Understanding Ford PCM ignition correction logic

Posted by Timothy Pownall on

What are ignitions corrections and why do people tell me anything over -2.00 is bad?

The Cobb Ford ignition correction logic (cobb defined parameter) is an offset to the final ignition timing curve (base borderline timing + temperature compensations + KOM/OAR BL compensations + KOM/OAR closed loop compensations).   The ignition correction logic is a real time function of keeping track of spark advance and spark retard offset of the final ignition timing.  The max allowed spark advance is configurable in the sense that you can limit how far the car can advance timing at a given load and rpm (see below).

 

The above table is from a factory Focus ST calibration which is vastly different than what most are used to as the cobb OTS maps allow for more advance than factory.  The factory logic uses octane adjust ratio (KOM on newer vehicles) to get the base timing as close to the knock threshold as possible and then relies on knock based spark advance to add additional timing at wide open throttle as long as there is no knock.

I see often that ecoboost owners say that they never have knock and their reason is because they only ever have positive ignition corrections.  Well I'm here to tell you that's wrong, not only is it incorrect, but unfortunately lots of common knowledge that's shared about this platform is incorrect, however widely accepted as that is what people are told by their calibrators/friends.  The lack of deep understanding of these ecu's is partly due to the fact that most pro tuners will start from a cobb ots vs. stock.  The ignition correction logic (max allowed spark advance), spark retard, and advance rates/clipping points on OTS are vastly different from OEM in the sense that cobb works through a lot of the torque limitations on OTS maps to make tuning the platform easier as the torque logic, and ignition logic on these cars is fairly complex.

Depending on how the spark advance is configured on your tune the car will advance timing offset until knock that in that specific cylinder is encountered and when this happens the subtraction of the correction will depend on the intensity of the knock and the configured retard rate on the tune.  All of our tunes utilize close to factory knock retard values to limit the chance of the cylinder knocking more than 3 times during a pull which is factory spec for the Focus ST. The ford PCM has two knock rates (Fast/Normal) and by default the knock fast rate is used when accelerating, however after the threshold of knock per cylinder exceeds the below value (factory value: 3 knocks), the knock rate will change to the configured normal so that ignition advance can level out and prevent the cylinder from knocking a fourth time.

 

One needs to be careful with adjusting knock rates/increments as it can result in the engine constantly knocking (see image of our Monster Tuned Stage X 93 vs. other competitor's tune on the same car and same day) 

Above you will see the "other" tune far exceeding the ford engineering decision of 3 knocks per cylinder and rate switch. Over time this can result in engine failure. You will also notice that with all of these knocks the ignition correction is still highly positive. This is due to the fact that the knock advance is too fast and the knock retard table has been modified to pull very little timing on a knock event.   Our tunes utilize the slightly modified factory knock retard values to favor performance and to ensure engine safety at wide open throttle operation.  This can result in seeing negative ignition corrections more often (even with way less knock)  But how can this be?

I'll explain how ignition correction logic works now. Depending on how fast your knock advance is configured, the car will start to climb positive knock advance until it encounters knock in that cylinder, which will trigger the spark retard table.  If the spark retard table has been altered to subtract less timing on a knock event and the knock advance has been configured to advance much faster you will end up in a situation where you have constant and many knocks counts on a single pull as shown above.  Picture this as trying to use a bucket to stop a sinking boat that's taking on water too quick.  The water (knock advance) is coming in faster then the bucket (spark retard) can remove the water.

Below is a knock event with slightly modified factory spark retard values.

Note the knock retard values below:

 You can see that ignition correction was +6 until it encountered a single knock event of lowest intensity resulting in a subtraction of 1.00 degree of ignition leaving us with a +5 ignition correction.  There was a knock event, however the ignition correction remains positive. With near factory retard values as shown above it would take cylinder 4 (7) knock events to show a -1.00 at wide open throttle. This is above the ford engineering values of 3+ knock rate change and should be a cause for concern.
With the factory retard logic rolling from a stop (0 positive ignition correction), a false knock from engine movement can become a -2 or -3 ignition correction very easily.

To sum all of this up we are educating users to monitor knock count in each cylinder as opposed to ignition corrections as you can have 14 knocks in a single cylinder on a pull and still have positive ignition corrections with other tuners.  Monitoring ignition corrections is not a reliable method for determining knock amount.   If you are exceeding 4-5 knock events per cylinder on a single gear pull you should reach out to your tuner to lower the amount of knock events and keep the engine safe.


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