Alternatives Re: <VV> Rebulding Retard?

[burkhard at rochester.rr.com]

I don't have a list, but I'm pretty sure most cars in the 50s - 60s 
took vacuum advance from a ported signal -- that's how it was done on 
the well pre-emission Rochester HV as well as the non-Corvair 1950s 
applications that used the Carter YH (Corvairs using the YH were 
turbos... a special case).  I don't have ready access to information on 
other 1950s-early 60s carbs, but I'm pretty sure ported vacuum was the 
norm on most applications.

As for the ported signal being used only to reduce idle NOx, that 
doesn't make a whole lot of sense. NOx produced at idle is soooooooooo 
small compared to NOx under load!  The NOx reaction is very strongly 
temperature (and hence pressure) related.  Idle (with very low peak 
pressures and temperatures in the combustion chambers) is the LAST 
place you would tinker with anything to reduce NOx.  Worry about HC & 
CO at idle? Yes!  NOx? no!

The reason to use ported vacuum is exactly as my father described -- 
you need to look at what the two types of vacuum do on tip-ins and 
driveaways.  Plain intake manifold vacuum takes a dive as the throttle 
is opened when you perform a driveaway. If this is used on most 
vehicles, you will get a sag that results from the sharply reducing 
vacuum advance.  Ported vacuum was used instead because it starts 
out "OFF" and then INCREASES as the throttle is opened.  In this 
manner, the base timing can be set a bit higher and then the vacuum 
advance coming in *adds* torque, not pulls it away.  Finally, it's 
common to calibrate idle spark under MBT timing. On modern cars, this 
is down to provide some spark-based torque authority to allow active 
spark control to control the idle speed; on older cars, the somewhat 
retarded spark at idle allows a better misfire resistance and idle 
stability.

So, why does your SS396 (which came AFTER the Corvairs which used 
ported vacuum advance!) not use ported vacuum for its vacuum advance?  
I don't know exactly, but it could be related to the MUCH HIGHER 
power:weight ratio of this vehicle. On a light tip-in the manifold 
vacuum probably doesn't sag nearly as much as with a Vair.  Also, I am 
guessing that that vehicle probably has a pretty wild cam, so it 
doesn't have very much idle vacuum in the first place.  In any event, I 
think vehicles like an SS396 were the exception, rather than the rule.

Jim Burkhard

----- Original Message -----
From: Andy Clark <slowboat at mindspring.com>
Date: Thursday, February 22, 2007 0:30 am
Subject: Re: Alternatives Re: <VV> Rebulding Retard?
To: virtualvairs at corvair.org

> Frank, this has not been my experience nor that of many others.
> Long before the smog regulations were enacted, most, if not all, 
> cars with
> vacuum advance took the vacuum signal directly from the manifold. 
> With the
> advent of the early smog regulations, NOX emissions at idle were 
> reduced by
> reducing the total engine advance at idle, hence ported vacuum 
> advance. I
> remember my '66 SS396 was ported directly to the manifold and the 
> last thing
> it had was stumble off-idle.
> My Cord 180hp engine uses manifold vacuum to a Dale vac/press unit 
> and it
> too comes off idle smartly and smoothly. The only effect of adding 
> the Dale
> unit using manifold vacuum was an increase in idle speed, easily 
> adjustedwith the idle speed screw.
> Andy Clark
> Camano Island, WA.
> 1966 140/4 Monza Sedan
> 1966 140/4 Yenko Clone
> 1966 180/4 Cord 8/10 #60
> 
> ----- Original Message ----- 
> From: <FrankCB at aol.com>
> To: <louis at suffolknet.org>; <virtualvairs at corvair.org>
> Sent: Wednesday, February 21, 2007 2:00 PM
> Subject: Alternatives Re: <VV> Rebulding Retard?
> 
> 
> >
> > Lou,
> snip...............................................
> >     However, this setup now provides manifold vacuum to  the 
> advance at
> idle
> > conditions, where you don't really want it since  it tends to cause
> "bogging"
> > as the car is accelerated above idle  speed.
> snip................................
> >     Regards,
> >     Frank "likes boost" Burkhard
> 
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