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View Full Version : Can the knock sensor malfunction and pull timing?



90tbirdsc
05-16-2013, 07:09 PM
Hey guys, basically what the title says. 90 sc auto, mp inlet/ported late model, 2.5 to single 3 exhaust with resonator, all stock internals, FMIC, 255lph walbro, 42# injectors and lmaf.

at 20% od with about a 30/70 mix of 110 race gas and pump 91 it would pull timing way back to around 17-18 degrees when I was commanding 25. now I lowered the OD back to 10% with the same fuel and It pulled to back to 18 degrees again.

Im stumped

XR7 Dave
05-16-2013, 11:44 PM
The knock sensor doesn't often malfunction but it's operation is often misunderstood.

The knock sensor simply listens for noise in a certain frequency range and when it records that noise, it looks at how loud that noise is. If it's louder than a certain threshold, it starts to pull timing. The noise that is heard from knock is also present in normal combustion, but at a lower volume. Ford dialed all this in for a factory engine at factory boost levels. Increasing boost and therefore cylinder pressure increases combustion noise of all frequency levels. Since the knock sensor is tuned to one specific frequency, it is possible to trigger the knock sensor when knock isn't actually present. The trick is knowing if you have knock or not. On a stock engine you can normally count on the knock sensor to be pretty accurate up to a certain boost level. But there does come a point when it can't be relied on anymore. Knowing where that is, is the trick.

On a motor with ported heads and cam, that point is increased. With stock heads due to some of the design the engine won't tolerate much timing so you have to be careful. On a modified engine you might get away with running 19psi and 26 deg timing - and make good power too. But on a stock motor 26 deg and 19 psi will just break rings. For example, my current ride is internally stock and at 20psi I'm running 16 deg on top. It makes 280rwhp so you know it's not a slouch but I can't run 26 deg, it would just blow crap up.

90tbirdsc
05-17-2013, 12:06 AM
see, mine is pulling timing and is very poochy, it runs nothing close to this http://youtu.be/Oy8levQlwEI he has the same mods but its a manual.

mine is this:http://www.youtube.com/watch?v=RJPU-0IS-uU&feature=share&list=UU_KBEm1RAchVUFGeEAj0i6A

ported late model, mp inlet, 75mm tb, lmaf, fmic, ported exhaust manifolds, raised top, dual 2.5 to single 3, coated rotors, 42#/255lph fuel, 3:55 gears


The knock sensor doesn't often malfunction but it's operation is often misunderstood.

The knock sensor simply listens for noise in a certain frequency range and when it records that noise, it looks at how loud that noise is. If it's louder than a certain threshold, it starts to pull timing. The noise that is heard from knock is also present in normal combustion, but at a lower volume. Ford dialed all this in for a factory engine at factory boost levels. Increasing boost and therefore cylinder pressure increases combustion noise of all frequency levels. Since the knock sensor is tuned to one specific frequency, it is possible to trigger the knock sensor when knock isn't actually present. The trick is knowing if you have knock or not. On a stock engine you can normally count on the knock sensor to be pretty accurate up to a certain boost level. But there does come a point when it can't be relied on anymore. Knowing where that is, is the trick.

On a motor with ported heads and cam, that point is increased. With stock heads due to some of the design the engine won't tolerate much timing so you have to be careful. On a modified engine you might get away with running 19psi and 26 deg timing - and make good power too. But on a stock motor 26 deg and 19 psi will just break rings. For example, my current ride is internally stock and at 20psi I'm running 16 deg on top. It makes 280rwhp so you know it's not a slouch but I can't run 26 deg, it would just blow crap up.

LIVipers
05-17-2013, 07:00 AM
Something isn't right there, it sounds like the car is going from 1st to third gear, or accelerating like it anyway

neverfastenough
05-17-2013, 08:32 AM
It's too hard to compare to YouTube videos. That car ran 13.50's all day . Get to the track and see what yours runs.

neverfastenough
05-17-2013, 08:38 AM
Also understand that my tune was about as mean as possible and the motor survived 2 years. Don't forget you still have a stock motor in there. My built motor making double the rwhp in that video, has already lasted a year longer than that video. And it's had some bad tunes too. Theres little wiggle room before boom in your motor, especially with 20% od

90tbirdsc
05-17-2013, 11:56 AM
Something isn't right there, it sounds like the car is going from 1st to third gear, or accelerating like it anyway


Yea it will go through 1st and go to 2nd for a moment and then go to 3rd. If I manually shift it it won't do it

LIVipers
05-17-2013, 12:14 PM
Yea it will go through 1st and go to 2nd for a moment and then go to 3rd. If I manually shift it it won't do it

So it sounds like your transmission is the problem and not the knock sensor...

90tbirdsc
05-17-2013, 01:39 PM
it is acting up as well as my gauges, http://www.sccoa.com/forums/showthread.php?t=130079

The knock sensor will pull timing from 19 degrees back to 12 one time and then another time it will pull it to 17-18. But there is no difference in power between the runs.