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Advice on where/how to mount a knock sensor on the block?

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  • Advice on where/how to mount a knock sensor on the block?

    I'm in the middle of a rebuild on my 2.5L S14 and will be adding a knock sensor to work with my megasquirt system. My knock sensor mounts using an 8mm bolt. This link shows a pic of the sensor:

    Does anyone have a suggestion on a good place to mount this sensor? I would prefer to pic up an existing hole in the block. If anyone knows of a good and safe place on the block to drill and tap a new hole, please speak up! Since I'll only be running one sensor, it makes sense to locate the knock sensor on the block between cylinders 2 and 3. The mounting hole for the plenum support arm is a good location for this sensor, but I would have to fabricate a new arm to accommodate my sensor. I would need a shorter arm to account for the sensor thickness. I'm not running a CF plenum yet, so can't just delete the arm. To clarify, I'm talking about possibly mounting the sensor on the block where number 7 attaches in the picture below:


    I could possibly mount the sensor using a smaller diameter bolt if there is a good, existing hole in the block that is less than 8 mm. I might be able to use a different sensor if that would make finding a mounting location easier. Finally, I could use two sensors mounted between cylinders 1 &2 and 3 & 4 if there happen to be existing holes there.

    Thanks for the input.


  • #2
    I was just looking at the realoem pic above a little more and had another idea. The simplest thing to do might be to use the location on the block where number 7 mounts and keep the existing plenum support arm in place. I could have a 10 mm bold (number 12) fabricated that had a 8 mm stud protruding out of top of it. It would be a 10 mm bolt with a 8mm stud welded to the head of the bold. Then I could just install the sensor right onto the top of number 12 in the pic above. It wouldn't be as good as having it mounted right on the block since the sensor would have a smaller contact patch, but it would be safer than putting new holes in the block.

    Thoughts? Suggestions?
    Last edited by indie_wannabe; 05-26-2011, 05:18 AM.


    • #3
      hey indie,

      I already did exactly what you were thinking about and even posted a drawing in a previous post.

      It works fine, but I know now that it's unnecessary. There are several tapped holes on the exhaust side of the block that could be used that are much easier to access. The sensor, from J&S' experience, does not need to be centrally located or even very close to the head.
      Last edited by Anders; 05-26-2011, 05:47 AM.

      "Objects in mirror are losing..."


      • #4
        Is there any chance that exhaust noise would be a problem when mounting on the exhaust side of the block? The header is attached to the head (of course) and not the block, so it is far enough away not to contaminate the knock signal, right? Are the frequencies coming from the exhaust different enough from the frequencies generated by knock that it is a moot issue? I'm guessing heat from the exhaust would be more of a problem than noise?
        Last edited by indie_wannabe; 05-26-2011, 06:07 AM.


        • #5
          Exhaust noise should not be a problem. THe frequency of knock on the S14 is around 6500 hz, which is pretty high. John from J&S also recommended the exhaust side after he saw a picture that I sent to him.

          I don't think heat-proofing should be very difficult.

          "Objects in mirror are losing..."


          • #6
            I mounted mine under the coolant pipe under the TBs. I used a Airbox to TB mounting dowel to fit the M6 bolt.

            I seem to be getting a signal, but I haven't done an analasys yet.

            Sport Evo No.47

            My Sport Evo Restoration


            • #7
              Mine is currently mounted exhaust side, front of the engine where one of the AC bolts used to be. Still need to do more testing to see how well it works, but no issues with heat so far.
              Track Car Build | COM Sports Car Club


              • #8
                Oh yeah I gotta get with you guys on how the MS works with the knock sensor input until I get the J&S to play nice...


                • #9
                  Thanks for the input guys. I have decided to mount the sensor on the exhaust side. I've ordered a Teflon coated cable to give me some insurance against heat issues. I think I'll also try to fab up a simple sheet metal heat shield to go over the sensor to protect it against gradual deterioration.


                  • #10
                    I fitted a Bosch (for a BMW M43 engine) sensor, which uses a 6mm bolt, on top of the driver's side engine support by using a longer one.
                    On Steve's picture right where the bolt is next to the dipstick tube.

                    I have no signal processing device yet apart from a velleman kit (mk136)

                    and hooked the knock sensor up to where the mics normally go.
                    I observed two things:
                    1. you hear exactly the same noises using a knock sensor as using the mics (glued to the head, one between cyl. 1&2 right where the ICV-bracket sits [drilled a hole through it to get to the head] and one between 3&4 )
                    2. you hear absolutely everything like the valves opening and closing, the injectors firing.

                    The problem with 2. is that I cannot discern between knock, normal noise or an input signal that is too powerful for the amplifier to deal with (resulting in distortion). It's also just way too loud at anything more than idle rpm to listen to with an ear bud type headphone, even at the lowest of the volume setting. Any lower and it's off...
                    In that respect it's not as convincing as made it appear in their article, where I got the idea from a couple of years back:

                    I must give it a try with a ignition map that is sure to cause knock just to know what to listen for when tuning the ignition as most probably I just dont have knock. (Not the least because I am using Brendan's ign. map and my WOT ign values are identical to the ones stored inside the Motronic).
                    I'll also dig into a MkII device autospeed came up with that uses a band filter:
                    Last edited by hardtailer; 05-27-2011, 09:43 AM.