pre-programmed curves on 123 ignition - help please?

[John ball]

Just a couple more comments. The vacuum on the 123 is not a real vacuum canister, just I believe a micro switch, so I wonder if the standard default MAP curve on my picture on post 70 is just a simple way it works as a switch as it is all straight lines ? Whereas the real version has curves.

Also, the standard MAP curve says vacuum only starts after 1500 RPM, should I change that as it can be changed to anything.

[christopher storey]

John : manifold depression may be affected to a very minor degree by altitude, but the princpal effect is that the higher you go , the lower the partial pressure of oxygen there is, and therefore the lower the degree of filling of the cylinders of normally aspirated engines. So, you can ignore that aspect of things for the purposes of setting up a 123

To answer your second question, yes indeed the combination of mechanical advance and vacuum advance can produce a total advance of 48 degrees at, say, 3000 rpm - but only at part throttle.This equates to normal light running at a constant fairly low cruising speed - say 45-50 mph . As you apply more throttle , the vacuum in the inlet manifold reduces, and as that happens the additional advance created by the vacuum capsule also reduces . The purpose of the vac advance is to increase part throttle efficiency when there is little or no risk of detonation which is usually heard as pinking

Without wishing to teach my grandmother to suck eggs etc, it sometimes help to understand the reasoning behind all this if it is appreciated that the flame front of combustion within the cylinder travels surprisingly slowly - generally speaking about 120 feet per second - and thus on any single firing from the central spark plug it takes about 1/720th of a second for the flame front to reach the periphery of the cylinder( and probably twice that to reach the farthest parts of the mixture once the piston has started down the cylinder ) and thus ensure complete combustion . This is why, as engine speed rises, it is necessary for the spark to be generated earlier and earlier to try and ensure efficient/complete combustion until such time as the point is reached whereany further advance would be counter-productive - in the case of these engines about 3000 rpm

[John ball]

Hello Chris, if I am driving at 70mph in my car in 4th gear it equates to about 3,000 rpm, but if I put it into O/D 5th gear it is doing about 2,400 rpm. What is worrying me now is that my throttle opening in the 5th gear is a lot less than in 4th gear, so does this mean it is likely to ‘Pink’ ? The problem being it is so loud with engine, exhaust and wind noise, that I am not likely to detect it !! What happens when I floor to overtake ?
Maybe I should disconnect the vacuum or buy a non vacuum version ?

Also see my comment on post 81 which may have crossed with your 82 timewise ?

Really do appreciate your time and effort trying to help me understand.

Raining again so what else can I do in Lockdown !!!!

[malcolm]

If it's too advanced it will pink when you accelerate. If it isn't, it shouldn't. You don't need a non-advance model. As Christopher and others have said, the advance comes in on vacuum when you are on light throttle, so won't cause pinking. Advance on vacuum goes when you floor it.

[max-it-out]

Would it be right to say that maximum vacuum in the manifold should be at fairly high RPM when the throttle is closed . eg. take your foot off the pedal at 60 in 3 rd gear .

Minimum vac would be the opposite , ie. wide open throttle at low RPM .

Just guessing now , but maybe 70 mph in 4 th would be somewhere in the middle , so a vac advance of around 10 degrees ( crank ) ?

[Tom W]

Yes, your assumption on the conditions for maximum and minimum vacuum are basically correct. Maximum vacuum under conditions of maximum engine braking, and minimum vacuum under conditions of maximum load. Any conditions where the engine speed is throttled by the throttle butterflies will see some vacuum.

To correlate any given driving condition to a specific amount of vacuum advance is more tricky though. You’d need a vacuum gauge, or data logger or similar to see what’s actually going on. The apps for the tuneable distributors should be able to do this too. For example, the engine could pull far more vacuum than is necessary to achieve maximum vacuum advance. So maximum vacuum advance could be seen more often than just conditions of high speed engine braking. Similarly, the vacuum advance doesn’t start at pressures immediately below atmospheric, so it doesn’t take only wide open throttle for it to disappear completely.

If you measured the manifold vacuum for any specific driving condition, you could work out what the actual advance is.

[Tom W]

Additional to the above. The pre programmed 123 has a gearshift vacuum advance cut off, so at vacuum beyond a certain point (low pressure) the vacuum advance is actually removed. And on cars with ported vacuum advance, when the throttle is closed, the vacuum port is no longer exposed to the manifold vacuum. So no vacuum advance applied in conditions where the throttle is completely closed.

[mgcjag]

Hi Tom.... the gearshift cutoff feature is on the programable 123....when you mention pre programed are you refering to the 123 with the swichable options.....Steve

[Tom W]

Hi Steve. Yes, I’m referring to the distributors with the 16 switchable curves.

On the programmable versions, presumably you can just draw a curve without it if you didn’t want it.

[John ball]

These are the two curves I am proposing to try based on the post 65 with Jaguar manual and on Chris’s post 78 of the maximum vacuum advance curve.

This is based on the static at 10 BTDC.

What are the opinions ? I am now waiting for a dry day to try out, during lockdown on the way to supermarket the long way round !!

[christopher storey]

Personally, I would set the rpm limit a little lower than 6000 rpm for longevity's sake - the red line began at 5500 for 3.8 cars, and 5000 for the 4.2 . Although the engines would go up to 6000, - and indeed beyond in some of the racing applications - it was not very wise to do so for road use, and there really is no advantage in doing so

[mgcjag]

Hi John....you can confirm that what you programed in is actually working.....use a timing lamp to check.....Steve

[John ball]

Hello Chris, the 6000 limit does not matter as I never rev the engine anywhere near that. It is just the way the graph shows maximums.

Hello Steve, Yes I will check with the strobe light.

I have also programmed on another page of the 123, the bottom end of both the crankshaft advance and bottom end of the vacuum advance corresponding with the lowest figures on the Jaguar manual. So, first I will try the one I have drawn above, if the car performs badly or Pinks to an extreme, I will try the lower version.

I still find it pretty poor that the standard default setting on the Bluetooth 123, both ignition curve and vacuum curve are nowhere near the original Jaguar manual. We already know from previous posts the older NON Bluetooth 123 system with adjustable 16 setting does not have the correct mid range advance settings.

As I said before, I have a CSI system on my Healey 100. If I cannot get this 123 to work properly, I will change out to either a CSI or back to original points and vacuum unit.

[Tom W]

I don’t think the Bluetooth 123 purports to come pre-programmed with a curve specific to a Jaguar. It’s not even specifically a Jaguar distributor, as it fits many 6 cylinder cars that used the Lucas 22D6. I think it’s expected that you adjust the curve to suit your particular application.

[malcolm]

I don’t think the Bluetooth 123 purports to come pre-programmed with a curve specific to a Jaguar. It’s not even specifically a Jaguar distributor, as it fits many 6 cylinder cars that used the Lucas 22D6. I think it’s expected that you adjust the curve to suit your particular application.

Not quite so Tom. When I bought mine, there was a choice - the general purpose 123 with 16 curves suitable for most classics, and a specific Jaguar one suitable for "all long stroke xk engines" I got the Jag one, so expected one of the sixteen curves might be close to a Jag one! But as per my previous posts, there wasn't one. In fairness, it does run very well on curve 2, but it would be nice to be correct!

[Tom W]

The switchable distributors with the 16 pre programmed curves are sold as manufacturer/model specific. Whether one of the 16 curves is suitable is another discussion.

I was referring to the 123tune and 123tune plus (Bluetooth) which aren’t manufacturer/model specific, besides the physical mounting, polarity and number of cylinders needing to be compatible with the car it’s fitted to.

[malcolm]

OK, get it.

[John ball]

Ok, so sunny day, took my shopping bag with me -

The curves that I had set on post number 90, I tried this afternoon. No good !!
Don’t know why as although they are at the top of the parameters from the manual on post 65 and Chris’s calculations from inches to kpa’s on post 78.

The revs at 3000 went up to 37 degrees and after that up to 40 degrees.

There was a previous post with an ignition curve from the distributor doctor, which I tried this afternoon and it seemed much better.

But, I may try the lowest figures from the Jaguar manual, which will be the lowest ignition advance and Chris’s original Moderate middle position vacuum.

[mgcjag]

John...dont just keep changeing maps......as i mentioned use a timing lamp to see whats actually happening and map the results.....then compare with your programmed map.....are you sure your static is set at 10dbtdc...how did you check this.......not just by looking at the pointer on the pully marks...have you actually set the pointer up to tdc when piston was at the top.....Steve

[Tom W]

That’s disappointing. How are you determining them to be no good?

The car didn’t perform as expected or you didn’t get the advance number you expected at a given RPM?

If I’m following this correctly (I might not be) at 3000 rpm you have 24degrees of mechanical advance, and possibly up to 18 degrees additional vacuum advance. Your app will show anywhere between 24 and 42 degrees depending on throttle position and load at 3000 rpm. On top of this you have an additional 10 degrees static from how you’ve installed the distributor. So the same conditions when measured with a timing light would give between 34 and 52 degrees (though you won’t practically be able to use a timing light on the car on the road).

Am I right in thinking your engine is a 3.4, but modified from standard somewhat? So the 3.8 or 4.2 E-type curve might not be appropriate anyway. Do you have an XK120 workshop manual?