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#41 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Thu Aug 02, 2018 7:45 pm
Sorry about that - my computer is not any good at displaying pictures, so I tried to draw a table.
The x axis is "rpm" and it runs from 501 rpm to 3700 rpm
The y axis spells "engine load" and runs from 35% (very light engine load) up to 100% which is very heavy engine load.
The numbers in the table are how open the throttle plate is.
During the half hour journey, every few milliseconds the computer has logged what the throttle position setting was and also what the engine loading was. It has then sorted all of the numbers in to the appropriate box in the table, added them up and averaged them. A figure in the table of of "100" means the throttle was 100% wide open - it never happened! A figure of "50" means the throttle plate is exactly half open, or for the pessimists out there, it's half shut..... A figure of "1" means the throttle is 99% shut.
So the numbers in the table are the average amount that the throttle was open during the ride, so when I was at 800 rpm and under a very heavy engine load, the throttle was on average 28% open. When I was at 2600rpm and the engine was still fully loaded up between 85% to 100% as before, the throttle was open 45% of the way.
I can show you a squigly line graph of throttle against time, but to demonstrate "full throttle" isn't always "100% open throttle", you have to also show the engine load against time and correlate the two. The top line of the table shows that for a fully loaded engine, "full throttle" at 800rpm means "28% open throttle" and at 2600rpm, full throttle is "45% open throttle". If you extrapolate the top line across to 6000rpm, then you'd expect to see "100", i.e. full throttle really does mean 100% open throttle.
Now for the interesting bit.... Engine load is often expressed as how much vacuum it pulls, so for 100% engine load, the manifold is at atmospheric pressure (because the throttle is as "wide open" as needs be and the engine is getting all of the air it needs, the air pressure in the manifold is the same as outside and there is no vacuum in the manifold so there is no vacuum signal which is why Philip said the static timing reflected wide open throttle), i.e. amount of vacuum is the exact inverse of engine load.
#42 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Sat Aug 11, 2018 4:19 pm
Another opinion on this matter
This article confirms much of what I’m saying. The only area where I would like to get clarity from John Hinckley (the author of the article) is that he refers to idle mixtures as being “lean”:
"is that lean mixtures, such as at idle and steady highway cruise, "
Idle mixture is certainly not lean!! None of my cars would idle with an AFR of more than 13.5 - exactly the same AFR that I feed them at full throttle (all rpms).
I’m sure though that what he calls “lean” is in fact “low manifold pressure conditions”.
#43 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Sat Aug 11, 2018 7:08 pm
Or maybe you agree with much thats already been said in the subject....
#44 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Sun Aug 12, 2018 6:17 am
mgcjag wrote: ↑
Sat Aug 11, 2018 7:08 pm
Or maybe you agree with much thats already been said in the subject....
Quite right Steve, absolutely.
#45 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Sat Aug 18, 2018 2:25 pm
For those who still remain convinced that Ported Vacuum is what is needed, here is another solution that will effectively provide the same as ported vacuum but does not require removal of carbs, drilling them, risk them being messed up, fitting ported tappings, refitting them, re-balancing them...
All you need is a standard Jaguar part EAC4013 (that you can find on a EFI’ed XJ-12/XJS/DD6). Connect 1 wire to ignition switched 12V, the other wire via a microswitch to earth on the throttle mechanism. Port 3 to atmosphere, port 2 to vacuum advance diaphragm, port 1 to manifold vacuum. When throttle is closed, port 2 & 3 are connected, hence no vacuum advance (and fast warm-up), when off idle, port 2 & 1 are connected and you have manifold vacuum advance.
#46 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Tue Sep 18, 2018 1:04 pm
Great mechanical engineering course for vacuum advance and engine optimal timing. For those not so mechanically inclined I ask for your advise on converting my stock 69 XKE 2+2 AUTOMATIC Transmission car equpped with the original 2 Stromberg carbs to the 3 SU carb setup. All the blogs say I will need to change out the distributor to be able to get enough vacuum advance to run the 3 SU's with the greatest performance. Now I don't know if this means the original distributor in the car now does not have any vacuum advance or it does not have the adjustment to set per the factory specs for the Vacuum to what say a 67 XKE with the 3 SU carbs ?
Question, can I add the vacuum advance mechanism to my original distributor, or do I indeed need to buy a different distributor? If so what distributor do I get. I am not a racer, just want to get the 3 SU's tuned to the factory suggested settings let's so again to what an off the boat 67 XKE equipped with the 3 SU's . Do I need a 65-67 XKE distributor to do this ? I read alot about the 123 distributor, do you recommend this? Another question is my origianl 69 XKE 2+2 with automatic transmission have an electrical tach or mechanical tach? If current tach is electric i would not need to get a new distributor with a mechanical tach fitting.
Thanks for your advise on changing the current 2 strombergs on my automatic car to the 3 SU setup and getting the performance it should get. I am not concerned with fuel economy but do want my tachometer to work with whatever distributor I go with. If I can add the mechanical vacuum parts to my exisitng distriburor to achieve factory suggested setting for the XKE originally equipped with 3 SU that seems to be the less expensive way to go.
Thanks if any of you are still awake and will take the time to advise me on this.
#47 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Tue Sep 18, 2018 1:57 pm
No E type has a mechanical tacho drive. The S1 cars had a self contained AC circuit driven by a generator at the back of the right hand cam bank, and your 1969 car will have an electronic tacho driven by a signal from a loop of the ignition supply lead ( white ) wrapped round a fitting on the tacho, and with the moving coil of the needle being powered by a green 12v wire which goes to a lucar terminal on the back of the tacho. The lack of vacuum is not going to affect performance much, but it will affect economy, as the function of the vacuum is to provide a much higher amount of advance at light part throttle running. Probably the cheapest way to solve this is to get a s/h Series 3 XJ6 distributor and amplifier from a scrapyard
#48 Re: Ported vacuum advance vs manifold vacuum advance
Posted: Tue Sep 18, 2018 7:11 pm
There are few things to check to match the setup appropriate to a triple SU engine.
Firstly, vacuum advance. Not all federal distributors had vacuum advance capability. Easy to check if your distributor has the vacuum module on the side, it’s visible from outside the distributor.
Secondly, there’s the total mechanical advance applied by the distributor. This is intrinsic in the mechanism, but can be changed by swapping out the base plate for one with a different total advance, by grinding the existing base plate to give more advance, or welding it to reduce the advance.
Thirdly, there’s the rate at which the mechanical advance is applied, this is the advance curve. This is dictated by the strength and free length of the advance springs.
Finally, there’s the advance setting, usually, for convenience, set at idle with the vacuum disconnected. Though this actually affects every other setting.
To convert your car, you need to get all these characteristics to match those for a uk spec distributor. It can be done at home, but it’s easier to buy a distributor from a uk spec E-type. The alternative is an electronic distributor like the 123. All the the above is controlled electronically. On the basic model, you have predefined curves to pick from. The trick is finding the one that matches the curve for a UK spec 4.2. Problem is all the curves built into the Jag123 start at a different point to the book curve. More sophisticated versions of the 123 can have custom curves drawn, either by plugging it into a laptop, or via Bluetooth. Here, you’ll be able to match the factory uk spec curve exactly.
The final curve ball is the original factory UK spec curve may no longer be right due to the different characteristics of modern petrol, so there’s still some experimentation to be done to get everything optimal.
The best option is probably to buy one of the programmable 123 units, and have it set up on a rolling road by someone who understands what they’re doing. This will give you the best performance, yet staying below the point where you risk engine damage.