[eschaider]

I believe the issue with the more aggressive cams is lobe separation angle (LSA) and idle reversion you were speaking to earlier that it fosters, Olddog. Lobe separation angles between 104˚ and 108˚ or so come with significant overlap especially with the longer duration cams.

That overlap and the low speed idle reversion is what provides the nasty idle sound that is so sought after by many. It also produces an engine with relatively bad low speed manners below 2000 rpm or so. The EFI helps some in alleviating the issue because the fuel delivery is no longer contingent upon a vacuum signal from the cylinder. On the other hand, while the particular cylinder is reversing its flow up the intake port, it will necessarily drag some fuel with it leaning out that particular cylinder.

During the next intake event there will be some of the fuel that never made it into the cylinder from the last induction cycle and it will be mixed with a full charge of fuel for this cycle (less some for reversion again). The upshot is the cylinders get a raggedy inconsistent supply of idle fuel and air that produces the raggedy idle (that every one likes) with the attendant bad low speed manners (that get old quickly).

Plumbing the intake ports together with a heat resistant tubing on the valley side of the intake manifold can mitigate a lot of the negative pressure pulses but will not eliminate them entirely. As a result once again we have a bad mannered low speed performance but not as bad as before the plumbing was put in place.

Tight LSA's reproduce the sound of the 1960's race versions of the engine remarkably well. They just force you to operate the engine above the ratty idle rpm range so the car drives better around town. EFI (and hidden plumbing) can help to mitigate some of the mischief but it will not eliminate it.


Ed