On Thu, 24 Nov 1994, Matthew Beaubien wrote:

> Why is it that the vacuum _advance_ would retard the ignition? Is this 
> because when you set up the curve, you have to take into account vacuum 
> and then when you go WOT, you lose the advance you had taken into account 
> at other throttle positions? Couldn't you setup the curve that you lised, 
> and then add vacuum advance for better part-throttle running? If this 
> leads to pinging, you could always reduce the amount of vacuum by 
> bleeding it off or modifying the vacuum advance unit itself.

Depending on the type of vac adv you either have no vac adv at idle and
then full vac adv at part throttle (the vac port is just above the
throttle plate at idle and any opening of the throttle plate exposes the
port to vac), and then decreasing vac as the throttle is opened further; 
or you have full vac adv at idle (the vac port is in the manifold) and
decreasing vac as the throttle is opened. Either way, at part throttle
(just off idle), light load cruise you have max vac adv and less than that
if you open the throttle further.  With the vac hooked up you would open
the throttle to get a little more power and loose some advance (and
therefore some power) untill the engine speed increased and the mechanical
advance bumped the adv up. This slight delay in adv does cause a slight
hesitation. If the curve is set right without vac, any added adv from vac
is going to lead to pinging problems, so you would have to "detune" the
advance so that timing is right for light throttle, full load conditions
with the vac adv. It would then be too retarded at full throttle (and most
other operating conditions). 

What I've just outlined is not exactly acurate, but it is close enough 
for this disscusion. My point is this; a mechanical advance is a speed 
(engine) sensitive regulator, a vac adv is a throttle position/load 
sensitive device. They tried to use the two to run the ignition right up 
to the edge of detonation. Problem is it won't work for that. There are 
other factors that will bring the vac adv into play that do not relate to 
the engine's proximity to detonation. To me the way to go is to run the 
optimum mechanical adv (optimum for acceleration) and run either A) gas 
with high enough octane, or B) water injection to control detonation (it 
has both speed (engine) and load (manifold vac) sensors so that it will 
function the same as the vac adv, but it does not have as detrimental an 
effect on performance), or C) an electronic anti-knock device (a piezeo 
sensor hears the knock and retards the timing, thereby relating timing 
directly to what you are trying to control, namely engine knock). These 
choices are in (my) oreder of preference by the way. The higher octane 
gas will let you extract max power and so is my first choice. The water 
injection does have the advantage of helping to keep the combustion 
chamber clean, and provides nearly the performance of the high octane 
gas. The anti-knock will prevent knocking (mostly) but engine power is 
limited to the knock point of the fuel used. Its one advantage is it 
would let you use varying octane fuels without having to adjust the 
timing, provided the advance was set up for the highest octane fuel, and 
the lower octanes did not cuase knocking at a point beyond the ability of 
the anti-knock to compensate for. I don't know what the effects would be 
from having the anti-knock functioning (retarding the timing for 
lower octane gas) most of the time. 

I suggested in a news group once that a piezeo sensor hooked up to a 
water injection system (and maybe to the reatrded point set if a dual 
point dist is being used) might be a good way to control knock. There are 
also cockpit adjustable timing devices that let you quickly and easily 
reset timing for varying conditions (temp, octane, etc.). This would also 
be good for a car that is driven daily on regular pump gas and then raced 
on higher octane gas on the weekend. I just don't see why anyone would 
trouble with a vac adv when it won't do what is required of it and other 
things will.

Marc Sayer
Performance Engineering
1070 W. 2nd Ave. 
Eugene OR 97402
(503) 484-0904  Fax 746-0863
msayer@efn.org

------------------------------------------------


On Sun, 27 Nov 1994, Matthew Beaubien wrote:

> 
> So why is it that manufacturers equip their cars with vacuum advance? 
> Can't an engine handle more advance at part throttle then at WOT (at the 
> expense of NOx) thus the reasoning behind vacuum advace?
> 
> Thanks for your explantions.
> 
> 
> Curious Matt.

Auto makers who used vac adv's weren't tuning their cars for performance. 
What they cared about was emmissions, mileaage, and low cost. Even the
auto makers had a hard time making that system work when the octane of
pump gas went down. Even think about how many cars from the 80's you hear
pinging? Most automakers are going to DIS systems now and have been using
electronically controlled systems for some time (like GM's later HEI
systems, where all advance functions are controled electronically. Those
auto makers who do build performance generally don't use the vac system
any more, becuase it won't let them get the performance they want with the
emmissions etc they must have. 

Marc Sayer
Performance Engineering
1070 W. 2nd Ave. 
Eugene OR 97402
(503) 484-0904  Fax 746-0863


----------------------------------

I have obviously hit a special spot for a bunch of people with this and I
am not willing to waste anymore of mine time on it. I believe you are wrong
about several points Andrew, but no mater you do your thing your way and
I'll do mine my way. Fact is that cars I build run fast and that's what
people pay me for. If you have never seen manifold vac on a single vac
advance, you don't have much experience with cars. If you think there is
no vac adv at light throttle cruise, with ported vac, you are missing the
point of the thing entirely. And I can't see anyway for a modification to
the ignition system that improves acceleration to hurt mileage. Any mod
to the ignition that improves acceleration would be improving combustion
efficiency and would have to improve mileage at the same time. You are
getting more power from the same ammount of fuel since all we changed was
the ignition and not the carb/FI. Anyway I am tired of this thread and
tired of trying to explain that the point I was initially making was that
there are better ways to control knock if that becomes a problem, and
that to make good power with Webers/Mikunis/SK's you need to run alot
more initial and probably not use vac advance. I have set up these carbs
with and without vac and the power output is always better without. So
why would someone who just spent some serious $ for more power want to
loose some of it to a system of knock control that could be replaced with
a system that works better? Hell most of these set ups will run great
with no knock control at all, and no one bought these carbs for their
mileage in the first place.

Marc Sayer
Performance Engineering
1070 W. 2nd Ave.
Eugene OR 97402
(503) 484-0904  Fax 746-0863
msayer@efn.org