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Old 07-05-2008, 06:54 PM   #1
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Do knock sensors really allow the engine to advance indefinitely? (Theoretically)

On the civics before 1996, they did not include a knock sensor, and the ones 1996 and above do because OBD-II requires a knock sensor. The advantage of a knock sensor is to prevent the engine from knocking, the ECU tunes the ignition timing until it knocks (supposedly). Now I'm wondering, does the ECU on the Civics have a limit to how much it can advance timing? Is there a top end to how much it can advance ignition timing?

Supposedly with the knock sensor, it should be advancing the timing as much as possible right before it knocks. But what if I were to remove the knock sensor from the motor so that it still "functioned" but wouldn't be able to detect a knock? Would it advance the ignition timing, causing it to eventually knock/ping and then blow up since it doesn't know when to stop?

Cause if the whole knock sensor allowing more ignition timing were true, we wouldn't need to tune our ECUs when we put in a turbo now would we?

One point I forgot to mention is that it is said if the car is "designed" for regular gasoline that it wouldn't benefit from premium. But if the car's knock sensor is the only thing preventing it from advancing timing, then wouldn't premium gasoline allow it to advance the timing further, therefore leading to better performance and possibly fuel economy?
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Old 07-05-2008, 07:52 PM   #2
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There's an upper limit. The timing map built into the ecu is used when there's no knock. When knock is detected, timing is reduced. How much timing gets pulled is determined by the severity of the knock, among other things.

I think the reason some people say that a knock sensor gives more timing is because engines designed without knock sensors have to be tuned with a greater safety margin, so they get less timing. An engine designed with a knock sensor can be tuned closer to the edge of knock since the knock sensor acts as a safeguard, so they get more timing. Or in other words, 'the knock sensor gives more timing'. It's just a matter of semantics, but speaking with less accuracy allows more room for interpretation.
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Old 07-05-2008, 07:55 PM   #3
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Oh yeah, higher octane can handle more timing. I've been using 'premium' octane for years. I tried using 87 octane for a while but my engine knocked anytime I used more than 10% throttle.
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Old 07-05-2008, 08:42 PM   #4
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Many ECUs pull timing in the 2000-3000 range to avoid generating NOx, so I'd figure that knock sensor or not, there's only going to be limited advance there.
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Old 07-06-2008, 05:07 AM   #5
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It is limited, and in fact, it's even limited so much that it can't usually take full advantage of higher octane gas or E85. If it weren't so limited, E85 could be used much more efficiently.
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Old 07-06-2008, 10:00 PM   #6
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DRW nailed it. On knock controlled engines, the ignition timing advance is relatively more aggressive than their earlier iterations. Timing is run off the ignition map and knock is used as one of several trims to pull timing. In other words, knock reduces timing advance. It's not the other way around with the ECU pushing timing until it reads knock. That would be a disaster.

Comparing 1996 Civic ignition maps to 1992 maps is very misleading because in 1996 Honda designed heads that were more efficient requiring less timing advance.

Knock control is extremely conservative meaning that it pulls a ton of timing. You will always get better mpg running the next grade octane than the minimum required according to the owners manual. The WRX is notorious for pulling ridiculous amounts of timing for a ridiculous amount of time, not only for knock but for boost. People have unlocked unheard of amounts of power simply by re-tuning the calibration.

Because engines are so noisy, knock readings are not very reliable. A knock sensor is continuously reading knock the entire time the engine is running. Factory tuners draw the line at how much knock voltage to ignore. My tuner ran several logs and then chose a knock threshold from very sharp spikes on the graph. It's never supposed to come into play. Normally when he tunes he runs no knock control at all.
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Old 07-07-2008, 07:13 PM   #7
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In a distributor ignition system (I'm not sure if 96+ civics have it nor not but my knock sensor equipped cressidas (85-92) all have) the advance is largely limited by the mechanical terminals of the distributor cap. narrow terminals inside give a very low range of advance/retard, large terminals give a wider range.
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Old 07-07-2008, 07:33 PM   #8
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Not all manufacturers run default timing maps unless knock is detected. Ever noticed that Fords seems to knock like crazy? They have a default map but they advance timing based on if there is knock detected and write the change into the map like most ecus do fuel trims as a +-xx% value.

My Tracker follows its default map and that's why I've advanced the timing via cam position sensor. Works great in town but floored on the freeway I can hear the engine start to knock and feel engine power fluctuate as it retards the timing then tries the default map again.
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