This is from the DYNOmite Forum. They make engine transducer and data recording kits.
I suspect that they know more than just a little bit about these measurements!
The coloured and resized text is my work.
Link here:
http://forums.land-and-sea.com/showthread.php?t=64 2-stroke crankcase pressures ________________________________________
How should I set up to measure pressures in my snowmobile’s crankcase? I already have the -12 to 22 PSI manifold pressure sensors but it seems to read rather erratically.
#2
07-05-2005, 08:49 AM
Tech
Land & Sea Engineering Join Date: Mar 2004
Location: Concord, NH
Posts: 210
Re: 2-stroke crankcase pressures ________________________________________
1) If you are trying to map the cyclical crankcase pressure changes that occur during each revolution of the crankshaft, you need more than just your sensor. This requires the same type of equipment and software Option Pack as combustion chamber pressuring mapping.
Our DYNOmite-Pro boards collect data at either 200 Hertz or 1,000 Hz (in USB mode). This is ideal for traditional dynomometer testing - where you are collecting data on Hp, Torque, RPM, fuel flow, etc. But, to map what goes on during a single revolution of the engine, you'll need individual data readings taken consecutively at each degree of crankshaft rotation. At high (two-cycle range) RPM's we are talking about board sets that can handle over 100,000 Hertz - like the 500,000 Hz board found in our Combustion Pressuring Mapping Kit.
You also need an encoder that provides an index for each pressure reading (to the exact degree position of the crankshaft when that data is captured). Again, the encoder in the Combustion Pressuring Mapping Kit is appropriate.
Note: It's mounting (which is designed for an automotive crankshaft dampener) would have to be custom adapted to your magneto housing.
Lastly, you will want to custom order the kit with a much lower range pressure transducer. Instead of the normal 2,000 PSI range, you
will want one that is appropriate for the -14 to +30 PSI readings of a normally aspirated two-stroke's crankcase.2)
If instead all you want is to monitor the average crankcase pressure (a relatively low positive number), then your existing sensor can do that. However, you can not just screw the sensor into the crankcase. Those erratic readings are caused by the arbitrary synchronization aliasing) between your DYNOmite's 200 Hz sampling rate and the position of your engine's crankshaft. You're never taking the pressure readings at the same index twice.
Instead, plumb it to your engine's crankcase using a restricted dampening orifice. A hose with about a 0.030" fuel jet stuck in it works well as the orifice. Also, put a small dampening chamber (plenum) between the orifice and your pressure sensor. A small in-line fuel filter makes a convenient chamber. Once your have the restrictor and plenum in place I think you will find your erratic readings cured.
Note: We recommend that the pressure sensor be oriented so any residual fuel droplets will gravity drain back into the plenum and crankcase between tests. That greatly extends the sensor's life.
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