Ported vacuum advance vs manifold vacuum advance

[Robsan]

jagwit I think that Jerrl [ nearly spelt his nme right] raises a very valid observation, plus his honesty is to be applauded

Idling the engine............... loosen distibutor clamp then swivel right to left and back again, repaet and repeat until the sweet spot is found !!!!
Yes its at idle rpm but why?
why is it the sweet spot at idle.?. Once me and Jerrl understand that, then can start to consider what are the varying pressure [Vacuum] profiles of INTAKE Manifold...........AND where the spark should fire [light the fire] at somepoint BTDC on the [compresssion stroke] of the 4 stroke engine

Back to basics for us :-)

love this thread btw

[lowact]

U get the most torque if the explosion is just as the piston starts on its downward stroke. It takes milliseconds for the explosion to develop (called flame propagation) so u need to fire the plug a little bit earlier. If u retard the timing (e.g. 8 degrees instead of 12 degrees before TDC) the explosion doesn’t develop until well after the piston has started descending so is too late to be fully effective, less torque is produced, instead more heat. When idling, torque causes rpm so retarding the ignition causes less torque therefore less rpm.
At the recommended 12 degrees BTDC the explosion is still slightly retarded, i.e. more torque therefore more idle-rpm can be achieved by advancing the ignition, e.g. from 12 degrees to 14 degrees before tdc. If you advance too far some of the explosion is trying to happen before the piston is ready to be pushed down so some is wasted, less torque is produced, rpm falls away.

Question 7.3, why not do as Jerome does, set timing at max idle rpm? The timing set at idle is the base setting. It directly influences the timing that occurs when driving. When driving, rpms are higher, to get the explosion happening just as the piston starts on its downward stroke u need the plug to fire even earlier. The greater the rpm the more advance req’d, for max torque. This is achieved by the mechanical (centrifugal) advance mechanism that is deep inside the distributor (see below). At 7000 rpm you should have not more than 40 degrees BTDC, i.e. yr 12 degrees static timing + 28 degrees caused by the (OEM) mechanical advance. My test results with different distributors on the E, blue is the retard distributor, orange is the advance distributor, I did this to check if the stock advance distributor had the same mechanical advance, yes it does:


At idle the cylinder pressures/piston stresses are very low, the effect of too much advance is reduced speed. At full song the pressure/stresses are bloody high, the effect of too much advance can be blown pistons. Fuel octane is also a factor, there other factors also. Because the consequences are potentially disastrous, and because there are factors that you cannot control, the static timing is set slightly conservatively. Originally it was 8 degrees BTDC, by service bulletin Jaguar allowed it could be increased to 12 degrees. That idle speed might increase if u increase this is not useful and the risk is too high.

vacuum retard system makes the idle situation deliberately less efficient

Not the full story. More efficient at warm-up, smooth running and killing toxins, less efficient at producing torque. I don’t need torque when idling and I felt (no evidence) that the lower idling speed achievable with retarded emission gave me better idle fuel economy. Despite this I don’t have the retard connected atm, reason being as per the following test results, I’ve measured vacuum at the retard tapping at rev’s much higher than idle! Being stationary is not enough to explain this imo so I’ve disconnected it. Also shows my engine is a little bit sick, should be getting 20 “Hg vacuum at idle, I’m believing I have a sloppy valve guide, going by the noise. Does mean all my test results are sus …

Did you measure this? I'm just trying to establish if all this has been quantified through measurement.

“Tested” often, long ago when all cars had carbs. On 4 cyl effect most obvious. Not on my v12 ‘cos I don’t have ac, in any case I would expect with v12, firing 3 times per rev, any ac operation should be almost indiscernible. And as above I have other issues that has stalled my testing regime.
What I have explained is one reason why manufacturers used to use ported advance. The amount of internet debate on this topic is proof that, if u don’t have a ported vacuum tapping, manifold vacuum can be used.

[jagwit]

jagwit I think that Jerrl [ nearly spelt his nme right] raises a very valid observation, plus his honesty is to be applauded

Idling the engine............... loosen distibutor clamp then swivel right to left and back again, repaet and repeat until the sweet spot is found !!!!
Yes its at idle rpm but why?
why is it the sweet spot at idle.?.

Robsan, I am REALLY pleased that you and Jerome raised this question for the opportunity it presents to bring more perspective on this topic. The shortest possible answer I can give you is:
when you do this at 800rpm, throttle closed, high vacuum, the engine is at IDLE, not full throttle.

Chances are this answer will sow more confusion than clarity UNLESS you now have grasp on the difference between full throttle at 800rpm and idle at 800rpm.

Please consider that you are now experimentally finding that value of centrifugal spark advance which renders the combustion process MOST EFFICIENT BUUUUUUTTTTT, you are doing so at IDLE when a high vacuum (Low manifold pressure) exists!!! If you lock the dissy down in this position, the CENTRIFUGAL timing advance for full throttle (when low vacuum / high manifold pressure exist) will be WAAYYYYY to far advanced, not just at idle but for the entire rev range (because you set the CENTRIFUGAL advance based on "IDLE"). Chances are the engine may detonate (aka Knock aka Ping) at wider throttle openings.

There was a very SPECIFIC REASON why I started this thread with a definition of "IDLE". The main reason was to highlight the fact that "idle" at 800rpm and full throttle (or the equivalent thereof) at 800rpm are two very different engine running situations that require two very different fueling and timing requirements (but on our distributor engines we can not easily meet BOTH idle and full throttle operating requirements (unless we implement manifold vacuum advance - the objective this entire thread is about).


Even at this point things might STILL not make sense if you don't understand the implications of low vacuum vs high vacuum .... Please read on then.

Its time to explain "Vacuum advance" in layman's terms:
Lets have a BBQ!! The entire forum is invited. Therefore we will need two fires to cook the meat on. John will do the one fire and Peter will do the other. I ask them both to please have both fires ready at 1pm. Only trouble is that Peter gets a bag of "wet" wood - not quite dry, but John get lovely dry wood.

What time will John start his fire to have his coals ready at 1pm? Shall we say around 12:30? This gives him 30minutes to get his coals ready.
What time shall Peter start his fire (with his wet wood) to have his coals ready at 1pm? Shall we say 12:00? He most certainly needs to start earlier, I'm sure we can agree on that.

So how does this barbie relate to our engines?
When you have the throttle far or wide open, lots of air and complimentary fuel is rushing into the engine. Once this air is inside the combustion chamber its like having DRY wood. Lots of air rushed in and is compressed, the air a fuel molecules are "close" together and when this lot is ignited, it will burn (NOT EXPLODE - see below) rather quickly. When the air and fuel molecules are pressed TOO closely together they spontaneously react and this causes "detonation, aka ping, aka knock". These are NOT "pre-ignition" I'll explain that later, not relevant now.

When you have the engine at partial throttle, the amount of air (and fuel) that enter the engine is being restricted. This is like making a fire with dry wood. Because there is less air and fuel molecules (than at full throttle), they are further apart and thus they burn (NOT EXPLODE!!!!) slower.

Another analogy:
We have two fields of grass, both fields have very dry grass, but the one field has dense grass and the other field has rather sparse grass. If I drop a match in the centre of both fields the fire will spread out in a circular fashion but it will burn gradually from the centre outwards. You will have areas where the grass is already burnt, the "flame front" and then the area yet to burn. The thick grass however, will burn quicker (flame will progress faster) than the sparse field. VERY IMPORTANT!! BOTH fields TAKE TIME to completely burn, the thick grass (dry wood) will just burn quicker than the sparse grass (wet wood).

It is very similar in our engines: With wide open throttle we have the thick grass (dry wood) situation. With part throttle we have the sparse grass (wet wood) situation. In both situations the air/fuel mixture requires TIME to burn (not explode)!! Full throttle requires less TIME to burn so we can light the fire at 12:30 eg 12ºBTDC. Partial throttle requires more TIME therefore we have to start this fire at 12:15 (earlier), say at 22ºBTDC.

Now at last we get to vacuum advance!!
Centrifugal advance only, Only, ONLY applies to WIDE OPEN THROTTLE operating conditions, from 800rpm (note I did not use the word IDLE here) all the way to 6000rpm.

Vacuum advance is an ADDITIONAL timing advance implemented ON TOP OF centrifugal advance. Its aim is to "start the fire earlier" when "the wood is wet", "the grass is sparse", the vacuum is high, the manifold pressure is low, so that the "BBQ fire" can be ready at 1pm!! 1PM, being the OPTIMAL point where the "fire" (the combustion process) can do the best work on the piston. When the fire is started too early (too much advance), we lower the efficiency and will get less torque. When we start the fire to late (too little advance = timing too retarded) the engine will also be less efficient and again we will lose torque (etc).

After all this, I'm hoping for a glimmer of understanding why Jerome's method is not a good method.....

UNLESS....

His engine is configured with a vacuum advance module on the distributor and its vacuum line is connected to MANIFOLD vacuum. Question 8: WHY????????

[jagwit]

Gents,

Tomorrow, I'm off the the bush as a volunteer for the "Tour de Tuli" cycling event. I'm coming back on the 7th, so please don't stress if I don't reply in this time.

Colin, I don't have time to also address your latest post because I have run out of time. I do believe that my posting above may go some way towards addressing yours.

In the mean time, don't hold back. Marek is watching this thread.

[christopher storey]

U get the most torque if the explosion is just as the piston starts on its downward stroke. It takes milliseconds for the explosion to develop (called flame propagation)

Being pedantic it should not be an explosion , but a steady development of the flame front. It is not widely known just how slowly this travels. A good deal of research has shown that its speed is only about 120 feet per second, and thus for the mixture at the periphery of the combustion chamber to start burning ( about 2 inches away from the plug) takes as much as 1/720th of a second or c 1.5 milliseconds . This is why the ignition needs further advance as engine speed rises, because at e.g. 3000 rpm ( 50 revs per second ) the engine turns through 50 / 720 times 360 degrees = 25 crankshaft degrees before the mixture is completely burning

[PeterCrespin]

Sadly, the efficiency of communication (number of muscle movements and energy required per concept over time) is worse when typing than talking. Reading and listening are about the same, except that retarded people like me can be idle and re-read a misfired line with less throttle than asking you verbally to repeat the cycle.

I like Colin’s line of thinking and I also think it is valid to cite near universal practice in support of a concept being adequately addressed. Never let the perfect be the enemy of the good enough etc. The fundamental existential efficiency automakers have to strive for is making profit from turning raw materials and components into sellable vehicles, and many have failed to achieve even that. Your approach seems more like that of blue sky faculty than a blood and sweat factory. We need both, of course, but they tend to speak different languages.

The few times when cutting-edge perfectionism and practical production intersect, you get the McClarens, Ferraris and Koenigseggs of this world, who operate on a different plane at huge cost where an extra few noughts on the price is an inherently attractive differentiator, rather than a sales obstacle, for a subset of the market. I suppose that’s like the Holy Grail of reducing components and complexity whilst gaining profitability and reliability. Maybe it’s more efficient to buy a new horse for your cart every decade and use the emissions on your roses, than smelt Bauxite and refine petrochemicals to make sure the recycling industry has plenty to chew on in twenty years time?

Pete

[jagwit]

if u don’t have a ported vacuum tapping, manifold vacuum can be used.

Thank you Colin.

May I add please?

For the benefits ported vacuum advance may hold, it certainly is not worth the cost and risk of:
- removing the carbs
- having them drilled and fitted for ported vacuum (hoping they don't get dropped or damaged in some other unforeseen way);
- Refitting the carbs;
- having them set and balanced (hoping the so-called specialist REALLY can and knows what he's doing);

when in stead, connecting a little rubber pipe to an existing manifold port in 10 seconds is all that is required (for what I 'm trying to show is a more appropriate result if it has to be one or the other.)

PS: Just as a reminder, I did open the thread saying to have BOTH ported (during warm-up) and manifold vacuum (when warmed up) is 1st prize.

[jagwit]

When I retard the engine timing below 12º, idle rpm drops. WHY#7.1?
When I advance engine timing to more than 12º, idle speed increases to beyond 800rpm WHY #7.2

Found a few moments....

7.1: When you retard timing, the fire starts too late, fire burns nicely by the time the piston is well on its way down, conversion of combustion to work on the piston becomes LESS EFFICIENT and more energy from the fire is not utilised to produce torque. RPM drops. Instead it just converts more fuel to heat. This is EXACTLY what the vacuum retard system on the V12s aimed to achieve. This situation results in rapid engine warm-up but continues to do so at every idle for the duration of the drive...
7.2 When you advance the timing the fire is started earlier and earlier (more and more BTDC) and becomes more optimal and more optimal and more EFFICIENT as you advance, resulting in more torque, until you get to a point where further advance no longer seems to yield any improvement. You have now found the optimal timing for IDLE at 800rpm (not for full throttle!!!!)

Again please note that optimal timing for IDLE at 800rpm is NOT THE SAME as the timing required for full throttle @ 800rpm.

What I aim to achieve with manifold sourced vacuum advance is to provide the engine with optimal timing for IDLE @ 800rpm AND full Throttle @ 800rpm.

[JerryL770]

Crikey!!

It's gorn right over me now. Think I will put me coat on ........

.... but I will keep watching. Most interesting, thanks all.

However, I feel the need to ask, what is the point of considering "full throttle at 800rpm"?

This is surely not a viable operating condition as if I dropped the clutch at this rpm, whatever the throttle opening, it will stop the engine dead!

[jagwit]

Jerome, this approach is wrong, wrong, wrong and I will make sure to explain WHY (actually I already have ), specifically on this matter, after questions 7 have been answered and I'm sure the answer will NOT be over your head. Hint: 7.2 relates to this matter...

Jerome, if you now read my previous post, does it now make sense to you why this method is not appropriate? Did I manage to explain in understandable terms? If not, feel free to say and I'll try again.

[jagwit]

Crikey!!

It's gorn right over me now.

Ummm, only saw this after I hit "submit". Clearly I did not succeed.

[JerryL770]

Sorry Phillip. (cross posting!!) Yes I did understand your explanation. Then I posted and went away, then editied my post and found you had already responded

No, really I'm wondering what it is all about, other than your opening intention to maximise your engine efficiency so as to minimise cooling requirement - with the added advantage presumably, of getting more mpg.

One thing I don't have a handle on is "Ported" as opposed to "Manifold", though I do really know what is the manifold

[jagwit]

After all this, I'm hoping for a glimmer of understanding why Jerome's method is not a good method.....

UNLESS....

His engine is configured with a vacuum advance module on the distributor and its vacuum line is connected to MANIFOLD vacuum. Question 8: WHY????????

Question 8: WHY????????
So, lets start again with our engine nicely tuned to 12ºBTDC at 800rpm.

I trust you now understand that the 12ºBTDC applies to a situation where the engine finds itself in a full throttle situation at 800rpm (easily done in a manual, this condition may exist very briefly in an auto). So far 12ºBTDC has also been used for "IDLE" @800rpm but only because we did not have a way of ALSO providing the optimal timing for "IDLE" a a 800rpm....

.... until Philip (Jagwit) started bleating about using manifold vacuum for the vacuum advance module that is.

So how would we tune an engine if configured with manifold vacuum for the vacuum advance diaphragm?
1) Disconnect the vacuum to the vacuum advance module and block the pipe;
2) set timing to 12ºBTDC @ 800rpm (the centrifugal advance has now been set for 800rpm full throttle)
3) reconnect vacuum advance to the vacuum advance module but NOW, the idle speed will increase! Because the vacuum advance will add another 12 degrees or so on top of the 12 already in place to make the low vacuum condition combustion more optimal. This additional vacuum advance now provides more optimal timing for the IDLE condition as such (as opposed to the full throttle condition also at 800rpm.
4) reset idle speed back to 800rpm by adjusting the idle stop screws on the carbs (making sure they are still balanced). This will require CLOSING the throttles more to REDUCE the amount of air (AND FUEL) going into the engine and this is why the engine will now consume less fuel during idle (as opposed to ported vacuum) although STILL idling at 800rpm!!

The ONLY "penalty" that has been incurred in this method is now we do not have the faster warm-up situation we had before. (But I can offer a solution to that as well - just two pieces of standard Jaguar hardware required...)

Voila! Cheers until the 8th - unless admin barred me for my sacrilegious preachings.

[MarekH]

I think there is only two things that seems to have been hard to explain here and that is "What's all this about wide open throttle?" and "Why did he say all that about setting the advance thingy stuff at wide open throttle and not at idle?"

So....

In the spirit of the thread, "What is wide open throttle?"

"It's when you put your foot to the floor and the throttle butterfly is wide open, isn't it mate?"

Yes and no.

The engine is designed to run at up to 6000rpm. When it does that, the cylinders are pulling in 6000/800 as much air as they are at idle speed. The intake plenum, runners, inlet valve and throttle butterfy are all sized to deliver that much air - the amount needed at 6000rpm. At 800rpm, you probably only need to open the throttle butterfly by about 800/6000 of the full 90' movement to let the engine pull in enough air for 800rpm running, so wide open throttle means just that at 6000rpm but it also means anything over ~20% at the lower end of engine speed is as good as wide open. (If that still doesn't make sense, then you can go up a hill and hold it steady at 800rpm with the throttle 20% open and the heavily loaded engine can tolerate less advance than when sitting at the lights, throttle closed at 800rpm which can happily run better with more advance, i.e. you can run at 800rpm and be doing more than just an idle amount of work.)

For the second bit, the v12 engine on the etype only came with a vacuum retard unit. This took its vacuum signal from the bottom of the left rear carburettor and was designed to give vacuum retard when the throttle was shut. As soon as the throttle is opened, the signal drops away and there is no retard added to the distributor setting. This also means that there is no variation in vacuum signal when the engine load is heavy (going uphill or accelerating hard), when it is light (going downhill, steady cruising), or when it is somewhere in between (light acceleration, slow changes in speed) so there is no advance or retard applied in response to the the engine load. Philip explained how and why more advance for a lightly loaded worked better, but Jaguar never gave v12 owners the benefit of having the choice so the only safe thing to do was to set the static timing to reflect a heavily loaded engine. This is done by proxy - you deliberately set a very conservative number for static timing under lightly loaded conditions. (There is no practical way to set it whilst driving up a steep hill....) If you set yourself more static advance under light load then this, then the engine will be much too advanced under heavy load. It also means that day to day, anytime you aren't running with your pedal to the metal, you are running an engine that is too retarded. You are not getting the most out of the car that you could and the difference is reflected in more heat and more friction.

"So should I drill a hole and do this ported thing?"

The ported answer, touted by many, gives you advance when you are moving (yippee!) and cuts the advance when you are sitting at the lights (with throttle plate closed). But that also means you are retarded and generating heat whever the throttle is closed. This is pretty dumb, because this coincides exactly with when you are in stop-go traffic, at minimum revs, generating more heat
than you need to at a time when the water pump is running as slow is it possibly can, when there is no cool airflow into the radiator and what there is is a hot exhaust gas from the car in front pumping heat into the engine bay and you phone me up and start another thread titled "the needle has gone past the N of Normal". At this point, if you want to go past the S of Stupid, remove your thermostats and make sure what little water that is circulating mostly goes right back into the engine and not via the radiator where it is needed and you can play double or quits with the big "O" of overhating. What is needed is to generate as little heat at idle as possible, because this is exactly the time when it is hardest to dissipate heat. To that end, running the engine at a more efficient operating point is key to helping with overheating.

"Can I have my cake and eat it?"

Yes you can!

If you have the current retard setup, you can have some crumbs by simply plugging the line at both ends. No retard at idle is a small improvement on sitting around burning fossil fuels just to run a fan to dissipate the heat you have just generated.

Next up, plug the bottom of the left rear carburettor and plumb in a vacuum advance capsule to your holy grail of ported vacuum port. That's great except when you are sitting at the lights, burning fossil fuels to....- oh, that'll be the R of repetition...

Next, plug the ported vacuum port and take your distributor vacuum advance feed from one of the four main manifolds.

For close to full marks, get a vacuum switch either electric wired up to work by temperature or if you are smart, from the right rear water manifold and plumb in manifold vacuum when hot and ported vacuum OR nothing (i.e. vented to atmosphere) when cold. What this does for you is give you the optimal fast warm up from cold advance and the optimal setup that good old analogue technology can give you when hot. (On the thermostatic vacuum switch, "D" goes to distributor, "C" would go to ported vacuum feed and either of "F1" or "MT" would go to manifold vacuum and the other would be plugged.) So before the car reaches operating temperature, if you stop at the lights, it'll warm up. The only downside I can see here is that you'll have a slower idle before you are fully warmed up as you'll be tuned for a hot advanced idle.

For top prize, gold star, fit a trigger wheel on the front use it to drive a fully programmable ignition.

kind regards
Marek

[christopher storey]

At 800rpm, you probably only need to open the throttle butterfly by about 800/6000 of the full 90' movement to let the engine pull in enough air for 800rpm running, so wide open throttle means just that at 6000rpm but it also means anything over ~20% at the lower end of engine speed is as good as wide open.

I think there is a misunderstanding here . At an idle ( light running ) of 800 rpm on a 6 cylinder E, the throttles should not be open at all, with the entire mixture being supplied by the bleeds . If the engine was under load, however, then depending on the amount of load , governed by gradient and gear , the required throttle position to maintain speed but neither accelerate nor decelerate will vary according to the load imposed

[JerryL770]

One thing I don't have a handle on is "Ported" as opposed to "Manifold", though I do really know what is the manifold

I've boned up on this now so I'm not quite as ignorant as before.

[MarekH]

Christopher,

Yes the illustrative example is simplified in the way it treats the air through the idle screw and what sneaks past the throttle plate when the throttle is shut which means it isn't a straight 1:1 relationship. Here are some real figures for the average %age of how open the throttle plate is from a half hour drive through Weybridge:-

E......100%....17.....28.....35..... 41...43....45
N..L....85%....16.....24.....29.....34...38....43....42
G..O....75%.....7.....17.....22.....29...33....39....41
I..A.....65%.....2......5.....18.....23...29....33....39
N..D....55%.....1......1.....11.....18...24....29....31....35
E........45%.....1......1......5.....14...20....24....27....32
.........35%.............2......3.....7....12....15....20....30

.................501..800..1100.1400.2000.2600.3100.3700 rpm

100% engine load means the engine is fully loaded.
"Idle" for this engine is roughly at 800rpm and 45% load
(Engine loads below 45% can happen when going downhill)

What the table illustrates is that at 800rpm, the engine gets all of the air it needs when the throttle plate is 28% open, but at 2600rpm it needs to be 45% open. If you extrapolate the 100% engine load line across to 6000rpm, then you'd expect to see 100% wide open throttle.

This engine sits "idle" at about 45% load and around 800rpm. As you point out, all of the air needed is that which gets through the small throttle plate gap set up by the throttle linkage and the air bypass - the computer says the throttle is on average only 1% open, i.e. 99% shut when idling at 800rpm

kind regards
Marek

[mgcjag]

Hi Marek...cant make any sens of your chart above......what are the letters on the left coloum...then you give a % figure.....what do the numbers after relate to eg in the first line 17..28..35 etc also where does the last line of numbers fit into the chart..for a 1/2 hour journey i would expect to see throttle %..opening against time...Steve

[christopher storey]

Steve :It took some figuring out with all the dots, but reading vertically , it's ENGINE LOAD !!!

[mgcjag]

Thanks Christopher....i expected it to be something far more complex....shame it all not that easy to understand.. Steve