Club 80-90 Forums

my head hurts

Is there any chance that you’ve got a vacuum hose T-piece, to find out what happens when you apply equal vacuum to both spigots on your spare DJ distributor?

probably the retard has a T piece to the fuel pump regulator and I have a spare set...

what is the logic?
And which engine?

CJH wrote:

itchyfeet wrote:I'm begining to think those WIKI graphs are not realistic

Well at the very least, the experimental method used to derive them must be different from ours. Doing it on a running engine isn't precise enough to reproduce the curves properly.

I can't see how centrifugal advance can vary with engine load so how can the graphs be so different?

Engine isn't involved - I had in mind repeating the bench measurements you made on your spare distributor, but with equal vacuum on both spigots, to try and find out what causes the advance function to dominate over the retard function.

If the sensor doesn't move at all when you apply equal vacuum on both spigots at the same time, then it tells us that the effect must come from different levels of vacuum in the injection system. But if it moves in the advance direction, then we know that it will advance the timing even when the ported and manifold vacuum levels are the same.

itchyfeet wrote:
I can't see how centrifugal advance can vary with engine load so how can the graphs be so different?

No, it doesn't, but I think engine speed isn't perfectly stable, and the timing light may not be precise enough, so we get error bars on our data points. But I agree, there does seem to be an offset.

CJH wrote:Engine isn't involved - I had in mind repeating the bench measurements you made on your spare distributor, but with equal vacuum on both spigots, to try and find out what causes the advance function to dominate over the retard function.

If the sensor doesn't move at all when you apply equal vacuum on both spigots at the same time, then it tells us that the effect must come from different levels of vacuum in the injection system. But if it moves in the advance direction, then we know that it will advance the timing even when the ported and manifold vacuum levels are the same.

easily done

I have a 123Tune dizzy you can borrow if you want to experiment to find the best curve (especially useful on the rolling road as it can be adjusted in real time).

https://www.123ignitionshop.com/gb/tune ... e4rvv.html" onclick="window.open(this.href);return false;

And here's a good graphical representation of how vacuum varies with load...

I use a vac gauge regularly as a diagnostic tool. It's very useful used in conjunction with other tools, but also reveals how vacuum collapses instantly on blipping the throttle. This is the main function of the vac advance on a dizzy, to smooth the transition with changes in throttle position and provide a smoother drive, especially in traffic. At full throttle under load (especially at lower rpm) the manifold is effectively open to atmosphere, and vacuum reduces to almost nothing. Therefore vac advance is reduced and pinking (hopefully) avoided under load.

That's an interesting bit of kit. Does it have a mechanical vacuum unit, or does it measure the vacuum and set the advance electronically? I think I might be stretching the limits of what the folks on the rolling road will do for me if I was to take that along as well - I'm already asking them to jet two carbs in the one session. And in any case, if I was to set up a curve for that distributor then I'd have to buy my own!

CJH wrote: Does it have a mechanical vacuum unit, or does it measure the vacuum and set the advance electronically?

It's electronic. The software lets you set the amount of vac advance (and retard) at various pressures and revs, so it can be used to simulate the retard function of a dual vac dizzy.

CJH wrote: If I was to set up a curve for that distributor then I'd have to buy my own!

Ha! You know you want one...

But where is this vacuum measured Andrew?
What we are trying to figure out is the different vac points of DG and DJ to see if running a single vac advance from under 2e3 throttle on a DJ is reasonable.

ajsimmo wrote:And here's a good graphical representation of how vacuum varies with load...

I use a vac gauge regularly as a diagnostic tool. It's very useful used in conjunction with other tools, but also reveals how vacuum collapses instantly on blipping the throttle. This is the main function of the vac advance on a dizzy, to smooth the transition with changes in throttle position and provide a smoother drive, especially in traffic. At full throttle under load (especially at lower rpm) the manifold is effectively open to atmosphere, and vacuum reduces to almost nothing. Therefore vac advance is reduced and pinking (hopefully) avoided under load.

I wonder if its worth trying a DJ dizzy on a dg with the advance connected to the second throttle vac port and the retard to the under throttle vac port.
Would that give results more similar to a DJ digijet?

I'm just thinking if your rolling road session is not going well because of ignition timing then having some different options with you might be worth it.
Personally I'd just get them to jet one bigger 2e3 carb and spend the extra time on playing with ignition timing, I think its pointless jetting a stock 2e3 it has to be lower power.

itchyfeet wrote:I wonder if its worth trying a DJ dizzy on a dg with the advance connected to the second throttle vac port and the retard to the under throttle vac port.
Would that give results more similar to a DJ digijet?

I'm just thinking if your rolling road session is not going well because of ignition timing then having some different options with you might be worth it.

I don't think it would work as intended, as there's no ported spigot on the 2e3. The advance would always overcome the retard. I'm pretty certain this is achieved mechanically by design of the internals of the dual vac can, not by a relative difference in vacuum signal.

itchyfeet wrote:But where is this vacuum measured Andrew?

I've used both ports on a Pierburg, and haven't noticed any discernable difference.

itchyfeet wrote:What we are trying to figure out is the different vac points of DG and DJ to see if running a single vac advance from under 2e3 throttle on a DJ is reasonable.

Yep, got that by wading my way through all of it, but it's useful to remind readers with such a summary. [WINKING FACE]