The XEDE piggyback is a sophisticated unit capable of more control than your typical piggy
When developing a tuning program, Shiv is as conservative as he needs to be. Each programming step must take a full load dyno blast for more than a minute. The car is run at full throttle for 20 seconds at each 1000-rpm increment without registering any significant knock sensor activity. The engine must not register a significant loss of power on subsequent runs, not just during the dyno pass! That tuning pass/fail methodology is a testament to the reliability of Vishnu's tuning.
For engine control, Shiv first flashes the factory ECU with a performance base program using the Xflash tuning system. The Xflash is an ECU reprogramming system for the EVO that is capable of revising all performance-related engine control parameters. The high- and low-octane fuel tables, high- and low-octane ignition timing tables, adaptable knock control logic, injector scaling, boost limits, rev limits and speed limits can all be modified depending on the application.
For our application, we needed something more than just an ECU reflash. We needed something that would allow us to have multiple maps for race fuel and water injection operation. To allow multimap capability, Shiv installed his user-programmable XEDE piggyback ECU to work in conjunction with our Xflashed stock ECU (which itself was only remapped for higher boost and rev limits). The XEDE is a sophisticated piggyback ECU capable of altering every critical signal input and output of the factory ECU. This way it avoids the typical pitfalls of piggyback ECUs, fighting the factory self-learning control.
The XEDE piggyback plugs into the stock EVO wiring harness.
The XEDE has some drawbacks though. It cannot raise the rev limit, speed limit or boost cuts (that's where the XFlash comes in). Like a typical piggyback, it must alter the MAF sensor's signal to trim for larger injectors. This used to be a significant disadvantage because altering the MAF signal can also put the engine in the wrong cell of the spark table. This, fortunately, is easily accounted for due to Vishnu's knowledge of the factory ECU tables.
We drove Project EVO to Vishnu's Northern California headquarters for a tuning session with hopes of exorcising our car's low-octane demons for good. At Vishnu, Shiv strapped Project EVO to the rollers of his Dyno Dynamics AWD dyno and went to work by first downloading an Xflash to the factory ECU. Next, Shiv installed the XEDE and revised the factory fuel and timing maps. Finally, he experimented with different cam timing scenarios only to determine that our prior settings were close to optimal for the Tomei camshafts.
Shiv at work Xflashing the stock ECU.
About then we ran into our first problem. After many pulls our air/fuel ratio started to get inconsistent, fluctuating between normal and lean. Turns out our fuel pump was overheating and losing efficiency. According to Shiv this is a common problem when the stock fuel pump is being pushed to its limit. The fuel pump was quickly swapped out for Vishnu's 255 liter per hour high-capacity pump and we were back in business.
With the stock ECU tune, our boost pressure would vary from run to run, from as high as 19 psi to as low as 15 psi depending if detonation was detected. Shiv set our boost at 20 psi and raised the rev limit to 8000 rpm, removing the speed cut. He also modified the fuel and spark maps and limited the amount of knock retard.