DIY_EFI Digest V4 #573

[Phil Lamovie]

> What about effects on exhaust back pressure caused by lower
> barometric... I'd think it would reduce the residual pressure
> in the cylinder at the end of the exhaust stroke, allowing 
> more air in for a given MAP.

> Orin.

Except for the fact that back pressure of the exhaust is not 
a function of the outside pressure as much as a function of
exhaust manifold configuration and pipe diameter/length/rotation.

Exhaust pipes are too long and the exhaust event too short for
the atmosphere to effect it much. The entire event at 280 degrees
of duration at 3000 rpm is 280/360/20 msec or 15.55 msec.
And of course this is no flow for the first or last 20 degrees
of cam lost in taking up the slack. now 220/360/20 or 12.22 msec

The in cylinder pressure at the time of the exhaust opening
would be in the order of 250 to 400 psi. (varies a lot with engine
load.)

As for allowing "more air in" the air flow is a function of the 
pressure differential between the in cylinder volume and the plenum
chamber pressure. The piston has been past top dead and the chamber
has been purged of exhaust. In fact about this time some new air fuel
mixture is following the exhaust gas out the still open exhaust valve
courtesy of a small "low pressure" event in the exhaust manifold.

This pressure event is exhaust gas speed/density/temp related
though not immune to air pressure out side the effects would be more
mechanical as the "spring" value of the air would alter and this
would detune the tertiary part of the wave trap.

Phil Lamovie

Injec Racing