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View Full Version : How sensitive are SR knock sensors?



Ovrated
09-02-2005, 06:48 PM
we are trying to tune my redtop on the AEM, and it seems for being a forged motor, that the knock is always kinda quiet.

my buddie who is tuning it said i should try to get a more sensitive one, like a dsm or something close to it.

what do you gusy think

Hollywood
09-03-2005, 06:42 PM
we are trying to tune my redtop on the AEM, and it seems for being a forged motor, that the knock is always kinda quiet.

my buddie who is tuning it said i should try to get a more sensitive one, like a dsm or something close to it.

what do you gusy think



Na. The stock one works just fine. You should even see some knock at idle in a really hot day waiting at a red light with values 0-10 depending what u have reading it.
I dont know aem, but perhaps the AEM is not calibrated properly to the sensor. AFC2's have a calibration.

Kingtal0n
09-03-2005, 08:20 PM
we are trying to tune my redtop on the AEM, and it seems for being a forged motor, that the knock is always kinda quiet.

my buddie who is tuning it said i should try to get a more sensitive one, like a dsm or something close to it.

what do you gusy think



You cant mix and match knock sensors. A knock sensor is tuned to the specific frequency of the motor it comes in. Even something subtle as boring a motor can reduce the effectiveness of a knock sensor, and going from motor to motor will not work the way it should...

"calibration" as its called, for knock sensors, is pretty much just the separation of engine noise from actual knock. Alot like electronic EMI, knock sensors pick up all sorts of things that hit that certain frequency, that "noise" needs to be accounted for and weeded out so when a "new noise" comes along the ECU knows it for what it is, knock. Note that once knock is audible, it is usually already out of the scope of the knock sensor's "hearing" and the ECU will register no knock whatsoever. Thats why they say "dont tune your timing by the knock sensor", because you may have pre-ignition and your knock sensor will be non the wiser.

_Def_
09-03-2005, 10:36 PM
Knock sensors pick up noise as well as anything else in their frequency bandwidth. The properties of the piezoelectric sensor that makes up the "hearing" portion of the knock sensor determine its frequency range. So you have to *ALWAYS* use some common sense when you're getting a voltage signal out of the knock sensor. It's only as useful as the nut between the wheel and the seat as they say...

Hopefully the original engineers were smart enough to pick the "right frequency" for the application, and that's also why you can't use any knock sensor you'd like. Even things like the block material and how thick the water passages(or how thick the sleeves are as was already mentioned) will change the frequency produced by preignition/knock.

I also wouldn't use it to find the "limit" of tuning, because depending on when in the combustion cycle you're getting preignition, it can change the frequency just enough to be outside the range of the sensor(audible knock is usually earlier in the combustion cycle compared to small amounts of preignition). It should be obvious that this is a very complex process, and a knock sensor is simply a cheap way to get *minimal* detection of preignition

So I'd give it a good tune from general "rules of thumb" that most EFI tuners use, then use the knock sensor as a failsafe device. That'll probably be more reliable than cranking up the timing until you do get knock.



I will say that I've seen audible knock register on my AFC2. It wasn't pleasant, but at least I know it works(didn't think I was getting the car into timing mode successfully, and I confirmed as much with a short test drive http://forums.freshalloy.com/images/graemlins/shocked.gif).