Engine Rebuild.. Fuel Economy?

[MechEngVT]

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.

[brucey]

I thought the flat 4's were pretty good for torque and had a very flat curve?



Mine's an auto with a 4.44 final drive, it's almost exactly 3000 rpm at 70 mph. At 60, its more like 2000. Mine's a SOHC version though, where as yours is a DOHC, if that makes too much of a difference.

[MechEngVT]

That curve is flatter than I remembered, but there's still a peak and 3000 rpm is slightly below it (funny, rated peak torque on the EJ25D is 2800 rpm). It's not exact, but peak torque typically occurs very nearly the same engine speed as the engine's minimum brake specific fuel consumption (BSFC). Since BSFC is usually rated at wide open throttle where there is a minimum of pumping losses (best volumetric efficiency) your best partial load BSFC while highway cruising is going to be a couple hundred RPM less than your WOT BSFC. The overall torque output is still not very high, which is typical when an engine is tuned for a broad curve.

My Subaru (well, it was technically my wife's before we got married) is also an auto, but don't know too much more specifics about it than that. It's still a stock ~145k engine with a blown head gasket.