The Next Step?

[Rope-Pusher]

The engine should feel and sound more powerful when VTEC kicks in.

[Shadow]

Fuel octane level isn't just important at peak power. The spark advance settings can be optimized for higher-octane fuel all over the engine operational map. This is often done to improve fuel efficiency on fuel economy test schedules. No mater what octane level the engine software was configured to support, a knock sensor is often used to retard spark timing if knock occurs. This helps prevent damage from high-speed knock and customer dissatisfaction and extra emissions from low-speed knock.

Typically, engine knock only occurs under heavy load on non-modified engines. That usually means wide open throttle under a variety of conditions, such as maximum acceleration when merging/passing a highway. Or maybe hard mountain driving....or towing up a hill. If you're cruising around with less than 50% throttle opening, knock is VERY unlikely to occur.

And besides, even if knock does occur, obviously the knock sensor(s) will immediately retard ignition timing to protect the engine against any damage. So the only thing likely to occur is a slight loss in engine power only when knock is detected. Not a big deal in the grand scheme of things.

[Shadow]

My understanding is that the knock sensor is used at all times to run at the ragged edge of allowable knock, not that conservative timing is used when running the recommended octane and the knock sensor only alerts the computer to an unusual condition.

I'm not sure if you misunderstand the way a knock sensor functions or if you just didn't word your response correctly. The knock sensor isn't used to keep the timing as advanced as it can be throughout the RPM range. The knock sensor is a relatively simple device that responds to knock (a frequency range) and only then does it factor into ignition timing. I'm not sure if there are any cars out today that have the ability to constantly adjust ignition timing from idle to redline. Even if there are, they aren't common and they definitely don't use the knock sensor as a way to keep timing as advanced as possible at all times.

I've actually done some testing on a few different vehicles by using the data stream function on my scan tool to record ignition timing using different octane levels under different driving conditions. The only time I've ever seen timing pulled by a knock sensor is under wide open throttle in a car tuned for 91 octane, but running 87 octane. I had a big debate with some friends and we had to prove it one way or the other.

[Shadow]

Well here's the thing - The Knock sensor in my car is currently malfunctioning. (Something I need to get replaced in the near future) I've gotten estimates that it'll cost around 250$-500$ to replace as well =X But anyway - should I use premium until I get my knock sensor fixed?

If you know for sure that your knock sensor isn't working properly, then try not to drive too hard if you're using 87 octane fuel. You should be okay. Or if you want to mash the gas pedal, then put the high octane stuff in the tank. The knock sensor is the only thing that's really going to protect your engine if you're driving like a maniac!

BTW, back when I had my old 5.0L Mustang, I would manually advance my timing to make more HP. There were no knock sensors on the old 5.0 Mustangs. So I'd advance the timing a couple of degrees at a time and then take the car out on the highway and run wide open throttle through all the gears. If I didn't hear any knock, I'd advance another degree or two and repeat the process. Once I got to the point that I could hear the knock, I'd back it off a degree and then lock it down. That's one of the ways we used to tune those old Stangs for maximum power.

[Rope-Pusher]

Fuel octane level isn't just important at peak power. The spark advance settings can be optimized for higher-octane fuel all over the engine operational map. This is often done to improve fuel efficiency on fuel economy test schedules. No mater what octane level the engine software was configured to support, a knock sensor is often used to retard spark timing if knock occurs. This helps prevent damage from high-speed knock and customer dissatisfaction and extra emissions from low-speed knock.

Typically, engine knock only occurs under heavy load on non-modified engines. That usually means wide open throttle under a variety of conditions, such as maximum acceleration when merging/passing a highway. Or maybe hard mountain driving....or towing up a hill. If you're cruising around with less than 50% throttle opening, knock is VERY unlikely to occur.

And besides, even if knock does occur, obviously the knock sensor(s) will immediately retard ignition timing to protect the engine against any damage. So the only thing likely to occur is a slight loss in engine power only when knock is detected. Not a big deal in the grand scheme of things.

And that is where you'd be wrong. The engines are timed to run very close to their knock limits for improved fuel economy under part-load conditions. It's not just a high-power thing, like in the daze of distributor-run ignition advance systems. There is a lot of micro-managing going on these days, like charging a cold battery at a higher rate than you would a warm battery, cutting back power to smooth slushbox shifts and using the knock sensor to pull back spark advance from a spark curve that is optimized for ideal conditions.
The same cannot be said for brake systems. There is still a lot of time and effort spent to balance the brake systems front-to-back. Sure, ABS could cover for a system that was unbalanced and prone to locking wheels on one or the other end, but ABS is not a transparent interdiction, and the driver isn't intended to hear and feel its operation each time he uses the brakes. If micro-managing the spark advance wasn't transparent to the driver, it wouldn't be relied upon on a full-time basis.

[theholycow]

(Still sort of annoys me that the engine light is on all the time becuase of it. And I've read rumors that if my engine light is on, the Vtec won't work.)

Could be. It's common to disable features if anything is not right.

[Shadow]

And that is where you'd be wrong. The engines are timed to run very close to their knock limits for improved fuel economy under part-load conditions. It's not just a high-power thing, like in the daze of distributor-run ignition advance systems. There is a lot of micro-managing going on these days, like charging a cold battery at a higher rate than you would a warm battery, cutting back power to smooth slushbox shifts and using the knock sensor to pull back spark advance from a spark curve that is optimized for ideal conditions.

Actually, I'm not wrong. I'm not going to say that there are not very advanced systems in use on some of the most modern high-dollar vehicles. Why wouldn't I say that? Simple--I'm not up on the most advanced engine control systems used in every brand new car. So there may be some very advanced system that I don't know about, but generally speaking there's no special magic happening when it comes to ignition advance. Most cars on the road work respond exactly as I described earlier in the thread.

And besides, knock sensors aren't ever used to advance timing beyond the base map...they are only used to retard timing. You can make an argument that they are used to advance the timing back to where it was before knock occurred (as the knock goes away), but then that wouldn't really be advance, would it?

Instead of telling me I'm wrong, how about backing up your opinion with something concrete? It's easy to tell someone he's wrong without supporting why/how he is wrong. How about some tech articles to show me that you know what you're talking about? I've actually done the testing....I've seen spark advance...I've tested it myself on several different cars (none more than 10 years old, and some as new as 2011 models) and I've never once seen ignition timing retarded during normal driving conditions. So yeah...go ahead and prove me wrong.

[Shadow]

Some light reading that supports everything I've mentioned so far:

http://www.cartalk.com/content/features ... ons.html#9

Disclaimer: This is pretty simple info intended for people who don't know a lot about octane/knock sensors and how it all works. Still though, it's good info...

[Rope-Pusher]

And that is where you'd be wrong. The engines are timed to run very close to their knock limits for improved fuel economy under part-load conditions. It's not just a high-power thing, like in the daze of distributor-run ignition advance systems. There is a lot of micro-managing going on these days, like charging a cold battery at a higher rate than you would a warm battery, cutting back power to smooth slushbox shifts and using the knock sensor to pull back spark advance from a spark curve that is optimized for ideal conditions.

Actually, I'm not wrong. I'm not going to say that there are not very advanced systems in use on some of the most modern high-dollar vehicles. Why wouldn't I say that? Simple--I'm not up on the most advanced engine control systems used in every brand new car. So there may be some very advanced system that I don't know about, but generally speaking there's no special magic happening when it comes to ignition advance. Most cars on the road work respond exactly as I described earlier in the thread.

And besides, knock sensors aren't ever used to advance timing beyond the base map...they are only used to retard timing. You can make an argument that they are used to advance the timing back to where it was before knock occurred (as the knock goes away), but then that wouldn't really be advance, would it?

Instead of telling me I'm wrong, how about backing up your opinion with something concrete? It's easy to tell someone he's wrong without supporting why/how he is wrong. How about some tech articles to show me that you know what you're talking about? I've actually done the testing....I've seen spark advance...I've tested it myself on several different cars (none more than 10 years old, and some as new as 2011 models) and I've never once seen ignition timing retarded during normal driving conditions. So yeah...go ahead and prove me wrong.

Stop trying to put worms in my mouth. I never said or implied that knock sensors advanced timing. They merely send a signal that the engine control computer interprets as an indication of knock and then it retards the timing from the value that was established in the spark timing map. Spark can be retarded without going all the way over from BTDC to ATDC, right? Even the lowliest engines are sophisticated enough that they run as much spark advance as is prudent and then retard the timing from that pre-established value as needed to avoid knocking. The stakes of improving fuel mileage are high enough to warrant such attention to detail.

[Angelbroken]

Hm - well I certainly don't rev my car that hard - so that would explain why I don't hear any knocking with my sensor not working.

Well I went and filled it with Premium just to be safe - and I also got a really heavy car wash done to it.

However, I noticed it sounds different now. Could it be the gas? Or just the fact it was cleaned heavily? (It looks AMAZING - only 10$ too)

[Shadow]

...

[Shadow]

Stop trying to put worms in my mouth. I never said or implied that knock sensors advanced timing. They merely send a signal that the engine control computer interprets as an indication of knock and then it retards the timing from the value that was established in the spark timing map. Spark can be retarded without going all the way over from BTDC to ATDC, right? Even the lowliest engines are sophisticated enough that they run as much spark advance as is prudent and then retard the timing from that pre-established value as needed to avoid knocking. The stakes of improving fuel mileage are high enough to warrant such attention to detail.

If you have worms in your mouth, it has nothing to do with me! LOL

As for the lowliest engines....you do realize that a significant number of active members here seem to drive older vehicles, right? I mean seriously....some of them still have carbs! And again, prove me wrong instead of trying to generalize. Demonstrate how timing is typically adjusted during normal driving conditions on most vehicles. Fact of the matter is that it is not and base timing is established by maps. In the old days, it was 100% mechanical. Anyone who has ever twisted a distributor knows that already...

[Shadow]

Hm - well I certainly don't rev my car that hard - so that would explain why I don't hear any knocking with my sensor not working.

Exactly. Keep the revs & engine load down and you should't have any worries at all, even if you're using lower octane fuel. If you decide it would be fun to run it hard, just use higher octane fuel to be safe. And once you get your faulty knock sensor replaced, then you don't really have to worry about it at all, as the knock sensor will protect the engine if knock does happen to occur.

[Angelbroken]

Well so far so good - I took my car in heavy traffic today for work - first time ever; I did well. I only had one stall, which was a tad embarrassing, but unlike practice, when I go out at night in very low traffic and I'm well rested, for work theres LOTS of traffic and I was very tired from my job, which might of been a factor to why I stalled.

Either way, how much should I be concerned over a stall? It's not really embarrassment that bothers me, its possible car damage - I've read that stalls, 'bucking broncos', and idling out; really don't do serious damage - but I'm still concerned. (I'm a tad obsessive with my car - its the first vehicle I ever owned and I'd hate to see my n00bish driving skills ruin my car early)

[Squint]

Unless the car doesn't start again after it stalls, I wouldn't worry too much about it. You're effectively turning the car off the same way you do when you turn the key back to electronics only.