Caveman Engine ManagementThe whole point of removing all these flow restrictions is to let the turbo operate more efficiently. Less backpressure means more energy to turn the compressor wheel, which means more flow for less work. We can now take advantage of running more boost, which means more power. Like most modern cars, the MR's boost pressure is electronically regulated by the factory ECU.It's programmed to build 19 pounds of peak boost near 4000 rpm and drop slightly as revs approach redline for safety reasons. Without reflashing the ECU, the boost curve can't be changed unless we use an aftermarket boost controller.
The simplest and cheapest way to change the stock boost curve is a ball and spring type manual boost controller (MBC). We chose just such a unit, made by Dejon Powerhouse. It's an adjustable spring loaded check valve that stays closed until boost pressure on the other side of the ball exceeds the set spring tension, allowing us to set our boost a little higher. More importantly, the MBC gets rid of that sissy boost drop at the top where there's real power to be made.
It's not electronic or fancy, but the Dejon Powerhouse MBC is the easiest way to override
We left the stock boost control solenoid plugged into the harness to avoid triggering an engine check light. And because the EVO does not use a closed loop feedback for its boost control, the solenoid will continue to click happily away in its own little world, without the ECU having a clue that the wastegate isn't doing what it's being told.
The real key to making more power on a stock turbo car is to lean out the air-fuel (A/F) mixture. Stock cars are tuned to use an excessive amount of fuel to cool the intake charge and prevent detonation. But the mixture gets so rich that combustion efficiency is sacrificed and overall power is lost.
A'PEXi's S-AFC (Super Air Flow Converter) II offers an affordable and tunable temporary solution for correcting the factory fuel map without reflashing or replacing the ECU. The S-AFC II splices into the factory ECU harness and intercepts the signal coming from the air flow meter, which is then corrected for the specified settings and sent to the stock ECU.
Although the stock 560 cc/min injectors flow enough fuel for our current power level, we u
By using a negative airflow correction factor on the S-AFC, the stock ECU is tricked into seeing less airflow going into the engine, so it injects less fuel, resulting in a leaner A/F ratio and more power. Simple right? Well, sort of, but the right correction factor must be entered for every 500-rpm increment, on both high and low throttle maps, without running the car so lean that it starts hitting the knock limit.
RRE has an informative web site for instructions on how to street tune with the AFC on the EVO (www.roadraceengineering.com/newafcsetting.htm and www.roadraceengineering.com/newafc.htm).
A leaner mixture is only half of making more power; the other half is ignition timing. For the same conservative reasons the stock car runs rich, the factory ignition timing is overly retarded at full boost. To save money and avoid the need to install another piggyback, we've resorted to other tricks to fool the ECU into advancing the timing.
This is why we replaced the stock 560 cc/min injectors with 660 cc/min GReddy/Denso units. What does injector sizing have to do with ignition timing? The ECU controls spark timing based on how much air flow it's detecting. By using larger injectors, we have to further increase the air-flow correction on the AFC, which makes the ECU think the engine is using even less air. The ECU will only see part-load when, in reality, the throttle is wide open and the boost is pegged at 21 psi.