[Diy_efi] OL operation - How lean at cruise?

gary gas-
Thu Mar 31 22:51:01 UTC 2005


----- Original Message ----- 
From: "Michael Richards" <michael at fastmail.ca>
To: <diy_efi at diy-efi.org>; <A6intruder at myo-p.com>
Sent: Thursday, March 31, 2005 10:40 AM
Subject: RE: [Diy_efi] OL operation - How lean at cruise?


> As Hugh mentions EGTs are important to watch as running too lean
> can result in burned valves. I see this a lot in the honda world where
> kids swap their heads and continue to use factory computers that are
> mapped for less VE.
>
Peak EGTs occur at the stoichiometric ideal AFR of 14.65-70: 1.
Making the air fuel mixture richer or leaner from that point, (all other
factors being equal)....will result in LOWER, NOT higher .... EGTs.
With the ignition timing set approopriately, how can you possibly
generate more combustion heat with less fuel?  Not logical.
>
> I think you will find the main economy gains under cruise will come
> from ignition and not mixture. Without some special features for lean
> running you will be hard pressed to cruise at 16:1 and leaner without
> mis-firing.
>
Actually, main economy will come from a leaner AFR, coupled with the
correct timing.

AFAIC, misfire from too lean an A/F mixture, is a false myth.  In reality,
continual leaning can be conducted until which time, the combustion flame,
due to a lack of heat, just goes out.  Sorta like turning down the gas valve
on a kitchen range, less and less heat (not more) is generated until the
flame, from a lack of fuel finally goes out.  The misdiagnosed so labeled
'misfire' is actually the result of the flame in one or more cylinders going
out ahead of the others, due to the normal lack of a precise equal AFR
in all cylinders.  The physical appearance/feeling of a misfire, is actually
in reality, one or more cylinders going dead, no different than if a plug
suddenly stopped firing.
>
> On many V8's you run alot of advance during cruise. I suspect this
> is related to their archaeic head design (insert other V8 digs here).
> In short if you've got a lot of exhaust left in the cylinder you will need
> more degrees in there.
>
The reason for the need/necessity to advance the ignition timing in a
lean burn application, is due to the slower burning flame front, which
consequently will take longer to complete.  Because the point of peak
combustion/heat is now later than what is closer to optimum, more
undue heat is generated/experienced in not only in higher EGTs, but
higher temps in the cylinder as well.  The AFR that generates the
quicker flame propagation is that which generates the most power,
appr. 12.5:1.  The flame travel is slowed either way, from that point.
>
> Realistically you need to put the car on the dyno to find MBT for
> cruise but you can try adding 3-4 degrees in the cruise part of you
> map and seeing how much of an economy effect it has.
>
> I worked on a 302 powered Ranger some years ago. The customer
> was running 16 degrees of advance during cruise and taking about
> 22l/100km.  I leaned it out a bit and found MBT to be up in the 40
> degree advance range. It's been a while but 44 degrees sticks out in
> my mind. End result was 12l/100km. Don't blindly try this but it is an
> example.
>
> -Michael
>
> > -----Original Message-----
> > From: diy_efi-bounces at diy-efi.org
> > [mailto:diy_efi-bounces at diy-efi.org]On Behalf Of hugh at sol.co.uk
> > Sent: Thursday, March 31, 2005 2:59 AM
> > To: diy_efi at diy-efi.org
> > Subject: RE: [Diy_efi] OL operation - How lean at cruise?
> >
> > I read on here a while back that an AFR of 16 : 1 was the most
> > economical cruise mixture.  I have experimented with 16 : 1 and
> > 17 : 1 without much to choose between on economy.
> >
> > My Exhaust gas temperature creeps up to 800?C when doing about
> > 90 MPH on the UK motorways at 16 : 1,






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