Ignition retard knock sensor.

[G. Scott Ponton]

Hi all,
    I have been following this thread for a while. I am going to put my .02$
in at this point. I am sure some will have "alternate" views of this but
........................

<snip>

>In the last few months, I've found that detonation is *not* directly
>related to boost.  On 89 octane and no water injection for instance, if
>I whack the throttle open above about 2000 rpm, boost jumps almost
>instantly to 5 - 6 psi.  Detonation is almost instant as well.  However,
>if I ease into the throttle and let boost come up over 2 or 3 seconds, it
>can go to 7 or 8 psi with no detonation.

>Also, if I'm in a situation where I want to accelerate and I'm at say
>3000 rpm in a high gear.  I can press the throttle up to a certain point
>(probably about 3/4) and get say 6 or 7 psi with no detonation.  If I
>then floor it, boost doesn't increase very quickly until the rpms come
>up a bit (takes longer in high gear) and the mixture isn't going lean
>according to the A/F meter, but detonation happens pretty much instantly.
>Still can't figure that one out.  Boost and A/F don't change very quickly,
>but detonation seems to be tied directly to the gas pedal.

    The A/F ratio isn't the only thing happening here. Yes, you can use
"extra" fuel to help cool the intake charge and to reduce detonation. It
only works to a point though. After a certain point you are just pouring
fuel out the t-pipe as the mixture becomes rich enough to be impervious to
pre-ignition or any ignition for that matter. The reason you can "ease" into
the throttle an let the boost climb slowly has to do with the density of the
air charge. It works the same way with a NA engine which likes to "ping" at
WOT. If you slowly step into it you reduce the density of the air charge and
by the time you get to the higher revs the "time" it takes for the piston to
move is reduced. Therefor, even though the cylinder pressures are "equal",
the uncontrolled burn doesn't have time to create "ping" like it does at
slower engine speeds.
    When you open the throttle suddenly to WOT you are imposing no
restriction to the density of the air entering the cylinders. Boost won't
necessarily climb quickly as it takes some time for the turbo to "spool up".
If it spools up too quickly the housings are mismatched for the application
and the waitgate will have to be a complete equal sized bypass to keep boost
under control. My experience with late model engines is that they are tuned
from the factory to be on the edge of pre-ignition. Add just 2 lb. boost and
you have pushed it beyond what the availible fuel can handle.
    Here is a question that has helped me many times. Assuming 100% VE and a
300 CID 4 stroke engine, how much air is swept through the cylinders in 2
revolutions at idle with the throttlr closed?

<snip>

>It seems that unless you can factor in rpm (at the very least) in addition
>to boost to more accurately predict knock, you have to *really* retard
>ignition to cover the worst case, which makes you suffer a power loss most
>of the time.  That's why I figured the active J & S system would be much
>better.

    Here you are nearly entirely correct. One other thing you are missing or
forgetting though. Once pre-ignition has started, you have to retard the
timing further to get rid of it than you would have if you prevented it in
the first place. IMHO the best way to handle this is to figure out how much
retard it requires to eliminate the ping then add a boost retard function to
the ignition system so it retards the timing to that level under boost
conditions. In other words retard the timing a couple of degrees at a time
until the ping goes away then set your boost retard to that level. Adding a
knock sensor from there will make sure that under abnormal operating
conditions you won't have to worry about it.

    I mentioned once before in another post I had added a ignition retard
device to one of my old Pontiacs. I finally have dug up the notes on it.
This may be a "cheap" alternative for many. The unit I used was not and ECM.
It is an ESC controller off a 82 Chevy P/U. This was a standalone system
that worked with the normal dissy and uses a Knock sensor with a board to
retard the timing until knock goes away. It is fairly easy to connect and
doesn't require much besides the sensor and the proper ignition module. I
forgot to bring the part #s home with me. If any are interested I can scan
the skitz and include the part #s I used. I had to manufacture a dissy for
my Pontiac as I was using a Tripower intake and a normal HEI won't fit
behind it. You should be able to hook it into your current ignition system
as the negative of the coil goes out to the module and as long as there is
no "knock" signal it just comes back. Otherwise it "delays" it until the
knock signal goes away.

Scott