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Thread: Knock sensors revisited

  1. #21
    Visit www.boostbutton.com... Turbo Mopar Contributor ShelGame's Avatar
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    Re: Knock sensors revisited

    Quote Originally Posted by wallace View Post
    Have you guys tried using higher octane fuel to see if this reduces or eliminates the knock? I have had 2 problems that were not knock cause the cel to flash. I traced one to bad alternator bushings and the cel went away. The second was knock on light load when engine was not up to operating temp. This second I believe was a rod bearing on its way out...I haven't taken the engine apart to see but I believe it spun while being flogged. With mpscan can we see which cylinder is detonating in the log?
    Yes, MP Scan can log all 4 individual knock retards so you can see which cylinder is knocking.
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    Re: Knock sensors revisited

    Quote Originally Posted by ShelGame View Post
    Ken, the current implementation of the CEL knock indicator is that the light comes only when the ECU is retarding timing due to knock.
    IE, any one of the 4 knock retard values is greater than 0.
    Knock retard is ignored below a set coolant temp (to reduce cold false knock).
    So, its not just simply looking at the threshold.
    I agree - the production minimum temp for knock reduction will definitely work to reduce false CEL triggers.
    How many coded versions/variations are presently available and are there additional parameters being monitored, beyond the knock cells greater than zero?

    The concern then becomes understanding how the following comment occurs.
    I have the knock sensor trigger my check engine light, and it comes on when I hit a pot hole.
    In other words, if a knock cell has a value stored; the CEL is illuminated until all the cells decrement back to zero.
    This event can be greatly extended if the return rate to normal spark advance has been changed to an improper value.
    This can also generate the impression that knock is still active even though it is not.
    As previously mentioned, when additional required conditions (knock sensor voltage currently above the threshold, active retard, etc...) are monitored, then the CEL is illuminated only when legitimate knock is actually occurring.

  3. #23
    Moderator Turbo Mopar Staff Force Fed Mopar's Avatar
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    Re: Knock sensors revisited

    All my testing has been done on 93 octane.
    Rob M.
    '89 Turbo GTC

    2.5 TIII stroker, 568 w/ OBX and 3.77 FD

  4. #24
    Visit www.boostbutton.com... Turbo Mopar Contributor ShelGame's Avatar
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    Re: Knock sensors revisited

    Quote Originally Posted by 5DIGITS View Post
    I agree - the production minimum temp for knock reduction will definitely work to reduce false CEL triggers.
    How many coded versions/variations are presently available and are there additional parameters being monitored, beyond the knock cells greater than zero?

    The concern then becomes understanding how the following comment occurs.


    In other words, if a knock cell has a value stored; the CEL is illuminated until all the cells decrement back to zero.
    This event can be greatly extended if the return rate to normal spark advance has been changed to an improper value.
    This can also generate the impression that knock is still active even though it is not.
    As previously mentioned, when additional required conditions (knock sensor voltage currently above the threshold, active retard, etc...) are monitored, then the CEL is illuminated only when legitimate knock is actually occurring.
    I see your point. Currently, all of the CEL illumination code is exaclty the same; in the cals I have posted.

    Adding a criteria to check the knock thrshold as well is easy, and I think it's a good idea.

    In my experience, the knock indicator works well as-is (only from knock retard > 0). On the SMEC anyway, the factory knock retard decay is fast enough to make it very effective.
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    Re: Knock sensors revisited

    Is it possible that the forged pistons add more operational noise even when warm? I have to say though that I have watched the voltage some and it doesn't seem to pick up anything except in boost or on shifts in a certain rpm range.
    Rob M.
    '89 Turbo GTC

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  6. #26
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    Re: Knock sensors revisited

    Am I the only one that driving around with a scanner? I look for knock voltage and retard when the light flashes.


  7. #27
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    Re: Knock sensors revisited

    Quote Originally Posted by Force Fed Mopar View Post
    Is it possible that the forged pistons add more operational noise even when warm? I have to say though that I have watched the voltage some and it doesn't seem to pick up anything except in boost or on shifts in a certain rpm range.
    I would say that they would add additional noise compared to a stock cast piston. I think a test with higher octane fuel would give you an idea as to whether or not the knock is real or related to some other noise present. If the knock is reduced significantly or eliminated then it really is detonation if not then I would look at finding the source of the noise and eliminate it

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    Re: Knock sensors revisited

    Quote Originally Posted by ShelGame View Post
    I see your point. Currently, all of the CEL illumination code is exaclty the same; in the cals I have posted.
    Adding a criteria to check the knock thrshold as well is easy, and I think it's a good idea.
    Thanks and I agree - this one is simple.

    Quote Originally Posted by ShelGame View Post
    In my experience, the knock indicator works well as-is (only from knock retard > 0).
    On the SMEC anyway, the factory knock retard decay is fast enough to make it very effective.
    Rob, I agree 100% that there are many things in the code/calibration that work fairly well as is.
    But whether they need it or not, having an adjustment 'knob' permits change.
    The challenge for everyone is knowing when they need to change and to what values.
    An example for requiring change is the use of a G-Head on later electronics and may lead to a cut and paste of the early cal values, from a 1985 Turbo-I.
    Although, with head milling and discontinuity between the early and later style single/two-piece intakes or fabricated intakes means the older T-I values may not apply.
    These conditions will affect the increment rates, decrement rates, long term knock constants and tables, and the knock sensor threshold table that triggers it all.
    With this, an indicator for things heading in the wrong direction is disabling the use of long-term knock.
    Rather, this should be a nagging question that asks "Why is the controller always trying to retard (de-rate) timing to a lower value spark table?" so the real problem can be addressed.
    This is why "When things are done properly, backing out of the throttle is no longer needed and the use of the CEL for knock indication is reduced to 'an event occurred but it's been handled."

    Once again, I applaud your efforts and always enjoy a good technical discussion - great thread !

    EnJoy !
    Last edited by 5DIGITS; 03-20-2014 at 11:25 AM.

  9. #29
    Supporting Member II Turbo Mopar Contributor Shadow's Avatar
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    Re: Knock sensors revisited

    Quote Originally Posted by tryingbe View Post
    Am I the only one that driving around with a scanner? I look for knock voltage and retard when the light flashes.

    Not the only one! lol I use a scanner on ever car I tune. Once tuned with zero knock I find no reason to monitor knock any further. Run the right fuel, Don't mess with Anything, and everything remains Safely tuned.

    Only issues I've run into; Some people can't leave well enough alone and just have to mess with stuff. (mostly because of what they read on the forums)

    For this reason I started installing knock lights in some cars..........

    The Charger has never had any knock gauge or anything to monitor knock that remains in the car. Last time I checked the tune for knock was probably 4 years ago, but as some have said, I run the best race fuel I can nowadays. That, along with reading plugs on a regular basis, and there's really no reason to monitor knock unless something is changed.

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  10. #30
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    Re: Knock sensors revisited

    Quote Originally Posted by Shadow View Post
    Not the only one! lol I use a scanner on ever car I tune.
    Once tuned with zero knock I find no reason to monitor knock any further.
    Run the right fuel, Don't mess with Anything, and everything remains Safely tuned.
    Only issues I've run into; Some people can't leave well enough alone and just have to mess with stuff. (mostly because of what they read on the forums)
    For this reason I started installing knock lights in some cars..........
    The Charger has never had any knock gauge or anything to monitor knock that remains in the car.
    Last time I checked the tune for knock was probably 4 years ago, but as some have said, I run the best race fuel I can nowadays.
    That, along with reading plugs on a regular basis, and there's really no reason to monitor knock unless something is changed.
    Thank you - Very well said !

  11. #31
    Moderator Turbo Mopar Staff Force Fed Mopar's Avatar
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    Re: Knock sensors revisited

    So then basically I am probably running the limit of 93 octane on a 2.5, hence why the light always seems to be on above 12 psi, with all values set to stock? FWIW I ran 20 psi on my 2.2 Daytona without these issues, using the same exact top end and knock sensor. Only difference being 2.2 cast piston and 2.5 forged piston, and the 2.2 was on a large fmic, the 2.5 is on a awic. Also the 2.2 had a stock exhaust manifold, and on the 2.5 I ported it. AFR is 11-11.5 under WOT.
    Rob M.
    '89 Turbo GTC

    2.5 TIII stroker, 568 w/ OBX and 3.77 FD

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    Re: Knock sensors revisited

    Quote Originally Posted by Force Fed Mopar View Post
    So then basically I am probably running the limit of 93 octane on a 2.5, hence why the light always seems to be on above 12 psi, with all values set to stock? FWIW I ran 20 psi on my 2.2 Daytona without these issues, using the same exact top end and knock sensor. Only difference being 2.2 cast piston and 2.5 forged piston, and the 2.2 was on a large fmic, the 2.5 is on a awic. Also the 2.2 had a stock exhaust manifold, and on the 2.5 I ported it. AFR is 11-11.5 under WOT.
    When you say "all the values" does that include fuel, spark, and knock sensor cals?
    What are your EGT's currently at ?

  13. #33
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    Re: Knock sensors revisited

    Quote Originally Posted by 5DIGITS View Post
    When you say "all the values" does that include fuel, spark, and knock sensor cals?
    What are your EGT's currently at ?
    I don't have an EGT gauge, just an AEM wideband AFR monitor. Fuel is scaled for +40's and is 11.0-11.5 at WOT at any boost level. Spark tables being stock except for continuing slope after 14psi (where the stock tables flatline when you scale for 3-bar map). Knock tables being stock MP values. at 14-15 psi or higher with these settings, it would always set the CEL off at WOT. Normal light boost acceleration would be fine, moderately heavy part throttle acceleration would set it off sometimes. At the time I did not have a scanner or a laptop with a good battery so I couldn't look at the knock retard.
    Rob M.
    '89 Turbo GTC

    2.5 TIII stroker, 568 w/ OBX and 3.77 FD

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    Re: Knock sensors revisited

    This is a stock MP 2.5 MTX cal, scaled for 3-bar and +40's, no other mods. It will flash the CEL for knock above 12 psi in my car, guaranteed, and it is audible.
    Attached Files Attached Files
    Rob M.
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    2.5 TIII stroker, 568 w/ OBX and 3.77 FD

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    Re: Knock sensors revisited

    This cal is based off the MP 2.5 MTX cal, but with highly modified knock threshold and the spark tables have the slope continued after 14psi instead of flatlined. Also the MultiplerOnMapAdvance has been flattened out. Seems like Rob L. recommended this for tuning the timing tables, but maybe I misunderstood him.
    Attached Files Attached Files
    Rob M.
    '89 Turbo GTC

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    Re: Knock sensors revisited

    Quote Originally Posted by tryingbe View Post
    Am I the only one that driving around with a scanner? I look for knock voltage and retard when the light flashes.

    I'd love to have a scanner...just never could afford one when they'd pop up.

  17. #37
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    Re: Knock sensors revisited

    fwiw - mpscan will log all those variables and more ..... and you could use your android phone if you want. so if you are already setup to burn tunes there is no additional cost for scanning. and actually i have been working on implementing the drb sensor/relay tests also.

    sorry - off topic comment finished.
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    Re: Knock sensors revisited

    maybe 2.5s want more fuel, stock at 15psi is running around 10:1 isn't it?
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    Re: Knock sensors revisited

    Quote Originally Posted by shackwrrr View Post
    maybe 2.5s want more fuel, stock at 15psi is running around 10:1 isn't it?
    The current concern is the use of a stock 'fast-burn' head, slightly milled, at 18+ psi.
    The fastburn head is already susceptible to knock, due to the lack of squish/quench, and without any chamber mods or bowl work the pressure rise at 18+ psi is violent.
    When the pressure curve is short with high amplitude, the burn rate becomes an essential spike.
    This leaves very little room for 'normal' spark advance and is shortened even further when combined with elevated boost levels.
    In short, the burn rate determines how much spark can be tolerated and exposes why there is such a significant difference between the blastburn and previous G-head spark curves.
    Last edited by 5DIGITS; 03-21-2014 at 09:49 PM.

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    Re: Knock sensors revisited

    With permission, I am providing the following discussion from a PM exchange that has been going on per this threads topic.
    Hopefully this information may also be foudn useful by someone else.


    I will check that. Would be worth going to an upgrade spring, or should I just stick with the stock roller springs? If I understand correctly from reading tech threads on places like Speedtalk, you only want as much spring as is needed to properly control the valve at maximum rpm and boost. Any more than that and you are just creating additional rotating friction which takes more power for the crank to turn.
    Absolutely true !

    It is a completely stock 782 swirl head, stock valve size, no porting, edge beveling or gasket matching at all.
    This is the problem.

    So what you're saying is that the swirl design swirls too much at higher boost levels? What does this cause?
    In part but yes.
    The head does not provide adequate squish.
    This is the event that occurs as the piston approaches TDC.
    The gases are squeezed between the outer diameter of the piston and the deck surface of the head, towards the center of the combustion chamber.
    This does two things - compresses the remaining charge and increases localized charge density and forces any residual unburnt fuel into a reduced volume to extend the burn process.
    Both of these events suppress knock and the fast burn falls significantly short in supporting this process.


    Uneven combustion?
    Yes, this is also part of the problem.
    The gases that are permitted to linger between the plug and the intake valve provides a non-centralized burn, in a horse shoe shape.
    Conceptually speaking, this allows the burn to collapse into each other and colliding flame fronts cause knock.

    All my problems do seem to be much worse above 14 psi. 10 psi was no problem, 12 and 14 psi needed a little tweaking IIRC, and once I start running 15 and above it gets exponentially harder to avoid the CEL illuminating due to knock retard.
    Exactly - As mentioned early on, the hardware arrangement will work well up to 13-14psi.
    After that, the returns/power gains are over-shadowed by the need to greatly reduce spark advance.
    Therefore, approx. 30HP is gained by raising boost from 10-13psi but raising the boost from 13-17psi only nets 10hp.
    Peak efficiency and acceptable operating conditions have been exceeded.

    BTW
    I'm convinced that this is also the cause for your head gasket leak - the cylinder pressures can double during knock.
    Don't bother milling the head again.
    Replace the gasket and lower the boost to 14psi or less.

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