Ignition advance at cruise rpm?

[panic]

A specific cruising speed needs about the same cylinder pressure regardless of static CR.
The amount of vacuum spark depends heavily on how well its controls function, and especially how lean you can make the cruise mixture. 18:1 A:F is possible but needs over 60 degrees total.

[hoodeng]

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.

[rustbucket79]

I just don’t see how you could blanket determine if a combo needs “x” then a half point lower needs “y”. Assuming optimized air fuel ratio, complete combustion is complete combustion. I don’t see a degree or two making a difference in a cruise condition. Maybe I’m out to lunch.

I am not looking for a simple universally applicable formula that gives you a final answer. I’m looking for a rule of thumb that I can use to get it into the ballpark. I guess I am just indirectly asking the question how does the combustion speed change with compression ratio in part throttle conditions. For simplicity, let’s say the ingested charge mass remains the same: how much faster will the higher compression ratio engine burn the charge?

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.

[Roadknee]

I tuned a fuel injected SB 400 in an early nova several years ago. The owner wanted to maximize fuel economy. We were on a level straight highway running 2000 rpm at 70 mph. I adjusted ignition advance from 35 to 50 degrees in 2 degree increments. There was no noticeable change in drivability, injector pulse width or duty cycle. In comparison, changing the target AFR 0.25 made a noticible change. This engine had 10:1 compression and Edelbrock 170 vortex style heads.

[exhausted]

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.

[panic]

As the cam passes (roughly) 240° @ .050" ported vacuum becomes more useful: the idle is more stable.

[Geoff2]

Exhausted, you got it right. Panic, you got it wrong.

Stable idle is....stable idle. If vac adv is connected to manifold vacuum [ MVA ] , as long as the VA unit fully deploys at idle, then the idle timing will be stable. This is where MVA gets a bad rap because people use a fixed VA unit with, say 12"of vac reqd for full deployment, with a cam that has only 10" of vac at idle. The problem is the idiot doing the job, not MVA.....

At cruise, there is no difference in operation between PVA & MVA, but you don't get the idle benefits with PVA that you do with MVA.

[RDY4WAR]

I always get a good laugh when people insist on running mechanical only on an 8:1 compression 454 daily driven and wonder why its gutless and constantly wants to overheat.

Changed that for a friend recently who had a 351W rebuilt for his Bronco. Mechanical only distributer with no vacuum provision and had just 25-28 degrees advance at cruise rpm. He had a big 3-core radiator, overdriven high volume water pump, 160 stat, and straight water trying to keep it cool and it would still creep to 205°F on the highway. His engine builder told him it was due to his engine being a .060" overbore and there was nothing he could do about it. I took that distributor out and put in an OEM one I had in my shop with a vacuum canister and put it on ported advance. Viola! The temp never went over 170°F on the highway. The spark advance was ~45 degrees at cruise.

[panic]

Panic, you got it wrong

I was about to say the same thing.
I'd explain, but your knowledge of the subject is a bit slow.

[Firedome8]

20201025_190521.jpg

20201025_190530.jpg

[ptuomov]

Thanks everyone.

So the car is very well tuned for high load and high rpms. It drives like a dream (or a terrifying nightmare if you think about it, really, but entirely as intended) at high rpms and high loads.

At cruise, I am trying to bring down the underhood temps and quiet it down a tiny bit. Based on this thread and some additional research, I added on average 10 degrees more timing in the cruise loads and rpms. To compensate, I made the large load or rpm change retard close to stock, it temporarily pulls 5 degrees of timing anytime something unexpected happens and then quickly adds that back.

Psychological butt-dyno perception effects are that it seems quieter and when in traffic I don't see as much hot air coming from under the hood. The engine temperature is well controlled by the thermostat so it's really mostly the exhaust manifold temps and underhood temps that I am concerned about. No idea about fuel consumption, I could compare the cruise ignition pulse widths I guess.

[BLSTIC]

I was going to suggest pulling the spark maps from various EJ25 engines that people have modified and posted online. There's a wide range of similar engines running different compression ratios because turbo, because vct, etc.

[ptuomov]

I was going to suggest pulling the spark maps from various EJ25 engines that people have modified and posted online. There's a wide range of similar engines running different compression ratios because turbo, because vct, etc.

The tune is perfect under boost, it was run to death (of the dyno) under boost. It’s only under vacuum and cruise that I believe I can improve on the maps.

I basically went to the normally aspirated higher compression ratio engine maps, adjusted the scaling to make it apples to apples, and added a couple degrees of ignition advance across the board. This ended up with about 5-15 degrees more timing that my engine used to have when we only thought about high load operation.

[Dan Timberlake]

Hi Calvin,

I'd understand if, in the emission era, ported vacuum was the norm for the hot exhaust reasons you mentioned.

The attached images are copies of technical descriptions of vacuum advance automatic spark control from two sources predating the emissions era by years or even decades.
1959 Edsel factory shop manual, and an Audels manual so old their reference to "engine side" and "carburetor side" of the butterfly coincide with updraft carburetors.

Note that Audels' comment is that "the most popular ............ vacuum connection is made at the carburetor side of the butterfly valve." Figure 6.
Ported vacuum.

Probably safe to say the choice was not driven by emissions in either case.

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.

I tried to reach him over on Corvetteforum.com in 2017 to discuss that article.
https://www.corvetteforum.com/forums/c1 ... 1595201905

John "Z" was in pretty poor health a few years back, but posted fairly recently (Feb 2020) over here -
https://gmauthority.com/blog/2020/02/ch ... nt-page-2/


Regards,

Dan T

[MadBill]

I was going to suggest pulling the spark maps from various EJ25 engines that people have modified and posted online. There's a wide range of similar engines running different compression ratios because turbo, because vct, etc.

The tune is perfect under boost, it was run to death (of the dyno) under boost. It’s only under vacuum and cruise that I believe I can improve on the maps.

I basically went to the normally aspirated higher compression ratio engine maps, adjusted the scaling to make it apples to apples, and added a couple degrees of ignition advance across the board. This ended up with about 5-15 degrees more timing that my engine used to have when we only thought about high load operation.

I can't recall if the chassis dyno you were using was inertial or brake, but if the latter (or you locate one that is), you could set it for various cruise range revs and dial in minimum spark advance for minimum pulse width in the applicable cells.

In a former life doing emissions (re)development to Canadian standards in the late seventies/early eighties, one of the keys to the typical 10% or more reduction that I achieved in fuel consumption vs. the matching US federal calibration (+17% for one model) was a 30° vacuum can. Another was cruise AFR of ~16.5:1 .