Active Knock sensor

[Fat Spider]

Does anyone have any info regarding whether the 86 runs an active knock sensor eg. the knock sensor has total control over the amount of ignition being used by the engine ????

[Kodename47]

Yes, same as any other Subaru

[Fat Spider]

Thanks for the input but I'm looking for a link or similar with reference to manufacturer explanation of knock sensor workings.

[Calum]

Paging Dr. @arghx7.

[Kodename47]

Here you go:
Nice and easy explanation.

[Fat Spider]

Thanks for the link, I'm sure I will need to read it through a couple of times to get a full understanding but in the brief read I have had it appears that there is a base map and the knock sensor is instrumental only in pulling advance out of that map and that the ECU never allows advance above the base setting even if there is no knock detected.

[Kodename47]

Opposite way. Base map and full advance map. You start at base values and then increase in increments towards the fully advanced map until you detect knock. Those jumps are controlled by the ignition Advance Multiplier - 0 to 1 on our car. 1 is full advance, 0.8 is 80% of full advance etc.

Out of interest, why are you asking?

[mad_sb]

Originally Posted by Fat Spider

Thanks for the link, I'm sure I will need to read it through a couple of times to get a full understanding but in the brief read I have had it appears that there is a base map and the knock sensor is instrumental only in pulling advance out of that map and that the ECU never allows advance above the base setting even if there is no knock detected.

Subaru knock control is probably one of the most sophisticated (and annoying) around. There are a couple of ways to make it a little easier to work with:
1. flattening the advance map to a fixed value by adding all of the advance into the base maps then pulling out the amount you fill the advance map with
2. manipulating the IAM (total timing = IAM*Advance + Base + comps)

In stock for the initial IAM is .7

BTW the FA20 has two sensors, one on each bank.

Here is another good link to read over and over again (same info as above but the forum thread with pages of additional info questions etc):
http://www.romraider.com/forum/topic1840.html

[Fat Spider]

Many ECUs seem to only use the signal from the knock sensor as a protection system with the ECU only able to drop the advance out rather than continually taking the advance up to the knock point to take advantage of better octane fuels.
I must confess I have had little contact with Subaru engines but have heard many stories of people with aspirated ones complaining of detonation particularly if they are only able to source low octane fuel which doesn't really indicate an active knock sensor.
I take delivery of a new 86 manual next week and was interested in whether there would be any real benefit in running 98 or 100 octane pump fuel which can be hard to source here in the country sometimes.

[Toma]

Originally Posted by Fat Spider

Many ECUs seem to only use the signal from the knock sensor as a protection system with the ECU only able to drop the advance out rather than continually taking the advance up to the knock point to take advantage of better octane fuels.
I must confess I have had little contact with Subaru engines but have heard many stories of people with aspirated ones complaining of detonation particularly if they are only able to source low octane fuel which doesn't really indicate an active knock sensor.
I take delivery of a new 86 manual next week and was interested in whether there would be any real benefit in running 98 or 100 octane pump fuel which can be hard to source here in the country sometimes.

Most current model ecus have adaptive timing.

[mad_sb]

Originally Posted by Fat Spider

Many ECUs seem to only use the signal from the knock sensor as a protection system with the ECU only able to drop the advance out rather than continually taking the advance up to the knock point to take advantage of better octane fuels.
I must confess I have had little contact with Subaru engines but have heard many stories of people with aspirated ones complaining of detonation particularly if they are only able to source low octane fuel which doesn't really indicate an active knock sensor.
I take delivery of a new 86 manual next week and was interested in whether there would be any real benefit in running 98 or 100 octane pump fuel which can be hard to source here in the country sometimes.

Probably not any gains with 98 or 100 on stocke tune as compaired with 93 - 94. Once IAM reaches 1 you won't get any more advance.

With a tune there are certainly gains to be had with 98 or 100, if the tune takes the octane into account.

[arghx7]

I don't know all the specifics of the FA20 knock control. I assume it's not that different from all the other Subaru stuff--all signs seem to point that way. The Rom Raider links explain how the final timing is calclulated pretty well.

One thing I will add based on my experience with other knock control systems is that inside the ECU is a bunch of signal processing that you don't see. There is a monitoring window in crank angle degrees so that noise from the valve motion is ignored. The signal processor dynamically calculates a background noise level based on another set of learning conditions. So basically, it learns the noise during operation that is not prone to real knock, and figures that into the calculation. It's certainly not perfect and whoever did the knock control at Subaru had to set signal thresholds and sensitivities that would be a compromise to account of production tolerances.

Originally Posted by mad_sb

Probably not any gains with 98 or 100 on stocke tune as compaired with 93 - 94. Once IAM reaches 1 you won't get any more advance.

With a tune there are certainly gains to be had with 98 or 100, if the tune takes the octane into account.

He's in Australia. That means he's used to RON, not RON + MON / 2 like in the USA. So his 98 is kind of like USA 93.

[arghx7]

Originally Posted by mad_sb

Subaru knock control is probably one of the most sophisticated (and annoying) around.

Ehh I'd put it in the middle of the pack as far as how "advanced" it is. Some older engines in the market have very very basic knock control, where there's a basic ignition advance and then some reactionary ignition timing retard with some timer logic to restore timing advance.

The most advanced ones I've seen are part of fully torque-based engine control systems (typically German). They actually model the MBT value and base timing calculations off of that. There's also modelling of the engine octane as well. Then there's "ignition efficiency" maps that calculate the effect of retarding the timing on the torque output of the engine which then feeds into the torque arbitration system.

[mad_sb]

Originally Posted by arghx7

Ehh I'd put it in the middle of the pack as far as how "advanced" it is. Some older engines in the market have very very basic knock control, where there's a basic ignition advance and then some reactionary ignition timing retard with some timer logic to restore timing advance.

The most advanced ones I've seen are part of fully torque-based engine control systems (typically German). They actually model the MBT value and base timing calculations off of that. There's also modelling of the engine octane as well. Then there's "ignition efficiency" maps that calculate the effect of retarding the timing on the torque output of the engine which then feeds into the torque arbitration system.

Indeed, I'll correct myself to say more complicated than some (mitsubishi, aem ems, etc) and less complicated than others.