At any constant level of RPM’s is the ignition advance angle greater or less as engine load is applied?
For example, my 305 TPI likes 26 degrees advance at 2000 RPM with no load. If load is applied at the same 2000 RPM’s, is the advance angle more or less than 26 degrees?
The base maps for my new ECU indicate that the advance angle is reduced under load, is this the right way to see it?

 

To avoid potentially damaging preignition, spark timing should be retarded as load increases.

More broadly, the point of power cycle initiation in any piston engine should vary in regard to speed and load, whether that engine uses gasoline, propane, compressed air, kerosene, steam, or other medium for power production.

 

Generally so, yes.

When the engine is under light load, let's say cruising down the highway, the cylinders are only partly filled. THIMK: if vacuum is, say, 15", then that's half of atmospheric pressure; the cylinders are only getting half as much air as they would if load was high (throttle is wide open) and vacuum is zero. PLUS, the mixture at such times is VERY lean, for fuel economy; no sense burning any more fuel than necessary if we're not trying to produce lots of power, right? Such conditions lead to a VERY slow burn in the combustion chamber. The slower the burn, the earlier it needs to be started. In a mechanical distributor this is called "vacuum advance"; it amounts to 12 - 15° over and above ALL OTHER "sources" of advance. The ultimate ignition timing at cruise can often be greater than 50° depending on the engine and power demands of vehicle at cruise (weight, aero, tire drag, etc.).

RPM-based "advance" is easier to understand if you forget about how a distributor operates, and instead, look at it from the point of view of what's going on inside the engine. So: consider an engine that constantly runs at high speed and full load; let's say, a pump or a generator or a lawn mower. Such an engine doesn't really need ANY "advance" since its operating conditions never change. We can just set the ignition timing to where the engine runs the best: some combination of best power, highest efficiency, longest life, coolest running, etc. Turns out, this setting is ALMOST ENTIRELY dependent on the FUEL being used; engine design (combustion chamber shape for example) affect it slightly, but FAR LESS than the FUEL does. This is dictated by how fast the particular fuel burns. Turns out that for typical pump gasoline, the spark needs to occur somewhere around 35° BTDC, at max load and full speed. Butt now consider what happens if we let the engine slow down. Well, we end up with a time-domain component as well as the degree-dependent one; that is, it takes no more than x milliseconds for the fuel to burn, no matter what. If the engine RPM is much lower than full-speed & full-load, the # of degrees that make up x milliseconds is MUCH LESS than the number of degrees that the engine wants at full speed and full load. Butt, we don't want the fuel to ever burn COMPLETELY, and then the piston to still be ascending and trying to compress these already high-pressure gases. IOW, when the RPM is lower, we need to RETARD the timing, so that the combustion still finishes right near when the piston reaches TDC but is moving much slower. Butt we can't set up a mech dist this way, therefore we talk about it in terms of "static" timing (low speed, fully retarded), "total" timing (full load, full speed), and ... there isn't a word, but the sum of "total" plus the "vacuum" (low load, high speed) related component.

Expect "static" timing to end up at around 12 - 15°, "total" to add around 20 - 25° more, and "vacuum" 12 - 15 on top of that; resulting from the physical setting of the dist body and the table. At idle that gives you 12ish of "static" plus 12ish of "vacuum". The RPM-based component should start up just off idle, maybe 1200 RPM or so, and should be "all in" by around 2800 or thereabouts.

 

Not sure what you mean by "my 305 TPI likes 26 degrees advance at 2000 RPM with no load"...

With no load (low MAP values), I'm running in the high 30's at no load. Some factory calibrations are in the low 40's.

Unless you're running very high dynamic compression ratios (i.e. over 10:1 static with a small cam) and on low octane pump gas, 26 is too low for low-load conditions.

 

Hi guys
great feedback.

The only ignition spec that I could find for my engine (LB9 305ci TPI build date 1985, modified with a 350ci combustion chamber - which I think is 58cc compared to the OEM 64cc) was 7* at crank and 34* max advance at 3200 RPM . Therefore I’m surprised that ULTM8Z is running a lot higher, which may be where it needs to be.???

Can anyone supply their ignition map for the 5 litre engine so I can compare this to what I am running, or provide more detailed GM specs for the LB9

 

That's just an elevated idle, its meaningless. Engine will run on almost any timing in that condition. Ignore that and focus more on driving conditions.

Get it to idle good first though, that's where the bedrock of the spark timing curve starts, and it grows from there. That's just the way distributors work, you can't get around that.

 

With the HEI handing back ignition to the ECU above crank RPM, it’s ultimately the ECU mapping that will control ignition. This is completely different from mechanical advancing off a good idle.

 

How much freedom do you have? Do you literally have a spark table tunable in software?

If so then tune spark for the numerically lowest MAP value during part throttle driving conditions. That's where engine is making most torque. ​​​​​It's very time consuming to do on the street.

Just keep timing similar at no throttle regions so your timing doesn't jump around when transitioning between low and no throttle. If the timing jumps around then the car is going to drive jerky.

Don't shoot for lowest MAP value at idle though. The engine needs some torque reserve so the ECM can control idle. If you maximize torque at idle then the ECM has no way to add more torque to control idle. This is why timing at idle is different than even just off idle, because the goal is different. You purposefully keep timing soft at idle so there is a torque reserve, but anything off idle you want to make torque!

 

Haltech ECU uses both RPM and engine load to control advance angle off base TDC timing. Coolant and timing correction maps take care of the early stages on cranking and post start.

If I don’t get any further replies, I am going to load in a base map that is 10* crank, 25* idle and 38* max from 3500RPM, using lineal curve and reducing slightly with engine load to account for increased (richer) air fuel ratio.

Still happy to see any comparable ECU ignition maps if anyone has one, or a GM engine spec for the LB9

 

Now wait a minute, you're not being forthright with what you have. Who puts a Haltech on a 305? The ECM is 10x more expensive than the engine. What else is unexpectedly different about this engine?
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Here's the 305 TPI spark timing map. Note that you have the main map plus the power enrich timing.

 

The engine has valve gear/combustion chamber from a 350, which fits exactly on the 305. The smaller combustion chamber and larger valves are a very easy upgrade that lifts performance.

 

The base ignition map has much higher advancing than I expected, and I think from other posts as well that I should lift the advance curve up to similar numbers.

I am not familiar with power enrichment but I assume this is a correction map setting that increases fuel/air ratio under certain conditions that may be signalled from either manifold absolute pressure (load)/throttle position sensor/RPM or all of the above. A max of plus 6* seems high though and I would have thought adding this would bring it close to predetonatation and engine damage?

 

Thanks for posting this! Can I ask where you got it from? I am new to TPI tuning and havent been able to load any ignition map that makes sense to me in tunerpro etc. I have been fighting what I think are bad ECM issues, bought the car a few years ago, its a 92 z28 convertible and recently started starting very rough, pouring black smoke, high idle, will come and go. Haven't been able to track it down, replaced the remanned ECM with another and now on the third, each seems to have things wrong with them. Got frustrated and put a MaxxECU on it wiring into the ecm connectors using header connectors I stole from the original ECM that died and wouldn't do anything.

My dumb question, I assume the spark advance table is above the static timing of 6 degrees or whatever it is.

You Tube

Wiring job not mean to be pretty or permanent just seeing how things works with factory ECM removed (much better LOL).

 

The Initial or Base 6° is accounted for by the ECM. The ECM thus commands 6° less timing to reach the values in that chart.

 

thx so much!