Todd Mullins wrote:
>
> I can ramble on about things like carburetion and ignition, sometimes
> even drawing on factual information, but I have exactly ZERO clues about
> ignition advance and its relationship to performance.
No clue? Great! My favorite audience!
Bob's Law: The crummier the carbs and intake system, the more advance
needed to make power! Yep, them SUs really suck but they're not very
good at it.
Seriously, I haven't owned a 'B' in years and find it frightful how much
initial advance you guys run. I can only think of a few good reasons:
1.) Your total advance sucks. Maybe the 'B' dizzy doesn't have much
centrifigual advance and you need all that inital advance to make up for
it.
2.) The great, late, and lamented, vaccum advance/retard debate. Some
models of SUs have the vaccuum retard port hidden under the edge of the
throttle plate at idle. When you first crack the throttle, the port is
uncovered and the vaccuum retard dumps in -- allowing you low RPM
acceleration without pinging while still providing decent total advance.
3.) Much like throwing a match on a woodpile with gas on it, there is a
momentary ignition and then a KA-THUMP as the vapor ignites. Same thing
happens in a motor. You want that KA-THUMP at precisely TDC + BCH for
maximum power. Perhaps our beloved SUs are dribbling out a pretty coarse
fuel droplet and then the droplets are getting confused in the siamesed
intake port -- so it takes awhile to get the fire going inside the
motor.
4.) Maybe it is something else.
5.) Vaccum advance don't mean diddly in 'normal' cars but probably comes
into play with the silly SUs. Their 'constant depression' design may
provide some vacuum advance at high RPM with the throttles wide open.
Dunno for sure and don't even wanna bring it up again.
With the DCOE Webers on the C/GT I am running 8 degrees of initial
advance and 38 degrees, total, at 3500 RPM. I can measure this with an
adjustable timing light (highly adivised) and can adjust it to anything
I want with the Mallory Dual-Point (also highly advised). There is no
vaccum advance (oh well) as there is very little vaccum left. I've
pretty much proven that any more advance causes severe detonation and
any less makes less power. Also, the car is a great barometer of gas
quality (cheap gas and ping!).
How about someone with a stock dizzy and carbs, with an adjuatable
timing light, provide us with a reading at idle, 2,000 RPM, and 3,500
RPM. I'd be curious. It this test is done along with vaccum readings, we
might really learn something.
And then in the BS-repellent category, I've heard people say that the
actual advance is double what you see on the timing light as the dizzy
is running half the speed of the crankshaft. I ain't buying this as I am
measuring the advance on the crank pulley, not the dizzy. However, I do
believe that my Webers see 36 degrees of total advance while the dizzy
is only having to manufacturer half of that - 18 degrees. Thus 'a
little' movement in your centrigual or vaccuum advance makes a big
difference in your total timing.
Which leads me back around to liking the Mallory distributor so much.
Mainly because it is new. You can put all of you panzy-ass computer
gizmos in your worn-out dizzies you want but you can still get erratic
ignition if the dizzy bushings are shot (or the camshaft chain is
sretched). You can't (to my knowlegde) buy the parts to rebuild the
dizzy and I have yet to hear anyone say "I just bought a brand new
distributor, threw out the analog guts, and installed a 'bug-zapper
1000' computer igniton module and now the car runs great!"
Liquid gasoline isn't very flammable. It's the vapor that makes the
KA-THUMP. So finely atomized fuel, thourghly mixed in the air flow, is
the best way to make power. Now you tell me how the SU design, with the
gas dribbling in off the bottom of the venturi bridge, getting jerked
around inside the log manifold, and then being prodded through the
siamesed intake ports, is going to be an efficient design! A lot of
advance is needed to get the coarse mixture lit off in time to greet the
piston.
I'll stick with my Webers and the six inlets on the 'C', thank you very
much. The minivans can still give me a run for my money but at least
they have to put the cell phone down for a moment and concentrate.
With a hesitant and begrudging nod to our Canadian FIend, modern Fuel
Injection systems don't use much ignition advance. The reason is that
the fuel injection nozzles (Port Fuel Injection) put out a really fine
spray right onto the intake valve (which cools the valve and atomizes
the fuel) thus the combustion process goes KA-THUMP real fast. Therefore
little advance is needed and, again I'll admit begrudgingly, the
combustion process is very efficient (most of the fuel is burned and a
real lean and hot exhaust flow is generated) and the engine makes nice
power.
An additional power factor with modern FI is that the intake manifold is
'dry' as the gas isn't injected until just before it enters the
cylinder. Thus the designers can make long, smooth, convoluted (and
plastic) tubes to get all the air lined up before it is rammed into the
motor. That is why most 'V' motors have that 'bundle of snakes' look on
the top -- you don't have to worry about the gass puddling out of the
mixture.
But I digress....
And as David Duetsch has suggested, I don't worry about using too much
bandwidth because I'm only typing with my two middle fingers!
Bob Allen, Kansas City, '69CGT with fine advance, '75TR6 with crummy
advance.
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