Stag timing, fuel consumption

[Tim Buja <buja@domain.elided>]

Brian Tink wrote:

> Tim, you say that on your trip you returned 18 MPG on US measured fuel.
> According to my way of thinking that figure gets worse on Imperial
> measurements..

Say I used a specific volume of fuel (129.5 US gallons) on my trip.
Since an imperial gallon is 25% larger than a US gallon, I have to
divide this by 1.25 to get the number of equivalent imperial gallons
(103.6) consumed in 2378 miles.  This makes my overall consumption
calculate out to 22.9 miles per imperial gallon, or 18.4 miles per US
gallon.

> What intrigued me is that you state that the timing was set to 4
> Degrees AFTER TDC.
>
> In all my years of playing with cars, and that amounts to over 40
> years, I have never yet set or seen a car set to  AFter TDC.
>
> This would mean that time of the bang the piston is half way down
> the pot instead of at the top or a fraction after, thus utiliasing
> the full effect of the bang.

The static timing was done per the book at 10 BTDC, with dynamic timing
measured at 4 ATDC (all vacuum hoses connected).  The purpose of having
a vacuum retard is emissions related.  Retarding the spark will reduce
the idle speed.  To compensate, the carb butterflies must be opened
further, giving better control of the fuel/air mixture than when the
butterflies are shut tight.  The vacuum source is taken from a point
on the carb that is on the manifold side of the butterfly only when
the butterfly is fully closed, causing the ignition timing to return
to its non-retard setting as soon as the throttle is opened.  If your
Stag still has the thermostatic vacuum valve on the radiator, the
timing is advanced (retard is removed, actually) when the coolant
temperature is above the setpoint in an attempt to increase the engine
speed, coolant flow, and fan speed.  This assumes that the vacuum
capsule is still working, of course.

Retarded ignition timing at idle was standard practice in the USA
for many of the early years of idle emission controls.  This was
done for the specific purpose of opening the forcing the butterfly
open further for improved fuel/air mixture control at idle and on
the overrun.

Tim Buja - Rockford, Illinois, USA - 80 TR8, 73 Stag, 72 TR6