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Re: S4 20v vs S2 10v
> Re: The center coils: my guess is this is a vital component when running
> hiboost with hi compression, not so key when using the low comp motor.
"Separate" coils have a number of advantages, not the least of which is
the simplification of switching 20KV (i.e., the distributor). It is a lot
easier to switch +12 to each coil than switch the coil to each cylinder.
Also, for a given rpm, you're only switching each coil half as much (as-
suming you take the shortcut of "sharing" alternate cylinders on a 4-cyl
engine -- e.g., 1&4 on one coil, and 2&3 on the other coil, which still
greatly simplifies the switching, gives most of the advantages of separate
coils, and costs less than truly separate coils) to one-fifth as much (if
you have, for example, a 5-cylinder engine where you can't timeshare coils).
This allows the coil(s) to more-fully saturate at high rpm, which means
you can deliver a stronger spark when you dump the coil's inductively-
stored energy. (I think it also means you can run a smaller coil per in-
dividual spark for much the same reasons, but I can't remember all those
niggly little details.) Finally, it also means you're running much shorter
high-voltage lines, which decreases radiated noise and makes for greater
(I suspect it also means you can charge five times as much for a tune-up
since you have five times as many coils to tune, right?)
my understanding is that the 9.3 compression on the S4 is for
off-boost response.. once the turbo spools up, CR is really quite a
meaningless number, because 2 bars of boost is 2 bars of boost,
regardless of what the geometric dimensions of the engine are. i.e. 2
bars in a 9.3 compression engine is equally likely to detonate as 2
bars in a 7.8 engine, all other things being equal.. no?
Um, I don't think so. 7:1 compression of 2 bars is 14 bars, while 10:1
compression of 2 bars is 20 bars of pressure. I believe that detonation
is more-or-less a direct (if not linear) function of gross pressure. So
higher-compression leads to quicker detonation, all other things being
equal. At 20:1 you can dispense with ignition altogether (you have a
diesel engine). I recall that detonation is a function of many differ-
ent factors, not all totally understood. For example, a dirty engine
with lots of carbon buildup can detonate when the carbon retains enough
heat (sitting there glowing cherry red, for example) to ignite the fresh
mixture (whose susceptability to such might also be a function of its
pressure). Now if you had direct injection . . .
I believe the general rule is lower compression can run more boost and
yield higher gross power, but at a lower total efficiency (certainly
off boost; you may get the same "efficiency" on-boost, given same bore
and stroke figures; the turbo may actually feed back more power into
the engine -- yielding better efficiency on boost than you lose with
the lower CR).