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Old 05-28-2014, 04:50 PM   #4 (permalink)
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MC SBX - '95 Chevrolet Monte Carlo LS
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Originally Posted by ever_green View Post
allow me to run a lot more timing during cruising
that's the wrong way of thinking about spark advance.

more advance means more time the piston fights a combusting air/fuel charge to get to TDC(and start going down in the correct direction for the power stroke). anything you can do to allow output to peak with less advance will return you more power per unit of fuel injected.

introducing EGR with no other changes(or the PCM accounting for it) will cause two really noticable things to happen: the engine will run rich and you'll lose power. it runs rich because the EGR gasses displace oxygen that would otherwise be in the cylinder, it drops power for the same reason.

with correction/accounting:

the PCM has a good idea of how much of the air/fuel charge is EGR gasses and reduces fuel flow accordingly to maintain whatever AFR it is targeting. a lot of factors contribute to how accurately the AFR is maintained, but the O2 sensor will dial it in pretty soon after.

the engineers who setup the calibration for the PCM know that once EGR is introduced, the amount of advance to get back to peak output per fuel unit is greater(because of a slower air/fuel mixture burn speed), so advance is increased.

for a given manifold pressure/vacuum, you CANNOT create the same amount of power with EGR compared to without it, since the EGR gasses will bring the manifold pressure up, requiring the throttle to be closed to bring manifold pressure back down. you can get back MOST of the power lost through EGR operation through EGR-added spark advance.

the economical gains are from the reduction of throttle losses and reduction of exhaust backpressure, since the throttle has to be opened further to maintain a given power level and when dealing with gasses, an area of higher pressure will flow towards and area of lower pressure.
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