Club 80-90 Forums

OK, so we mean the same thing at least! But I always thought earlier means advanced, later means retarded. So the mechanical advance changes the timing from 10ºBTDC at idle to something more than 10º before (i.e. even earlier) at higher revs.

itchyfeet wrote:Dunno but if the second vac port won't open surly there must be a difference

I guess it might depend where the vacuum comes from - if it's above the throttle butterfly maybe it can't see the full vacuum coming from the cylinders at idle, whereas below the butterfly it won't matter whether the butterfly is open or closed. From memory I can't remember where the vacuum advance port comes from in the 2E3, but I guess in the injection setup the throttle opening will have an effect on one or other of the two vacuum ports at least.

Maybe the second vac port in the 2E3 is above the throttle butterfly, so that it only sees enough vacuum when the throttle is wide open, which may not be needed when there's no load. But even so, on my drive it would still be possible to open the throttle fully, even if it wasn't necessary to achieve maximum revs. I'm getting a bit confused now.

I've checked where the vacuum ports originate in the 2E3. The distributor vacuum advance originates below the butterfly for the second venturi. I think that means it sees the full vacuum even when that butterfly is closed.

The vacuum that opens the second venturi originates above the butterfly for the primary venturi, which I think means it will only see the full vacuum when the throttle is open far enough. Interestingly there's also a smaller opening for this vacuum supply above the second venturi butterfly - maybe that's to help keep it open once it's opened, somewhat independently of the throttle opening.

Where does this get us? Not sure. I think it means that for the carb setups I can possibly measure the proper vacuum-assisted advance curve regardless of engine load, because the signal for the vacuum advance is not blocked by the throttle butterfly and therefore depends only on engine speed. But it's largely guesswork on my part.

photos edited

itchyfeet wrote:photos edited

I bet that was more work than annotating them originally.

Ok, so the vacuum advance (full scale I assume) is the same on both distributors, and the vacuum retard on the DJ distributor is about the same value as the advance. But the numbers for the advance don't match what the various sources tell us, namely 12-16 degrees. I think there's a factor of two missing - the rotor arm goes around once for every spark cycle, whereas the crankshaft goes around twice (the spark fires every other stroke), so I think your measurement of 7 degrees actually corresponds to 14 degrees of the crankshaft - bang in the middle of the expected range.

I guess it isn't easy to check how the advance and retard work together on the DJ vacuum unit. The story seems to be that the retardation disappears quickly once the throttle opens, so perhaps the feed for the retardation is a smaller channel than the one for advance. If you block one, does it prevent the other one moving in response to the vacuum?

I still think something is missing, 4000 rpm stationary the throttle will barely be open, up a hill it will be wide open for the same revs

Anyway whats really important is the differerences in DG and DJ ignition and why they are different? is it because if the injection system or because of the higher compression

itchyfeet wrote:I still think something is missing, 4000 rpm stationary the throttle will barely be open, up a hill it will be wide open for the same revs

Agreed. But what I think this *might* mean is that vacuum ports originating above the throttle will see more vacuum when the throttle is wide open (so the secondary throttle signal is greater when going up a hill), whereas vacuum ports originating below the throttle will see the same vacuum regardless of throttle opening (so the vacuum advance doesn't depend on load). It makes sense to me, but I have no idea whether it's correct.

Do you suppose the same flow at same revs?
same fuel consumption staionary as going up a hill at same revs?

itchyfeet wrote:Do you suppose the same flow at same revs?
same fuel consumption staionary as going up a hill at same revs?

Clearly not, but flow isn't directly equivalent to vacuum I think. Imagine sucking on a straw, and then pinching the straw half way along. The vacuum between your mouth and the pinch point would remain, whereas the vacuum beyond the pinch point would disappear.

CJH wrote:

itchyfeet wrote:Do you suppose the same flow at same revs?
same fuel consumption staionary as going up a hill at same revs?

Clearly not, but flow isn't directly equivalent to vacuum I think. Imagine sucking on a straw, and then pinching the straw half way along. The vacuum between your mouth and the pinch point would remain, whereas the vacuum beyond the pinch point would disappear.

I think there os more too it, time read up on fluid dynamics.

Been reading up on vacuum sources. The crucial difference seems to be between 'ported' and 'manifold' vacuum.

Manifold vacuum is used for things that need to be activated regardless of throttle position, i.e. at idle as well as at other throttle openings. Manifold vacuum originates from below the throttle butterfly, i.e. it sees the same vacuum pressure as the manifold above the cylinder.

Ported vacuum is for activating things that should not be activated at idle. Ported vacuum originates from a port above the throttle butterfly, so the vacuum signal is blocked by the throttle butterfly, and hence there is no signal at idle.

There's a video which explains these here. Bear with it - the description of manifold vacuum comes after about 8:30 minutes, and ported vacuum is described after about 11:30 minutes.

Most vacuum advance systems work on ported vacuum apparently. But not ours. The 2E3 takes the vacuum advance signal from below the throttle butterfly, so it is activated by manifold vacuum. So it operates even at idle - hence the need to disconnect that vacuum advance when setting idle timing.

There's a nice description of distributor advance here, and it includes a nice plot of manifold vacuum, ported vacuum and throttle position.



Notice how ported vacuum drops to zero when the throttle is closed. Notice also how manifold vacuum drops away as the throttle opens. I think this is because the manifold is exposed to ambient air pressure when the throttle is wide open, so the vacuum gets 'filled' (like unpinching the straw in my earlier example - air rushes in from the end of the straw to fill the vacuum created by sucking). So this implies that the vacuum advance gradually unwinds as the throttle opens - vacuum advance is therefore only helpful at mid-range throttle openings.

Once the throttle opens, even a little, manifold and ported vacuum are the same. So the 2E3's secondary throttle, which seems to be activated by ported vacuum, will also see a reduction of vacuum as the throttle opens wider. I still haven't figured out the implications of this, or indeed how any of this explains why the secondary throttle won't open unless under load. At this point I"m a bit sceptical of the load argument - I think throttle position and engine speed are the major factors, which would imply that you could get the secondary throttle to open on your drive if you could bear to have your engine running at full tilt.

Veryyinteresting

CJH wrote: Once the throttle opens, even a little, manifold and ported vacuum are the same. .

not totally convinced

also rememer its the differences between DG and DJ you are looking at DJ has advance above throttle, DG below

Thinking about it the amount of vacuum needed for the control system is tiny compared to the amount that an engine can generate so it may create more vacuum one side of a half open throttle than the other but if its past the limit needed the control system sees them as the same.
On a DJ with advance and retard its using the difference between them which may react differently at half throttle

itchyfeet wrote: not totally convinced

also rememer its the differences between DG and DJ you are looking at DJ has advance above throttle, DG below

The author of the article addresses the small offset between ported and manifold pressures:

I've had several people comment that ported vacuum is not the same as manifold vacuum because at low throttle positions the throttle blades act as venturis which makes ported vacuum stronger than manifold vacuum. Referring again to the graph, this can be seen in the middle when the throttle was held around 10%. Ported vacuum does pull an inch or two more vacuum but the vacuum advance is already maxed out at this point so it has no effect on the timing. Ported and manifold vacuum will deliver the same amount of vacuum advance any time the throttle is open. The advance only differs between the two when the throttle is closed.

So the increased airspeed past the butterfly causes a small drop in pressure at that point (the venturi effect), which is where the ported take-off is located.

itchyfeet wrote:Thinking about it the amount of vacuum needed for the control system is tiny compared to the amount that an engine can generate so it may create more vacuum one side of a half open throttle than the other but if its past the limit needed the control system sees them as the same.
On a DJ with advance and retard its using the difference between them which may react differently at half throttle

I think that's it. Having the DJ advance port above the throttle, so 'ported vacuum', means that it disappears at idle, whereas the retardation port, which sees manifold vacuum all the time, will stay activated at idle. Correction to my earlier version of this post - the difference between them ought to be the right way round, because ported vacuum is stronger than manifold vacuum because of the above venturi effect. But there are times on that graph where manifold is the same as, or slightly greater than ported vacuum, so I think there must be something else that guarantees that vacuum advance always dominates when both signals are present - maybe the retardation drilling is smaller, or maybe there's some mechanism in the vacuum can that ensures this.

(that's it - I'm done editing this post now )

The graph has no units what are the left right and lower scales do you think.

the article also says

There are two major factors that effect how much advance is required, engine speed and load. You increase advance with rpm and decrease advance with engine load.

so it brings me back to the thing I don't understand ... is revving the engine with no load giving the same timing as with a load