Knock Sensor question
 

After talking tonight and after letting Evil 1 drive my car I have a question.

If my knock sensors are reading vibrations from the blower and causing the engine to pull timing what prevents me from disconnecting them? I would throw a CEL obviously. I do know that I am creating no knock in my current condition. Jim ( evil 1) was saying that he always pulled the knock sensors on the cars he built to prevent them from inappropriately pulling timing.

given that they could potentially save your engine in the event that it is actually experiencing knock, why not try relocating them to a different part of the engine? what makes you think that the s/c is imparting a load on the engine block that resembles a "knock"?
-Bill

Because of their proximity to the blower and the amount of noise the blower makes....

Also... I dont recommend ANYONE pulling the knock sensors unless they are on the safe side of their tune and monitor the engine VERY closely.
I have done it on almost all of my turbo engines because the ECU was VERY aggressive about pulling timing and the sensoers were bad about picking up a false positive on detonation.
Relocating them also works, but as I am still learning about the SVX I cant offer an alternative location to mount them.

However, that being said... I have pulled/disconnected them and picked up 30+ whp on a daily driver pushing 25 psi... that showed no sign of detonation when the engine was pulled down to upgrade the head gasket in anticipation of more power.


Jim

There should be no reason for the knock sensors to react to noise from a blower. A weight is incorporated so that they are not sensitive to anything even remotely within the audio frequency range. Furthermore they a mounted to a large mass which would damp out extraneous indirect vibrations.

I don't know how advanced the ECU is on an SVX, but most newer cars, if the ECU detects knock, it ignores it unless it occurs within a window around the ignition pulse, in order to prevent false signals from the knock sensor from affecting engine performance.

well. Our cars are pulling timing in a big way. Not sure how to explain it but with an eye on the AFR and an ear to engine, running without just to test how the car runs is a good idea. I already gave him precautions about what to expect when it comes to audible knock but we should not be getting this with the new motors and 93 octane fuel. A better test (now that I am thinking) would be to run close to empty on the tank of fuel you have and dump $50 worth of 103octane race fuel in it to see what happens.

Tom

Knock sensors detect knocking that is not audible to the ear. They detect a very specific frequency. I would not reccomend disconnecting them, my experience tells me that's not a good idea. They are there to prevent damage to the engine. Disconnecting them would most certainly gain you power, but it would sacrifice engine longevity. I do know that the knock sensors that our engines use are considered outdated and modern engines no longer use the peizo type knock sensors. I would think that adapting to a newer style sensor would be the answer to this problem. But, of course, this is much easier said than done. Does anyone know what sort of sensor the new turbo EJ motors use? I would also assume that knock sensor logic would be significantly different in a forced induction engine.

I dont understand how I would be sacrificing engine longevity. I will be carefully monitoring the engine when I do the test AND do not misunderstand my intentions. The plan is not to disconnect them and find that there is a bunch more power and then leave it that way. If I get the results that I anticipate this will just mean the knock sensors need to be relocated.

Making a few dyno pulls with high octane in the car and monitoring it for AFR, audible knock, and temp *engine, induction, and exhaust would be best*... shouldnt cause damage but would give further insight to the performance of the KS and possibilities to work around false positives.
Driving it you feel the car starting to respond and then essentially falling on its face, compared to the pull it should have.
It isnt even my car and all I could think of was how to get past this stumbling block.
I can imagine how it must be for Bobby and Tom.


Jim

Please advise in full regarding the knock sensors now in use. I have not seen any information in this regard and the subject is of considerable interest.

well using the higher octane with the knock sensor still connected should yield interesting data. Meaning, if it is detonation pulling timing, this will be reduced and timing should not be pulled. If it is noise that the sensor is picking up (possibility I will not rule out) it will continue to happen. Its a test worth while that will not risk much of anything except some $ for race fuel

Tom

Relocating them sounds like the winning ticket.

Could they be relocated to the bottom of the engine? This would allow them to pick up on the same area to ensure they get a proper signal. Granted the wiring may have to be extended, but this sounds like the most reasonable course of action.

There are two knock sensors. It very unlikely that there will be differences from on e bank to the other. Therefore one sensor could be safely disconnected and the output monitored to ascertain exactly what is going on. A resistor of approximately 550K ohms as a substitute, should prevent an error code.

Unless the car is on a dyno it will be difficult to check sensor output using a scope. Being from a number 8 wire background I have a suggestion.

The manuals show an output impedance of 560k ohms, so that if the sensor output was hooked into the input of an audio amplifier, any signal occurring could will be audible, even if only as a series of clicks or thumps. Any sort of simple portable audio amp. or something incorporating same will do the trick.

The gain available will surely far exceed what is required, so that a 500k potentiometer should be connected across the sensor output, and the amp. fed from the wiper. A low gain input should be used if one is available. Audio fanatics will know what I have in mind. :D

Obviously I am suggesting an experiment with no guarantees. ;)

I am calling the dyno tomorrow to see if they have 103ish octane gas