Why Vacuum Advance Works Re: <VV> Turbo Hesitation - More Info

FrankCB at aol.com FrankCB at aol.com
Wed Oct 12 11:35:28 EDT 2005

In a message dated 10/10/05 7:40:07 PM Eastern Daylight Time, joew at diveaz.com 
Thanks for the reply Frank...Do you know specifically why a vacuum advance
would cure the problem?

On 10/10/05 4:15 PM, "FrankCB at aol.com" <FrankCB at aol.com> wrote:

>     If you want to eliminate this problem install a vacuum advance on the
> distributor, just like EVERY OTHER Corvair has.  When I did this years ago 
> my 180, the problem 
    Well, according to my "consultant" (son Jim) the timing of the spark 
should be such that the peak cylinder pressure occurs at around 14 deg. AFTER TDC 
(top dead center).  This gives the most power/efficiency for the engine.  
However, the speed at which the flame front progresses across the combustion 
chamber depends on the DENSITY of the air/fuel mix.  This density, in turn, depends 
on the mass flow (weight) of air/fuel entering the chamber which is 
controlled by the throttle valve.  The DENSER the mix, the FASTER the flame front 
proceeds so the LESS ignition timing advance needed.  Conversely, the less dense 
mixtures from the throttle being open only a small amount (as in low speed 
cruising) require MORE timing advance.  All Corvairs EXCEPT the turbos accomplish 
this with the ported vacuum advance which provides a range of timing change 
varying with mixture density.  The Corvair turbos, in contrast, have FIXED timing 
from idle to around 4000 rpm until boost comes in.  This leaves much to be 
desired in terms of throttle response as well as poor fuel economy for highway 
    I've often wondered why the factory didn't correct this situation 
sometime during the few years that the turbos were produced.  Maybe they were too 
busy designing the big valve 140 engine.(:-)
    Frank "if it don't go, blow it" Burkhard

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