D.I. COLUMNIST
Tuned Up
with Will Hanna
A
nybody who has ever
heads-up raced has heard the
saying “lean is mean.” It’s 100
percent true. It can be mean on the
competition, mean on your motor
and mean on your pocketbook. The
last two, almost always.
My dad gave me a very good les-
son on rich versus lean one day out
in the shop with an oxy-acetylene
torch. He was also teaching me
how to cut with it. Too “rich” and
you have a big, cold, orange flame
that doesn’t cut anything. Keep on
“leaning” the flame with oxygen and
you get this blue flame that will cut
metal. Too much and it goes “pop”
and goes out. That was a pretty good
analogy because racing engines do
the same thing from rich to too lean.
EFI is great because it gives you a
lot of control over how rich or lean
you are. MFI has less control but has
been refined for many applications
over the years. Air-to-Fuel Ratio
(AFR) is something you will defi-
nitely program in an EFI applica-
tion, but it’s a target with a MFI or
carbureted application. It is often
assumed that there is a “target” AFR
that is correct and should be main-
tained for an entire run to make
peak power. In the world of drag
racing, I don’t think that’s always
the correct assumption.
“It was hauling ass until it blew
up.” Why would it haul ass if it was
“too lean?” The closer you get to the
true optimal AFR for your applica-
tion, the more heat is created from
that flame. That heat is absorbed
by the piston, sleeve, chamber and
valves, and can eventually change
the ignition point. Without proper
adjustments, this can lead to pre-
ignition and detonation.
Another factor to consider is in-
creased boost from ram air in super-
charged application as the car goes
down track. Higher intake air temps
as the car goes down track is not
uncommon as well. There are obvi-
ously a number of factors that make
the optimal AFR a moving target.
In the MFI world, we use “lean-
outs” to basically “lengthen the wick
of the bomb” as it goes down track.
They can also be a useful tool in
power management. In a perfect
world, the AFR would stay optimal
and timing would be used to man-
age wheelspeed. However, some-
times running the motor a little rich
through the shake zone will give you
a wider window.
Timing is a key ingredient to any
AFR discussion. We often think of
it as a horsepower knob. After all,
it’s what is primarily used in power
management. It’s tempting to push
the limit on max timing because
they can really haul ass with timing
in them, but as I tell people, timing
has torn up more parts than lean
ever did. A little over center it goes
fast but beats up parts. A tick more
from there and it wrecks engines.
For timing, we need to remember
it is exactly what it says – it’s the
timing of the start of the combustion
process. Any given amount of fuel
is going to take a certain amount
of time to fully combust. More fuel
takes longer, so you can get away
with more timing advance. There
are a number of different factors
that affect how much timing is op-
timal, including compression, rpm,
head temp, charge temperature,
camshaft, cross flow – the list goes
on and on.
Sometimes if we get married to a
certain amount of timing or pushing
the timing envelope, you have to run
too much fuel to keep it from hurt-
ing parts. Yes, burning more fuel
makes more power, but not when
you are starting the burn too early.
This can obviously turn into a
much deeper conversation, but the
moral of the story is you are chasing
a moving target all the way down
the track. You can’t just set the AFR
or Boost/Fuel ratio to “X” and try to
maintain it the whole run.
One last bit of advice – it’s way
easier and cheaper to work from
rich to lean than it is lean to rich!
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