Smallblock efficiency or How to have your cake and eat it too

T56MaxTorq said

RAMM, I think if your cam is too large like the 250 you mentioned, you’ll bypass to much fuel during overlap on the highway, and fuel mileage will always suffer.

I think you’d want a mild, high torque cam with the most flat torque curve possible with a higher ring gear.

Click to expand...

If the RPMs are kept high enough that *shouldn't* be a problem. With a high-overlap cam what happens during part-throttle low-load situations (below the effective RPM the cam is meant to work at) is exhaust gases get pulled back into the intake manifold because the pressure in the intake is always at least slightly lower than in the exhaust; unburned fuel gets spewed out the exhaust because of misfiring and incomplete cylinder filling which is caused by the exhaust gases diluting the intake charge, like uncontrolled EGR basically. At WOT or heavy load things change completely, as the RPMs get into the "tuned" range of the intake, cam and exhaust headers there is the vacuum on the exhaust side which pulls extra intake charge into the cylinders (we know it as scavenging).

So in short if there is any vacuum in the intake from the throttle plates being anything but wide open the concern is for exhaust going into the intake, not intake charge flowing into the exhaust. Of course I'm sure there are exceptions and this is a big generalization, and I'm not trying to be a know-it-all just adding to the discussion.