At 13:53 10/11/98 +1100, you wrote:
>Dennis Plunkett wrote:
>> In this case the rev limiter can cause the exhaust to be blow off
>> when it gets filled  up with unburnt vapors (Then you may get some nasty law
>> suits!)
>
>Sounds good though :-) It's a good trick as long as you know how to weld
>up mufflers.
>
>I used my trusty ol' FJ ute as a guinea pig while developing the
>software to test different times at missing sparks, and while I concede
>that backfiring could be a problem, it won't occur unless the engine has
>been off for at least a few seconds. I don't think it's worthwhile
>debating the subject here, because there are too many engines out there
>with different levels of performance, and I don't think there is a
>single solution for all of them.
>


Agreed on that, on modern engines the intake valve is open while the piston
is on the rise, thus causeing a slight back pressure on the intake. This has
some other nasties and side effects when the engine is not firing on all
cylinders. Yea ol FJ is a simple engine with not too many ticks under the
rocker cover.


>> One thing to do is retard the timing, thus reduce the engine power. Other
>> options are to control the fuel in-take, but this is not as easy.
>
>I've heard that retarding the ignition is not as good for the engine
>either, especially at high RPM. I've tried to retard the ignition, but
>unless you retard it a long way back, there is not much noticable
>difference. Unless you know what you are doing, reducing fuel flow could
>also be dangerous to the engine at high RPM.

Retarding is exactly what is done on the high performance stuff. It is just
that now with fuel injection one can keep the optimuim power while at high
RPM. As for dammage? Well excessive fuel over a period of time will cause
the oil to be washed off the walls of the cylinders, this simply promotes
high wear, not much else I think.
Retarding does work, but will require anything up to 17 degrees, this then
may cause a cross fire, but is generaly not a problem. Also in this case the
engine has to be loaded so that a notticable difference can be seen, if it
is not loaded, then the engine will free wheel.

By reducing the fuel flow, I ment that not all cylinders receive fuel. If
the engine is run lean, then it will create "Hot spots" on the cylinder
crown, this heat is then transfered to the rings and cylinder walls. This
then causes the rings to "Weld" themselves to the walls and causes scaring.
Ford did have a system that caused the fuel to be cut off if the engine was
not loaded and running at over 2400RPM. With simple controllers, one could
imagine some of the results (If I remember it was dissabled, someone else
may know a bit more about it, I never had the chance to fully investigate)


>
>I can't say exactly, but I've seen late model V6 engines trying to out
>rev themselves, accidently of course :-), and it appears that the fuel
>may be cut for short periods because the engine starts to sound like it
>is 'motor boating'. This is probably an easy task for the engine
>management system.

Yes this is correct, the late model engines with simple (We make em by the
thousands) engine controllers just stop fuel metering-> this then causes
"Knee Jerk". Try a VN Commadore (Opal, with the Buick 1962 engine block that
was 3.7lt converted into a front wheel drive V6, then converted into 3.8lt
with a rear wheel drive )(Assuming that you can get the poor darling to go
over 6700 RPM), but hang on! (No offence to GMH, its just that the HC11
can't handle it very well at all :-) )


Dennis