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FUELforTHOUGHT
by Dave Koehler
12/9/03
QUESTION 1
I run a blown alky Hemi with a port Hilborn and I am having trouble getting it to return to idle. I have sufficient springs and I have centered the butterflys and I lube the shaft and barrel valve before every race. Any suggestions?
Chuck
Chuck,
This is one of those deals
that can make you a trifle crazy. You are a
little short on details so I will do some guessing
and throw up more questions than answers.
Duplicate this problem by getting
the engine as warm as it is when the rpm problem
happens. Shut it off and immediately recheck
the butterfly gap. If your butterfly clearance
is close to the same initial setting, then it
is not a mechanical problem. If is has changed,
then you will have to find the binding problem
and adjust accordingly.
If it is not a mechanical problem,
I will then assume that you are not trying to
achieve an impossibly low idle rpm and that
the rpm comes up after the engine is hot. I
will also assume that the throttle is coming
down to the idle stop like it should but the
rpm is up.
The most likely thing is that
you are dealing with a too lean barrel valve
setting. The engine just wants more fuel when
it is nice and toasty. Adjust it to what the
engine wants. Once you find what the engine
likes, you can take a barrel valve leak down
reading and record it for a future baseline
reference. Other unlikely things would be a
shut off valve that is trying to rattle closed,
a sticky idle circuit valve, or a nasty air
leak between the hat and the blower. I hope
this gives you some things to look at.
Have a safe race,
Dave Koehler
QUESTION 2
What is the difference between lowering your fuel pressure and changing the fuel jets to a smaller size? Is the pressure and volume going to mix that much differently or is the pressure needed so it gets the right amount of atomization with the nitrous through the fogger nozzle for mixing?
Andrew Wenzl
Andrew,
When all is said and done there
is no difference in flow. You can have a smaller
jet with higher pressure or a larger jet with
lower pressure and get the same flow through
a nozzle. X amount of fuel flow for a given
potential HP setting is the important thing.
Higher pressure does create a finer mist IF
it slams against a restriction like the end
of a nozzle. This finer mist will allow more
of the fuel molecules to catch and burn thereby
creating the potential for more horsepower or
at the least, a cleaner tune up. Watch DRO for
an article on this in the coming months.
Have a safe race,
Dave Koehler
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