Subaru Knock Sensor

Scooter

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I found the following paragraph at this site, http://www.sdsefi.com/aircraft.html.<br><br>11/20/00 Knock Sensors on Aircraft<br><br>We don't recommend the use of knock sensors for engines powering aircraft. Generally speaking, air cooled engine have too much mechanical noise to be able to filter it out to hear the knocking. The second problem on opposed engines especially, is placement of the sensor. Most auto manufacturers spend countless hours determining optimum placement. You probably can't. The third concern is that the engine needs to actually knock before the ECU retards the timing which isn't good for it. Fourth, if you have the sensitivity set wrong, the ECU may retard the timing the full 25 degrees because of mechanical noise. You would lose over 50% power and this could cause an accident. Extremely high EGTs could also damage the engine. Finally, if you are running on 100LL and have reasonable manifold pressure limits, compression ratio and ignition timing, the engine WILL NOT knock in the first place. <br><br><br>I am installing a 2.5 in a fixed wing and am trying to decide if I want to use the knock sensor or O2 sensors, (2.5 uses two O2 sensors).  I am inclined to by-pass them.  I would be interested if anyone else has not used them and what were the results.
 

rfi

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Re: Subaru Knock Sensor

I am in the business of manufacturing and modifying accessories for the Subaru engines.  In addition to building over 500 direct drive/reduction drive systems I have modified over 300 stock Subaru EFI harnesses.  <br><br>Whoever made that statement about knock sensors doesn't know what he is talking about.  I have never heard of a problem with a knock sensor on an aircraft installation.  In fact, without the knock sensor the engine will go into LIMP HOME mode and rpm will be limited to 3600 rpm.<br><br>You can run the engine without the O2 sensors but it will run on the rich side.  You only need the front one for mixture control.  The rear sensor (behind the Catalytic converter) is used to determine if it is functioning properly.<br>The stock system works just fine.  The only thing you have to do to the EJ25 is wire the inhibitor switch wire so that the ECU thinks that the transmission is in DRIVE and implement a SPEED SENSOR SIMULATOR which is simply a 5 volt pulse generator.
 

PW_Plack

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Re: Subaru Knock Sensor

It should have been a clue that the guy wasn't up on Subarus, when he started talking about O2 sensors on air-cooled engines!
 

hasse

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Nov 12, 2003
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Thailand
Re: Subaru Knock Sensor

To not use knock sensors is just a load of bull..<br><br>Knock sensors are basically small microphones that are tuned to "listen" for a certain frequency (the knocking) and if they are properly tuned (and they are or your car would stop) then they are trouble free, also why would an air-cooled engine be noisier than a water cooled? there are less moving parts!<br><br>And yes, the engine does knock before the sensor picks it up, otherwise it would not be able to pick it up.<br><br>That guy has no idea what so ever.<br><br>Don, would love to get a link to your site or your e-mail as I am about to re-build a Subaru for use on a gyro.<br><br>Hasse
 

Scooter

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Oct 30, 2003
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Cabot, Arkansas
Re: Subaru Knock Sensor

Over the past couple of years I've read many postings about  the pros and cons of using O2 sensors.  Some say that without them the engine just runs a little richer. no harm done  It's really hard to know what to believe, but being as I don't have a clue about Subarus, I believe to be on the safe side, I will just follow Mr. Parham's advise and use the knock sensor and front O2 sensor.<br><br>Oh, Mr. Plack, if you would reread my post you will see that what is underlined is what the author of the paragraph about knock sensors had to say.  What is not underlined is what I had to say in referring to my 2.5 water cooled Subaru engine,  which uses two O2 sensors.  I don't believe I mentioned air cooled engines, he did.
 

PW_Plack

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Re: Subaru Knock Sensor

Scooter, sorry! My mistake. <br><br>If a Subaru in a car successfully uses a knock sensor at high loads and RPM, I'm not sure why an aircraft conversion couldn't. His point about the right tuning and fuel precluding knocking from happening in the first place sounds valid, as long as you're not running a turbocharger.
 
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