924Board.org

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

Hi folks,
I finally found out what blew my headgasket- looks like I've been getting some pretty nasty detonation (pounded the spark plug electrodes flat against the tip- yikes!). I'm cracking the head off over the long weekend and I've already got an extra headgasket sitting on the shelf. Thing is, I don't want to make this into a habit. Besides the obvious- reduce boost pressure (apparently 11psi and 8.5:1 on 93 pump gas doesn't do the trick) I was thinking of finding a way to install a knock sensor to only serve as a warning to tell me when I'm getting detonation- since I'm apparently not hearing it in the cabin.

Any thoughts on this? I'd like something as simple as just making the sensor light a warning light in the cabin when it hears detonation. Not only will this help me find what max boost I can run now, but it will also help when the heat of summer comes back around and when I get around to adding an intercooler.

Any info would be appreciated.

best,
nick

Lizard · Posted: Sun Oct 10, 2004 2:56 pm Post subject:

ouch, sorry to hear that Nick, I am not sure exactly how the knock sensor outputs there signal, there should be a way to hook it up to a light, but I dont recommend that,
if you are getting detonation do 1 of 2 things, run 10PSI, or get a DIS that you can hook a knock sensor upto and try to get one with a memory so that it remembers what advance it can use at what RPM.
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-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

Yep, I'm going to set the boost back down to stock for the moment (~7psi). If I mess with DIS then I want to spend the time to do it up right. For now, I just want a quick way to determine how much boost I can run without cooking headgaskets.

I'll be sure to post anything I come up with. It looks like knock sensors can be had pretty cheaply off ebay or the like. If there is an easy way to hook one up to a warning light I think it would benefit anyone who wants to run boost up on the ragged edge.

nick

wdb · Joined: 02 Nov 2002 Posts: 2024

-nick are you sure thats the problem , cause I would think the electrode would have to be overheated before it would bend like that , its suggest to me your running lean . maybe have a look at this spark plug diagnosis page to see if anything looks familiar . are you using hot or cold plugs , I think the hot plugs are longer ,stick out farther into the comb. chamber and get hot faster and generally run hotter , are typicaly used for mostly short trip driving , cold plugs are generally for longer highway driving .

http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

I dont know the 31 well , maybe you should ook at your air/fuel ratio under boost .

also the flat plug electrode might be from water getting into the combustion chamber from the gasket leak . the piston could slam the water into the head and pound the electrode down.

Smoothie · Posted: Wed Oct 13, 2004 8:37 am Post subject:

From what I've seen (read) you can't just hook up a knock sensor to a light or even go something like knocksensor-amplifier-light because that'd just leave you with a meaningless and annoying flashing light. Apparently there are plenty of non-detonation, non-pre-ignition, and non-knock-related vibrations going on in an engine that a knock sensor will pick up. -So the signal from a knock sensor has to be interpretted by something with digital signal processing capability... something like this - http://focus.ti.com/lit/an/spra039/spra039.pdf
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

john h · Posted: Thu Oct 14, 2004 2:31 pm Post subject:

You could try using a GM knock sensor and mount it below the inlet manifold - there is a couple of holes that it can be mounted on. Over here one of my friends has used the GM sensor and then used a link detontaion display (see http://link-electro.co.nz/knocklink.html) not sure what the cost was but it seems to be quite effiicent.
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Smoothie · Posted: Fri Oct 15, 2004 1:29 am Post subject:

They seem to be selling at approx. £150 GBP
http://www.dpmotorsport.com/parts/knocklink/home.htm
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Paul · Posted: Fri Oct 15, 2004 8:40 am Post subject:

Found this at

http://www.autodesignhaus.com/tuners/hks/ipm.htm

Knock Monitor

For high level engine tuning HKS offers the New Knock Monitor. Engine detonation is one of the most damaging effects an engine can encounter during high level tuning. One of the few ways to actually detect detonation is by the specific sound the engine makes while detonation is occurring. The only problem is other external noises makes it very difficult to hear the "knocking and pinging" associated with detonation. With the HKS Knocking Monitor it utilizes one small sensitive microphone to be placed in the engine compartment so that through your vehicles radio system or with the use of standard headphones you can hear all the "noises" your engine is producing under the hood. And because detonation noise produces a certain frequency, HKS has built in a noise filter that when activated (by pushing the sound button) will silence all engine noises except the frequency that detonation occurs at. Therefore only revealing the detonation noise.

The HKS Knock Monitor can also be used to "listen" to specific noises your vehicles suspension or turbocharger is making during driving conditions. All top level tuners know the importance of being able to hear the noises the engine and other parts are making during their normal operation. The noises that the parts produces can sometimes reveal a lot more than can be seen!

http://www.autodesignhaus.com/tuners/hks/ipm.htm