Bit of a techie issue, and a bit long - sorry...

I've been drawing up an Excel graph from a MJ log file I created in October... and I'm confused. Worse still, the information I've been getting from the MJ forum has confused me even more. What's even worse than that is that they may well be correct!

If you look at the graph below, as I push the pedal the throttle opens and the rpm increases (red line - left-hand scale), resulting in an increase in the vacuum reading (white line - left-hand scale). Note that increasing vacuum is indicated by a lower value, as it represents the pressure in KPa - 100KPa is approximately atmospheric pressure, 20KPa is a fairly strong vacuum. The net effect is to give extra advance on a wide-open throttle, and some retardation when the throttle is closed. Apart from the risk of over-high full-throttle advance, insufficient advance on a closed or part-closed throttle can lead to flat spots when the pedal is floored quickly - which is something I have experienced.

My problem is that my results seem to be the opposite of what everyone says should happen, as far as I can tell! Theory says that with a wide-open throttle the vacuum should be near-atmospheric (over 90KPa) that slightly retards the ignition timing, and when the throttle is shut the vacuum should plummet to 20-something KPa so that the timing is advanced again. This extra advance means that the weaker, slower-burning fuel-air mix is ignited earlier, and also gives a slight kick of power when the throttle is opened, until the loss of vacuum retards the advance again.

I've got a few checks to make to ensure that I haven't got a problem with my MJ unit, but in the meantime, can anyone look at this graph and tell me whether it's right or wrong?

David

BTW: It's a crossflow with a Weber DGV down-draft carb. The vacuum take-off is the same one used for the original dizzy, at the base of the carb where it meets the manifold.
Rescued attachment vacuum-graph.png

It is true when pressure is high, (wide open throttle) there is more air in the cylinder, compression is higher, and the combustion spread is much faster so you need less advance.

Opposite is true of a low pressure intake situation.

You can see the load vs. Rpm trace are linked correctly, more rpm, more advance

load vs. Adv looks correct also. More load, more Adv.

The resulting Advance, is a balance between rpm adv + load adv


The data log looks fine to me.

It looks correct to me also, does the 100 figure indicate kpa or load ? either way at max load it is on mimimum advance which is correct.
It does look as though the RPM is out of step with the load however, could this be the way it is logged or made into a graph?
Paul.

[Edited on 21/12/07 by paulf]

Was this log taken whilst driving the car, or just blipping the throttle with the car stationary?

The load value is in KPa - so a value of 100 is at atmospheric, i.e. no vacuum.

Remember that the majority of the advance is set from the rpm, with the vacuum used to fine-tune the advance.

I believe that the graph really does show load, not the vacuum, in which case it's spot on. However the author, Brent Picasso, assures me that the load line is in KPa, in which case it's inverted. I'll test this as soon as I get power back on the car.

The log was taken on the road, not static.

cheers,
David

The load line does look inverted to me. When you have load at 100 your revs are dropping which suggests a closed throttle not a fully open one.

What load do you see with engine off and with it idling? Also what happens to the load value if you pull the vacuum line off your manifold and suck on it?

Chris

David

Roughly what time frame does the graph represent?

Gary

quote:

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Originally posted by chriscook
What load do you see with engine off and with it idling? Also what happens to the load value if you pull the vacuum line off your manifold and suck on it?

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David,

Are you absolutely certain that this graph was plotted "on the road".

I can't understand how you would get an instantaneous change in RPM every time you lift off the throttle.

I would have thought that if you were in top gear, you should be able to jump on and off the loud pedal and see the advance changing solely due to the change in load, with the RPM staying pretty much constant.

Apart from that, the graph looks ok to me.

Drew

What does the advance do on the normal run time screen?
I have never bothered datalogging the megajolt so dont know what the results normally are, but may try and have a go on mine tomorrow, I have a MAP option megajolt controlling my ignition so should get the same results .I know that it actualy functions correctly so any error will be the data log, could it be to do with the choice of TPS or MAP settings?
Paul.

quote:

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Originally posted by David Jenkins

Chris,

According to the log, at idle I see 99 - presumably 99KPa. I was planning to pull some vacuum on the pipe with a syringe I use for oiling things... we'll see what happens then. I plan to watch the MJ console and record a log as I do it.

cheers,
David

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Rats!

Just had another look at the actual log file, rather than the Excel sheet. When I turned on the ignition, but before the engine started, I got a load reading of 101KPa (atmospheric pressure) which is absolutely correct - no vacuum expected at this time.

So my problem is with the engine, carb and/or the point where I'm taking off the vacuum.

Anyone got any bright ideas?

David,

I have just re-read your post. You are correct. If the white scale on the RHS is kPa, then the line is inverted.

I have a TPS system and TBH this whole MAP thing confuses me somewhat, as things tend to work back to front.

Presumably once your engine is started and idling the KPa gauge reads a low figure, and when you open the throttle the needle indicates a higher pressure?

Surely if that bit is working correctly, then your system is working correctly and it is just the graph scale which is wrong.


Drew

[Edited on 22/12/07 by lotustwincam]

[Edited on 22/12/07 by lotustwincam]

IIRC map sensors normally take the vacuum from the inlet manifold which may give a different reading than a vacuum advance take off??

The original dizzys vacuum pipe will be a ported vacuum, which usually detects atmosphere or there abouts at idle and a vacuum when opening the throttle lightly, which is what you are seeing. This is NOT where you should be taking a MAP reading from as it is not MAP. See here:
http://www.extraefi.co.uk/MAP_Alpha_N.htm
to be honest your better off using the TPS for a carb setup, try it using MAP in the correct place (e.g. actually in the manifold rather than the carb) but it wont give you much resolution, TPS will be better
Phil

Phil,

I think you're pretty close to the answer - I 'think' that the port I'm connecting to is a straight-through link to the top of the manifold. However, the results suggest otherwise.

I've finally made my mind up - when I change to bike carbs in the next few weeks I'm going to change to TPS, as there's a pot ready and waiting on the carb spindle. MAP is technically ideal, but TPS is so close it makes no difference (allegedly).

This means some extra effort - modifications to the MJ board, and some re-wiring - but it's easier than making connections to my new manifold and fitting pipes, chambers, etc.

cheers to all,
David

David is it worth mounting a tps with your present carb until you fit the bike carbs?. May be easy to mount on the throttle linkage

Not worth the effort, Mel - everything's ready to swap over, so no point in spending time on the old setup.

Anyway, I hope to sell the old carb and manifold as a set, complete with cables and linkages, so I don't want to muck it about (and before anyone asks, it's not for sale yet! )

cheers,
David

quote:

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Originally posted by daxtojeiro
to be honest your better off using the TPS for a carb setup, try it using MAP in the correct place (e.g. actually in the manifold rather than the carb) but it wont give you much resolution, TPS will be better
Phil

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Another member of this forum has a x-flow with a Weber DGV, very similar to my setup. The only significant difference is that he took his vacuum off the manifold rather than from the dizzy vacuum outlet. Having just fitted MJ he made a log and sent it to me (see graph below).

Now it looks to me like the load value is falling as the revs rise - which is what I was getting (the lower the load value, the higher the vacuum).

Although this discussion is a bit irrelevant to me as I'm soon going to bike carbs with TPS, it does annoy me that I don't know the answer!

I have read the theories that state that the KPa should increase (i.e. get closer to atmospheric) as the butterflies open, and I expect that it would happen with straight-through carbs like the bike ones I'm about to fit, but I'm wondering if there's something special or unusual about Weber down-draft carbs that's giving odd results.

There is this recurring image in my brain - on one side of the point where the MAP is measured there is a stonking great air pump trying its hardest to reduce pressure. On the other side there is the carb with its venturi that must give a certain amount of restriction to the airflow. As the revs rise the engine sucks harder, so I'd expect the vacuum to increase (lower KPa value). Countering this is the fact that the carb is opening to the normal atmospheric pressure (100KPa approx), but there's a lot of carb gubbins between the outside world and the inlet manifold.

Maybe I'll just give it up as a bad job and get on with my carb conversion!

David

P.S. If the owner of the graph wants to make his name known then he can speak up - otherwise I'll preserve his anonymity!
Rescued attachment Screenshot.png

Hi there,
OK, Im guessing that thats a log of a reving engine without any load on it. It does look a little odd but as a very small amount of throttle will increase RPM without any load (out of gear) then the KPa will not really increase much at all, in fact as the rpm increases the vacuum will increase too. As you can see the idle KPa isnt very low, thats typical of that type of inlet, thats why I recommended TPS,
Phil