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    Thread: Spark plug knock under boost

    1. Member
      Join Date
      Aug 23rd, 2011
      1979 Mk1 Scirocco
      03-19-2017 08:42 PM #1
      Hi all,
      So I am having some pretty bad knock under boost. Sounds like a bad rattle or tapping under WOT and when the turbo seems to be spooling.
      I have checked my timing, vacuum lines and fuel air mixture; all is well. Note that F/A mixture test was only done at idle and high speed. I do not have an O2 Sensor.
      The engine is running really well under all other conditions, including high rpm with no boost.

      Here are what I think could be possible culprits:
      Intake temperature:
      The intake manifold gets hot as hell.
      I am also having hot start problems.

      I am also wondering if the Audi 5000 WUR is not operating correctly and not delivering extra fuel under boost and start ups.

      How hot should my intake manifold actually be getting?
      Is there anyway to check the operation of the WUR?
      If the knock is fuel related should I add a extra injector? How should I do that?

      Thanks for the help!

      Here are some quick specs on the engine:
      79 scirocco
      1.8l block, 2l crank, stock 1.8l rods, "squished" pistons, 2 over bore, stock compression (sorry I don't have the measurements on me), hydraulic lifters
      BAE Rajay turbo good for 5-7psi or boost
      Volvo fuel distributor, Audi 5000 WUR
      Volvo Oil cooler, Euro GTI Oil filter Manifold, Porche 944 Thermostat.
      Stock everything else

      Here is a picture, because post kind of suck without at least one:

    2. 03-20-2017 10:54 AM #2
      It sure appears you no intercooler. So, if that is true, you pretty much stuck at 5-6psi and that is with a stock of 8 to 1 compression.

      You could add an intercooler, but I would add water injection first. That should be fairly easy to install and certainly could be done in a couple hours.

      You could back down the ignition timing, but that would give you less power all the way around.

      Without a doubt, unless you want to be replacing pistons soon, I would back down the boost to prevent further damage. You could back down the throttle, but I lacked the control back in the day and I would always end up replacing pistons.

    3. 03-20-2017 02:33 PM #3
      I think there is going to be a bit of confusion here about what your compression ratio is. more details needed fella

    4. 03-20-2017 08:32 PM #4
      I would be looking into fitting an intercooler before anything else. maybe you could have the 5th injector activated by a boost pressure switch??? but to be honest, you might be better with a mappable efi engine management set-up in the long run.

    5. 03-21-2017 12:19 PM #5
      Compression ratio is probably it, because it should not be that bad at 5-6psi.

      Check the compression and find out what it is, if you're pushing above 190psi, then you may have something higher than 8 to 1. I think it should be more like 160ish.

    6. Member
      Join Date
      Aug 23rd, 2011
      1979 Mk1 Scirocco
      03-21-2017 01:55 PM #6
      I will try to dig up those exact numbers for you guys.
      That is right, no intercooler. I have never needed it; I had this set up on my 1.6.
      I have done a compression check recently and it was reading at about 155psi

      A friend of mine once gave me a Jetronic controller. Could that set up to help out with the fuel management?

    7. Yesterday 07:19 AM #7
      what kind of ignition dizzy are you using? and what's your timing set at? if it's a standard 1.8 k-jet dizzy, then maybe the advance curve needs adjusted. good chance there is too much advance when the turbo boosts. so maybe stronger springs on the advance weights???

    8. Yesterday 09:41 AM #8
      With a properly running CIS system, you should be good. You have the larger fuel dist. and turbo warmup regulator so adding more fuel is most likely not the answer. Of course, if your system is not working properly, then anything is possible.

      With 155psi, I think you are correct with your assumption of 8 to 1 pistons. So that leads to the ignition system. MSD makes a boost retard system for the ignition system, it may be a thing to look at.

    9. Yesterday 09:58 AM #9
      do you think it could be a big-end catching something inside the block??? i'm in the process of building a 3a based 1.8 block, and there really is very little clearance in there. just wondering if a little internal flex is causing something to catch and make the rattle you're hearing

    10. Member
      Join Date
      Aug 23rd, 2011
      1979 Mk1 Scirocco
      Yesterday 01:58 PM #10
      I am running the stock Vacuum advance points distributor. I recently replaced the condenser and points, adjusted dwell.
      How does that MSD Ignition work? Does it replace the distributor all together?

      Could that WUR be tested by running the car and removing the 5th injector, then apply "boost" with air pressure to see if any fuel squirts out?
      That part is likely 30+ years old.

      Good question about the clearances, things are super tight in there with that 2l crank. Had to shave down the intermediate shaft gear a lot!
      There is plenty of clearance now, I had that same though after hearing that noise so I checked to see if there has been contact
      Here is a pic of it when it was being machined:

    11. Yesterday 02:41 PM #11
      have you tried retarding the ignition timing to see if that stops the noise?

    12. Yesterday 02:59 PM #12
      are all your fuel injectors flowing freely? also, how much of this set-up were you running on the 1.6? I could be wrong but is the 5th injector on them not just a cold start???

    13. Member
      Join Date
      Aug 23rd, 2011
      1979 Mk1 Scirocco
      Yesterday 03:08 PM #13
      I don,t know why I did not try doing that?!
      I will see if I can squeeze in some road time this week with that distributor retarded.

      I think that you are correct that factory the 5th injector is a cold start, with the WUR that I have, boost flexes that diaphragm to use that injector as a extra injector under boost

    14. Yesterday 04:37 PM #14
      to be honest, retarding the timing would only ever be a compromise, as there is no way the standard advance cure for an n/a engine would be perfect for an f/I engine. have you got a live going to the fifth injector? you could try pulling the injector, then revving the engine to check if it sprays (away from your hot intake manifold of course)

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