Ignition advance at cruise rpm?

[Mattax]

I think the likely perpetrator of the "emissions only" explanation was John Z (John Hinckley ) who used to post on various Corvette forums.
http://www.camaros.org/pdf/timing101.pdf
He was a manufacturing/process Engineer at GM and later at Chrysler.
His mostly good "TIMING AND VACUUM ADVANCE 101" unfortunately stated flat out -
" Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. "
I also think John may have conflated lean mixtures and low density intake charge, but perhaps to simplify the discussion.
Regards,

Dan T

Dan,
Thank you for posting that. Those are great examples of pre-emissions use of ported vacuum. From my own experience in automotive manufacturingI long suspected the author was probably not involved in engine design and performance even if he was an engineer.
A classic example of a well meaning person beleiving his knowledge and therefore understanding being greater than it actually is.

Shrinker and a couple of others set me straight on this emissions control myth, and I don't understand why some have so much trouble recognizing the myth is based on a false premise.
Here's is the archived thread with a few more.

As I looked through factory lit, I found my own examples, such as this from Chrysler's 1959 Master Technician's Conference.

And then, recognizing the original 3310 carbs have timed vacuum ports, found examples of 49 state pre-1968 Chevy high performance engines.

1965-L78-3324-ps.jpg

Its really not a matter of good or bad. Generally its a better design to use base for initial, mechanical advance to account for timing needs that change with rpm, and vacuum for addressing timing changes that occur with load. Idle was not traditionaly a 'lean' condition until emissions concerns tried to reduce both CO and HC. Whether to use vacuum advance at idle or not comes down to what can be done to shape the mechanical advance, how stable the vacuum is at idle, and how slow the burn is at idle. A lot guys don't want to get into shaping the advance on an emssions era distributor (esp 1968 to roughly late 70s), and sometimes an easy work around is to switch to manifold vac.

I'll load up an example of non-smog vs. smogged timing specs so folks can see how switching to manifold vac can work.

1967-426-Timing.png

CAP = Clean Air Package (in 1967 only used for California cars)
The other change that will likely have to be made is to richen the idle mixture from the CAP settings.

[Tartilla]

Higher comp [within reason] will give greater power/efficiency for a given capacity at full throttle.
At part throttle higher comp is not really playing a part here as we are not filling the cylinder with fuel air mixture, burn efficiency is at play here, chamber design, an active charge and fuel/ignition control will impact more at this point.

Cheers.

Higher Compression for part throttle is a benefit for sure. Better and more complete combustion, less timing required, more pressure and less throttle/fuel required, than with a lower compression.

Running lean cruise with an optimal FA mixture (fuel droplet size etc) and better combustion means less heat into the ex valves + wasted energy.

More timing at 60° area will mean lower cyl pressure to get the spark kernel going and flame front moving. Anything that let's you reduce timing will reduce BSFC and BSAC.

In my.view, part throttle operation doesn't get the attention it deserves.

[Tartilla]

I have always wondered why so many people continue to run their vacumn advance off of Port vacumn instead of manifold. And even fewer know that it is possible to adjust how much advance they can have in their vacumn advance units...etc. Engines always like as much advance as you can deliver whether at WOT or Part throttle or idle. Wake up. Manufacturers made and utilized them for a reason. They resorted to port vacumn to help with emissions as late timing at idle and high vacumn times kept things burning on the exhaust side which helped and kept catalytics hotter etc.

Great point on how things change, often people forget why years later.

Cats have been prioritized in many factors.

[RW TECH]

Here's a very common timing curve from a mid 60s GM 350 motor with 10.25 comp and 180 psi of cranking compression.

The picture in Mag2555's post is a good reference.

Unless it's a boat or a loaded truck, the engine will only have about 30-40 ft lb load against it at low throttle cruise on the freeway, so it'll want A LOT of timing in that operating area. And, at such a low load it's unlikely that it'll ever knock.

[RW TECH]

In my.view, part throttle operation doesn't get the attention it deserves.

Right, exactly. A street car with distributor ignition should NEVER run with no vacuum advance, locked centrifugal advance, etc.

All the time I see EFI cal files created by "tuners" that don't have nearly enough spark at low load, sometimes they have same spark at low load as they have at high/full load regions in the map.

Tuning at WOT is the easy part that pretty much anyone can do. It's a LOT more difficult to get a good cold start because you only get 1 of those over a 8-12 hour period, and it's also harder to get cruising load areas dialed in because it requires quite a bit of driving at various loads to get it right.

When I say right, I mean really right, not just "It doesn't stall after it fires up".

[Walter R. Malik]

Higher comp [within reason] will give greater power/efficiency for a given capacity at full throttle.
At part throttle higher comp is not really playing a part here as we are not filling the cylinder with fuel air mixture, burn efficiency is at play here, chamber design, an active charge and fuel/ignition control will impact more at this point.

Cheers.

Higher Compression for part throttle is a benefit for sure. Better and more complete combustion, less timing required, more pressure and less throttle/fuel required, than with a lower compression.

Running lean cruise with an optimal FA mixture (fuel droplet size etc) and better combustion means less heat into the ex valves + wasted energy.

More timing at 60° area will mean lower cyl pressure to get the spark kernel going and flame front moving. Anything that let's you reduce timing will reduce BSFC and BSAC.

In my.view, part throttle operation doesn't get the attention it deserves.

Better ? Only until detonation rears its ugly head.

[hoffman900]

Higher comp [within reason] will give greater power/efficiency for a given capacity at full throttle.
At part throttle higher comp is not really playing a part here as we are not filling the cylinder with fuel air mixture, burn efficiency is at play here, chamber design, an active charge and fuel/ignition control will impact more at this point.

Cheers.

Higher Compression for part throttle is a benefit for sure. Better and more complete combustion, less timing required, more pressure and less throttle/fuel required, than with a lower compression.

Running lean cruise with an optimal FA mixture (fuel droplet size etc) and better combustion means less heat into the ex valves + wasted energy.

More timing at 60° area will mean lower cyl pressure to get the spark kernel going and flame front moving. Anything that let's you reduce timing will reduce BSFC and BSAC.

In my.view, part throttle operation doesn't get the attention it deserves.

Better ? Only until detonation rears its ugly head.

I know a combustion engineer at an OEM. They do all their mapping with in-situ pressure sensors at all loads / throttle openings, and in many different conditions. Going by cylinder pressure takes a lot of the guess work out of it, and opens up the ability to tune individual cylinders, with some needing more or less of a something depending on load, rpm, throttle opening, etc. Nothing is fixed.

Look at all the effort OEMs do to have more geometric compression for a given octane / quality fuel. If you told someone that your regular commuter car would have 12:1 compression on junk octane / quality fuel, and be beat to crap by the average driver, for 100s of thousands of miles, no one would believe you.

[Tom68]

I have always wondered why so many people continue to run their vacumn advance off of Port vacumn instead of manifold. And even fewer know that it is possible to adjust how much advance they can have in their vacumn advance units...etc. Engines always like as much advance as you can deliver whether at WOT or Part throttle or idle. Wake up. Manufacturers made and utilized them for a reason. They resorted to port vacumn to help with emissions as late timing at idle and high vacumn times kept things burning on the exhaust side which helped and kept catalytics hotter etc.

Great point on how things change, often people forget why years later.

Cats have been prioritized in many factors.

Poor Man's start retard.

[hoodeng]

At part throttle we are not filling the cylinder. We put in enough fuel air mix to equal the power reqd to maintain a steady speed/load.

Burn efficiency is what matters here not the mechanical compression ratio. Even then, cam timing will determine what max cranking comp can be achieved, this test is done at full throttle.

If you did a comp test and got 200psi at full throttle you will not get anywhere close to that with the same test and a part throttle opening,

To initiate economic combustion at part throttle these days is a lot easier with closed loop ECU's ,knock sensors, O2 sensors, etc. In a points ignition the vacuum advance was as good as it gets to optimize part throttle. Hence the increased amount of advance at part throttle,to initiate burn earlier as the fuel particles are further apart.
A good dyno tuner can sort part throttle tune easily.

The only engine that has a cylinder full of air at all running conditions is a diesel. The only thing varied is the fuel delivery.

I understood that the rule was to match Compression, Capacity, Cam, and Capability [air flow].

[Dan Timberlake]

Back in the crappy driveability carbureted 1970s Exhaust Gas recirculation was a powerful NOx reduction tool.

When EGR was happening, a bunch of ignition advance ( at part throttle) was added to get the forest filled with dead trees to ignite in time to push the pistons down in a reasonably useful fashion.
Later in the car's life the EGR valve might malfunction, calling for extra advance, but not R-ing and EG.
The part throttle detonation was impressive, and could be "cured" by flooring it.

[Dan Timberlake]

1990 Smokey passenger van advice

[hoodeng]

Dan, the reason the dets were overcome with a full throttle application was because the vacuum advance was dropped out of the total advance curve, and it went back to mechanical advance only.

[Tuner]

The engines were not initially calibrated to knock, but engine vibration, heat cycles, constant vacuum fluctuation, etc. caused the adjustable Ford and Mopar vacuum cans to back off to the most relaxed spring tension and this shifted the range from start at 6" or 8", total at 14" or 18", to start at 2", total at 6" or 8", so the near full vacuum advance was all in at heavy load.

Combine that with carb calibrations that were near stoichiometric or leaner, some much leaner, and the engines would detonate continuously at part throttle.

Some of the vacuum cans not only dropped the vacuum range down into the heavy load range, they also increased the range of travel. Some would increase from 15-18 degrees to 25 or more.

Many aftermarket replacement vacuum cans were commonly worthless because they had too soft a spring and could not be adjusted properly, so someone who didn't know any better would replace one with a ruptured diaphragm that didn't work at all with one that waaaay over-advanced the timing.

The adjustable vacuum advance cans available today still have this same problem, they will adjust themselves and always add more timing when they do.

In those days (mid '70s) I frequently traveled a stretch of freeway that was a couple miles long incline where I routinely saw soccer moms in tuna boat station wagons with 360 Mopar and 400 Ford engines rattling along merrily with detonation at 8" (or so) with 50 or 60 degrees where there should have been only 25 or 30.

Disregarding some of the silly calibrations with too much or too little advance they used for emissions before catalytic converters, the most important thing about spark advance for performance and fuel economy is MBT, "Minimum timing for Best Torque", the least advance that provides best torque for the condition of load and RPM.

[hoodeng]

Tuner, when i did auto elec after my trade one of the machines that proved more wrong than right was the 'Sun Distributor Tester'. And yes the amount of dashpots that were nowhere near what they were supposed to be was more often than not. And as you pointed out any wear and aging seemed to always give more advance than specified than less.

When i started auto elec Chrysler Lean burn was pretty much established, the trade school i went to had a set up for testing these ignitions on the Sun machine. First thing noticed was virtually no spark scatter.

[Mattax]

Perhaps the answers to the question you are looking for are in service manuals and web searches. In terms of ancient engines, there has to be a reference to both initial, mechanical advance, and vacuum canister advance on say the 325, 350, 375 HP versions of the 396 Chev and equivalent in the small block. Small block reference would be good as there were so many different compression ratios through the years to analyze. Look also for timing tables for the various LS engines for the same reason.

In the hunt for fuel mileage, nobody spends more resources than the manufacturers.

Although the OP has presumably long ago ended his efforts, your suggestion is well worth the effort for anyone interested.
Either Sun cards or the Service manuals almost always have the timing info for all the flavors being offered each year.
I have done this mostly with pre-cat, pre-EGR mopar passenger car engines and even that limited set of data is pretty interesting.
Now a Chrysler timing isn't going to be the same as a similar size Chevy etc because the dwell time of the piston at the top is different, the chamber is a bit different etc. But in general, for light throttle, the vacuum advance brings the total to around 50* to 52* at highway rpms for the LA small block, a little mor efor the B/RB. What we see is for early emissions, initial timing is reduced, but the mechanical advance was altered so the off idle timing is pretty close to the pre-emissions curve.

Re:EGR I'd have to look if there are some years with both EGR (Calif) and non-EGR (Fed) to see if they really made any adjustments to the timing. My experience with pinging part-throttle EGR equiped engines led me to conclude the problem was in part carbon-build up due ot poor maintance and 'repairs'. EGR can be hot so that ought to assist in the burn rate, even though the left over HC and depleted O2 should slow the burn rate. A problem with out modern US fuels, especially the E10, is the distillation curve.