The 'E' Type Forum

Hi,

I’ve finally bitten the bullet and am fitting vacuum advance. I chose ported vacuum and drilled the LH rear carb as guided in several sources. As the difference between ported and manifold vacuum has been debated so extensively elsewhere, I will not go in to the reasons why, save to say that the evidence presented indicates this is best, at least to me.

With that in mind, I wanted to ask what degree of timing advance to set, what rpm this should be set at and should the sensing line from port to advance unit be disconnected when I set the timing?

I am guessing there will be many opinions for this and I’ll probably have to choose between them, but I am looking for opinions and a starting point. The car is very early UK standard with no emissions gear, Strombergs with original pattern stainless exhaust using original manifolds, Re-Opus ignition, a 5-speed Getrag gearbox, alloy radiator and modern fans, although much of that should not really have any bearing on the issue. The engine was rebuilt and is in good condition as far as I know. The vacuum advance unit is 54405202 from British Vacuum Unit in New Hampshire, USA.

With lockdown, I can’t do much testing so I am not in a great rush to finish, so I am taking the opportunity to fix several minor issues and leaks as I go - nothing major, but the V12 has a few more potential areas for these minor issues to arise and that engine bay, which looked so huge when empty, is surprisingly cramped when working on the unit!

Regards,

Simon.

Hi Simon,

I have a very similar setup to you. The only difference being the use of the SNG HEI ignition amp, from the later V12s. I went down the Opus, reopus Path till finally arriving at the HEI amp which has been superb for a number of years, but I digress from your question.
I have the same advance unit as you, and also had my carb drilled for vacuum pickup.
When I spoke to Rob, from British Vacuum, his recommendation was.

Static timing 12deg btdc This assumes use of at least 95 RON octane petrol
3000rpm 28deg Btdc

Vacuum is disconnected And plugged during the timing procedure.
Buy one of those timing lights with the ability to angle the head, it makes it easier to shine the strobe on the timing plate. I found marking the 12 deg and 28 deg marks with some white marker made reading much easier.
Cheers

Hi Mark,

Thanks for the quick response. I have one of those lights - you’re right, an absolute godsend. Just to clarify, the setting of 28 degrees btdc is at 3000, with all vacuum holes plugged? I know, that’s what the answer said, but I have misinterpreted things before! Actually, mine may vary slightly, since the distributor is actually an XJS one and I think the centrifugal weights are for 18 degree advance, rather than 16 degrees, a fact I forgot in my original post and likely to have more effect than 5-speed gearbox etc.! I’ll see what happens. Thanks again.

Regards,

Simon

Yes all vacuum disconnected.
I also run a 5 speed and a 2.88 diff, which the V12 handles effortlessly.
As far as the extra advance goes, why don’t you email Ron at British vacuum and ask him. He is very approachable.
Cheers

Hi Mark,

Many thanks again, I’ll get in touch with him again.

Regards,

Simon.