Large displacement engines & fuel economy

[Jody Shapiro]

Here's an idea/thought I've been kicking around for awhile and wanted to 
see what others thought about it:

A common reason for going to larger displacement engine is for the gain 
in torque, particularly at low RPM, without having to sacrifice high-RPM 
power (by changing intake runner length).

The downside is a drop in fuel economy at part/light throttle since you 
still have to "feed" the extra displacement.  A good example is a 
350" motor stroked to 383".  You get all of the benefits of the extra 33" 
of displacement at WOT, but highway mileage will decrease now.

Unless I'm misunderstanding something, the drop in mileage is because you 
need to maintain the same A/F ratios that you would have maintained for 
the stock displacement motor.  You can lean the mixture out as much as 
you like, but there's a limit to how lean you can go.  The stock fuel maps 
will try to lean out the mixture as much as they can for a 350" motor.  
Since you now have a 383" motor, if you inject the same amount of fuel as 
you did for a 350" motor the mixture will be too lean since the motor 
will be sucking in more air into each cylinder now than it did before.

Given that you can't lean out the mixture anymore, what is the best way 
to boost part/light throttle fuel economy?

How about a system similar to Cadillac's on the NorthStar engine whereby 
they don't inject -any- fuel into some cylinders.  The reason Cadillac 
does this is in the event of cooling system failure.  Cylinders are 
starved of fuel in a preset order to allow them to just pump air in an 
effort to allow things to cool.

This obviously has an adverse affect on power (I believe the NorthStar 
engine has problems going faster than 55mph or so when in this state).

I don't know how frequently the Cadillac system cuts out cylinders, or how
many it cuts out in a given firing sequence, but couldn't this idea be
applied to a large displacement port-fuel injected motor?  In other words,
to conserve fuel, cut out a cylinder in every firing sequence (but make
the cut-out cylinder a different one each time so no cylinder gets cut out
twice in a row). 

I don't know what kind of effect this would have on RPM stability, or 
engine smoothness (hopefully not too bad), but is this a feasible 
alternative so that you can have your cake and eat it, too?  You get all 
of the advantages of extra ci at WOT, and you don't have the gas guzzling 
at part throttle.

Obviously, this is something that we can only do via a custom programmed 
EFI system (like EFI332), but would it work?

Comments?

-Jody
-- 
http://www.token.net/~jshapiro/z28/