Ignition Timing Theory - Port Source vs Manifold Source

[Captain Obvious]

Thanks Leon, that's good stuff there. That thesis paper was definitely deeper than I needed to go, but I made it through. Good to know that there are smart people out there still working on trying to eeek out more usable power from the same amount of gasoline. I'm surprised this topic so quickly included calculus.

Two things in that paper that really stood out to me...

The first was that there's actually (what I consider) a pretty wide timing window while still attaining 99.6% of max torque. Seems you can miss the perfect spark timing point by +/- 2 degrees without making that much of a difference in the final outcome.

The second thing that hit me was that his original purpose for the whole paper was an attempt to use the ionization current as an indicator of the perfect spark timing point, and it seems that there was an experimentation error that prevented him from actually making that conclusion.

Maybe I'm misunderstanding the paper (which is quite possible), but my interpretation is "I tried to this, but there was something wrong with my test setup which made my experiment results unusable". Seems unfortunate to spend that much effort on the subject, only to have that let-down conclusion at the end.

[LeonV]

Thanks Leon, that's good stuff there. That thesis paper was definitely deeper than I needed to go, but I made it through. Good to know that there are smart people out there still working on trying to eeek out more usable power from the same amount of gasoline. I'm surprised this topic so quickly included calculus.

But of course, calculus is the foundation of physics!

Two things in that paper that really stood out to me...

The first was that there's actually (what I consider) a pretty wide timing window while still attaining 99.6% of max torque. Seems you can miss the perfect spark timing point by +/- 2 degrees without making that much of a difference in the final outcome.

The second thing that hit me was that his original purpose for the whole paper was an attempt to use the ionization current as an indicator of the perfect spark timing point, and it seems that there was an experimentation error that prevented him from actually making that conclusion.

Maybe I'm misunderstanding the paper (which is quite possible), but my interpretation is "I tried to this, but there was something wrong with my test setup which made my experiment results unusable". Seems unfortunate to spend that much effort on the subject, only to have that let-down conclusion at the end.

Yeah, the main topic of the paper wasn't up to snuff but the reason I brought up the paper was to help explain MBT. I think he did a pretty good job of it.

There is definitely a window of spark advance where torque doesn't change much, this is evident during dyno tuning. However, as you advance more, you bring yourself closer to the knock limit. That's why when dyno tuning a spark curve, you will see max torque over a certain spark advance range, but set it at the lowest advance needed to hit that mark. It's better to be on the approach to MBT timing rather than overshooting.

[Captain Obvious]

Yeah, the main topic of the paper wasn't up to snuff but the reason I brought up the paper was to help explain MBT. I think he did a pretty good job of it.

I understood that, and I think he did a great job of explaining MBT as well. It's just such a shame to spend all that time and effort only to have something like that happen at that end. Grades are grades. I suspect he ran out of time or contracted senioritis else he would have fixed the issue and run the test again.

It's better to be on the approach to MBT timing rather than overshooting.

Yes, and he made mention of that in the paper as well.

Something I forgot to ask you about earlier. I was asking about using a knock sensor and always advancing until right before knock, and you said:

The thing is, the knock limit can stop you before you reach MBT. However, if you reach MBT but have not reached the knock limit, then you can easily over-advance the timing.

I now understand how you can over-advance beyond MBT if you haven't yet reached the knock limit but have reached MBT, but why would a knock limit stop you before MBT? Is that because of false triggering? Seems from that paper that the peak pressure is pretty short lived, and I'm not sure how you could possibly knock before you have even reached MBT.

What am I missing?

[LeonV]

I now understand how you can over-advance beyond MBT if you haven't yet reached the knock limit but have reached MBT, but why would a knock limit stop you before MBT? Is that because of false triggering? Seems from that paper that the peak pressure is pretty short lived, and I'm not sure how you could possibly knock before you have even reached MBT.

What am I missing?

It's simply because the conditions for knock were right. It might be too low octane, too high intake temperature, engine running too hot, etc. There are many factors that affect when knock occurs, and it just may be that it happens before MBT timing. This is more likely to happen around torque-peak, where cylinder pressures are highest (max. volumetric efficiency).

[Captain Obvious]

Gotcha. I was assuming after reading that paper that since the pressure peak to achieve MBT usually ended up around 15 degrees or so after TDC that you couldn't possibly knock if the wave occurred that late.

Thanks again for all the education and the willingness to help the ignition challenged.

So back to my original question, I don't see any significant advantage to changing the Z's original port source to a manifold source. Once you're off idle, the port source is the same as a manifold source anyway, and I might be gaining some idle stability with the port source. I will also have lower idle NOx with the port source.

Sound about right?

[LeonV]

Gotcha. I was assuming after reading that paper that since the pressure peak to achieve MBT usually ended up around 15 degrees or so after TDC that you couldn't possibly knock if the wave occurred that late.

Thanks again for all the education and the willingness to help the ignition challenged.

So back to my original question, I don't see any significant advantage to changing the Z's original port source to a manifold source. Once you're off idle, the port source is the same as a manifold source anyway, and I might be gaining some idle stability with the port source. I will also have lower idle NOx with the port source.

Sound about right?

No problem. Sounds good to me!

[siteunseen]

So my 240's vacuum advance has been plumbed off the balance tube, manifold vacuum, right?  It holds at 17 on the vacuum gauge at idle then goes down to nothing with RPM increase.  The front carb vacuum has been plugged off until I rebuilt my carbs.  It barely has anything until the RPMs raise then shoots to almost 20 on the gauge.  The butterfly opens exposing the hole.

 

That's the way I want it, right?  Port vacuum for the vacuum advance on the distributor? 

[beermanpete]

So my 240's vacuum advance has been plumbed off the balance tube, manifold vacuum, right?  It holds at 17 on the vacuum gauge at idle then goes down to nothing with RPM increase.  The front carb vacuum has been plugged off until I rebuilt my carbs.  It barely has anything until the RPMs raise then shoots to almost 20 on the gauge.  The butterfly opens exposing the hole.

 

That's the way I want it, right?  Port vacuum for the vacuum advance on the distributor? 

 

Ported vacuum for the spark advance is the correct connection on the Z (and most cars).

[Captain Obvious]

The ported vacuum signal is how it was originally designed and I would recommend re-connecting it that way.

 

The bottom line from the discussion above is that there doesn't seem to be any significant advantage to changing the Z's original port source to a manifold source. Once you're off idle, the port source is the same as a manifold source anyway, and you might be gaining some idle stability and have lower idle NOx with the port source.

 

BTW - The ported vacuum readings you got above sound perfect. Highest just off idle and acts pretty much like a manifold source as the throttle plates open more.

 

So why were you using a manifold source anyway? Did you do that or did the car come to you like that?

[siteunseen]

It was done that way by the PO.  I'm guessing the vacuum advance will work properly now?  That float jet should be here Friday, and if it's not raining Saturday I'm going to take it for that "spirited drive" Blue talks about when adjusting the nozzle height in his Quick and Dirty tune.  I've been chasing that drive since I bought it. 

[240260280z]

My understanding is that at cruise in 5th on a highway, the throttle valve is not opened greatly so there is a very high venturi effect near this partially opened throttle valve. This is when max vacuum advance is in play.

Moving towards WOT causes the venturi effect to drop off a bit until the rpms are high enough to match the localized flow in the cruise area.

[siteunseen]

So advancing the timing through vacuum advance is for cruising speeds, not romp on it speed?  That's why everybody sets theirs to max advance to begin with?