so recently, i installed a midpipe on my car... the boost was creeping....so i had a restrictor plate made. is has a 2.5 inch opening....just installed it and took the car down the street. my boost didn't creep, but the knock sensor on my PFC hit like 71....i went through my PFC manual and it doesn't really explain what knock is....please give me some help....porfavor, Zach

 

i think the max knock reading should be 60. sombody that knows more than me will confirm this. knock is a way of measuring detonation. high knock readings can come from faulty sensor/ loose fasteners/etc causing '' noise'' around the sensor. probably a crappy explination but i tried lol

 

The knock sensor is a microphone that listens for detonation. The stock one isn't know for being overly accurate......each FD seems to have a different baseline, so to speak. On my old engine I saw peak readings around 50, on my new engine it is much lower, around 10. 70 may not be a problem if you're seeing it all the time.

Bottom line, it's interesting to track it but I wouldnt hang my hat on it. If you're that worried either install a water/meth injection setup or put the main cat back on

 

would porting the wastegate end this? i only see the knock go that high under heavy acceleration. but also, there is that small cross member that is beneath the exhaust tunnel and it didn't touch the pipe, because of the exhaust hanger. but after i installed the midpipe, it now rests on this crossmember, because there is no hanger on it. could this contribute to it? but it wouldn't make sense because then the vibration would occur all the time, enabling the knock sensor at any rpm...but the knock only gets excessivly high when i boost further along in the rpm range at wot. also the car will only backfire after i really get on it.....i know this is newbish....but i realy need the help....deployment is only a month away and i want my car to at least run right until then

 

If you are only seeing those readings under wot/high boost situations then that could be a sign of true knock. Many cars get readings that high even in the cruise sections of the map which can pretty much be ignored, but its the high boost and high rpm knock readings you really have to worry about. Do you have a wideband on the car, datalogit? Porting the wastegate wont help if you dont have a problem with boost creep, but it might let you get rid of that restrictor plate. What other mods do you have, and what are you actually boosting?

 

i have 99 twins, streetport, apexi cai, dp, mp, cb, power fc, y pipe....possibly some i'm forgeting too..but thats pretty much it..no wide band or boost controller. when i had the catalytic converter on there....my boost pattern was 12-10-12

 

did you have your car tuned with all that other stuff on it before you went mp?

 

good move on solving the boost creep.

the OEM knock sensor, as Rich stated, is a band limited microphone and i have found them to be quite accurate. i was chasing a knock and tried four of them and all four produced the same reading.

as to your situation.... you do not disclose what your readings were before the restrictor. i will just assume they were lower. the sensor is probably picking up a mechanical noise perhaps relating to your midpipe.

often this noise happens around peak torque and drops after. how does your reading relate to rpm?

hc

 

The engine knock sensor is very much like a vibration analysis program for industrial machines - you must have a baseline idea of the levels in order to detect problem conditions. So you diagnose conditions by looking for changes in knock, all else being held equal. You start with the system running well, and take note of the knock levels. The actual level of knock will depend on just how "noisy" your engine is at the frequencies picked up by the sensor. (I'm not sure offhand what frequency range it is, but I doubt it corresponds to how the engine sounds by your ear). Mechanical looseness, the stiffness of the motor mounts, etc can affect the normal knock level.

If your boost is under control now (no creep), fuel delivery is good, car is running well, etc it's a good time to pay attention to the knock levels at no load, medium load, and full load. Like the others said if it's in a reasonable range just note the number and don't worry about it.

Now you can say something about knock when you make changes to the car and detect problems by comparing it to the baseline values.

Howard Coleman has made it perfectly clear that knock can be reduced to very low levels using an auxiliary injection system. (That's not the only purpose of AI systems, but one of it's best side effects).

Dave

 

This is the definition of precision, but the sensors aren't very accurate, in that they can be thrown off by ancillary noise.

Many years ago when WOT and shifting fron 2nd to 3rd I heard a horrible noise under the hood and my knock reading spiked to over 200! I think my BOV malfuncitoned, to this day I'm not really sure. The noise didn't come from inside the engine though.

In the past when test driving customer cars I've noticed some have a peak reading at cruise at over 100, which would cause some serious alarm on my setup. Some cars are 'louder' than others

 

the car still has yet to be tuned, since there are no tuners close to me, i have to wait for either kan or bdc to come to my area. but before i put on the mp, my knock readings never went above 20. if it is a microphone, the car is very loud compared to what it was before, before i added the restrictor plate, the knock would only go to high values when under WOT and boost. now that the restrictor plate is on there, the knock value will hit the mid 50s while it is idleing. i haven't heard any crazy noises yet.....so maybe i'll just continue watching it. i just don't wanna blow the engine before i deploy in a month...lol. but i appreciate all the help you all have given me, thanks again and keep it going

 

a streetport, 99 spec twins, full exhaust, and 12 psi might be asking a little much from the base tune. Even tho you are not boosting alot more than stock all of your mods create a decent amount of extra flow, I would take it pretty easy on the car and try to get it tuned.