I am considering increasing the VTEC switch-over point in my 2007 Honda Accord. The reason I want to do this is to improve the MPG of the car.
In this question I am ignoring the open loop versus closed loop effects because all the driving I plan to do, as well as the tests I have and will perform will be oriented towards the closed loop functioning of the ECU.
The switch-over point defaults to ~2400 RPM off the assembly line.
I ran a few tests over the summer to simulate this. The tests consisted of driving on the highway for at least 100 miles at 2100 RPM (60 MPH). 2 tests showed that during the summer the MPG was above 43 MPG. The same routes at "normal" speeds (70 or usually above) yielded a maximum of 33 MPG. (The vehicle is rated at 31 MPG highway by the EPA.)
I have repeated the tests several times as well as tested in "city: driving conditions by paying strict attention to the RPM at all times, versus ignoring the RPM. The MPG gains are similar but not exactly the same. In the "city" driving tests the MPG is ~31 MPG when I am not strictly controlling the RPM and +35MPG when I am strictly controlling the RPM.
If you research VTEC (in this case i-VTEC) you will see that the ECU uses numerous variables to determine when to enable the VTEC solenoid. But the switch-over point is still within a range determined by static/persistent variables in the code the ECU is executing. i.e. the range is set, but the point within that range can vary. This means that the switch-over point may not be exactly 2400 RPM but will be within some number of RPM of 2400 RPM. If I floor it (WOT) from a standstill the switch-over may be ~2200 (estimated) or so, but if I accelerate very gradually the switch-over may be ~2600 RPM.
At RPMs above the somewhat fluid vtec switch-over the MPG decreases due to drastically increased air flow into the combustion chamber.
So I would like to run tests of highway and city driving with either the vtec switch-over disabled or set to as high as possible. I realize there are aftermarket ECUs available that might allow this but they are all oriented towards performance, so they do not suit my interests.
I suspect that if the VTEC switch-over did not occur, and the ECU were driven in such a way to stay in closed loop mode as much as possible, the MPG could be significantly higher than the EPA numbers of 27/31.
It is also a fact that the maximum speed limit in Japan was 55 mph up until this past year. This speed allows the car to run below the vtec switch-over point, and would explain why the vtec switch-over point (IMO) is set so low.