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Old 03-27-2009, 10:02 AM   #23 (permalink)
TomO
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The VX - '92 Honda Civic VX
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You are correct that Lean Burn works when the motor is in 12Valve mode, which is when the VTEC-E is NOT engaged. at 2500RPM and above that certain throttle threshold, the ECU grounds the VTEC solenoid and then fully opens all 16 Valves, Lean burn does not happen when VTEC-E is engaged, mostly because the combustion dynamics turn to a tumble type system which doesn't support a lean mixture. When VTEC-E is not engaged, the vehicle is in 12 Valve mode where both exhaust valves are opening normally but only one of the intake valves are opening normally. The other intake valve is only opening enough to release any fuel that is sitting on top of it. This makes the fuel mixture swirl inside the combustion chamber. That makes it easier to ignite a lean mixture.

So, basically, Lean burn is dependent on throttle position, engine temp, and engine load below 2500RPM, but once VTEC-E engages, there is no Lean Burn.

SVOboy would be able to verify if Lean Burn is available above 2500RPM as he has had his motor in Lean Burn up to 70mph I believe. But the circumstances for that would be a light engine load and very small <10% throttle position. That is one way that you can go above 2500RPM and still have Lean Burn. VTEC-E will not engage until 3000RPM then as the driver is not demanding the full power of the motor.

Hope this clears some things up.
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