not quite how I was thinking of attaching the gauge, If I attached it into the manifold I would only record inlet pressure fluctuations which the mercury tube would probably be very good at, not too dissimilar to balancing tubes when setting carbs up on multi cylinder bikes..156 D wrote:...i have done so much reading that i may have the wrong end of the stick J1MS , but is it just a normal vacuum guage with -20 to + 20 inches of mercury as the scale that it would measure be what you need it doubles as a pressure guage on the + side as you are probably aware...if this were fitted by way of a small spigot threaded and of course 3mm piping, to the inlet manifold, as near to the carb. as possible would this not give what you are looking for???.......it has to record plus or minus vacuum depending how far the throttle is open!!.....i have always believed in higher primary crankcase pressure with tapered transfer ports, wider at the crank end than the head end......
To record the Actual peak working primary pressures of a running engine at X revs, the gauge needs to measure air pressure behind the piston skirt or after the reeds not before it, as compression in the primary will only fully happen after the inlet closes in the crank case, this is when & where the primary gasses compress. Also as the transfer ports are "opened in the bore" and the piston decends, then the gasses in the crankcase are subjected to the pressures and vacuums in the Cylinder above the transfers port and also the rise and fall in pressures in the open exhaust port prior to the exhaust port closing again.