YH Vacuum Advance? Re: <VV> Vacuum, and the YH (Long)

[FrankCB@aol.com]

        Yes, you are correct. There is no easy way to get ported vacuum from 
a CORVAIR YH carb.  That's why you can use a Dale vacuum advance/pressure 
retard chamber which uses intake manifold vacuum on your turbo distributor in 
place of the stock pressure retard.  The problem with this setup is it provides 
vacuum advance at IDLE which is NOT recommended.  (Just look at all the non-tubo 
Corvairs and all other cars - NO vacuum advance at idle.)  The solution is to 
add a solenoid valve and microswitch so that at idle the solenoid valve is 
closed, shutting off vacuum advance.  As the throttle is opened off idle, the 
microswitch on the throttle linkage activates, opening the valve and letting the 
manifold vacuum then correctly provide vacuum advance on the distributor.  
See Ray Sedman's   www.American-pi.com  for this setup.
       If you examine the separable throttle body section of the Corvair YH 
carb, you'll find that there is only a single shiny metal plug on the exterior. 
 The hole that this plugs is NOT a vacuum advance port since it goes to a 
slotted opening in the bore which is PERPENDICULAR to the throttle plate.  So 
using this opening will provide vacuum even at idle since the opening is located 
both upstream AND downstream of the closed throttle plate at idle.  A correct 
vacuum advance port is a small slotted opening (typically 1/8 in. by 1/32 in.) 
PARALLEL to the closed throttle plate and located UPstream of the throttle 
plate when it is closed at idle position.  The slot is then exposed to 
atmospheric pressure and therefore provides NO vacuum advance at idle.  However, as the 
throttle is opened, it moves to the other side of the slot thereby exposing 
the slot to manifold vacuum since the slot is now located DOWNSTREAM of the 
throttle plate.  If you examine the bore of the Corvair YH throttle body, you 
discover there are NO openings that match this description.  Therefore, NO vacuum 
advance opening is provided in the stock Corvair YH carb.
       But remember that the YH carb was also used on early 6 cylinder 
Corvettes and Nash engines (at least us "mature" guys remember when mothers wouldn't 
let their daughters go out with guys with Nashes since many of them had seats 
that reclined into BEDS(:-).  But returning to the matter at hand, these 
NON-Corvair YH carbs certainly had vacuum advance ports to provide correct 
ignition timing operation.  In fact throttle bodies from these carbs not only have 
the shiny metal plug, but they also have an additional external threaded opening 
which leads to the correct vacuum advance slot as described above.  When I 
rebuilt my YH years ago I used one of these NON-Corvair YH throttle bodies to 
provide vacuum advance to the turbo distributor using a "pot" from a NA Corvair 
distributor.  This functions perfectly providing NO vacuum advance at idle, 
but bringing in vacuum advance once the throttle is opened above idle speed.  
This not only significantly improves part throttle response but also improves 
mileage for street driven turbo Corvairs.  (Talk about having your cake and 
eating it too.)  My 180 gained over 20% mpg increase in the same daily trip back 
and forth to work.
       However, unless you plan on driving your turbo Corvair UNboosted, you 
have to add some provision for preventing detonation under boost.  I used an 
electronic knock eliminator (EKE unit as available from Corvair vendors) to 
provide retard when it "heard" pinging/knocking.  Ray Sedman's SafeGuard is an 
even better way of achieving this since it retards only the individual cylinder 
needing it instead of all 6 as mine does.  
       Further details can be found in the Modifications section of my 
Turbocharging chapter of the Corvair Basics Manual.
       Frank "180s need variable timing" Burkhard      
          
In a message dated 7/25/04 3:22:58 PM Eastern Daylight Time, 
m.j.murphy@comcast.net writes:

> All of this talk about vacuum and ported vs. manifold and all inspired me 
> to
> check my car today (OK, so I was bored).  I'm running a turbo with 140 heads
> on a powerglide.  I run minimal base advance with an advance unit and a
> (recently expired) water injection setup, and it does alright.  My question
> for the experts is this:  I have heard that there is no accessible ported
> vacuum on the Corvair YH, but I found a tap on mine that I use to signal
> both the VA and the water.  It reads minimal vacuum at idle, and increases
> with the throttle as it should.  There was a lead plug in the throttle plate
> housing, located just in front of the "back under" carb mounting nut that I
> tapped a nipple into and thought nothing of it.  Do I have a non-Corvair YH
> or does everyone have this?  The only numbers I could find on the carb (no
> tag) were "146-254" cast into the bowl cover.  Thanks for any ideas!