comments on ignition timing

[Ken King]

>The thing sucks +- 950 liters/second @ 6000rpm, so it needs BIG air channels.
my math: ((950 litres/sec)*(60 sec/min)) / (6000 rev/min) =9.5 litres/rev!
however, 4 stroke engine takes 2 revolutions to complete, 19 litres or
1178 cubic inches!  yowsa!  even pre-supposing a huffer/turbo doubling the
atmospheric pressure, that makes for a 580 cubic inches!  mid-engined!  if
i can get out there, can i get a ride!?!  :)  i'm presumming one of us muffed
the math... :)   still, sounds like quite a car.  a gt40?  just curious...

>The first thing that I would like to start playing with is ignition timing.
sounds like a great place to start... consider how a normal distributor
does timing & use it as a baseline to make additional refinements.  perhaps
using something like msd's boost timer (timing adjustment from drivers
compartment) for twiddling with the curve...

>My thoughts are as follows:

>* The air/fuel mix takes a constant _time_ to burn.
not true (by my sources... :)  the denser the charge, the faster it will
burn.  why?  not sure.  as the rpm increaces, the number of crankshaft 
degrees spent burning will increace, so timing must be adjusted to get peak 
pressure at teh proper time (see below), timing must be advanced.  also, as
the load increaces, the density of the charge increaces, speeding the burn,
which requires *less* timing (again, see further below).

>* The max. pressure point has to occur when the piston is on it's
>  way down, and when the conrod has a decent leverage on the crank.
correct.  recent mag. article (hot rod?) stated 14-18 degrees is 'optimum'.

>* Assume idle = 1000rpm, and advance = 10 degrees @ idle. 1000rpm =
>  6000 degrees per second, so the spark occurs 1/600 second btdc.
sounds reasonable.  our math matches on this one.  :)

>* Assume that we want the maximum pressure at 10 degrees atdc. (Please,
>  this is the wildest guess I've made in a long time)
closer than you think (at least by my sources).

  [ munch ]

>* And the next tricky point - vacuum advance. The vacuum is an indication
>  of engine load. But why is it a good thing to advance the timing when
>  the engine is loaded? By how much should the timing typically be advanced
>  for a given load? (And I mean _typically_, I know that the Bosch systems
>  amongst others use complicated, dyno-derived maps. However, I am more
>  interested in deriving the kind of curve a standard distributor gives.)
higher loads (low intake vacuum signal) helps to compress the charge, which 
causes the charge to burn much more rapidly, requiring less advance.  
without *less* advance, the location of the crank at peak pressure moves 
towards TDC, and could even occur *BEFORE* TDC, which is not a good thing
tends to make for unhappy cranks! :(  also the higher cyl. pressure can 
help a local hot spot (carbon deposit, flashing seam,...) to pre-ignite 
the charge (again, not a good thing :)

so how does it work?  in the vac advance canister, a vacuum diaphram works 
against a spring.  when there is high vacuum (low load), advance is made
because the vacuum diaphram pulls on a rod that rotates the contact plate
(for points distributors (proper term?)) or pickup plate (electronic)
oposite the direction of rotor rotation, causing the event to occur sooner.
when high load conditions exist (lots of pressure on the loud pedal :), the
vacuum in the manifold drops off, and the spring gains the upper hand over
the vac diaphram & pushes the plate back where it 'belongs'.  this is part
of the reason why you are suposed to disconnect the vac advance hose when
doing a tuneup (to allow for the setting the base timing).

hopefully i haven't propagated too much mis-information.  corrections are
expected if i blew anything, flames cheerfully ignored. :)

later,
kc