Re: etc.

["Melissa & Jim" <jadney@vwtype3.org>]

On  7 Jul 97, Keith Park wrote:

> 
> JIM..
> 
> in regards to the vacuun advance (retard)  my meaning was that when
> you floor your car the vacuum goes down, you lose the vacuum advance
> and the timing is actually retarded.. hence vacuum retard.  yes the
> vacuum diaphragm gives advance in most circumstances but the retard
> is necessary under full power to reduce knocking and let the #3 run
> cooler.  with a 009 you lose this feature and need to set the whole
> curve lower to prevent knocking.

Okay, then at least we agree on what it is actually doing and when.  
The thing to understand is that the vac advance gives you a bit of 
extra advance when there is less mixture in the chamber, not lean, 
just "thin."  Under these circumstances the flame front burns slower 
and extra advance is useful.  The result of this is a little higher 
efficiency at high speed, low throttle (cruising) because it allows 
the peak combustion pressure to be reached closer to TDC.  Note
that this is not the regime where knock occurs (low speed, full 
throttle, high chamber pressures.)

Note also that this gives an amount of advance at high speed cruising 
that the engine could not tolerate under high oupput.  The only thing 
that keeps it from burning up is the fact that it is loafing.

You can omit the vac advance and never see a change in performance, 
only a loss of gas mileage, because the vac adv only comes into play 
when the engine output is low.  So I don't think it is necessary to 
drop back on the advance with a 009 due to the lack of a vac advance.

I say this as a result of all the vacuum advance units I have 
replaced over the years where the owners (myself included) had 
noticed no change in performance, just a loss of gas mileage.

Jim
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       Melissa Kepner                                    Jim Adney
       jadney@vwtype3.org              jadney@vwtype3.org
                             Laura Kepner-Adney
                             Madison, Wisconsin
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