Alan,
I will try my best to answer this but it really gets complex, all of my info
was substantiated and verified with dyno pulls. To understand detonation you
must really
understand the type of fuel you are using. Gasoline is not the most
forgiving and ideal fuel but we need to live with it. It is really quite
fussy. Ideal combustion
occurs at approx. 14.7/1 fuel air ratio, and max. power occurs with a 12.5/1
fuel air ratio. These values are approx. values and they do change and are
effected by
many variables (head design, piston design, temp. fuel mixture, etc, etc,
etc, etc, etc). Ratios leaner then 14.7/1 and richer then 12.5/1 tend to
slow the burn
process. Octane rating is the fuels ability to resist spontaneous combustion
(detonation). A change in any variable will have a direct effect on the fuel
and whether
it will detonate. When we put an engine together, one of the most important
variables that we need to consider is something called pumping compression.
In an
ideal world where complete even combustion could occur, we would like to
compress the fuel air charge to approx. 165 psi (given the use of fuel with
an octane
rating of 92), anything less and the fuel air charge burns to slow (wasted
power) and anything greater and you stand the chance of detonation. Again
this is ideal
world conditions, all of the above variables may cause you to raise or lower
the pumping compression. The three major variables that effect or change
pumping
compression are: compression ratio, cam duration, and volumetric efficiency
(ability to get fuel and air into the cylinders). When you place a
supercharger on an
engine, what you are actually doing is increasing the volumetric efficiency
of the engine. If the engine already has a pumping compression in the 160's
then you
need to control fuel detonation. We can raise the ability of the fuel to
resist detonation by cooling the fuel air charge, raising octane, reducing
timing, richen up
the fuel air charge or a combination of any of the above. In the case with
my truck with boost pressures around 4 lbs in the 2,000 - 3,000 RPM range
there was
nothing we could do with pump gas, timing, intake temperature to stop
detonation. Timing figures were right on, and we even used a boost retard
feature of the
ignition and could not suppress detonation. The fuel air ratio under boost
was already rich (12:1). Even though you may not hear it, with a stock
Magnum engine,
boost pressures above 4 lbs between 2,000 and 3,000 detonation occurs I
guarantee it. All you need to do is run the truck on a dyno and look at the
exhaust and
you will see it. With My engine, we reduced the static compression, altered
the cam duration and raised the torque peek, used larger injectors, modified
the
engine controller and many other things. A lot of work and down time.
Pumping compression at crank speeds is approx. 155 PSI. I am really quite
skeptical
of someone telling me that they can run 4 lbs boost in a stock Magnum engine
and not have detonation and if someone told me 7 lbs, well they would need
to
show me. Remember I tow with my truck and run 4 lbs boost sometimes as long
as 30 minutes at a time. Even now I can feel the difference between tanks of
gas. Add on top of this the need to comply with OBDII and it really becomes
a tuning nightmare. At minimum, if you consider this avenue, ask for a list
of
customers and talk to them. Better yet, find someone that will take you for
a ride and listen. Remember talk is cheap. My thoughts.........
Frank
> -----Original Message-----
> From: Mok, Alan (SPB Brentwood) [SMTP:AMok@spbank.com]
> Sent: Thursday, September 17, 1998 7:50 AM
> To: DML
> Subject: DML: detonation with only 4 lb or boost?
>
> I'm seriously considering a Vortech for my RT but I just read a post
> about problems with detonation with more than 4 lb or boost. In my
> experience 4# is very little pressure for a 9:1 compression engine. Even
> if we do have iron heads it should not detonate with a 5 lb blower.
>
> Is the computer's timing advance too aggressive? What's the static
> timing on a 5.9? I would hate to retard the timing at this level because
> you're just dumping some of the HP you sought with a blower in the first
> place.
>
> Are you guys using a colder spark plug? Vortech tend to calibrate their
> FMUs to run on the rich side so I don't think you're running too lean.
>
>
>
> Alan
>
> '98 Dakota R/T (Headers, FABM & waiting for Shift kit)
> '89 Mustang GT (347, Griggs, S-trim...)
> Ducati 916 (someday)
>
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