clarkefarm
Active Member
I have been running a Motec M4 ECU in a number of Subaru EJ25 engines for quite a few years. The M4 was an older generation ECU which amongst other things was not generally recommended to be run in closed loop at higher power settings.
I have recently changed to a new generation Motec M130 in which the fuel tables are broken down into a Volumetric Efficiency Table (how much air passes through the engine at a given inlet MAP and RPM) and a Fuel Aim Table which sets the desired fuel air ratio. The new CAN to Lambda exhaust gas sensors are very fast and have a sensor checking ability so closed loop (automatic fuel adjustment based on exhaust value) is accepted.
However, I noticed that there was a considerable variation in the values I was seeing when tuning for Volumetric Efficiency (VE) depending whether I was in flight or on the ground. Possible reasons were lower pressure in the exhaust system due to either airflow or lower ambient pressure due to altitude.
The actual numbers from an initial review are quite startling and would explain why, over the years, I have had trouble with rod/crankshaft bearings seizing.
At 75% power (inlet MAP 75 kPa) on the ground the VE was 96.7 with an ambient pressure of 95 kPa. At an altitude of 2850' above the field and still at an inlet MAP of 75 but an ambient pressure of 88 kPa the VE number had moved out to 104.9 or an extra 7.55% of air being mixed with the fuel charge (before closed loop trim).
With the older ECU without closed loop the lean condition resulting from lower ambient pressures would have led to detonation and excessive pressure on pistons, rods and bearings.
While the new ECU has closed loop and also some ability to program for ambient pressure, the reliance on an exhaust gas sensor which would be adversely affected by the use of leaded fuel, is less than ideal.
Some aftermarket auto ECUs are capable of using MAF (mass air flow) rather than inlet MAP to control engine load and if anyone is planning an auto engine installation these might be preferable.
Rupert Clarke
RV-9A
I have recently changed to a new generation Motec M130 in which the fuel tables are broken down into a Volumetric Efficiency Table (how much air passes through the engine at a given inlet MAP and RPM) and a Fuel Aim Table which sets the desired fuel air ratio. The new CAN to Lambda exhaust gas sensors are very fast and have a sensor checking ability so closed loop (automatic fuel adjustment based on exhaust value) is accepted.
However, I noticed that there was a considerable variation in the values I was seeing when tuning for Volumetric Efficiency (VE) depending whether I was in flight or on the ground. Possible reasons were lower pressure in the exhaust system due to either airflow or lower ambient pressure due to altitude.
The actual numbers from an initial review are quite startling and would explain why, over the years, I have had trouble with rod/crankshaft bearings seizing.
At 75% power (inlet MAP 75 kPa) on the ground the VE was 96.7 with an ambient pressure of 95 kPa. At an altitude of 2850' above the field and still at an inlet MAP of 75 but an ambient pressure of 88 kPa the VE number had moved out to 104.9 or an extra 7.55% of air being mixed with the fuel charge (before closed loop trim).
With the older ECU without closed loop the lean condition resulting from lower ambient pressures would have led to detonation and excessive pressure on pistons, rods and bearings.
While the new ECU has closed loop and also some ability to program for ambient pressure, the reliance on an exhaust gas sensor which would be adversely affected by the use of leaded fuel, is less than ideal.
Some aftermarket auto ECUs are capable of using MAF (mass air flow) rather than inlet MAP to control engine load and if anyone is planning an auto engine installation these might be preferable.
Rupert Clarke
RV-9A