unshrounded the
valves a bit. To find out what you need to shave or can shave you need some
measurements and some calculations. As noted in other replies, you can shave
quite a lot from this head, but you can run into a can of worms doing too much.
The stock combustion chamber is 44.5 cc. After my mods, it ended up at 46 cc. I
need to plane the head down to make up for the gained volume and maybe raise the
compression. Just changing to the flat top pistons is supposed to change the
compression ratio from 7.5:1 to 9.0:1 and that is accomplished by eliminating
the extra volume in the dish of the piston. Even if you don't plane your head,
you will end up with 9.0:1.
There is a great tool at the SmokemUP.com:
http://www.smokemup.com/utilities/calc/cyl_head_cham_vol.cfm
You plug in some numbers and it calculates the combustion chamber volume you
need for a desired compression ratio. If you want to change to a higher
compression ratio, then repeat the calculations of the new desired compression
ration and subtract the two volumes to find the difference. Then you calculate
the height of a cylinder (really stubby little cylinder) to make up this volume.
You also have to consider that the combustion chamber is not a cylinder as the
squish area takes up part of the volume. I estimated that the chamber takes up
about 85% of the cylinder area, so you need to cut off more to make up for this.
Here are some numbers you need:
Bore 2.9"
Stroke 3.44"
Deck height (I don't know the flat top pistons but the dished are .017 below the
deck)
Head Gasket Bore ? (I used the same as bore 2.9", close enough)
Gasket Thickness .030" (measured on my old squished gasket)
Piston dish 0 cc for plat top, 6 cc for dished
You can find a calculator to find the cylinder height here:
http://www.ex.ac.uk/cimt/res2/calcs/calcyl.htm
I wanted to get at somewhere between 9.0:1 and 9.5:1 after planing my head. I
needed to cut enough to make up for the chamber mods (1.5 cc) and maybe a little
more to get over 9.0:1. Using the 85% factor and converting the cm to inches, I
cam up with .040". After planing the head, I cc'd them again and found the new
volume to be 43.5 cc. Plugging in the numbers to find the compression ratio that
matches the closest to 43.5 results in a compression ration of 7.8:1. When I put
.020 over flat top pistons in, this will rise to 8.8:1 or 9:1 if the piston deck
is 0.0" rather than .017".
The net-net is you are safe in going .010 to .050 off the head. Just plug in the
numbers to find the compression ration you want.
Standard planing at the machine shop is .010", so mine needed four passes. The
extra time means more cost. The machinist did a double-take when I asked for
.040" off. Was I sure? I went through my calculations with them and they said it
sounded right and agreed with me. They will also recommend new valve guides as
these tend to become sloppy over time.
I learned a lot going through this process and really enjoyed it. In hindsight,
I should have taken a little more off but will be happy with the increase when
the flat top pistons (and cam) get installed on the next winter project.
Vic Whitmore
76 Spitfire
Thornhill, Ontario
http://www.vicwhit.com
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