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Old 09-12-2012, 08:35 AM   #1 (permalink)
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Getting the most out of lean burn (Honda)

I've had this idea for a long time that our civic leanburn engines didn't have to be handicapped due to emissions, I believe they have a lot more potential than the EPA allows them to have.

The subject will be the 92-95 Civic VX, D15Z1 or D15 vtec-e equivalent, that is what I'll be doing here. The idea is to maintain a leanburn under all - or almost all - conditions except for mid and max power. Unfortunately, the US P07 ecu limits leanburn to 2600 rpm and the only way out of that is to find a UK version P07 which is extremely rare.

A full retune with another engine management system would be best, but aside from the cost I do foresee some possible implications - a monetary risk would be at stake.

Goal -

1. Allow leanburn to activate at approximately 140 degrees coolant temp and in all gears; 1st gear leanburn is blocked and 2,3, and 4 gears have coolant temp restrictions.

2. Leanburn at idle and when stationary.

3. Allow leanburn to remain engaged at nearly ambient manifold pressures.

To achieve #1 the ecu must see the car as being in neutral and coasting - neutral uses same tables as 5th gear.

To achieve #2 the ecu must see the car as being in neutral and coasting, the TPS sensor must also output a off-idle signal, but must be switchable back to idle values in order for the fuel cut to activate while in gear. Additionally, a greater volume of air must pass through the throttle body to maintain a 500+ rpm idle speed due to the higher manifold pressure required under a leanburn condition.

To achieve #3 a false map sensor signal must be sent to the ecu. The ecu must see a lower manifold pressure than is actually present. The ignition timing will be advanced because of this - every manifold pressure value has a corresponding ignition and fuel value. Distributor compensation may be required and closed loop fueling may need compensation due to potentially exceeding the ecu's limitations on the max percentage of O2 correction, this will be corrected by increasing the fuel pressure. Fueling under full throttle is a lesser concern since the air/fuel ratios are already 12.3-12.6. This engine will produce more power more efficiently at approx 13.5 AFR with its low compression, but with the onboard devices this will be trimmed to its optimal value further lowering the BSFC.

Just for the record - here's a quick and crude vid I shot on the route to establish a baseline for the STOCK leanburn limits. EDIT - I lost this video...

UPDATE EDIT - Here were the results -->


*Leanburn in neutral and stationary while throttle is depressed -

*Leanburn in 1st gear

*Leanburn in all gears (rather than 5th only) at cool engine temps - engages about 1 mile sooner than before.

*Leanburn at/near ambient manifold pressure

*More usable torque in leanburn

*VTEC does not engage until 3000 under any condition

*Max power air/fuel ratios can be held at 13.5 for optimal efficiency.

*A solid 2mpg gain.


Added parts - Apexi fuel controller, ebay square wave generator

Last edited by greasemonkee; 09-26-2013 at 07:36 PM..
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