Quote:
Originally Posted by serialk11r
Definitely possible. Running the engine slowly and at low load works great on tiny economy engines with low valve lift, but for engines with greater "breathing" capability combustion efficiency can be a serious problem.
Porsche said that they were able to decrease idle fuel consumption by 20% when they moved from fixed cam profiles to Variocam Plus, and that was with low duration cams (high pumping losses).
My own FR-S has this problem too, below 2000rpm the efficiency drops like a rock. I can drive at 50mph and get 48mpg, or I can do 60 and get 42mpg. Even dropping the speed to 40mph, I can't really break 50ish. If I'm going 35mph, dropping down to 5th gear and 2000rpm engine speed vs. 1500rpm in 6th doesn't even drop the mpg by 10%.
Increasing cam overlap by just 5 degrees to introduce a little bit of EGR past what it was designed for from the factory caused some misfiring and a massive drop in fuel efficiency.
I imagine on your older engine, the engine could be struggling to burn the mix quickly at low speed.
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That is a very good point!!! Something that isn't talked to much about.
I wish my cams weren't fixed. I tried to find a cam profile that would be give me the high rpm flow with low overlap for FE. After doing some WOT pulls the engine lays down around 6000 rpm. This is making me run a much higher octane full more then I wanted to at WOT high boost and WOT mid boost pump gas only do to my low rpm shift points dropping from 6500 rpm to 4400 rpm. This will get better with cooler ambient temperature as Fall approaches but it still drives me crazy.
So I'm going to see what the cooler temps bring.
Trying to make awesome power and still get great fuel mileage is a major pain and has been a lot harder then I would have ever thought.