Knock Sensor - Miata Turbo Forum - Boost cars, acquire cats.

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Old 06-20-2013, 10:37 PM   #1
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Default Knock Sensor

I'm currently running a Saturn knock sensor on my Hydra 2.7. When originally dynoing the car, we checked logging the knock and saw virtually nothing on it -- 0.15V or so at the highest.

Recently, just driving the car around, even at part throttle cruise, I'm seeing 0.15-0.2V, and just barely dipping into boost (a few PSI, with correct AFRs and weak timing around 15 degrees), I'm getting 0.4+V of knock. This seems ... wrong.

While it could just be some REALLY bad gas or something that I just put in the car, that seems pretty unlikely. I had an autocross last weekend where one run the car had just absolutely no power, and when I checked the datalog, it was showing 0.6-1.2V of knock for 40 seconds straight, both on throttle and off throttle. That also seems ... wrong. Especially since I checked the plugs and there's zero signs of anything bad. 40 seconds of straight knocking, even with the ECU pulling 12 degrees (max retard) the whole time, and it's crystal clear? This seems pretty phantom to me.

Reading around, it seems that the saturn knock sensor is better on the AEM (why I bought it in the first place), but on the Hydra everyone just uses a stock 99-00 knock sensor. Has anyone had any issues similar to this? I need to dig around to see if I can even find my damn 99 knock sensor now to try that out, but I'm curious if anyone has seen weird **** like this before..?
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Old 03-21-2014, 05:16 PM   #2
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You need to calibrate the knock map to your sensor.

When parked, rev car to to redline while watching the knock values. Then, set the knock threshold so it's just above the levels you're seeing while reving in neutral.

See FM's instructions for details.
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Old 05-22-2014, 11:39 PM   #3
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My knock sensor registers zero volts. Even under boost. I think the factor sensor is resonance tuned for knock. Which means zero volts until it resonates... But that is just my theory
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Old 06-11-2014, 11:16 AM   #4
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My knock sensor registers zero volts. Even under boost. I think the factor sensor is resonance tuned for knock. Which means zero volts until it resonates... But that is just my theory
I'm also on 2.7 and see 0 knock voltage at all times. Even at WOT in boost with the knock sensor amplifier 1 enabled in the knock sensor settings. If the factory sensor were to be resonance tuned for knock how would we set our threshold...
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Old 06-13-2014, 01:01 AM   #5
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beats me man. I don't trust the sensor since it registers zero. But on the other hand if it is resonant tuned maybe the engine has no knock. Just really unclear without some volts coming through it. I did give it a few raps with a hammer prior to install and I did see voltage output from it. So it is good for sure.
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Old 06-13-2014, 01:05 AM   #6
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Sounds like someone needs more timing in their tune.
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Old 06-26-2014, 10:00 PM   #7
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yeah, upped the timing by 3 degrees and went from 200 hp to 220 hp. Still no knock!!! Stopped at that level around 11 pounds of boost.
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Old 06-29-2014, 06:57 PM   #8
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I also get nothing off my knock sensor save perhaps .02v under boost. If I tap it with a wrench I get some voltage, so it's definitely working. Hard to gauge it now, prior to my supermiata harmonic damper I'd get numbers up to .45 all the time.
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Old 06-29-2014, 10:19 PM   #9
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Originally Posted by Der_Idiot View Post
I also get nothing off my knock sensor save perhaps .02v under boost. If I tap it with a wrench I get some voltage, so it's definitely working. Hard to gauge it now, prior to my supermiata harmonic damper I'd get numbers up to .45 all the time.
Cool. Nice to know it's making a measureable difference in damping bottom end shenanigans.
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Old 06-30-2014, 08:23 PM   #10
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If I tap it with a wrench I get some voltage, so it's definitely working.
Just did your test, aaand nothing! Guess my sensor is dead
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Old 07-01-2014, 03:28 AM   #11
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Tap it a little... more firmly. Specifically the allen bolt.
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Old 07-09-2014, 07:15 PM   #12
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Does anyone know if the knock sensor is tuned to a resonant frequency? That could explain why we see hardly any voltage by doing a simple TAP test... I think it will take a pretty good tap if it is still installed.
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Old 07-14-2014, 11:12 AM   #13
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It's very possible; I used a 20-22mm wrench and while I wouldn't say I beat on it, I did show that allen bolt some love.
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Old 07-14-2014, 03:52 PM   #14
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Is it possible there is some vibration, like the exhaust hitting the body that only happens when the car is rolling down the road? My knock sensor goes apeshit whenever I get violent wheel spin in second gear.
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Old 12-17-2014, 08:49 PM   #15
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Knock sensors (at least the ones we use) are piezoelectric. I just tap the bolt firmly with a screwdriver or a small hammer to test function.
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Old 12-21-2014, 09:24 PM   #16
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I've had to "Tap" mine pretty hard to get it to register anything. Even under boost it never goes over .08 volts.
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Old 01-27-2015, 02:07 PM   #17
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I sent out a 2.7 owner's list email the other day talking about the new firmware's knock detection, and how you really should re-calibrate your knock threshold due to how low the knock values are now. Some important functions have been updated.

In addition, I highly recommend reading (in the current 2.7 R36 software) Help -> Help Topics -> How Tos -> Knock Control. This will answer many questions. For example, in the latest firmwares, "Important note: the Hydra Nemesis 2.70 system only samples knock while the engine is running, so don't take a hammer to a stopped engine to try to produce a knock value."
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