How is ignition timing controlled?
 

I am wondering how a car's ECU controls ignition timing, in a general sense. I am well aware of the fact that the ECU advances and retards timing based on load, RPM, speed, intake air temp, coolant temp, throttle position, etc. But what I am wondering is if the ECU tries to advance the ignition timing as much as possible without the knock sensor detecting knock, or if the ECU just sets the timing to what it is programmed to and retards timing if it detects knock.

For example, if the car is cruising under a steady load and all other conditions stable, will the ECU advance the timing until it detects knock and then retard it slightly so the timing advance is always as much is possible, or does it just set the timing to what it is programmed to run at for the given conditions and leave it alone unless it is detecting knock? Thanks in advance!

It is more or less pre programmed.

Tuners can change it.

Thanks. So it normally just runs at what it is pre programmed to and only retards timing if the knock sensor detects knock for some reason? Also, presuming the recommended fuel octane is used and everything is working correctly, would the knock sensor ever detect knock under normal conditions?

In older fuel injected vehicles the knock sensor was used as a kind of a fail safe to keep them running smooth, only triggered by abnormal operations causing knock.
In newer engines it seems that the knock sensor is used as a real time instrument to provide vital computer input. It allows engines to get the best fuel economy running near stochometric air fuel mix buy running up to 50 and even 60 degrees of advance.
Modern engine control systems will keep adding timing advance while cruising down the road until a little knock is detected then the computer pulls timing back.

Also, some of the older electronic fuel injection cars, such as a Civic DX, CX, OR LX from the 1990s, did not even have knocks sensors. Timing on suchvehicles must be entirely by pre-programing, I would guess.

Great information, thank you! What would the knock sensor be used for on an 05 Civic? Would that be considered "modern"? Is it just a failsafe in case for some reason it knocks, or would it actually be used for realtime data so the ECU can optimize the timing?

To ask this another way, could simply using a higher octane fuel than what is recommended result in more ignition timing advance, or would nothing really change since there wouldn't be knock under normal conditions anyways? Thanks in advance

You have confirmed that your 05 Civic has a knock sensor? An 05 is the last year of the d-series engine. Not all d-series engines had knock sensors. You will want to check with a good source, or better find it on your block.

Yes, I know for a fact that my 05 has a knock sensor. I have seen it on the back of the block when I had the engine out to rebuild. Good point though!

If the engine uses mass air flow to regulate the air and fuel going into the engine it's more likely to actively use the knock sensor.
I think the easiest way to tell would be to use a live obd2 monitor rig get an idea of what the timing looks like. If it only ever goes to around 40 degrees or so it probably runs a fixed timing table. If it goes over 50 it's probably using the knock sensor.
Then to confirm put a few gallons of premium gas from empty in it and it should run even more advance.

Thanks. Mine doesn't have a MAF sensor, and the timing advance doesn't seem to go much past 30 degrees. Using premium fuel doesn't seem to change the timing advance significantly, but it moves around so much that it's hard to say for sure. For some weird reason the timing always runs at around -8 degrees at idle, not sure why. But it has always done that, both with the stock ECU and with the JDM Honda Stream ECU that I am using now, so I guess it's normal.

Nowadays, besides MAP, MAF, TPS, and eventually knock sensors, the Lambda probe also has an important role for the control of ignition timing. Times were much simpler when vacuum was the most relevant parameter for ignition timing...

Magic!

I can confirm the Honda K series (at least the A and Z engines) use fixed ignition timing. They use MAP rather than MAF, and ignition happens based on a set of tables.

Something I haven't yet figured out is how far ignition timing should be advanced at part throttle, and it's quite hard to test for even with a load dyno. More advance is not always better. Ignition advance refers to how many degrees *before* the piston reaches TDC it fires, meaning the piston is still moving toward the top of the compression stroke. Pressure is building and during that time it's actually doing negative work, trying to spin the engine backwards.

On most engines you can't over-advance ignition timing at WOT on pump gas. Before you have advanced timing enough to get peak pressure at the most efficient angle (which I've read is typically 14-16 degrees but may vary with flame speed), pressures get high enough early enough that the remainder of the charge in the cylinder ignites from compression ( = knock) while the piston is still rising. However at part throttle cylinder pressures are lower, and with lower compression engines (especially turbos) or those with great combustion chamber designs, it's almost certainly possible to over-advance ignition timing without running into knock.

https://i.imgur.com/JblsO8o.png

That is very helpful, thank you for the information! I presume that the D series engines uses fixed timing tables too then. Another question- Since the timing tables are fixed, there would be no benefit whatsoever to using a higher octane fuel than recommended since the ECU won’t advance the timing anymore anyways. Is that correct? Also, under normal conditions, would there ever be any knock for the sensor to pick up to retard the timing?

Also, does the IAT affect the ignition timing? If it does, I presume maybe I could get a little more power by installing a resistor in the IAT wiring harness so the ECU thinks the air is colder than it is and advances the timing more if I use high octane fuel?

No reason for higher octane fuel under normal conditions, no, since the ECU will only ever subtract (retard) timing, and never advance it.

Knock sensors are good to have especially on higher performance engines because they allow the manufacturer or tuner to tune advance at higher throttle much closer to the point of knock, since 1) you can't typically advance ignition enough on pump gas to get maximum economy and power at WOT, 2) under these conditions, knock can be very damaging to the engine, and 3) engine knock can vary based on conditions.

Just as an example, the TSX motor in my car calls for 91 octane. It has tables for ignition advance at 6 different cam angles which it interpolates, as well as tables which add or subtract timing from those other tables based on coolant temperature, intake air temperature, and other factors. But, they can't make a table for everything. What if the intake air sensor goes bad? Or, I get a bad batch of fuel? A knock sensor both allows them to be less conservative in their ignition tables and provides protection in case anything goes wrong.


It does, and what you suggest might work, but chances are good that part throttle ignition timing is already very close to perfect and more advance is not necessarily a good thing. Cooler air is more dense, and a higher density charge burns more quickly, so at part throttle it may actually pull timing for cold air, and it might add or subtract at WOT (or have tables for both) depending on whether the faster burn or higher cylinder pressures are more relevant.

I'll take a look at the factors I can modify on my Hondata ECU and get back to you.

Thank you for your help! That is a lot more complicated than I thought. I knew ignition timing was complicated, but I had no idea it is this complicated. If the ECU could be tuned, what kind of power/MPG gains could I possibly get by tuning it to run on 93 octane fuel? Maybe nothing if the timing is already ideal from the factory?

Also, since my ECU is from a JDM Honda Stream, is the timing likely to be any different than the stock US ECU because of Japan’s supposedly higher octane fuel?

Part throttle, probably little to nothing. For 3/4 throttle or greater you might see some decent increases in both horsepower and efficiency; often timing in these areas is compromised for knock resistance.

Came across a great writeup on this topic as pertains to Honda ECUs:

NOTE: This is not taking other compensation factors into account, such as temperature. Graphs that I've found I can adjust that affect ignition timing are:

-Idle ignition control (idle is controlled by changing ignition timing in many cars)
-Air temperature idle to light engine load retard
-Air temperature medium to high engine load retard
-Low water temperature idle and light engine load advance
-Low water temperature medium to high engine load advance
-High water temperature idle and light engine load retard
-Low water temperature medium to high engine load retard
-Ignition dwell angle (based on RPM)
-Battery voltage compensation

The main tables are typically found experimentally from a load dyno, and will depend mostly on cylinder pressure (which there's no way to measure directly), RPM and AFR. Higher cylinder pressure requires less advanced timing since the burn is quicker, and higher RPM requiring more advanced timing. Because cylinder pressure can't be measured, there are additional tables for air fuel ratio and cam angles, with X and Y axes typically RPM and manifold pressure.


Whew.

~

My takeaway? You can probably advance timing a couple of degrees and get slightly better fuel economy (and power). But how to do this without a programmable ECU?

Looking at the tables, by default my ECU is not advancing or retarding timing at light engine load for air temperature, so spoofing that won't help during typical cruising conditions, and to make matters worse it DOES compensate by adding more fuel. It also only advances ignition timing when water temperature is below (in my case) 9 degrees celcius, and I don't think you want the ECU thinking the engine is always running cold. If you spoofed the battery voltage somehow it would also affect tons of other things, such as the O2 sensor and fuel injectors.

Typically, if you add some reactive component to the timing sensor circuit, you could shift the signal to the ECU. Probably about 20 mfd across ground and signal wires. Might round the leading edge a bit too much, I dunno. Could be inductive instead, I am not fully functional this am. Net result is the signal arrived at the ECU before required so the timing advances by default. Haven't seen a obd2 car where you can still move the sensor. Sometimes the sensor is on the crank, so making a new bracket works.

Once a former neighbor of my grandparents from mother's side got a 5th-gen Civic VTi and attempted to convert it to ethanol believing it would respond better to forced induction. When he told me about the electronics of that engine I inquired him if it wasn't actually rendering his car with flexfuel ability, and it actually kept running better on gasoline...