SAFC II and knock sensor readings.

[Bronze MFP]

How much should the knock sensor reading be trusted on the SAFC II? When its cool out, it stays at 0, when its warm it might get up in the 25 range. what readings do you guys usually get, and at what point would you start to worry about engine damage?thanks,

-Neil

[S14tat]

there will be a setting 1 and a setting 2. usually when your in the teens to lower 20's, that is just random engine vibrations when the engine warms up fully. usually i would say at the 2nd point on the knock setting will range from the 60's to 90's. as long as your below 60, you'll be fine. this thing is very accurate. when i was tuning my timing, as soon as i heard it knock by ear, i looked at the peak knock number and it was at 76. my 2nd knock value was in the upper 60's so the proved the function works.

[ziggy682]

Hmm, when I had my SAFCII (before it got stolen) I decided that the knock sensore readings were completely useless. They varied sooo much depending on outside temps. In the middle of the Texas summer, I would hit 200 on the knock readings after I had been driving for a while, and this was before the car was even turbo. However, it would take about an hour worth of driving to get up that high. In cooler weather it would stay in the teens or 20s. Maybe my knock sensor is just screwy, but I wouldn't use it as a "tuning" tool, as I would trust my ears to hear knock more than the knock sensor reading on the SAFC.

[S14tat]

well in the north east region it works fine. when the car is running for like 5 hours, then it would go up to the lower 20's but still waaaay below the upper limit.

[Nismo_Freak]

Hmm, when I had my SAFCII (before it got stolen) I decided that the knock sensore readings were completely useless. They varied sooo much depending on outside temps. In the middle of the Texas summer, I would hit 200 on the knock readings after I had been driving for a while, and this was before the car was even turbo. However, it would take about an hour worth of driving to get up that high. In cooler weather it would stay in the teens or 20s. Maybe my knock sensor is just screwy, but I wouldn't use it as a "tuning" tool, as I would trust my ears to hear knock more than the knock sensor reading on the SAFC.

Duh, hotter weather = hotter coolant temps = hotter fuel temps = hotter running temps = more detonation

As long as it stays under 60 you are fine according to Apexi.

[Bronze MFP]

Yeah, i havnt had any readings above 25, and when i spray, it drops to 0. I'm not trying to use it as a tuning, tool, just checking to see if everything is in spec.

[jab11185]

huh....mine was fluctuating like mad before it got stolen....i never paid attention to it because i never heard any knocking...

[Bronze MFP]

You sure you tapped the right wire?

[DRIFTEADOR]

interesting stuff from aemforums:

[quote=""mx5""]Here is some VERY important info about knock sensors and your AEM EMS.

There are two types of knock sensors - broadband resonant knock sensors and flat response knock sensors

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- Broadband Resonant Knock sensor

This type of knock sensor is also called a tuned one. It is the old generation of knock sensors and they are tuned for the knock frequency for the particular engine. The sensor output signal is about 1000-2000 mV/g for the tuned frequencies and silent for the rest of the spectre.

Here is a document from Delphi with technical info about the Broadband Resonant sensors they make:

http://delphi.com/pdf/e/int_broadband.pdf

This is how a Broadband Resonant sensor usually looks like:

The Broadband Resonant knock sensor is the type which the AEM EMS can work with. The AEM EMS is "dumb device" knockwise so it needs the knock signal already filtered and at high level (because its knock sensor input is just a 0-5V input).

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- Flat Response Knock Sensor

Here is the bad part for the AEM EMS. The majority of the new cars these days use OEM this type of knock sensors - Flat Response ones. Also, all Bosch knock sensors are this type.

These sensors pick up the whole 5KHz to 20KHz range. And their output signal is very low compared to the Broadband Resonant ones.It is around 30 mV/g.

Here is some tech info about the Flat Response knock sensors Delphi makes:

http://delphi.com/pdf/e/int_flat_knock.pdf

This is how the Flat Response sensors usually look like (usually there is a bolt going through the sensor instead of bolting the sensor directly to the block):

The AEM EMS can NOT work directly with this type of knock sensors. These sensors need external device analyzing the engine noise signal and recognizing the knock frequencies (sometimes more than one frequency is monitored - these days the algorighms get better and more complex) - this is usually done by the stock ECU of the car. The AEM EMS can't do that - at least the current firmware - I am not sure if a future firmware could be made to do bandpass filtering (and does the AEM EMS have enough compiting power left to spend on such filtering process).

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So. If you have an older school car with Resonant type knock sensor the AEM EMS can use the signal from it directly. But you have to be sure this Resonant sensor is tuned for the main frequency of the knock sound of your particular engine. Otherwise it won't work! You can't just pick any resonant knock sensor and assume it will be ok. You need either the OEM sensor for this engine (if it exists) or to calculate the main knock frequency (there are some SAE papers talking about how to do it - i.e. according to it the 1.8L Miata engine has a main knock frequency of 6.5KHz) and pick a resonant sensor tuned for that frequency.

If you have a modern engine with a Flat Response knock sensor you are out of luck (I guess no matter even if the AEM is correct application for your engine). Even if the AEM EMS is hooked to the knock sensor and you get readings when hitting the engine block with a hammer, it doesn't mean the AEM will pick up the knock sound from the rest! - actually it won't recognize it. The AEM EMS can't work directly with Flat Response knock sensors! Hitting the engine block with a hammer and getting readings on the AEM EMS side doesn't mean it is working - it will pick the hammer signal but it won't recognize knock later

If you have a Flat Response knock sensor and you want to make it work with the AEM EMS you need an extra device which does bandpass filter (adjusted for the particular knock frequency of your engine) and amplification of the signal. You have to connect this device between the knock sensor and the AEM EMS.

Using the info above check the knock sensor type your car uses and if it is a Flat Response one don't expect the AEM pick the knock sound from the engine noise even if hitting on the block shows some readings! This is even if you use the correct AEM EMS unit for your car - AEM may say it will work with the OEM knock sensor but it will not.[/quote]apparently ours is a flat response sensor which doest work off noise, but rather off frequencies. so unless you have the time/testing equipment/money to figure out what at what frequencies knock occurs for your motor, the reading is useless.

[Bronze MFP]

wow. awesome find, I thought a knock sensor was a knock sensor...You learn something new every day.

[Jookmasta]

that info should definitely become a sticky to all who plan on going the aem ems route. i dont think half of us knew of that info on this board..............any comments from the mods or any others?

[DRIFTEADOR]

if someone wants to try it, you could test what the knock frequncies are using something like this (http://cgi.ebay.com/ws/eBayISA...&rd=1) while datalogging timing on the stock ecu. I think someone mentioned before that the ecu only reads knock up to a certain rpm, but maybe with the info gathered, an educated estimate could be plotted to redline.

...or a broadband knock sensor off a car with similar engine characteristics (4cyl, 2.4l) could be used