Ported vacuum advance vs manifold vacuum advance

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jagwit
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#1 Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Mon Jul 30, 2018 1:59 pm

I have decided to "take on the world" on this subject and cleared the idea of this thread with admin before posting this.

I want to show that :
a) ported vacuum is an effective method for rapid warm-up BEFORE the engine is fully warmed up ... and WHY;
b) manifold sourced vacuum its the most EFFICIENT solution for vacuum advance for the engine AFTER the engine is warmed up ... and WHY;
c) that the best solution for our classic cars is therefore to have BOTH ported vacuum (during warm-up) AND manifold vacuum (at operating temp), but,
d) in cases where the practical choice is one or the other, that manifold vacuum would be the preferred choice for vacuum advance ... and WHY.

This is a subject that relates to ALL petrol cars that has a vacuum advance diaphragm on its distributor. Eg, I know of at least one car, a 1987 Honda Civic 1500 (Carb), that has manifold vacuum as the vacuum source for its vacuum advance diaphragm on its distributor, so I KNOW that the case I am presenting has been implemented at least once, by a reputable manufacturer, on a car sold in vast numbers.

PLEASE NOTE: My aim is NOT to prove anyone wrong or to MAKE anyone believe or implement what I do. All I ask is an opportunity to state what I believe, but more importantly, WHY I believe so.

It saddens me to see how unsuspecting folk can be misled by
a) the sheer number of voices on a forum (the "pack mentality");
b) the "this is how its always been done" argument ;
c) the "this was good enough for the manufacturer, therefore it is good" argument;
d) the unwillingness of the above to ask WHY?
e) how much opinion is presented without base;
f) that a reference to web sites (which may, or may not, also be in error), somehow lends credibility to their argument.



My credentials:

I have a BSc degree (Electronic Engineering) from the University of Pretoria 1987 - but I will not claim to know it all. I will claim to have no idea how much I have yet to learn and how much of my degree is now forgotten!
I have extensive EFI experience gained since I started playing with Megasquirt in 2005: I have fitted megasquirt (myself, with my own hands, programmed and tuned by myself) to more cars than I can remember but Jaguar V12 in particular:
1) My own '80 pre-HE XJS (which I still have and is still running);
2) My ex S3 OTS E-type (which had the pre-HE engine when I fitted EFI before I upgraded it to the HE engine)
3) An HE-engine based Cobra replica I built and sold.

Non Jag includes more than 10 Rover V8's both 3.5 and 4.6, one my own. Lost count. Most recently I EFI-ed my 1974 Jensen Interceptor with the Chrysler 440ci engine, first low compression, now high compression.

It is this background which has led me to the conclusion I'm now at with regard to vacuum advance.

Right, shall the games commence?? :bigrin:
I invite robust debate but be ready to provide measureable evidence of any statements you make.

I'll start by posing a few questions hoping you will participate with enthusiasm:
1) If we could build an internal combustion engine with 100% efficiency, it would not need a cooling system. Correct or false?
2) the less efficient the internal combustion engine, the bigger/better the cooling system needs to be. Correct or false?
3) Define the condition: "Idle";
4) Let's pretend "the book" states that the timing for an engine must be 12ºBTDC at idle and that idle speed must be 800rpm. What exactly does that 12ºBTDC apply to?
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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Robsan
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#2 Re: Ported vacuum advance vs manifold vacuum advance

Post by Robsan » Mon Jul 30, 2018 4:21 pm

provide measureable evidence of any statements you make.......... Oh no :roll:
but I will try and respond in the good spirit of you post...........

1) If we could build an internal combustion engine with 100% efficiency, it would not need a cooling system. Correct or false?
if efficiency is a ratio of [energy calorific value] of fuel input vs output [say at flywheel] and that efficiency figure was 100%, then proposal is correct

2) the less efficient the internal combustion engine, the bigger/better the cooling system needs to be. Correct or false?
Cooling system is dealing with Frictional losses [also] therefore engine efficiency is not a direct influence of cooling system capacity and power requirement. therefore the proposal is False

3) Define the condition: "Idle";
The maximum speed that the engine will rotate for a given ammount of Fuel/ Air [ specific flow and regulated flow ] Idle can be 1000rpm or 3000 rpm ?

4) Let's pretend "the book" states that the timing for an engine must be 12ºBTDC at idle and that idle speed must be 800rpm. What exactly does that 12ºBTDC apply to?

XºBTDC defines an exact rotational alignment when ignition coil/ spark should be fired. Before Top Dead Centre............... in a circle [like flywheel] there are 360 º....... but 4 stroke engine needs two revolutions for a cycle. Therefoe 720 º. Thus for No 1 cylinder to fire at 12 ºBTDC should be 24 º before [as witnessed on the Flywheel]


Answered with less % Confideance than % Efficiency of an Internal Combustion Engine :wink:

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JerryL770
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#3 Re: Ported vacuum advance vs manifold vacuum advance

Post by JerryL770 » Mon Jul 30, 2018 4:23 pm

Hee hee, I like it! :wow:

My responses:
1) Define 100% efficient. This would appear to mean that all the heat energy produced by burning the fuel is turned into work. No friction in the engine. With no friction itself producing heat, then no cooling system should be required. Therefore correct. It doesn't happen.

2) Correct. The less heat turned into work, the more must be dissipated some other way.

3) Ah! Difficult. :scratchheadyellow: Perhaps ..... the minimum speed at which the engine must turn over using energy stored in its flywheel to compress the next combustion charge?

4) 12° of crankshaft rotation BTDC is my understanding. I wonder why you ask.

2p
Jerome Lunt
1970 S2 FHC - Dark Blue, Red Interior, MX5 Seats

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MarekH
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#4 Re: Ported vacuum advance vs manifold vacuum advance

Post by MarekH » Mon Jul 30, 2018 5:16 pm

You might be able to have your cake and eat it.....
Investigate vacuum capsules which had both an advance tapping on one side and a second retard tapping on the other side. What that might achieve is something similar to ported vacuum but with additional retard at idle depending on from where you hooked up the vacuum sources to each side of the capsule. Once you then decide where you pitch your static advance, it may then get you even closely to where you want to be but using manifold vacuum. I know these dual advance/retard capsules were fitted to some Triumphs.

To scientifically answer your question, I think you need to look at an engine which was in production at the crossover from carburettors to fuel injection. The reason for that is that fuel injection typically then begot mapped ignition. If you can get hold of an OEM ignition map for an engine which originally ran carburettors, then you can reverse engineer the optimal ignition profile using static, vacuum and centrifugal technology. If at the end of the day, a combination of ported, static and centrifugal advance most closely matches the mapped ignition then you will be proved right.

To do this, rather than an OEM ignition map, which may have been made with emissions legislation or reliabilty or another reason in mind, you may actually want to take a programmable ignition car to the dyno and ask the man to create the best igntion maps for economy, power, emissions, reliabilty or what have you.

Next, take a look at the available distributors and map the centrifugal advance response to rpm using the weights and springs of your choice.

Then take a look at the spec of available vacuum capsules to see where they cut in, at what rate they add/subtract advance and where they are "all-in".

Lastly, consider the available vacuum switches which dump or cut off vacuum at various levels.

If you now measure at what ports you can take how much vacuum from the engine (manifold, ported, the bottom left hand carburettor, other tapping points(?)) and combine all of these with static advance of your choice, then you ought to be able to make an analogue ignition map and compare it to the relevant mapped ignition from the dyno

It may well be that the ported vacuum gets you to the best easily reachable answer, but it is only one factor in the determination of the overall advance profile you really want.

People who just add "advance" in place of retard, without considering the total advance already added by the other components, may seriously do themselves some damage by over advancing the engine at some other part of the rpm/load profile other than the one static point they measured. OEMs will also keep it conservative as they don't know what grade fuel you will use (is 4star even still available? - I think not) or how much carbon deposit is sitting on the pistons.

What I think you will flush out is that the how and why it was set up on day one is different from what it has aged to be and different again to what may be achievable now.

My only suggestion is that if you change a car from stock then please document the changes as they are often difficult to spot and may have unintended consequences.

A good candidate for study may be the last of the v12 HE engines. That had fully mapped ignition (apparently the programming language was similar the javascript I was told) and has an analogue counterpart as explained on the AJ16 website and can be analysed and interrogated easily on Megasquirted cars.

kind regards
Marek

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#5 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Mon Jul 30, 2018 5:25 pm

Robsan wrote:
Mon Jul 30, 2018 4:21 pm
1) if efficiency is a ratio of [energy calorific value] of fuel input vs output [say at flywheel] and that efficiency figure was 100%, then proposal is correct

2) Cooling system is dealing with Frictional losses [also] therefore engine efficiency is not a direct influence of cooling system capacity and power requirement. therefore the proposal is False

3) The maximum speed that the engine will rotate for a given ammount of Fuel/ Air [ specific flow and regulated flow ] Idle can be 1000rpm or 3000 rpm ?

4) XºBTDC defines an exact rotational alignment when ignition coil/ spark should be fired. Before Top Dead Centre............... in a circle [like flywheel] there are 360 º....... but 4 stroke engine needs two revolutions for a cycle. Therefoe 720 º. Thus for No 1 cylinder to fire at 12 ºBTDC should be 24 º before [as witnessed on the Flywheel]
Thank you Robsan!! Thanks for being a sport!

I was expecting "lateral" thinking type asnwers and you did not disappoint!!
1) Sorted.
2) The question did not focus on a "direct relationship" but your answer does point out that things are most certainly not linear.
3) Its also the minimum speed as Jerome points out? "Idle" is most certainly defined, in part, as a desired rpm. You are correct in pointing out that "idle" can really be any speed of your choosing but depends very much on the nature of the engine.
4) You were doing very well until you got to "24º". I suspect you were thinking of distributor degrees vs crank degrees? Dissy rotates half the speed of crank, therefore dissy shaft needs to be at 6º for crank to be at 12º. OK, I'll accept that the way I phrased my question may not have pointed in my intended direction. I did not want to "give too much away" with my question. So you focused on the meaning of "BTDC" whereas my question assumed "BTDC" was understood.

But hey, if this thread serves no other purpose than for 1 person to learn 1 thing, it will have been good. Thanks for your input! :salute:
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#6 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Mon Jul 30, 2018 5:54 pm

JerryL770 wrote:
Mon Jul 30, 2018 4:23 pm
Hee hee, I like it! :wow:

My responses:
1) Therefore correct. It doesn't happen

2) Correct. The less heat turned into work, the more must be dissipated some other way.

3) Ah! Difficult. :scratchheadyellow: Perhaps ..... the minimum speed at which the engine must turn over using energy stored in its flywheel to compress the next combustion charge?

4) 12° of crankshaft rotation BTDC is my understanding. I wonder why you ask.
Another good sport Jerome!

Right, here are my views on those questions:
1) Sorted
2) Sorted. The less EFFICIENT and internal combustion engine, the more demand will be placed on its cooling system. This is the basis of my entire proposal in this thread. I am aiming to make this old engine run as EFFICIENT as realistically possible and the more efficient I can make it run, the less demand will be placed on its cooling system.
3) Definition of "idle" (the definition I wish to work with in this thread): It is a desired RPM, sufficiently high to enable smooth running, good coolant flow, sufficient charging, but not higher than is necessary. It is a condition where the engine is under extremely light load and needs to produce just enough torque to keep itself running, and the ancillaries connected to it. Being at such a light load the manifold pressure will be very low (high vacuum) for a typical road biased engine. The important bits in this definition in glow font.
4) You have spotted that I am going somewhere... :twisted:
My answer to #4:
12º BTDC has NOTHING to do with "Idle"
12ºBTDC has EVERYTHING to do with the timing required by the engine at idle SPEED (800rpm) at FULL THROTTLE.

This then brings me to my next question:
5) If 12º is timing appropriate for FULL THROTTLE, what timing value WOULD then be appropriate for "idle"??
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#7 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Mon Jul 30, 2018 6:17 pm

MarekH wrote:
Mon Jul 30, 2018 5:16 pm
You might be able to have your cake and eat it.....
You are pre-empting me Marek :bigrin:

Marek has inside knowledge of where I'm going with this thread, so just ignore him - for now :lol:
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#8 Re: Ported vacuum advance vs manifold vacuum advance

Post by MarekH » Mon Jul 30, 2018 6:40 pm

All I actually did was read the thread title and the opening paragraphs and then set out how I'd go about demonstrating, with the desired proofs, what your stated aim was. The answers to the subsidiary questions at the bottom will clearly help navigate a way towards demonstrating the desired proofs.

Good luck

kind regards
Marek

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Robsan
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#9 Re: Ported vacuum advance vs manifold vacuum advance

Post by Robsan » Mon Jul 30, 2018 10:12 pm

therefore definition of Idle speed Must have referance to throttle position........... throttle like a garden tap but quite a bit differant.
Whatever engine you start, all you can do is vary the spark and vary the ammount [pressure] of fuel air............. n muck about wi exhaust [ back pressure]
isnt it all aboout fancy ways of doing above ?

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#10 Re: Ported vacuum advance vs manifold vacuum advance

Post by lowact » Tue Jul 31, 2018 4:07 am

jagwit wrote:
Mon Jul 30, 2018 5:54 pm
[ idle SPEED (800rpm) at FULL THROTTLE.

This then brings me to my next question:
5) If 12º is timing appropriate for FULL THROTTLE, what timing value WOULD then be appropriate for "idle"??
You've lost me. Idle speed is by definition, minimum throttle.
Regards,
ColinL
'72 OTS manual V12

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#11 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Tue Jul 31, 2018 6:28 am

Robsan wrote:
Mon Jul 30, 2018 10:12 pm
therefore definition of Idle speed Must have reference to throttle position
Absolutely correct Robsan! Your comment has also prompted me to include reference to sources of idle air and fuel and spark timing. Will amend the definition. Please keep the suggestions coming!!

"Idle" definition rev1:
It is a desired RPM, sufficiently high to enable smooth running, good coolant flow, sufficient charging, but not higher than is necessary. It is a condition where the engine is under extremely light load and needs to produce just enough torque to keep itself running, and the ancillaries connected to it. Being at such a light load the manifold pressure will be very low (high vacuum) for a typical road biased engine. This desired RPM is achieved either by closing the throttle to let just enough air (and OPTIMAL ratio of fuel) into the engine to achieve this RPM or with a dedicated idle circuit through the carb or through a positive crankcase ventilation (PCV) system or some combination of all these. Along with the correct amount of fuel and air, the ignition timing should ideally be that which results in the MOST EFFICIENT combustion of the air/fuel mixture in order to extract as much torque as possible from this idle condition whilst consuming the lowest amount of air and fuel necessary.

Is that better? Please keep the suggestions coming!!

It should now become clear that "idle" is a very unique and somewhat complex running condition.

My Land Rover Discovery 1 3.9 V8 had a setup where the throttle was completely closed and most idle air was supplied through a dedicated idle air circuit but a smaller portion of idle air came through the PCV system.

My Jensen's idle air comes through the throttle plates AND a dedicated idle air bypass circuit controlled by a PWM valve AND the PCV system.
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#12 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Tue Jul 31, 2018 6:59 am

lowact wrote:
Tue Jul 31, 2018 4:07 am
You've lost me. Idle speed is by definition, minimum throttle.
Thank you for your question Colin!!

You are absolutely correct. But I'm sure that you can see that "idle" at 800rpm and full throttle at 800rpm are two very different engine operating conditions that SHOULD require two very different fueling and timing requirements? As little as 20% throttle from idle can change the engine's operating condition from "idle" to full throttle BECAUSE, at those low RPMs , as little as 20% throttle is enough for the manifold pressure to become equal to barometric pressure (this condition is equivalent to full throttle from a fueling and timing perspective)

With an ECU mapped ignition system, it is very easy to provide the two fueling and timing requirements (and everywhere in between) but on our old Jags, the reality is that with the ignition system it is more difficult to provide specifically the OPTIMAL timing requirements for both "idle" and "full throttle" where both conditions exist at 800RPM. (this is easy to demonstrate...) This is why we end up with only one "timing setting" for "idle".

So, given a choice must be made of what timing to provide the engine with at "idle", can you see that the choice HAS to fall on the timing required at full throttle?

Have I managed to explain why "12ºBTDC" applies to full throttle at 800RPM more than it does to "idle" (at 800RPM)?

It might become even more clear once we have (...again...) explored the principle of vacuum advance vs centrifugal advance and why an engine benefit so much from vacuum advance. Then I hope to show why manifold vacuum advance IS a positive step towards a situation where the engine will get more optimal timing for "idle" as well as 12º at full throttle (the timing most appropriate for the full throttle condition at 800rpm).
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#13 Re: Ported vacuum advance vs manifold vacuum advance

Post by lowact » Tue Jul 31, 2018 7:56 am

Idle speed is the engine rpm when the throttle plates are being held open by the idle adjustment screw?
At idle, efficiency is zero, there is no work being done (the car isn't moving). U mean fuel consumption?
jagwit wrote:
Tue Jul 31, 2018 6:59 am
"idle" at 800rpm and full throttle at 800rpm are two very different engine operating conditions that SHOULD require two very different fueling and timing requirements?
More than that, on my car, for 800 rpm at full throttle, i need to disconnect 9 spark plugs.
jagwit wrote:
Tue Jul 31, 2018 6:59 am
As little as 20% throttle from idle can change the engine's operating condition from "idle" to full throttle
On my car I need 100% throttle to change from idle (0% throttle) to full (100%) throttle. Using 20% throttle only gets me to 20% throttle. I'm being a smart-arse, apologies, is only 'cos its more efficient ... :bigrin:

I suggest idle speed setting is 99.99% for smoothness and quietness (this is Jaguar), all other mentioned requirements are provided by even lower speed?

Maybe because V12s were new, smoothness was the marketing distinction, coin balanced on it's edge kind of smoothness. With the carbs and manifolds that Jaguar were forced to adopt at the last minute, that sort of smoothness could only be achieved reliably by rich mixture and retarded ignition at idle?
Regards,
ColinL
'72 OTS manual V12

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#14 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Tue Jul 31, 2018 9:21 am

lowact wrote:
Tue Jul 31, 2018 7:56 am
Idle speed is the engine rpm when the throttle plates are being held open by the idle adjustment screw?
Yes and no.
Yes, when the engine is running at 800RPM
No when the engine is running at eg 5000RPM (This condition is called "overrun" again requires its own unique fueling and timing. I want to touch on this engine load condition but later. Please remind me if I forget.)
lowact wrote:
Tue Jul 31, 2018 7:56 am
At idle, efficiency is zero, there is no work being done (the car isn't moving). U mean fuel consumption?
At idle efficiency is not zero (or 100%). There IS work being done to overcome the compression, the friction of the engine, and to drive all the ancillaries and the gearbox, more so if its auto and even more so if its in a gear. So, fuel IS being consumed but if the Air-Fuel-Ratio (AFR) is not OPTIMAL, it will consume more fuel than is necessary. If spark timing is not optimal it will consume more fuel. The more fuel being consumed during idle, the more INEFFICIENT the engine is, the more heat is being generated, the harder the cooling system has to work to maintain engine temperature. So, the more we OPTIMISE the engine(within practical, economical limits), the less strain will be placed on the cooling system.
lowact wrote:
Tue Jul 31, 2018 7:56 am
More than that, on my car, for 800 rpm at full throttle, i need to disconnect 9 spark plugs.
Do you have an auto? In that case, of course to HOLD the engine there is impossible but the engine WILL spend a few milliseconds in that 800rpm/full throttle condition before it starts revving up. In case of manual cars, it is quite possible to drive the car lazily in eg 4th at 800RPM and applying just 20% throttle puts the engine in full throttle condition for all intents and purposes which is why the timing must be suitable for 800RPM / full throttle.
lowact wrote:
Tue Jul 31, 2018 7:56 am
On my car I need 100% throttle to change from idle (0% throttle) to full (100%) throttle. Using 20% throttle only gets me to 20% throttle. I'm being a smart-arse, apologies, is only 'cos its more efficient ... :bigrin:
Smart-arse and all, it was valuable input because it allows me to explain the difference in "throttle position" and "engine load".

Throttle position is exactly that. 20% throttle is 20% throttle. But...

What I've been saying is that throttle position is NOT EQUAL to "engine load". Engine load is essentially "what % of maximum power is delivered AT THAT RPM and throttle position". What I have learnt from my EFI doings, is that at low RPM, small throttle openings produce the equivalent of 100% engine load. So, at 800RPM, 20% throttle produces 100% engine load, meaning, once the throttle has reached 20% (at 800RPM), depressing the throttle further, makes no (or very little) difference in the amount of torque being produced at 800RPM.
lowact wrote:
Tue Jul 31, 2018 7:56 am
Maybe because V12s were new, smoothness was the marketing distinction, coin balanced on it's edge kind of smoothness. With the carbs and manifolds that Jaguar were forced to adopt at the last minute, that sort of smoothness could only be achieved reliably by rich mixture and retarded ignition at idle?
From what I have read a V12 is inherently very well balanced - best of all engine configurations. Smoothness most certainly requires a suitably rich mixture (but again, not richer or leaner than is necessary!!). Smoothness also requires a stable spark. Not just stable, but again, the more OPTIMAL that spark is timed, the more smooth the engine will run, because the combustion process will be more EFFICIENT.

From what I have read, the V12's retard system was very much intended to the meet emissions regulations of the day, MAINLY a rapid warm-up (possibly the TYPE of emissions being generated) but nothing to do with smoothness. Besides, my car idles smoothly with MANIFOLD vacuum advance, with spark timing in the high 20's (forgot the actual figure) and I can also balance the coin on its side. Did so just this weekend. Will post a youtube video if you want.

So, conveniently this brings me to the next question:
#6: WHY is the vacuum retard system on the V12 so good at achieving a rapid warm-up from a cold start?
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#15 Re: Ported vacuum advance vs manifold vacuum advance

Post by lowact » Tue Jul 31, 2018 1:28 pm

Wife says I should make the v12e electric like Harry’s. Son says if I do he’ll disown me.

When talking about efficiency we maybe need to be clear about what it is that we are trying to be efficient about.

E.g.1: If the purpose of a car is to move, at idle (stationary) efficiency is zero or worse, more efficient if u turned yr engine off. Burning petrol to overcome parasitic loads, friction and other losses, that wouldn’t exist if u weren’t burning the petrol, is maybe not a holistic enough def’n of efficiency for this purpose?

E.g.2: Retarding the ignition makes the engine a more efficient heater, which is why it warms up faster. Also answers yr question. Less efficient at producing torque but you don’t need more torque at idle. E.g, say yv got retarded ignition, at idle burning 2 parts of fuel, 1 part is producing torque that is driving ancillaries, the other part is producing more combustion heat so yr exhaust gases are hotter, engine warms up faster. Ok but u only need to warm up once per trip and because u spend hours idling in dead traffic u think to save overall by disconnecting the retard. Result is u now have 2 parts of fuel producing torque. Ancillary load is the same so net result is idle speed increases, causing increased friction and losses to balance the increased torque. But u don’t want increased idle speed so u back off the idling screws until yr burning only 1 part of fuel as before. Whatever yr definition of efficiency you are now using less petrol when idling in traffic, aren’t u clever.

Except then u suddenly notice that yr car is not idling quite as smoothly as it was before …

What u can’t notice is that u have also become more toxic. The additional combustion heat caused by retarded ignition is mostly out the exhaust pipes (more than CW system). Hotter exhaust, together with air pump, reduce the nasties that form in the exhaust because of the overly rich mixture required at idle because of the carbs and manifolds that had to be adopted because efi wasn’t ready in time …

One reason u can’t notice this is because, to make more than a theoretical impact on emissions, every element of the solution had to be working perfectly and yr air pump has been knackered for years, also the reflux valve is completely burnt out. Parts are not available because nobody buys them, because efi+ecu is infinitely superior solution. An even better solution would be to make it electric like Harrys …

As I understand it, the main reason vacuum advance is NOT applied at idling is because it is inherently unstable. E.g. say yr idling at the lights and the air-con cuts in. Revs drop, vacuum reduces. With manifold vacuum the reduced vacuum reduces the advance, amplifying the reduction in revs. With ported vacuum advance (no advance at idle) there is no secondary effect. With vacuum retard at idle, the reduced vacuum reduces the retard, speed increases to cancel the effect of the air-con load – smoothest antique solution?
Regards,
ColinL
'72 OTS manual V12

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christopher storey
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#16 Re: Ported vacuum advance vs manifold vacuum advance

Post by christopher storey » Tue Jul 31, 2018 2:01 pm

Well, my recollection is ( AFAIK ) prior to about 1960, most if not all vacuum advance systems were based on manifold vacuum. It was only with the advent of emission limits, particularly of Nitric Oxide , that ported vacuum started to be used , along with other bits of trickery such as ignition vacuum retard systems

Incidentally, all engine timing information is always given in crankshaft degrees , and all distributor specifications are always given in distributor shaft degrees

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jagwit
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#17 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Tue Jul 31, 2018 2:19 pm

lowact wrote:
Tue Jul 31, 2018 1:28 pm
When talking about efficiency we maybe need to be clear about what it is that we are trying to be efficient about.

E.g.1: If the purpose of a car is to move, at idle (stationary) efficiency is zero or worse, more efficient if u turned yr engine off. Burning petrol to overcome parasitic loads, friction and other losses, that wouldn’t exist if u weren’t burning the petrol, is maybe not a holistic enough def’n of efficiency for this purpose?
I thought we were discussing our classic Jags, not the modern plastics? Likewise the definition of idle should pertain to the realities of our classic Jags?
lowact wrote:
Tue Jul 31, 2018 1:28 pm
E.g.2:
Do I understand you correctly that
1) the vacuum retard system makes the idle situation deliberately less efficient to achieve a quicker warm-up. So instead of idling at 12ºBTDC, the vacuum retard system now has it idling at, lets say 2ºBTDC. I agree with this and therefore we have an answer to Question6.
2) you do agree that disconnecting the vacuum retard module on the V12 is more energy efficient than with it connected?
3) The initial warm up will take longer with disconnected vacuum retard module but at least for all idle situations to follow, idle will be more efficient?
lowact wrote:
Tue Jul 31, 2018 1:28 pm
As I understand it, the main reason vacuum advance is NOT applied at idling is because it is inherently unstable.
The way you describe the situation in sure sounds like it is inherently unstable. Bit is it in practice? My car says no. Sure, when my aircon cuts in the revs drop, when I go into Drive, the revs drop but so will a ported vacuum setup.

"Revs drop, vacuum reduces." Does it? Does the vacuum reduce THAT much IN REALITY that the vacuum advance is ACTUALLY affected to the extent that timing is now a bit more retarded? And even if it is, does it affect idle to the extent that it can be noticed? Did you measure how much vacuum drops and how much timing is affected by this? Likewise with the retard system. Did these vacuum changes REALLY affect timing? Did you measure this? I'm just trying to establish if all this has been quantified through measurement.
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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#18 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Tue Jul 31, 2018 2:28 pm

Question 7:
Right we have "our engine" idling nicely at 800RPM with 12ºBTDC. No vacuum retard or vacuum advance connected.

I trust you will now keep in mind that this 12ºBTDC is 12º not because this is what is needed at idle, but this is the timing needed when the engine goes from "idle" to full throttle (or the equivalent threreof) and all this happens at 800rpm.

Here's the question:
I'm a dumb blonde, blue-eyeed, African backyard so-called mechanic and I decide to play with the engine timing. I find as follows:
When I retard the engine timing below 12º, idle rpm drops. WHY ? #7.1 (it actually does, go test it yourself if you don't believe me)
When I advance engine timing to more than 12º, idle speed increases to beyond 800rpm WHY ? #7.2(it actually does, go test it yourself if you don't believe me)
Last edited by jagwit on Tue Jul 31, 2018 6:54 pm, edited 1 time in total.
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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JerryL770
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#19 Re: Ported vacuum advance vs manifold vacuum advance

Post by JerryL770 » Tue Jul 31, 2018 5:25 pm

jagwit wrote:
Tue Jul 31, 2018 2:28 pm
When I retard the engine timing below 12º, idle rpm drops. WHY#7.1? (it actually does, go test it yourself if you don't believe me)
When I advance engine timing to more than 12º, idle speed increases to beyond 800rpm WHY #7.2(it actually does, go test it yourself if you don't believe me)
:seeingstars:

Well, I have to say that most of this is way over my head and knowledge so please forgive me if I write out of order :salute:

It was my understanding that one way of finding the "proper" timing (adv BTDC - when you don't know the advised figure) was to rotate the dizzy one way and/or the other to find the max rpm position at tickover. In other words, when you advance past this position, the tickover speed will reduce.

Otherwise, all very interesting.
Jerome Lunt
1970 S2 FHC - Dark Blue, Red Interior, MX5 Seats

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#20 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » Tue Jul 31, 2018 6:54 pm

JerryL770 wrote:
Tue Jul 31, 2018 5:25 pm

It was my understanding that one way of finding the "proper" timing (adv BTDC - when you don't know the advised figure) was to rotate the dizzy one way and/or the other to find the max rpm position at tickover. In other words, when you advance past this position, the tickover speed will reduce.
When you take this approach, tickover will increase aand you will find the max rpm and at some point will start to decrease (along with some misfiring) but....

Jerome, this approach is wrong, wrong, wrong and I will make sure to explain WHY (actually I already have :geek: ), 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...
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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