# curious: what does a knock sensor actually do?



## Colraindub (May 29, 2003)

I've heard of people taking knock sensors of their cars, just curious as to what the advantages are. Also just curious as to what a knock sensor actually does, I know it 'keeps the motor from knocking', I want more than that though.


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## gearhead455 (Oct 30, 2001)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

A knock sensor is a piezoelectric microphone that generates a 0-5mV signal to the ignition controller (knock box). When a millivolt input is received to the knock box the ignition timing is retarded to suppress ignition timing. Ignition timing is advanced to the most degrees without knock for optimum performance.


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## Inssomniak (Mar 5, 2003)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

Well I dont run one, so here is my best explaination. It does just that, it retards timing when a sensor detects the knocking. But what it also does (in a VW case) is it will advance your timing to the point of knocking and bring it back a few degrees.. This allows for the most power output. Also they have rev-limiters in them.. and that varies between the cars they were installed in.. (8v, 16v) Other than that, I dont think it gets any more complicated than that.


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## Inssomniak (Mar 5, 2003)

*Re: curious: what does a knock sensor actually do? (Inssomniak)*

Looks like we were both writing replies at the exact time


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## Rocco_crossing (Dec 19, 2002)

*Re: curious: what does a knock sensor actually do? (Inssomniak)*

Very good question as I want to know too since I may be doing an RD swap in the future and will need to add the knock box according to what everyone has told me.


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## Inssomniak (Mar 5, 2003)

*Re: curious: what does a knock sensor actually do? (Rocco_crossing)*

I have been contiplating it up and down all summer, adding one, they are covered all over the place at the wrecking yards.. But I dont know if I want to mess with a good thing now.. The vacuum system is proven, and works well, but it does nothing for you if you get knocking. Im not sure the performance increase is worth the work, and possibly not getting anything out of it, or even hurt performance afterwards.. Its a must for high comp.. but Im not quite over 10:1+ they say it needs to be there for. I would just add it for the performance gain if it actually did that for me.


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## Colraindub (May 29, 2003)

*Re: curious: what does a knock sensor actually do? (Inssomniak)*


_Quote, originally posted by *Inssomniak* »_ Its a must for high comp.. but Im not quite over 10:1+ they say it needs to be there for. 

is that the only reason to take it off? Someone said something about it also acting as the rev limiter. Is that another reason?


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## 86gtibigbump (May 10, 2003)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

i have a cis e and just unplugged my knock sensor and drove around block and im not lying when i say i could feel the diffrence







i sware to god


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## saporter (Sep 10, 2003)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

Using a knock sensor will allow you to run a static timing of up to 12 deg. BTDC. 
When the engine begins to ping, the knock sensor will retard the timing until the ping goes away.
The rev-limiter is controled by the knock sensor. But you can bypass this.
Sean


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## slumberbunny (Feb 10, 2003)

*Re: curious: what does a knock sensor actually do? (saporter)*


_Quote, originally posted by *saporter* »_Using a knock sensor will allow you to run a static timing of up to 12 deg. BTDC. 
When the engine begins to ping, the knock sensor will retard the timing until the ping goes away.
The rev-limiter is controled by the knock sensor. But you can bypass this.
Sean


On what injection system? I know Digi is controlled by the ECU (perhaps the signal from the KS)... if there's a way to defeat the rev... let us know.


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## Colraindub (May 29, 2003)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

I didn't really get what I was looking for though. I wanted to know what the performance part of taking it off was. Like what does it do/ let you do? From what you said all I could tell was that it took the rev limiter off so you can rev higher.


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## 84_GLI_coupe (Apr 3, 2001)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

I put the KS system from an 85 GLI into my 84 GLI, since I'm running 11:1 and some other stuff. There is NO rev limiter when you put it onto a CIS car, seeing as I've taken it to 7000rpm (still making lots of power, but I needed to prepare for the corner so I didn't take it higher).


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## slumberbunny (Feb 10, 2003)

*Re: curious: what does a knock sensor actually do? (Colraindub)*


_Quote, originally posted by *Colraindub* »_I didn't really get what I was looking for though. I wanted to know what the performance part of taking it off was. Like what does it do/ let you do? From what you said all I could tell was that it took the rev limiter off so you can rev higher. 

Basically, the knock sensor (I think it was stated earlier)... retards the timing WHEN detonation (ping/knock) occurs. Without a ping situation, the timing will be advanced as far as it can go - so the performance gain is full timing advance.


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## Colraindub (May 29, 2003)

*Re: curious: what does a knock sensor actually do? (slumberbunny)*

ok, I get it


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## GolfDetailer (Jul 27, 2003)

*Re: curious: what does a knock sensor actually do? (Colraindub)*

Hello,
I'm just asking a question. Do Diesels have the knock sensor in them????? Thankyou


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## Colraindub (May 29, 2003)

*Re: curious: what does a knock sensor actually do? (GolfDetailer)*


_Quote, originally posted by *GolfDetailer* »_Hello,
I'm just asking a question. Do Diesels have the knock sensor in them????? Thankyou

Lol, good question (I also have a diesel)


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## onewiper (Aug 29, 2002)

How could you put a knock sensor in a motor that is all knock, those things sound like a washing machine full of rocks. No you can't put a knock sensor on diesels, they have no ignition systems.


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## Colraindub (May 29, 2003)

*Re: (onewiper)*


_Quote, originally posted by *onewiper* »_How could you put a knock sensor in a motor that is all knock, those things sound like a washing machine full of rocks. 

LOL. I didn't think they would. Hehe


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## 84_GLI_coupe (Apr 3, 2001)

*Re: (Colraindub)*

They're not all knock. Diesel burns slow enough to negate any knock. It requires compression for combustion, and it can't get that unless it's at/near TDC, therefore it's kinda hard to pre-detonate.
The higher the octane, the slower it burns. Diesel is kinda slower than all that.
And the KS system follows ignition timing maps, which are pre-programmed into the boxes. According to volkswagen.org, cutting the white wire between pins 3/11 on the connector at the KS box will access a different timing map which gets you a few more hp and ft.lbs.


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## onewiper (Aug 29, 2002)

I was just trying to be funny. I owned a diesel a few years back I kinda think of them as being acoustically challenged, I didn't think they were to bad, just a little slow. Diesel fuel uses an additive that is the opposite octane called cetane that increases the rate of burn to aid in combustion.


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## Vdubs (May 16, 2001)

*Re: (onewiper)*

Cetane number is actually a measure of a fuel's ignition delay.
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites.


_Modified by Vdubs at 7:35 AM 9-17-2003_


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## Domin8v (Feb 1, 2006)

*Re: (Vdubs)*

I highly suggest reading all the write ups included in the FAQ/DIY threads. There is a lot of information that will seem irrelevant to you at the time, but then somewhere down the line you'll realize there are parts that relate to your situation. 
It's also really good to read the multiple point of views. A lot of questions on here boil down to opinion. Too often we take the first posts we see as the final answer. Often times it is, but there are plenty of times where you will end up reading the other side of the story and change your opinion on the matter http://****************.com/smile/emthup.gif 

_Quote, originally posted by *The Difinitive Cheap A$$ Build Up!!!!!!!* »_
You can eliminate your rev limiter in a CIS car by 
replacing the fuel pump relay, which is a 5 prong realy for the standard turn signal, 4 prog relay. This keeps the 
car from telling the fuel pump to stop pumping, giving you the ability to rev it out a little more. I only recommend 
this mod for people that have solid lifter heads, or have replaced the valve springs with aftermarket ones, as you 
CAN DAMAGE THE MOTOR BY DOING THIS. I WILL NOT BE HELD RESPONSIBLE FOR ANY DAMAGE TO YOUR MOTOR BY YOU DOING THIS. 
IF PROLONGED HIGH RPM DRIVING SITUATIONS OCCUR, YOU MAY INFACT THROW A CONNECTING ROD!!!!! PLEASE ONLY DO THIS IF 
YOU KNOW WHAT YOU ARE DOING, OR DON'T CARE ABOUT THE WELL BEING OF YOUR MOTOR


_Quote, originally posted by *The Difinitive Cheap A$$ Build Up!!!!!!!* »_ 7. The ABA engine has 10-1 compression ratio and a knock sensor ignition system should be used. If your car wasn’t 
originally equipped with a knock sensor you can retrofit one from a ‘85-’87 GTI or GLI that uses CIS injection. You 
can lower the compression by machining the dish in the piston crown to a larger size. If you drop the compression 
ratio to 9.2-1 you can run w/o a knock sensor.

http://forums.vwvortex.com/zerothread?id=1584170


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## atoson (Dec 29, 2002)

*Re: (ScarredWithStars)*

Another way to look at it regarding positive benefits is it gives VW added insurance from people over revving or missing a shift in aspirated driving while using cheap/low octane fuel while the car is still under warranty. That was how VW was marketing the GTI's (pocket rocket) to it's consumers besides the increase in horse power from 90 to 110/15hp. Unless your daily driving environment demands heavy engine loads and operating a CIS vehicle in California. (91 oct premium fuel), you're prolly better off watching nudie pole dancing girls chugging your favorite brewski.


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## stucain (Sep 27, 2011)

*cant believe i just read this!!???*

i cant believe i just read on here that you cant get knock sensor's on diesels!!!??? i say this as they do actually have them. it does not work in the same way as petrol engines due to the lack of ignition, they work along side the rev limiter, ecu and injectors by adjusting the air/fuel mix and reducing the rev limit due to safety reasons, as it can cause serious damage to your engine!! it tells the ecu how much fuel the injectors need to send so that it can adjust the air/fuel mix to prevent engine detonation!! also, if there is a fault like your glow plugs, knock sensor or injectors are faulty or knackered then the ecu automatically goes to a safe mode set by the factory which reduces power drastically and prevents further damage to the engine. limited fuel for the engine leaving you with a sluggish drive and very weak air/fuel ratio.

so, to clear up all confusion, i hope that you now know what a knock sensor does. (on a diesel anyway)


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## Glegor (Mar 31, 2008)

stucain said:


> i cant believe i just read on here that you cant get knock sensor's on diesels!!!??? i say this as they do actually have them. it does not work in the same way as petrol engines due to the lack of ignition, they work along side the rev limiter, ecu and injectors by adjusting the air/fuel mix and reducing the rev limit due to safety reasons, as it can cause serious damage to your engine!! it tells the ecu how much fuel the injectors need to send so that it can adjust the air/fuel mix to prevent engine detonation!! also, if there is a fault like your glow plugs, knock sensor or injectors are faulty or knackered then the ecu automatically goes to a safe mode set by the factory which reduces power drastically and prevents further damage to the engine. limited fuel for the engine leaving you with a sluggish drive and very weak air/fuel ratio.
> 
> so, to clear up all confusion, i hope that you now know what a knock sensor does. (on a diesel anyway)


VW diesels dont have knock sensors.. any of them.


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## atxmatt2510 (Jul 7, 2013)

Weird question, but would a faulty knock sensor prevent a 1.8 from starting? Just trying to do some diagnostics here.


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## Mr.loops (May 27, 2010)

atxmatt2510 said:


> Weird question, but would a faulty knock sensor prevent a 1.8 from starting? Just trying to do some diagnostics here.


No Sir it wouldn't

Common symptoms of a faulty knock sensor is; poor gas mileage, lack of power, misfiring 

Start checking the big three. Fuel, Spark, and Compression


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## atxmatt2510 (Jul 7, 2013)

Mr.loops said:


> No Sir it wouldn't
> 
> Common symptoms of a faulty knock sensor is; poor gas mileage, lack of power, misfiring
> 
> Start checking the big three. Fuel, Spark, and Compression


Thanks for the reply. I will do that. Suspect I have a warped head :/


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## crapWagen (Oct 12, 2015)

Colraindub said:


> I've heard of people taking knock sensors of their cars, just curious as to what the advantages are. Also just curious as to what a knock sensor actually does, I know it 'keeps the motor from knocking', I want more than that though.


A: they make greedy corporations LOTS of money. See the car manufacturers are really good about doing useless **** that doesn't serve any purpose but make them money. Oxygen sensors and MAF sensors are another great joke they've played on the public. Just **** that breaks and breaks and breaks and breaks, requiring YOU to buy and buy and buy and buy!


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## Butcher (Jan 31, 2001)

Fortunately you have only 10 posts. Hopefully they are way more informative that what your wrote above.

Maybe changing your name to crapInfo, it would match a bit better.


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## venom_vdub (Jul 6, 2009)

Hey guys, hate to dig up an aged thread lol 

Haven't found much to confirm 

I'm having a sensor 1 circuit low input code. (i cleaned sensor and area of block / torqued it properly)
Are the 2 sensors the same or differ by there placement on the block.(I assume there the same)

Possible to swap places, to diagnose the harness or faulty sensor.
See if the code comes back on the same one or moves to #2


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## Butcher (Jan 31, 2001)

I would too assume that they are the same. I would also agree by swapping them, it would help in the diagnosis of your issue.


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