Lean burn engines

[Greg Hermann]

At 6:47 PM 10/17/01, Ray Drouillard wrote:
>I have heard that running an engine too lean will cause it to run hot, and
>will burn out the valves.  I have never seen an explanation of this.  Why
>does it happen.
>
>Generally speaking, how lean can I run a standard (normally aspirated)
>engine.  Specifically, (if it makes a difference), I want to improve the
>mileage of my Jeep Wagoneer, which has an AMC 360 V-8.  It is fed with
>Holley Pro-Jection, and is fired with a Jacobs Omni-Magnum ignition box.
>
>What can be done to an engine to allow it to run leaner.  The engine is
>currently at the rebuilder (though he hasn't started yet), so I might be
>able to get stellite valves or something like that.

Stellite faced valves and hard seats in the heads are a good start. A
thermal coating on the chamber surfaces and the piston crowns will also
help. You will likely want plugs a bit cooler than stock. A lean mixture
burns slower, so timing will need a bit of attention in order to optimize
things, as well. Crane sells adjustable vacuum advance pots---(Your timing
at WOT should NOT need to change)

As long as you don't rev it way high, setting the squish clearance between
the piston and the squish/quench portion of the head is a HUGE help.
.035"--.040" clearance is a good number so long as the revs are kept
reasonable. This can be obtained by skimming the surface of the block decks
(of course, you must also know the thickness of a compressed head gasket to
get it right).

If you have seriously "open" chamber heads--with no squish area--you're out
of luck on this one.

Best thing to do is tune leaner and leaner until you start getting lean
mis-fire, then back off perhaps .3 or .4 AFR richer. The more turbulent the
chamber is, the further you will be able to go.

Greg

>
>
>Ray Drouillard


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