Cincy: hotrod plans/what to do? ACK!

[Larry" <rocco16v@netzero.com (Larry)]

Scott-
  First, the engine you speak of is quite under-square.  This may only be a
problem if high (and I'm talking 10K+) rpm is your goal.  Since it probably
is not (because cams, airflow, and bottom-end 'robustness' will be your
limiting factors) I wouldn't be overly-concerned.  It indirectly limits the
breathing efficiency...the higher the rpm, the more limiting it is. But
there is nothing you can do about that.  We're working within the
limitations that VW has given us.  Get the motor.
(easy for me to say, it ain't my money.....)

 Combution chamber size (or, more specifically, CC diameter) influences an
engines tendency to detonate.  Visualize this:
Detonation is the phenomina that occurs when the pressure in the CC,
immediately after ignition, rises to the point that it causes the unburnt
mixture that is in the far reaches of the CC to spontaneously ignite. You
then have two (or more) separate flame-fronts going in the CC.  When these
flame-fronts collide, they cause a large 'spike' in CC pressure.  You hear
this as 'knock' or 'ping' (it's more like a rattle) and you see it in
collapsed ring lands, broken rings, and piston crowns that are literally
eroded away.  This is more evident when you 'lug' an engine, since lower rpm
gives each 'firing' more time for this to happen.  Upping the rpm will stop
this, even though the same octane, same CR, same CC design are in
play....another reason GSXRs, CBRs, ZX's, YR's can use higher CR: at 5000rpm
they are just barely into their working range.   I'm undoubtedly preaching
to the choir, since you probably know this.

Now, since this flame-front travels at a constant speed (determined by CC
design), if the CC is smaller, there is less time for this to occur (at any
given rpm), since the original flame-front (the good guy) will reach those
far boundries of the CC before the mixture awaiting there has time to
self-ignite(thereby becoming the bad guy).   This is why smaller CCs can get
away with higher compression ratios than larger CCs....all elso being equal
(which, of course, is never the case!).
HTH.

Larry


> Larry, what does combustion chamber size have to do with detonation
> characteristics? I know that there is a relationship; I just don't know
what
> it is.
>
> Btw, this 95.5mm crank and 83.5mm bore scares me. Can you say "horrible
rod
> ratio"? I knew you could. :^) The extremity of the rod angles and the
> potential for wear and tear in the bores seems pretty crazy. Then again, I
> know that people build these motors and are happy with them. So... the
> feeble engineer part of my brain is in conflict with my moderate-level
> hotrodder knowledge yet again. :^)
>
> -decisions. -decisions.
> --
> Scott F. Williams
>