Modifying Subaru's VVT system or Dual AVCS to optimize fuel economy and torque. read more.....
Hi guys, this weekend I have started logging and tuning my car. I will be providing details for people interested in this thread. Basically I will be using opensource tuning software such as Romraider and ECUFlash. I will use romraider for logging and ecuflash to read and write to the ECU. read more.....
Atikinson Cycle Tuning
I have spent lots of time researching methods to reduce fuel consumption for cruise events on modern engines. VVT is something that has existed for years and it is very very helpful when it comes to fuel consumption and power as well. I spent most my time on Subaru's AVCS (its own version of VVT) tuning past week. I realized after some analysis that subaru uses the early/late VVT strategy at pretty much anywhere below 3500 RPM. This means early intake valve closure and late exhaust valve closure. Allows for a crap ton of overlap and EGR. I'm not much interested in EGR as it kills throttle response. There is another way to improve part throttle economy without sacrificing response. The late/early strategy is utilized by toyota and other manufacturers trying to employ atkinson cycle functionality on VVT equipped cars. When you delay the intake valve closure, the intake valve remains open after the piston is travelling back up from bottom dead center (compression stroke). This pushes back some of the air/fuel mixture into the intake manifold which reduces amount of fuel and air in engine. SO the engine burns less fuel. However I have the AVCS tables setup in a way to quickly switch to early valve closure for better torque on increased throttle. Stock AVCS cam profile: Stock VVT Tuned AVCS at cruise: Tuned VVT The intake remains open 30* longer well past BDC reverting more fuel mixture into the manifold. The exhaust cam is also advanced so as to not coincide with IVO. The stock cam profile is actually underestimating the intake advance... but you can see in an atkinson style combustion we only have overlap on EVO. All this leads to the following AVCS settings (note we are only talking about the red highlighted area in this context, the rest of the powerband has been tuned purely for torque): AVCS Tables. For comparison, here is stock AVCS Tables.