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It is a widely known fact that putting a 12G295 head on an otherwise
stock 998cc engine will automagically increase the raw power output
by around 6bhp (source: David Vizard's yellow bible). However in order
for this to work, you must bring the compression ratio back to
something reasonable as the 12G295's chamber volume is
bigger than that of a stock Mini1000 head.
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Measure the volume in cc of one of the combustion chambers by setting
the head level and pouring in water plus a little detergent (assuming
that the head has valves and spark plug fitted, of course). Do the
same to your 998 head, or one in a scrappers, or ask the list to check
Vizard (I think he quotes the stock volumes). The 12G295 volume minus
the 998 head volume is the amount per chamber you need to lose to get
back to the 998 stock compression ratio. To work it out, draw round
one of the 12G295 combustion chambers on metric graph paper and use
the squares to calculate the area. The amount needed to skim off is
the volume required to be lost (in cc) divided by the chamber area (in
cm squared). If I've got this wrong, someone will hopefully say so
before you wreck a nice head (gulp).
Finally, if it is a lot, check the depth of the accessible oilways to make sure you have enough metal on the head!. Stephen Marley |