In order for detonation to be detected, it has to occur. So yes, knock sensors and computer programming will reduce engine knock, however it still occurs.
Also, the computer combats detonation by retarding timing. Pulling timing reduces power and efficiency. That hurts performance and fuel economy. Perhaps by negligible amounts, but it occurs no less.

 

Yes...but where are all the blown engines as a result then??
Low octane in, detonation, knock sensors pick it up, ecu adjusts in real time, timing retards, detonation now not happening.
A Very small amount of stress and wear.
Engines are wearing out from the first time they are turned over.
Detonation causes wear, absoloutely, as compression is igniting the lower octane gas before the spark can, while the pistons are still traveling up they are prematurley getting force to push them back down.
I get it. I understand engines.
My original question was, how can an engine with knock sensors destroy itself from detonation?
As long as it's functioning normally...it can't.

 

if the detonation is from the combustion shockwave you are correct, but if its from hot spots in the cylinder head then the ecu cant do anything about it. Probably not as big a deal now with lower btu ethanol diluted fuels, but a legit concern in the unleaded e-00 fuel days with a 60s era combustion chamber and no egr

 

Read my last sentence: "Perhaps by negligible amounts...". I don't think its good for the motor, but I don't think its causing them to explode either.

 

$2 per fill up? Try $30 to $40. And the ECU will adjust as long as it needs to. There are people who have run 87 octane in LR3s for 150k miles. Only in a hot, like LasVegas hot, would i dlsay 91 is absolutely mandatory.

 

Personally I run 91 because I want full power from my engine. I also use brand name gasoline like Chevron or Shell. There is a fellow in my club who has always run his LR3 on regular and he has had no issues. I would hope that Land Rover has built an engine that can run on regular since you are supposed to be equipped to go places where premium fuel is not available.
If an engine is going to knock it will usually do that when operating the engine in too high a gear while under heavy load, for example trying to drive up a hill in high gear. An automatic trans will not allow this. I dont think knocking in these engines is going to be very common.

 

For the knock sensor to trigger a timing retard, it has to sense detonation. That means it already happened. The computer will try to go back to “ normal” operation when the detonation stops.
Let’s say you’re driving down a highway, you pull out to pass a car and punch it. The computer sees the throttle sensor input and thinks “Monkey wants to go faster,.. sigh” and commands the throttle plate to open, and starts advancing the timing. As the RPM increases, the computer dial in more timing advance. It is also changing the cam timing, overlap, and on newer engines valve lift (overlap is when both the intake and exhaust valves are open at the beginning of the exhaust stroke) cylinder pressure skyrockets. Pre-ignition starts happening in one or more cylinders. The computer senses the detonation/pre-ignition and rolls the ignition and cam timing back to stop it. When it stops, the computer thinks “everything normal now!” Still sees the ”monkey wants faster” input, and throws all the high power adjustments right back in. Process repeats.
It doesn’t just stay in “ low octane” mode. There is no “low octane” mode. The computer is just as stupid as the monkey, it’s just a lot faster at the stupid.

There are several causes of pre-ignition, many of them are temperature related and/or transient. It’s not just fuel quality. The computer is programmed for a certain quality and octane of fuel and it thinks it’s getting it. When it senses pre-ignition, it has a single routine to stop it, and it has to sense it first.
Any pre-ignition can damage internal engine components. If you’re continually running low octane fuel, low order pre-ignition is happening all the time, and it is shortening engine life. You’ll probably never hear it.

If you’re live in a mild climate, never tow, never heavily load the vehicle, and drive like your grandparents, you can probably get away with 87/89 octane fuel without causing noticeable damage. But if that were the case you wouldn’t be driving a Land Rover. You can say, ”I never (fill in the blank) so I can get away with it.” but that’s just copium. Yes, the vehicle will run on low octane fuel, and yes it WILL damage the engine.

 

I must be the odd one out, as I have regulalry used 87 since I've had the most recent LR3 with the 4.4 with no issues. There have been a few times I've tried premium (91 or 93) and found a negligible increase increase in perofrmance and economy, or some recreational fuel which here is 90 octane and no ethanol, which works the best in mine but it's a $1.50 to $2.00 premium over regular so day to day i don't do that very often.

It's not for the sake of a couple dollars a tank. It's a $1 more per gallon than regular, or more sometimes, that's $20 a fillup more. That's a third again more in cost and I do not get a third more fuel economy or performance. My butt dyno can't tell the difference. My LR3 is more sensive to energy density, and some brands are denser than others. BP is the worst and Sunoco is the best.

 

All gas is the same besides additives. Literally the same base fuel is used at every brand pretty much, meaning one refinery will ship out to on distribution hub and from there the additives are added often right into the tanker headed to the gas stations. As for performance, economy. I certainly can feel with I put low octane in on trips out west where premium is rare. Economy takes a hit too, that is for sure. But I can say that I do save money. I think on my last trip I saved almost $200 in fuel because I had to go with a lower octane, which is cheaper. But I always put premium when I can.