<VV>spark plug miss (longish thought process - valve guides maybe)

[Padgett]

>Generally, increasing the gap improves idle quality because
>the higher voltage and larger spark area improve charge
>dilution tolerance.

In an ideal cylinder, the charge (gasoline) would be perfectly atomized and 
evenly dispersed in the chamber. No matter where the plug was, an ignitable 
charge would be in the same place. Unfortunately the real world is not the 
same as movie cars which burst into flame any time they fall off a cliff.

Carburetors, also knows as "poorly controlled leaks" are more likely to 
drip gas into a puddle on the bottom of the intake from which surface 
evaporation makes its way to the cylinder than as perfect atomization. Fuel 
injection, particularly port injection is much better.

Hokay. high vacuum cruise, particularly with a lot of advance, is really 
the worst case to a spark plug since the charge dilution is at a maximum 
and for best economy you want a high a/f ratio - more like 17:1 or 18:1 
than the 11:1 for maximum power.

So light load cruise and idle is really the worst case from some aspects 
(not going to get into high rpm leanouts at this point, just understand 
that carburators, ANY carburator is a kludge though as they went, 
Quadrajets and ThermoQuads with tiny primaries and giant secondaries plus 
both jets and rods were pretty good).

WOT, at least until you hit certain limits at high flow and high rpm is 
really the easiest place to fire since the charge going to the cylinder is 
maximised and the likelyhood of a combustable charge existing at the plug 
gap is maximum.

Now idle, having the minimum air flow so the venturi is not really working 
is a special case. Unlike fuel injection, part of the mass flow energy of 
the air is used to pump gas from the well to the airstream. At idle there 
just is not much energy in the Rochester HV (are some constant velocity 
carbs like the SUs that worked very well but had their own issues). For 
idle, a special circuit (the "idle circuit") is designed to provide enough 
gas to support the idle condition but being at the throttle plate rather 
than in the venuri, atomization is even worse than at speed.

So at idle you have a relatively high vacuum (which helps the atomization a 
bit) but a really inefficient means of introducing the gas into the 
airstram. As a consequence a combustable mixture at any given spot in the 
chamber is more by chance than by design. Add to the fact that old style 
points ignitions require narrow point gaps  and the chance that the spark 
will ignite something is less than gaurenteed. Corvair adds to the problem 
by placing the spark plug in the corner of the chamber rather than the 
center so the distance the flame front has to go if it ignites is maximised.

Hokay. so the bottom line is that the size of the gap and the duration of 
the flame is proportional to the probability of ignition since the charge 
is not evenly dispersed.

Early points ignitions were limited to about 30KV which in turn limits the 
size of the plug gap. In 1966, gaps ranging from .025" to .035" were common 
and enough for daily driving. "Hotter" coils were available but confined to 
high performance applications because they stressed the rest of the 
components (particularly the points which led to dual point distributers).

When electronic ignitions appeared in the early 1960s, they were expensive 
but worth it for competition. The first ones were transistor switches (e.g. 
the Delcotronic) that essentially did the same thing as a Pertronix Ignitor 
though at a much higher cost. By 1967 these were replaced by CD (capacitive 
discharge)  ignitions still at quite a high price, followed in the 1970s 
first by the unitized and then by the HEI ignitions that became quite large 
to reduce the arcing problems with very high voltages (48KV-60KV) until 
multi-coil distributerless ignitions eliminated the problem.

Now what I *think* is going on is that my engine at 74K really needs a 
rebuild or at least a valve job and specifically may have worn valve guides 
which would explain the fluttering in the vacuum gauge at low rpms while 
the compression in all cylinders is quite high. This could also explain the 
plug fouling if by crankcase gasses. Valve guide leaks would have maximum 
effect at very low air flows with high vacuum (idle) and would not affect 
compression.  If both intake and exhaust, the high pressure exhaust 
pulse  (amplified by the single exhaust) would leak into the crankcase and 
be sucked through the intake forming a sort of free EGR at idle.

One way to find out would be to see if the blowby goes away and it runs 
differently if the valve covers are off. Do recall the covers seemed quite 
clean when off before

This "dirty" component in the intake further dilutes the charge and acts as 
a small vacuum leak making it harder for the stock ignition, which would be 
fine if everything were up to snuff, to reliably fire and eventually fouls 
plugs. Revving the engine overcomes this leakage which is why there is no 
problem at 800 rpm in N.

What I am trying to do is to overcome the problem instead going into group 
red to fix it properly the same way the general did - with a High(er) 
energy ignition and wider gaps. Am actually planning to go to a 48 or 60 KV 
ignition first, with W8ACs at .030" and then widen to .040" if necessary. 
Hopefully this will not exceed the distributor cap limitations. Already 
have 8mm silicone plug wires.

Certainly, none of this will hurt once a proper fix is done. I am also 
slowly learning all of the operating characteristics of the engine again 
but need to get this fixed first before trying some other intake 
configurations (and the single exhaust is good if I wind up needing an O2 
sensor).

Perhaps I should be looking for 95 hp heads to remove the need for 93 
octane. Is the cr reduction done with the heads or the pistons or both ?

Turkey Rod Run is in Daytona this weekend and I have quite a list of parts.

Padgett