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Knock Sensor picking up headers?

bigburban383

bigburban383

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Will a knock sensor pickup anything other than a knock? Like headers?

The computer picks up a knock at idle and when your cruising at some speeds mostly. I have 4 gallons of turbo blue 110 octane, 4 gallons of 93 octane, 1 gallon of Toluene, and about 15 gallons of 87 to start with. Thanks for help /forums/images/graemlins/thumb.gif
 
I know a knock sensor will pick up a larger cam sometimes /forums/images/graemlins/dunno.gif /forums/images/graemlins/dunno.gif /forums/images/graemlins/dunno.gif
 
When you say it picks up a knock, how quickly is the knock count increasing? I've noticed that the knock count will slowly creep up (like 1 to 5 counts per minute) on a couple of my stock TBI engines while idling in the driveway. /forums/images/graemlins/eek.gif If you aren't seeing a huge number of counts then you may be chasing a ghost.
 
It is certainly POSSIBLE for the knock sensor to pick up something else, but since the knock sensor is "tuned" to a frequency, it's very rare.

LOTS of people run gear drives, headers, etc., and have no problems with knock. Not that it can't happen, it's just not common at all. If it were, *every* OBD1+ vehicle with headers would be having driveability problems, and there are a whole heck of a lot of thirdgen camaros, Corvettes, and TBI/Vortec trucks running around with headers.
 
I have quite a bit of experience with false knock as I routimely do modifications to late model performance cars.

Headers do not typically induce false knock, but I have seen it in modified cars with increased compression due to the stronger exhaust pulse. Remember each vehicle has a specific sensor for that particular engine. Once modified, it may be too sensitive & pick up false knock.

For example, the LT4 Vettes had a different knock sensor/module from the LT1 models due to the noisier roller rockers.

Often poorly installed/fitting aftermarket exhaust will cause false knock. Exhaust rattles will often show considerable knock.

Certain Roller rockers, almost all solid lifter cams & most gear drives do cause some false knock.

Most of this can be tuned out by programming.

Often the sensor can be detuned slightly as well by
removing it, adding teflon tape & reinstalling it at a lower torque.

Hope this helps.
 
Well, as was mentioned, the knock sensors are "tuned" to the applications. I would *suspect* even a .030" overbore likely changes the knock sensors "perception" of the engine.

Anyways, even a 305 and 350 of the same year/application don't share the same knock sensor, so you know it is quite application specific.

I never did quite understand the GM service manual "tests" (although this could have been out of an 80's service manual) that states you can hit the exhaust manifold with a hammer and watch for knock counts. If it is tuned to specific engine harmonics, how does it detect that? Again, that could just be due to the age of the manual I was reading, but on some applications the knock sensor uses a "stand off" from the block to desensitize it.
 
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With the type of fuel you are using, one would think that you are not getting any pinging, but you might want to check your timing with a vacuum gauge anyway. I keep trying to tell people that with an aftermarket camshaft A TIMING LIGHT DON'T MEAN JACK SH!T because aftermarket camshafts are not indexed to a GM factory crankshaft when machined, but I do not know if this message is getting through to people. If you have an aftermarket camshaft in your engine, and you are using a timing light to time it, then the timing on your engine is not what you think it is; unless you have gone to the great time consuming effort of using a degree wheel to find True Top Dead Center (TTDC) on your engine, dial in an off-set camshaft timing chain gear, attach a timing reference pointer on your engine, and use degree tape to mark TTDC on the Harmonic Damper.

When I worked on Funny Cars, degreeing in a camshaft is exactly what we would do because you can certainly not use a vacuum gauge on a blower; at least with any degree of accuracy, thus the only method of accurately timing a Blown and Injected engine on Alcohol was to degree in the camshaft, and use a timing light; so that we would know for sure what the timing light said was what we actually had for timing.

Then of course what was said about an aftermarket camshaft setting off a Knock Sensor is true.

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Please enlighten me on the method of using a vacuum gauge to find the timing /forums/images/graemlins/thumb.gif
 
This may have been stupid I don't know, but I disconnected the knock sensor connector and drove home. I got a code for near WOT no knock detected, as in the engine is supposed to knock /forums/images/graemlins/dunno.gif
 
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. I keep trying to tell people that with an aftermarket camshaft A TIMING LIGHT DON'T MEAN JACK SH!T because aftermarket camshafts are not indexed to a GM factory crankshaft when machined, but I do not know if this message is getting through to people.

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Well get this messege through--valve timing and ignition timing are too totally different things. Reguardless as to when the valves open and close, ignition timing is referenced to TDC on the compression stroke which occurrs no matter what camshaft is in your engine.

There are a number of factors that go into what the correct timing is for an engine, and you're correct in thinking that an aftermarket camshaft will change those factors, however, to tell someone that setting their ignition timing at least to start out with that a timing light is worthless is complete crap.

You might not find the ideal spot for your timing with a light, but it will certainly give you something to start with. If this is a TBI engine, the factory spec is zero anyway.
 
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I got a code for near WOT no knock detected, as in the engine is supposed to knock

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Yep! Part of the self-diagnostic is to occasionally force the engine into a state that should produce a knock for a very short period of time. If the knock sensor doesn't "hear" a knock during this time, then the ECM sets a code to let you know that the knock sensor circuit has failed. /forums/images/graemlins/cool.gif

It does something similar to test for proper EGR operation. While cruising down the highway at a steady speed it will briefly turn the EGR valve on and off, while watching for the appropriate changes in response from the O2 sensor. If the changes don't happen as planned, then the ECM sets a code for no EGR. /forums/images/graemlins/eek.gif
 
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If timing is locked down to zero, and the computer controls all advance, then both timing light and vacuum gauge are useless.

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Oh really? Can you explain to me how you'd find zero without a timing light if you wanted your timing set to the factory spec then? If you know of another way, I'm open to suggestions. Last I checked, a timing light was the only way to set your timing at 0*.

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To adjust the timing with a vacuum gauge connect it to a manifold vacuum source, and advance timing, until you reach maximum vacuum, then back it off 1/2 inch of vacuum, lock down the distributor and your set.

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If this really was an acceptable method to set ignition timing, the manufacturers would have suggested it a long time ago, even for stock engines. This method is flawed in a number of ways, the biggest one is emissions. Although your vacuum gauge may tell you that it is running better, it still may produce more emissions at that point.

Also, depending on if you're running vacuum advance at idle or ported vacuum to the distributor this may not be the ideal timing setup.

I strongly disagree with this method. It makes absolutely no sense to reference anything but where the piston is at any given time to set timing.
 
I am not mad at you either, I just find it hard to believe that if your method was really better, more people, especially the OE's, would be doing it.

I do also find it /forums/images/graemlins/confused.gif /forums/images/graemlins/dunno.gif how the OE's can spec the same ignition timing for a number of different engines of varying displacements/compression ratios with different heads to boot. Kinda seems like a band-aid they've been working with for a long time.
 
the reason must the stock stuff is just that "stock" it's so they can pass emissions and beable to mass produce cars.

and whos to say when your truck left deerborn that it was actually set at 8*.. GM is just as bad as a pervious owner as the next guy.
 
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[ If you want better or "exact science", with an aftermarket camshaft, then you will need to DEGREE in the camshaft, in order to get a correct reading from a timing light, which is time consuming, and takes some knowledge to do.

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Once again, valve timing has nothing to do with ignition timing. A timing light flashes when the cylinder that the pickup is on fires, and the timing marks are on the crankshaft.

The timing light doesn't care whether your cam has been degreed or not.
 
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