At the risk of making false statements, I am presenting this post based on input I received from the previous owner of the car I now own...and my memory (Warning!! ALtz light). If it transpires that anything I presented here is incorrect, I will edit and correct this first post as the information comes.
The E-type V12 - in standard form - thus also comes with its oddities. One of them is that the car is equipped with a Vacuum RETARD system - not a vacuum ADVANCE system. I believe that this situation is directly related to the emissions regulations that existed at the time. Regardless, this vacuum retard system renders the poor old V12 MUCH LESS EFFICIENT than what it can be most notably when it comes to fuel consumption.
This Vacuum retard system is implemented as follows.
- Vacuum is applied to the vacuum diaphragm (the flying saucer thingamajig on the side of the distributor) WHEN THE THROTTLE IS CLOSED;
- This vacuum causes the vacuum diaphragm to apply timing RETARD - when the throttle is closed;
- by "sucking" on the dissy side of the diaphragm;
- which causes a rotation of the pickup in such a direction as to affect a retardation of timing
So WHY am I raising this issue?
Well, this vacuum retard system renders the engine INEFFICIENT at ALL TIMES when applying throttle. The net effect is that when driving the car normally (light throttle), the engine is receiving spark timing as if it is running at full throttle. At full throttle, timing for the engine can be presumed optimal BUT NOT AT PARTIAL THROTTLE.
For the V12 to run optimally AT ALL TIMES, it is better to implement a vacuum ADVANCE diaphragm, such that timing is advanced at all times when the engine is running at light throttle. The VAST MAJORITY of modern road cars uses the vacuum advance principle.
Other than improving fuel consumption, vacuum advance should also contribute to a significant reduction in heat generation and thus a reduced demand on the car's cooling system.
Vacuum advance is easily implemented by obtaining a suitable vacuum advance diaphragm (which still works - confirmed by sucking on the vacuum port and should hold vacuum) and fitting that to the distributor and then connecting the vacuum port to permanent manifold vacuum.
Here is a pic of the vacuum advance module on my E:
This car did a 3500km trip with me in my Jensen and another friend in his '80 Corvette to attend the George motor show in April. This E-type consistently did better than 6.3km/L (14.8mpgUs, 17.8mpgUK, 15.9L/100km) on this trip, with Aircon on (38ºC every day) and doing 120km/h or more whenever possible. Whilst fuel was accurately recorded I don't know how accurate the odometer is (will find out soon....) Still, for a pre-HE, 4-carbed, 5.3L V12 running at 3300rpm when doing 120km/h (GPS measured), I think is remarkable. I ascribe much of this to the vacuum advance system implemented on this car. Carbs also exceedingly well tuned by the then owner.
I should also mention that the original OPUS ignition was replaced with REOPUS which was also found to be a disaster (another story). The car is now fitted with a LUMENITION pickup and complimentary coil.