SNG B. V-12 ignition upgrade

Wanted to report on the SNG Barratt ignition upgrade kit I have recently installed on my ‘74 OTS.

The kit came with a rebuilt pre-HE dizzy, upgraded electronics fitted into the original type amplifier case and upgraded ballast resistor. The instructions are very detailed with about 100 steps and photos where needed. It probably took me about 6 hours to fit as I went very carefully through the steps.

The engine started immediately on first crank over and has not run that well in the 21 years I’ve owned the car. No hesitation at tip-in anymore, revs easily to 6K + with no stutter. In fact the tach literally jumps between 4,500 to 6,000. Pulled the center plug for each manifold this a.m. and all were perfect grey-white instead of irregular sooty black to tan previously. Obviously the old unit couldn’t provide a hot enough spark.

Looking forward to seeing how it performs long term but so far the results are remarkable. It’s not cheap at $600 but at least for me has solved a lot of problems I’ve been trying to work around for a long time and made the car really enjoyable again.


Congrats! I’m about to do the same. Glad to hear the difference is noticeable. I have been bedeviled by what turned out to be rotting wiring on my car, but I recently heat shrinked several wires and the difference has been great. I can’t wait to see how the SNG replacement performs.

Any BTDT advice now that you have done it?

I did my car several years ago and would concur with the improvements.
I do still find after a couple of weeks without running I have to crank for 4-5 secs to get a start.
Hot start is improved, but still a need to put accelerator hard to the floor and crank a couple of secs if she doesn’t start on the first attempt.
If I remember correctly the sng notes call for setting the static timing to 8deg BTDC, on the advice of others including this forum I ignored this and set it to 12 deg BTDC.
It is worth noting that the SNG unit is faithful to the original in that it has a vacuum retard unit fitted to it, which is worth just plugging off and ignoring.
The final improvement you could consider in the future is to replace the vacuum retard unit with an advance unit, this will involve creating a new vacuum takeoff port on your left rear carb. There is plenty of advice on the forum on this subject.
check out his is a home based approach, but there are professionals who could do the work for you.
Mark .

The instructions are very complete and easy to follow, be sure to go through them carefully. It’s not difficult and really a fairly enjoyable experience. The only issue I had with old wiring was one of the leads to the ballast resistor, only a few strands of copper were still intact and I just snipped and replaced the connector. Prior to buying the SNG kit i’d considered buying a new engine harness as it’s only about $100 and easy to install but it seems like all is working well with the original.

Yes, the instructions did suggest 8 BTDC for the octane gas available nowadays as premium. I decided to go with 10 BTDC thinking that without any vacuum advance I’d get better throttle response at tip-in. That had been a problem for a long time with my car bogging initially and the new system solved it. The remarkable thing to me is that the plugs now show uniform color whereas before three from the same manifold set would all look different. Also no hesitation at all revving the engine to 6K rpm. Smooth as glass.

I will check into the advance module. I’ve thought about it before but was never clear if I could use the tap already under the left rear carb that was used for partial retard on my ‘74 model - since plugged off.

For my clarification.
Why will you need an other vacuum take off to operate an ignition timing advance distributor ? Vacuum is vacuum, unless it’s taken from a place where the amount of vacuum is different.

Secondly, to get vacuum advanced timing as opposed to the standard retard timing on the V12’s, would that not call for a modification to the distributor or maybe an other distributor ? Or is the retard/advance function built into the vacuum unit on the distributor ?

Cheers … Ole

In a bit of a rush but in a brief answer to oles questions,
The retard unit is the capsule on the side of the dissy, it ca be removed and changed.
The vacuum pickup needs a new position on the carb to only supply vacuum when the butterfly is partly open, the situation in mid rev cruise. The idea is to supply more advance in the cruise, and none with the butterfly closed. Some will argue it doesn’t matter where the vacuum comes from, but Jaguar disagree.

I’m not sure of the answer to your vacuum unit viz a viz the dizzy. I do know that advance modules have the vacuum port on the opposite side of the unit from the retard modules.

I did a little more reading which is dangerous but found that the port under the left rear carb cannot be used for advance because it is on the manifold side of the butterfly. Same goes for those manifold ports on the V12s. That’s why it has to be drilled into the carb on the outside of the throttle.

So, I’m confused now. The original set up is a vacuum retard for emissions purposes correct?

Group think right now is to get rid of the vacuum retard and change to vacuum advance (does it not have mechanical advance?), but to do that, you have to (i) port it from a carb somewhere in front of the throttle plate; and (ii) switch from a vacuum retard module on the distributor to a vacuum advance unit… and also replace the entire distributor because the port for the advance/retard module is on different sides?

That’s what i have deduced from this thread, but I’m not at all sure I have got this right. As I have not started to install the SNG Barratt dist., I would like to get this right. Thanks.

As do I: it matters greatly, towards what timing of which vacuum one needs.

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Hi guys
To go to vacuum advance you need to
Change the capsule on the side of the dizzy, you do not need to change any thing else on the dizzy, and you certainly don’t need to get a new dizzy.
I think there is some confusion between mechanical and vacuum advance. The mechanical advance is built into the dizzy and can’t be changed easily. Vacuum advance is added in at part throttle settings by the vacuum capsule, which is the unit on the side of the dizzy.
Regarding the vacuum port. You need to plug the port underneath the left rear carb and drill a new one on the upper surface. It’s easier to understand if you follow the link I posted earlier to Paul Clarksons website, he has pictures showing the port drilling.

I am one of those who are of opinion that you do NOT need ported vacuum. The main motivation for ported vacuum is for the MANUFACTURER to meet emissions regulations that existed in those days, to achieve things like fast warmup (WHY does the engine warm up faster with ported vacuum? Because combustion is less efficient with ported vacuum = less advance at idle).

It is common knowledge that a petrol engine is more efficient with vacuum advance at part throttle. The same principle applies at idle (which is just even lower rpm and even more part throttle). Because the engine is more efficient, it produces less heat, thus takes longer to warm up.

My car runs & idles happily on permanent vacuum (with vacuum advance module on dissy and Lumenition ignition system). Video: Enthusiastic hill climb with my E-type

Southern, your car will run just fine with no vacuum advance or retard at all applied to the dizzy. Most here, included me, plugged it off long ago. The retard module was only for emissions primarily to give a quicker warm up. An advance would help a bit on throttle tip-in and bit on mileage at cruise. But you don’t need it. Install the SNG system and rely only on the mechanical advance built into the distributor. At the end of the installation set your advance at idle to 10-12 degrees BTDC and you’ll be fine.

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Yep, agree…put over 4k miles a year on my etype, have not run any vacuum to the distributor in over 20 years…only vacuum Im using is to the brake servo. JS

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Great fun video Philip…Wild man !! :smile:

Being driven like they should be!!

Great fun! Is that Pikes Peak?

Mt. Evans: 14,255’: Highest paved road in Nth America!

Did that on a Harley about 10 years ago. Amazing drive and chilly on the top.

Enjoyed that! Last time I was there (Aug. 1978) I had to coast my motorcycle down the “hill” about 2000’ in order to get air dense enough for the cycle to start. If I ever get my jag to Colorado, Mt. Evans will be at the top of my list too.

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