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The Suter MMX 4 cylinder 500cc Fuel Injected Two Stroke
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https://youtu.be/X42r7fRdtWE
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https://youtu.be/i31XkNO89OU
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I think the Suter has a common single intake for an air box that feeds all four cylinders. I think you can see a small intake just below the front number board. If that is true I would imagine a common four stroke air flow measuring method like an air vane or hot wire would be the way to measure the mass air flow. And would be the way to go with any multi cylinder bike.
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An air box for a single might be possible but I have not tried that because of space restrictions. For my single cylinder two stroke it had to be done a little differently. Common four stroke methods like manifold pressure don't, hot wire or vane style air flow meters don't work.
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I started looking at changes in crank case pressure. Common 2.5 bar pressure sensors output there measured reading every mill second for one mill second. You can see the stepped pattern to the pressure curve in the attached pictures.
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The important thing to look for is the change in difference between maximum pressure near BDC and minimum pressure near TDC. (2 strokes are about face to 4 strokes). The greater the difference the greater the mass air flow.
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With a multy cylinder it may be possible to measure the change in one crankcase and assume all the cylinders are behaving in the same way.
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Things to remember:-
1) The average crank case pressure is highest at idle. Very near ambient. (crankcase is full of smog and backflow air from the exhaust.
2) The average crankcase pressure decreases a little when the engine is on the pipe. Makes sense when you think about it. Because the pipe is sucking hard out.
3) If you are using an oxygen sensor for testing, mount it as near the stinger as you can get it.
4) Mounting an oxygen sensor near the header is a mistake. Wash through will kill it.
5) When you are not on the pipe back flow air from the muffler and short circuiting air from the cylinder makes a oxygen sensor look lean.
6) At low speeds off the pipe a Wide Band O2 air/fuel reading will be like 18 or 20:1
7) As you come up onto the pipe the air/fuel reading will be like 13 to 14:1
8) At max torque where trapping efficiency is at its best you should see 12:1 (or richer).
9) As you go into over rev the air/fuel reading will trend back to 13 or 14:1
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It is the magnitude of change (delta) between the max and min crankcase pressure that indicates the change in mass air flow through the motor. Bigger difference = More air mas flow.
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Now that Speeduino can handle two maps and switch between them.
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I started by making an Alpha-N map and this works well at any RPM for anything above 20% throttle and on the pipe. The plan is to measure the outright delta of the crankcase pressure and with a bit of mathematical magic out put it as look a like ordinary four stroke MAP value to the Speeduino for use in the VE table for anything below 30% throttle..
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I started to get some good results with the Alpha-N table and was looking to develop the VE table when my engine cried "Enough" and quit. Not sure what went wrong, main or big end bearing don't like 14,000 rpm maybe. Or pealed a bit of chrome off the bore. I did a lot of porting without re chroming the cylinder. Hopefully I will get it fixed over the Xmas holidays.
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The pictures attached below are recorded on my actual engine while it was running on the dyno.
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