Ported vacuum advance vs manifold vacuum advance

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

Post by MarekH » 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.

kind regards
Marek

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

Post by jagwit » 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”.
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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mgcjag
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#43 Re: Ported vacuum advance vs manifold vacuum advance

Post by mgcjag » Sat Aug 11, 2018 7:08 pm

jagwit wrote:
Sat Aug 11, 2018 4:19 pm
Another opinion on this matter

This article confirms much of what I’m saying.
Or maybe you agree with much thats already been said in the subject.... :bigrin:
Steve
1969 S2 2+2 & Building a C type replica

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

Post by jagwit » 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.... :bigrin:
Quite right Steve, absolutely.
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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jagwit
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#45 Re: Ported vacuum advance vs manifold vacuum advance

Post by jagwit » 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...
Vacuum solenoid.jpeg
Vacuum solenoid.jpeg (132.15 KiB) Viewed 27 times
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.
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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