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22 Hp Harbor Freight engine evaluation + other V tiwns

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pictsidhe

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The 555 varies pulse width approximately with the signal from the MAP. The MAP pressure is approximately equal to the inducted air weight. You should get approximately the right amount of fuel with this scheme at any throttle opening. It is non linear and imperfect in many areas, but with a WBO meter and a leaning knob, should be OK for our use. The circuit could be changed to make it more accurate, but then it will get increasingly harder to set up. At a guess, a straight 555 and MAP sensor might be accurate to +/-10% across the full power range. It will likely be leanest around peak torque, which you need to watch. If you really want to make it more accuratre, it's probably easiest to build the basic circuit first, test it and work out what correction syou need where, then ask a friendly geek to design a correction circuit. But it still won't be dead nuts.
 

pictsidhe

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Could the leaning be done with a separate controller using EGT?
Theoretically, but it would be tricky to implement. I wouldn't try it myself...

WBO2 is the way to control mixture strength. Even that will go haywire if faced with rich misfires, which will show as lean...
 

Vigilant1

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Could the leaning be done with a separate controller using EGT?
Part of this gets down to the design philosophy. Every sensor is a failure point, and not a simple one (they can fail to "null" they might fail "high", or they might just get rapidly or slowly less accurate, etc). So, every sensor leads to more complexity in failure proofing the system. A potentiometer in the cockpit could fail, too, but it's less likely and easily diagnosed. Also, a manual mixture control lets us run rich to control CHTs, if needed (could be pretty handy if we are refining the cooling/baffling, etc).
An EFI like this one is cheap enough that 2 completely independent systems should be considered. Test 'em both during runup, like a mag check. The only common part would be the throttle body.
 
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