4-cyl HEI in GM Autos?

[Clare Snyder]

Garfield wrote:
> 
> On Thu, 27 Nov 1997 10:16:33 -0500, Clare Snyder <clsnyde at ibm.net>
> wrote:
> 
> >Why would the advance be different? It is X number of degrees, period.
> >The six cyl unit should work just fine. Even an eight should work. Check
> >with NAPA, you will likely find the same module used on the 305 and the
> >4.3 carbureted engines. Mabee even the same on 4.3 and 5 litre efi.
> >Take your 6 and a timing light, and check the advance curve with the ecm
> >disconnected. Does the advance slowly advance, or does it jump?
> 
> Uh, I already said, I DID that. Once more with feeling: It jumps +8deg
> when it starts the advance ramp at about 2Krpm, and then "ramps", not
> jumps, up another +10deg as I open the throttle. Just fineNdandy what
> I'd like.
> 
> As to it working on a 4 just same as a six because "it's the same # of
> degrees", think about it. The module doesn't know the rpm except by how
> many pulses it's receiving. And it doesn't know "degrees" either, only
> pulses and how far apart they are in absolute time. Doodle some on paper
> to check yerself on this. A degree is a degree is a degree, but a pulse
> isn't a pulse isn't a pulse, ifyaknowadamean.
> 
> Garfield
> 
> P.S. Easiest way to see the module CAN'T work the same on 4s, 6s, and
> 8s, is to consider at what rpm the advance is gonna kick in at all. Eh?
> If it's at 2Krpm on a 6, at what rpm will it be on a 4? You've got 2
> less pulses coming to the module per rev, on a 4 vrs a 6. But the module
> doesn't know this. It's just counting pulses per time period. So how's
> it gonna know the rpm? if it doesn't know the #cyls? it's dealing with.
> N'est ce pas?

I don't know about the GM HEI, but the B&S Magnetron functions on the
rate of rise on the signal. The faster the engine turns over, the
quicker the pickup signal rises. Because the pickup signal must reach a
particular value before the ignition triggers, the faster the engine
turns the sooner, in degrees, the ignition fires.
At low RPM the ignitor does not fire untill the reluctor actually passes
the core of the pickup coil. At high speeds the voltage reaches the
firing point about 20 degrees before the reluctor passes the core of the
pickup coil. This is how that ramp happens.
At least that is how it was described to me by a B&S expert. The whole
thing makes sense, as it can only work with magnetic pickups, not hall
effects or point triggers, which are self controlled switches.
Think about it, then try it and see if I am right.
Just connect an old HEI dizzy to either a dist machine or a variable
speed drill and crank it up with a timing light connected to one cyl,
and a white mark on the chuck. Check the timing advance at several
speeds, then knock a couple teeth off the reluctor wheel and try it
again. Just don't knock off the tooth triggering the wire you have the
timing light on. I think you will find the advance does not change much.
Otherwise the module would require a time base, which it does NOT have.
The module is totally passive - simply a level controlled switch, or
amplifier with hysteresis, from what I have gathered.
Now, as usual, I could be wrong, but now you know how to find out, and
why it works if it works.
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