|
Brucey:
Which vehicle are you talking about? I would think a Subaru would have higher rpms than 3000 at 70. I have a 99 Legacy DOHC 2.5 in the midst of it's head gasket replacement (don't they all need them eventually?) and I could have sworn that it used to turn 3400+ at 70mph.
Those boxer 4s are not very good torque engines, plus their packaging constraints limit the sroke length. The engines already have a larger bore (the 2.5 is a 99.5 mm bore with a 79mm stroke) than stroke. This makes it mathematically unlikely to achieve high torque at low engine speed, but has the inverse effect of decreasing the mean piston velocity at a given RPM. Engine friction is an exponential function of mean piston velocity just the same way that aerodynamic friction is an exponential function of vehicle velocity. In an engine with a very small stroke like Subies it doesn't necessarily hurt your economy just because you're turning 3k when someone with a "stroker" engine is turning 2k, both engines could have equal piston velocities and be producing equal internal friction losses.
A .040 overbore will increase your rotating mass, increase the ring/wall contact and thus internal friction, increase the volume of fuel consumed per cycle, but also increase thermal efficiency. It's hard to say if its hurting you or not.
Blue07Civic:
Be careful what you wish for. My truck's 5spd is set up with a pretty tall 5th so that at 70 mph I'm turning 2000 rpm with my 3.55 axle ratio (the only other factory option was 3.92, which would put me at 2200 rpm at 70mph). I can regularly get better fuel economy by foot than by Cruise Control because the cruise will aggressively attempt to hold speed up a hill in 5th despite falling down the weak side of the torque curve, whereas by my foot I can speed it up a bit in anticipation of a hill and let it drop below cruising speed up the hill. I'm sure if I could down shift without canceling the cruise it would work better, but short of that I get the same mileage if I speed up to 75-80 mph which puts the engine just above a localized peak in torque, and it can hold up a hill much better.
__________________
|