…Then I'll switch over the ignition to the CB Performance Black Box programmable ignition with a TFi distributor and the Crane. The question for me is how to develop the spark timing base map with the Duraspark II, its vacuum advance and the variable retard capabilities of the Crane Hi-6S.
While it's good to do some things in steps or phases; I would avoid tuning the DSII (or any other mechanical/vacuum advance) except as a casual educational exercise, and go straight to programmable timing.* You haven't had fun until you are watching your sweat sizzle on the intake the nth time adjusting the distributor mechanisms during cyclic testing of this type. With programmable, you just test, click, test again from the driver's seat. It's certainly not just convenience and comfort, as you will get better and clearer results, and much quicker — an important consideration when stuck with the asphalt dyno.
That said, you will rapidly progress to an inexpensive controller (ECM) that combines full fuel control with full ignition control, data logging, etc, in order to read, interpret, and adjust the functions together. Using the FiTech for full control is a good way to "get your feet wet", though ease of setup and operation tends to limit the advanced control you will need to feed your addiction. *See related stuff below, as control and analysis can be in one package that suits your
goals better for similar cost to the good CB ignition-only box.
…As you indicate, relying on finding the timing advance triggering the onset of detonation is a key point to know but its independent of the actual MBT timing value for a given load, AFR and RPM. I was also surprised at the number of research papers that focussed on 2 degrees less that knock limited MBT.
Correct, though in your last sentence you may have interpreted the data 2° retard to avoid knock as relating to MBT value. Your statement is correct that it does not, except by coincidence on any given day and tank of fuel. You can have less than knock, or you can have less than MBT, as it would be rare to have knock and MBT coincide perfectly. So yes, knock is a limitation on reaching MBT, and has nothing to do with finding MBT for that engine package and fuel.
…My problem is trying to find a measurement that directly relates to MBT given my limitations to establish variable load at constant rpm.
There are many creative ways to load the engine in order to find MBT. On a dyno is simple, predictable and stable. On the road, it can be creative driving with hills, brakes, or other drag or load; while monitoring things like MAP, RPMdot (rate of RPM change), MPHdot (commonly used by drag racers, but it doesn't have to be at WOT
…The only thing I can think of is maximizing manifold vacuum at a constant rpm and load as I vary AFR and timing.
And MAP is one of the good references, used as I described in phases 2 and 3. As-stated, there are other indicators, and some are easier than others when you have double-duty driving and monitoring. You will eventually experience police pullovers as they just have to know what the hell you're doing, looping the same highway and hill for the 4th or 14th time.
…I have a reasonably accurate manifold sensor capable of 32 bits output. I can link up serial AFR output with analog engine RPM and vacuum signals input to my laptop. I could include the filtered output of the knock sensor as well.
Here we go. We cannot possibly monitor and compare various responses, data, etc, while driving, with any reasonable accuracy or consistency. Very difficult and equally unsafe. The dyno guys can't either, and so plot their data on displays and printouts, often overlaying one result with others for direct comparison. Enter the data logger and analysis software, your personal live-monitoring assistant and record keeper. You can configure your laptop to do this and create/design a hardware board for all of your inputs from scratch
, or you can use a system designed for it, with compatible software also designed for engine tuning.
Without going into it at this point, there are such systems available at low cost that will make a huge difference in the quality and progress of your tuning. Even with your commitment to the FiTech; these systems may be added for this purpose, or even replace the FiTech ECM for full control of FiTech hardware while assisting analysis of the effects on tune, correcting within the same unit, sometimes automatically. That's another step, and I'd suggest relaxing to familiarize with your FiTech's operation, so you're comfortable when you go to have a talk with your engine and play it like a piano.
…The question is, is there any demonstrable relation ship between manifold vacuum and instantaneous torque?
Yes, and only by relationship (it is not a torque meter), but very useful for indications including relationships with other indications and factors. As MAP relates to most factors, it becomes a primary reference. Using data logs and enough coffee, the relative effects of various factors become apparent. That's where tuning really starts, with a recognition of relationships and how they are helping or harming the engine's happiness.
Yes, I am giving expanded responses to you, as it is uncommon to find individuals that not only thirst for the how and why, but are open-minded for new information and forming new (to them) connections of data and theory to help their engines perform best under any mode and conditions. Welcome to the addicts circle. You have my sympathy and support.
[EDIT] Fun PS: I see you are occasionally using the word "vacuum", and you may as well get used to thinking strictly in manifold absolute pressure. Everything from zero (outer space) through idle and cruise to max boost is just another absolute pressure number. At some point, it may become irritating to deal with translating vacuum. This is one reason that tuning superchargers and such is not concerning, as above atmo is just another pressure and the world does not rotate differently.