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Old 03-05-2007, 12:10 PM   #1
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Default What does knock sound like?

I've heard audible knock before, mostly in full-size trucks pulling a load on low octane gas. I don't think I've ever heard it on a small 4-cyl. I was getting a slight waver (just a jiggle really) in my oil pressure gauge but never looked at it before now, so I don't know if what I'm seeing is normal.

I'm gonna run it tonight with the wastegate actuator disconnected and see if the gauge moves naturally at high RPM's out of boost.

But back to the question. What does it sound like on a small 4-cyl? I've got all sorts of new noises coming from the car thanks to this new turbo and can't pick out any obvious knock. I may pull the timing back to 8* until I get a WB for safety.

Any observations?
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Old 03-05-2007, 12:17 PM   #2
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it's sound like a muffled can full of marbles shaking around. gotta listen closely, already enough rattles and shakes
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Old 03-05-2007, 12:19 PM   #3
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You've probably heard it a million times and just never realized it, especially if you drive in traffic with your windows down. Usually you'll hear it (from other cars) under slight acceleration or maintaining speed going up hill. Some people say it sounds like marbles, but there's really no accurate comparison.

Knock/ping/detonation/pre-detonation wouldn't have any effect on your oil pressure. (EDIT - I take that last statement back, read on) You say you've heard it on a big gasoline engine? It sounds the same on a small engine.

Once your ear is trained for it, there will be a neuron rearrangement in your brain going from your ear to right foot and you'll be amazed at how fast you let off the gas when you hear it

[mr mackey]Detonation is bad, mkay[/mr mackey]

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Old 03-05-2007, 12:21 PM   #4
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Originally Posted by SloS13 View Post
Knock/ping/detonation/pre-detonation wouldn't have any effect on your oil pressure. You say you've heard it on a big gasoline engine? It sounds the same on a small engine.

actually, The 90-94 oil pressure sensors are carbon resistance sensors. As the carbon is compressed, the resistance changes. Detonation shock waves travel through the carbon sensors with massive changes in resistance that is indicated by the gauge needle flickering wildly to the left.
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Old 03-05-2007, 12:37 PM   #5
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you're ******* with me, right? lol
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Old 03-05-2007, 12:39 PM   #6
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no go try it. boost 12psi without an intercooler and 10* timing.

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The sensor is a ( I think, guru help here please) a carbon capsule much like the early telephones that changes resistance with pressure. Thus it will respond to vibrations too. I do seem to remember that other cars did use compatible sensors, so a search may be in order for cross references that may be a lot cheaper than OEM.
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If you have a '92 as indicated in your profile, the oil pressure gauge is a knock sensor. If it's jerking to the left, you have knock.
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It's not a joke, knock will interfere with oil delivery from putting excessive load on the crank bearing and blocking the flow of oil, hence it serves as a knock sensor of sorts. Without knock the oil pressure will stay very stable, when knock persists it is very noticable on a sensitive oil pressure gauge.
quick search on m.net
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Old 03-05-2007, 12:43 PM   #7
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you're ******* with me, right? lol
Nope. I've seen it myself. However I think it quite prone to giving one a false positive.
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Old 03-05-2007, 12:45 PM   #8
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What happened to the good old days when an oil pressure gauge was an oil pressure gauge? Guess I learned something today...but that's just plain odd.
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Old 03-05-2007, 01:25 PM   #9
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Quote:
Originally Posted by SloS13 View Post
What happened to the good old days when an oil pressure gauge was an oil pressure gauge? Guess I learned something today...but that's just plain odd.
It's an interesting side effect of how the sensor is constructed. I didn't realize (as Braineack pointed out) that the pressure sender was a carbon transducer but it makes sense- that would be a relatively inexpensive way to construct a reasonably accurate pressure sensor. It's also a good way to construct an inexpensive microphone, which is exactly how they did it in old-style full size telephones.
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Old 03-05-2007, 01:29 PM   #10
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What does knock sound like?
"rattle rattle rattle BANG BANG BANG boohoo boohoo boohoo. Oh man, fucked up my engine! :( "

The thing that really started to worry me wasn't the knock I could hear, but the knock I couldn't here. Friends I know that are running a link with a knock sensor show many knock events that they could never hear.
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Old 03-05-2007, 03:13 PM   #11
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In my experience, it's sounded like rapidly-moving marbles in a can...almost like a mini Gatling gun.
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Old 03-05-2007, 03:25 PM   #12
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yeah, ironically to me it sounds like pouring all my loose change in a coffee can so I can afford a new motor.
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Old 03-05-2007, 04:27 PM   #13
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What you're telling me is that knock on a big motor sounds the same as knock on a small motor... that's good to know. I'm not hearing anything like that. How much should my oil gauge move when my engine is knocking? Will it make full left throws? or is it more of a slight 10psi flicker?

Now the next question. Is there an actual reliable/cheap/easy install/aftermarket knock sensor, maybe with a dash-mounted light that's worth looking into. It doesn't have to integrate with anything, just function as a standalone. For some reason, I'm really paranoid about this knock thing all of a sudden.
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Old 03-05-2007, 04:30 PM   #14
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just watch the oil gauge, it will flicker pretty wickedly.
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Old 03-05-2007, 04:31 PM   #15
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Quote:
Originally Posted by spamnavy View Post
Now the next question. Is there an actual reliable/cheap/easy install/aftermarket knock sensor, maybe with a dash-mounted light that's worth looking into. It doesn't have to integrate with anything, just function as a standalone. For some reason, I'm really paranoid about this knock thing all of a sudden.
I'm sure there are more out there, this is the one I've heard most about.
http://search.ebay.com/search/search...tSearch=Search
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Old 03-05-2007, 04:39 PM   #16
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Old 03-05-2007, 04:50 PM   #17
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That looks pretty spiffy, and less than $110 shipped on ebay.
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Old 03-05-2007, 05:21 PM   #18
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Quote:
Originally Posted by SloS13 View Post
That looks pretty spiffy, and less than $110 shipped on ebay.
does it work?
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Old 03-05-2007, 05:24 PM   #19
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From what I read on NASIOC a while back it does. You will need to by a proper knock sensor to go with it. The prices listed do not include one.
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Old 03-05-2007, 05:26 PM   #20
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eh theres always a catch :P
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