Hey Scott,
There was a good description of vacuum advance and the whole advance issue
on Two Guys Garage this week on the Speed Channel. I think there are other
showings later in the week.
I have a BN-2 4 cylinder so I don't think I can help you much on specifics.
Your question has prompted me to check my advance again. I set it earlier in
the year. Base advance was set at 10 degrees and then the combination of
mechanical advance and vacuum got it up to 40. The mechanical alone gets it
up to just 30 degrees. This is reving the engine almost as fast as it will
go. I can't see the tach from where I stand with the timing light so I'm
guessing its around 4500 RPM. My Tach is old and I wouldn't trust the
reading anyway. I basically rev it up until I don't see any further advance
and so i'm getting the max. Kind scary in my garage
On the Two Guys episode they seemed to associate better mileage with vacuum
advance component, meaning that the top end advance gets you more complete,
and efficient burning of fuel at higher rpm conditions. Burn rate of fuel
stays the same but the time it takes for the piston to move up to TDC is
faster at higher engine speeds (of course) so you need to start the burn
earlier in the stroke to have the max pressure where you need it on the down
stroke.
My distributor is original and the springs were worn (Stretched) when I set
this up in the spring. I think i was getting too much advance (Like above
50). SO I bought new springs from McMaster Carr, close to the size and
material I needed. Nobody makes replacement ones for the Lucas distributor
(At least not what I found). I had to cut and bend the springs. I opened one
spring coil at a time, shortening the spring, installing the pair into the
distributor and running the engine to measure the advance. I basically kept
shortening the advance one coil at a time until I got the right advance. I
think one spring was a little longer than the other which would give you
another step in the advance curve. It sounds like it took a while but it
really didn't.
Engine has run great. I just checked it and it is still set where I had it.
I may switch to electronic ignition next spring just to see how that
improves things.
Rick Neves
Millis, MA
56 'BN-2
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