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#1 Optimising the V12

Posted: Tue Oct 25, 2016 7:13 pm
by jagwit
These cars come from a wonderful era which also had its oddities.

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.
  1. Vacuum is applied to the vacuum diaphragm (the flying saucer thingamajig on the side of the distributor) WHEN THE THROTTLE IS CLOSED;
  2. This vacuum causes the vacuum diaphragm to apply timing RETARD - when the throttle is closed;
  3. by "sucking" on the dissy side of the diaphragm;
  4. which causes a rotation of the pickup in such a direction as to affect a retardation of timing
Since vacuum retard is only implemented when the throttle is closed, this timing retard only exists during idle and during overrun (higher rpm with throttle closed). This is why the vacuum pipe to the vacuum diaphragm should be disconnected when setting the engine's 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 (38ÂșC 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.

#2 Re: Optimising the V12

Posted: Tue Oct 25, 2016 7:52 pm
by MarekH
To compare like with like, you'd need to take the retard distributor and spin it up to get the advance curve and compare it to a v12 distributor designed for a vacuum advance capsule.

It may be that with a combination of different static timing, different weights and springs inside, the result may not be a sub-optimal as you may like to imagine.

The tan coloured US Owners Manual has details of the retard mechanisms and their cut-in and cut-out points and the ROMs have the factory advance curve tables.

Perhaps if you can get Martin to delve into the ignition computer on a 36cu equiped car then we'll know what advance Jaguar ran on their last v12 cars. It is written in a form of javascript apparently, according to a lister on the j-l forum.

kind regards

#3 Re: Optimising the V12

Posted: Wed Oct 26, 2016 2:26 am
by 42south
Hi Guys
Just to add a little fuel to the fire on this topic.
My car had a retard vacuum unit on it, which I decided to disconnect. The car ran fine but obviously sub optimally, as there are times at part throttle when more advance could be tolerated.
Searching around the net I found a guy in the states called Rob Medynski who has run a company called British Vacuum unit for some 30 years, specialising in rebuilt distributors and vacuum units. I contacted him to get a vacuum advance unit suitable for the V12. Talking to him he informed me that he had done several V12's and recommended a vacuum unit that is rated 5-15-8, on his website its part 54405202. Rob also recommends changing the advance curve, to
no mechanical advance below 1100 eng rpm figures are at the crank,( halve for Dizzy advance)
10 deg mechanical @2400 eng rpm
18 deg mechanical @3600 eng rpm
so with the initial static advance of 12 deg BTDC it gives a total advance of 30 deg at the crank, at 3600 eng rpm, with wide open throttle. The vacuum unit will put in a maximum of 8 degrees advance at part throttle.
I know this is somewhat different to roger Bywaters recommendation, but on my car its working very well. I'll step well outside my knowledge here and say that Robs recommendations are tied in with modern fuels as well, I want to run 95 RON unleaded fuel here in NZ, which I think is 90 MON in the USA. Back in the day, the engine ran 98RON leaded fuel.
The hardest part was finding a template to drill the carb for a vacuum takeoff in the correct position, underneath the carb, for part throttle operation. I would have just got a carb from another car already drilled, but had just spent good money getting mine rebuilt. However Rob can do this for you at minimal cost, as he has built a jig for the purpose.

If you look at posts on this and the JL website, and Pauls Daimler diary site, you will find that it is easy to get pinking in the engine with the wrong vacuum advance unit fitted. However by the sound of it you found the right one.
Boy I love the sound of your road trip!
I also dumped my Opus and Reopus in favour of the later CEI amp and Dizzy as fitted to the later V12's.

#4 Re: Optimising the V12

Posted: Wed Oct 26, 2016 10:12 am
by PeterCrespin
All fine Philip but why have full vac advance at idle? Mark has the right idea.

Pete -

#5 Re: Optimising the V12

Posted: Wed Oct 26, 2016 4:03 pm
by jagwit
Peter Crespin: "but why have full vac advance at idle?"

TWO reasons I can think of:

1) Physical constraints:
I understand the term "ported vacuum" as meaning that vacuum only appears on that port when the throttle plates rotate to the extent that the port is subjected to either the vacuum that may exist below the throttle plates or the vacuum that is created by virtue of a venturi effect of the air rushing by the port.

IIRC, from the PO, the ZS carbs do not have "ported vacuum" connections unless you are willing to drill holes into the carbs and fit the required ports.

He decided (as I would) to rather T into a line that already had manifold vacuum present.

2) Emissions vs efficiency

Again, emissions often play a bigger role than we think ie, "lets rather meet the emissions criteria albeit with an inefficient idle". The '95 Land Rover Discovery 1 3.9 V8 was a good case in point. Although it did have fuel injection, timing was still compliments of a good old dissy with vacuum advance. Vacuum came from a "ported vacuum" port on top of the throttle body. Thus at idle, with 0 throttle, no vacuum was applied and thus no timing advance was implemented other than that commanded by centrifugal advance. Simply sucking on the vacuum pipe would cause engine rpms to increase briefly - until the idle air stepper motor would close off idle air to return the idle speed back to where it should be. Disconnecting the stepper motor and then sucking on the vacuum pipe would cause a clear increase in idle speed simply due to advanced (more efficient) timing.

One can easily prove to yourself that the E V12 engine DOES idle more efficiently with more advance. Lets assume we have an engine that is as perfectly tuned as is realistically possible (fuel and timing). Note the rpms the engine is idling at. Simply remove the vacuum line from the diaphragm and block it. Removing the vacuum line should cause idle timing to advance (IF that diaphragm is still intact and operational) and engine rpms SHOULD rise. WHY would the engine RPMS rise? Because the advanced timing is causing the engine (combustion) to be more efficient.

The XJS V12s and XJ12s and Daimler Double Sixes all have highly complex timing advance elimination mechanisms which is (should be) ACTIVE ONLY during engine WARMUP. Why? An inefficient combustion process makes the engine warm up quicker (emissions again....) Once at operating temperature, they activate and rely on vacuum advance to run as efficiently as possible.

#6 Re: Optimising the V12

Posted: Wed Oct 26, 2016 4:22 pm
by jagwit
MarekH wrote:It may be that with a combination of different static timing, different weights and springs inside, the result may not be a sub-optimal as you may like to imagine.

Lets take a V12, bolt it to an engine dyno, fit it with ECU programmable ignition (as you have done :bigrin: ) with the sole aim of finding the advance curve at full throttle.

Consider that :
1) the vacuum RETARD system on the E SHOULD not play ANY role at full throttle (due to the absence of vacuum). LIKEWISE
2) the vacuum ADVANCE mechanism on the later V12s does not play ANY role at full throttle either (also due to lack of manifold vacuum)!

Both systems should therefore present the same timing curves as found by the programmable system to get maximum full throttle performance from the engine.

Running an engine at partial throttle but with timing such as at full throttle (ie advanced ONLY as per centrifugal advance) WILL cause the engine to run less efficient and increase fuel consumption. This is easily observed and proven from cars with broken vacuum advance modules and/or perished vacuum pipes.

The issue I'm on about only comes into play at PARTIAL throttle, where a vacuum advance system WILL cause the engine to run more efficiently.

The E ONLY has a RETARD mechanism which SHOULD only come into play at rather high values of vacuum - such as seen during overrun (VERY light throttle) and idle. Thus the standard E V12 runs all the time with timing determined only by centrifugal advance and hence, optimal for full throttle only but not at partial throttle.

#7 Re: Optimising the V12

Posted: Wed Oct 26, 2016 5:23 pm
by MarekH
From my notes, you should have centrifugal

0-700rpm 12' adv
1100 14-19'
1500 23-27'
1900 24-28'
3500 28-32'
5800 34-38'
7000 (god forbid) 35-39' advance

plus the vacuum capsule retard

Whilst that does give you an advance curve such that it delivers an unfairly high advance at and below max torque/mid rpm compared to a straight line from 0-5000rpm, you are asking for an extra hump at or below max torque rpm.

Time to fit a Megajolt and DESC mon brave?

kind regards

#8 Re: Optimising the V12

Posted: Wed Oct 26, 2016 6:29 pm
by MarekH
If you want to do some experimenting, there were some vacuum capsules (maybe for a Triumph?) that had both a retard and an advance port on them - on opposite sides of the diaphragm.

kind regards

#9 Re: Optimising the V12

Posted: Wed Oct 26, 2016 7:24 pm
by 42south
Hi Guys
I made a slight error in my original post . The vacuum retard port comes from underneath the carb, and the advance port is on top of the carb.
By the way you can get advance ported ZS carbs from some of the XJ series cars.

#10 Re: Optimising the V12

Posted: Wed Oct 26, 2016 7:42 pm
by jagwit
MarekH wrote: Time to fit a Megajolt and DESC mon brave?
Megasquirt-2 now offers 6 ignition outputs - thus can handle V12 wasted spark and hence no more DESC required. Megasquirt-3 offers 12 individual ignition outputs.

Whilst I am tempted Marek, the car I now have deserves to be kept as original as possible.

For those who don't know, "Megajolt" is a very elegant programmable ignition timing controller that works in conjunction with EDIS-4 or EDIS-6 or EDIS-8 wasted spark ignition modules to offer a 4-cyl or 6-cyl or 8-cyl distributorless ignition system. "DESC" is little electronic box I developed (for my XJS initially) to use dual EDIS-6 modules in order to build a V12 distributorless ignition system for the XJS (also implemented on the V12 OTS I EFI'ed).

#11 Re: Optimising the V12

Posted: Thu Oct 27, 2016 5:03 pm
by Whitact
Just a couple of thoughts on this.
If the vacuum is plumbed as per the parts manual the ignition retard is only applied until shut off by the thermo- switch, i.e only during warm up.
The Lucas vacuum advance units fitted to some of the later V12s appear to provide a max of 4 deg, so similar to the range of distributor manufacturing tolerances according to remarks attributed to Roger Bywater.
As such the 8 deg of vacuum advance mentioned in Mark's post might be necessary for any noticeable effect.
FWIW my OTS averaged 22 mpg over 2,800 miles on the RBCD, fully loaded including some spirited runs. Its on the original Strombergs with the SNG CEI ignition system. However, I do not know what the vacuum or mechanical spec of that system is. Its perfectly happy on 95 RON.