There are a couple more important measurements required here in the
calculation of your swept and unswept volume.
- deck height of the piston in the cylinder, measured when the piston is
at tdc you measure the distance from the top of the piston to the deck
of the engine block
- compressed head gasket thickness
With out these 2 numbers your calculation can be off by a fair amount.
Shaving 0.005 off the deck of your block can increase your compression
more than 0.010 off your head will and with these old motors having been
rebuilt god knows how many time before we lay hands on them so you don't
know if the block has been decked as well the head being shaved. A
decked block would be less likely to find on a wet sleeved tractor TR
motor than a six pot or Spitfire motor.
For example on my Fiat motor I went from 8.25:1 to 9.1:1 compression
with a 0.040" over bore, 0.010" shaved head, and 0.005" decked block.
Doug Hamilton
1960 Triumph TR3A
1963 Fiat Cabriolet
1967 Chev C/10
>From: Dave Massey <105671.471@compuserve.com>
>Subject: RE: engine
>
>Message text written by "Randall Young"
>
>
>>>I'll let others comment on the effect of mixing components, as I just
>>
>>
>don't
>know that much about the 6-pot. But according to Piggott, the HE engine
>was
>never more than 9.5:1; and if I run the above formula backwards then
>forwards (assuming a 7.47cm bore & 9.5cm stroke, also from Bill), I get
>that
>a .020" overbore will increase it to a whopping 9.6:1. Of course I am
>using
>a Pentium ... so YMMV <g>
><
>
>My HP 32S confirms the calculations so rest easy.
>
>The heads are interchangeble but the early CC heads need the early intake
>manifold and the later CF heads need the later intake.
>
>The recommended procedure to determine compression ratio is the "cc" the
>head. Assuming the valves don't leak and the spark plug seals well, mount
>the head upside down and as near perfectly level and using a graduated
>cylinder, determine how much fluid it takes to fill the combustion chamber
>flush with the head mating surface. You can use whatever fluid strikes
>your fancy assuming it doesn't evaporate fast enough to throw off your
>measurements. This will give you the "U." Divide that into your "S"
>(approx 420 cc) and you have your current compression ratio. If it comes
>in near 8:1 I would recommend shaving the head when you get it rebuilt. If
>it is close to 9.5:1, don't.
>
>Dave
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