Car in bits again...
AO Silver Member
Join Date: Feb 2007
Location: Hampshire, UK
Hi Mark - the vacuum advance is all about fuel economy. The mechanical advance is used to control the timing during hard acceleration and at maximum speed.
The vacuum take-off is used to advance the ignition during part throttle operation (deceleration, steady state cruising etc.), where the timing can be advanced as far as 50° without danger of pinking/detonation. The inlet manifold vacuum is high at these times, as the throttle butterflies are partly closed. Advancing the timing improves the fuel economy at these speeds, and because you don't drive everywhere at full throttle (do you?), the savings can be quite significant.
When you floor the throttle, the butterflies snap open and the inlet manifold vacuum drops to nearly zero - this causes the vacuum canister to drop the timing advance back onto the mechanical advance weights, which limit the maximum advance to about 36° BTDC. This maximum advance is set according to the fuel octane rating and engine load conditions.
So, provided that the vacuum stub is blanked at the carb, you can happily drive around with just the mechanical advance to keep everything ship-shape at maximum rpm, but at part-throttle openings, where you do nearly all of your driving, the fuel economy will be poor. Racing cars are not interested in fuel economy (unless they are endurance cars), so often this feature is disabled/removed, but it doesn't increase engine performance.
My old Vauxhall Droopsnoot Firenza (ahhh, the good ol' days!) had a 2.3 litre Group 2 rally engine with the vacuum advance disabled. I was lucky to get 16 mpg out of it. My 5.2 litre V8 Barracuda, on the other hand, with all the economy devices recommissioned (exhaust heat crossover, vacuum operated air cleaner diverter valve, vacuum advance and twin point distributor) regularly returned 18 mpg, and on one trip 22 mpg!
So it does make a difference!
Alfanauts are GO!!!