I am not sure how the ecu works in this respect. But I was under the impression that the ecu would only alter the timing in the presence of ping. I don't think the ecu will constantly test the knock threshold limits to see how advanced it can operate without pinging. That doesn't seem like a very smart way to build an engine, to be right on the threshold of knock. But take my opinion for what you will, I have never studied how an ecu deals with timing.

 

it's not instaneous.. it takes a bit of driving for it to relearn the curve.. on my two cars it's been about 2 or 3 tanks of gas

-joe

 

An ECU will fidget with that stuff for the one reason that everything's an estimate when you're designing stuff like this. Considered from an engineering perspective, they have tolerances on everything. Maybe the ideal octane for our engine's fresh ECU is 87.6 or every Lancer varies about an octane either way. When you take into account all the different aspects of an engine, environment, etc that decide optimum octane, you've got to realize those tolerances can go a long way. So they make the ECU trainable to your car, your driving, your climate, etc.

So while there is a theoretical ideal for everyone, Mitsu designed the engine so that all the engines will be fine with 87 octane, even though some might have their tolerances aligned such that they can take advantage of more.

As for not thinking that the engine will test its thresholds, it's got to. If for whatever reason, your ECU retards timings to reduce knock, it's got to try and get back to an ideal state - it will just go slowly as sillypuddy said and keep a close eye on things as it does. So it's already got the mechanism to advance timings - there's no reason it can't advance at least a little ahead of it's starting point just the same. And keep in mind, the post someone else mentioned where a little knock probably won't cause issues. I'm not sure I agree with that, but a knock here and there as the ECU finds its ideal certainly won't have an effect especially when it would be so insignificantly little.

Realistically, this is all splitting hairs in details. Ultimately, the gains from a higher octane will be minimal at best and I'd just as soon save a couple hundred dollars a year even if I were to see some gains. You probably get more HP/$ with even a cheap intake than with a higher octane gas.
-N

 

Most modern car's don't knock though. Unless your car's knocking pretty harshly anything higher then 87 is a waste of cash. That whole essay on gas kinda said the same. You're basically contradicting what you said earlier on this thread.

But, how about we just agree to disagree. I'm tired of this arguement. Here have a freshly brewed Canadian beer

 

how did i contradict what i said.

-joe

 

The ECU will constantly retard timing if insufficient octane is in the tank. Period. Each car is different, and it would take a pocketlogger to figure out the optimal octane level for the vehicle.

Refer to http://evo.jedinite.com/toluene.html

It is about Toluene, an octane booster of sorts, however, it gives a good over view on the effects of higher octane on both tuned ( boost esp ) and non tuned cars.

 

Dude, I'm not even agreeing with SillyPuddy's view here, but this statement is just weak. He isn't contradicting himself and you can't accuse him of that and then just leave it as "agree to disagree." If you have no opinion on the subject or don't want to 'argue" anymore, then don't, but a statement like that is just childish.

It is important I think for people to know what octane means and most don't. Anyone reading this thread now should. They should also pick up that it is feasible for a car to perform better/cleaner/whatever with a higher octane. They should realize that there are disadvantages as well as advantages and then they can make up their own mind. You've made yours, I've made mine, and SillyPuddy's made his.

The point is that the information from this discussion is here now and that's what matters. Nothing annoys me more than someone who tries to make an argument out of a discussion so they can get in a final word like this and "win."
-N

 

The ECU is programmed with a particular map. That tells it the limits or endpoints it can work with. It can adjust downward from there if need be to reduce/eliminate knock. It can adjust upward to it's programmed limit if there is an absence of knock and it isn't at it's programmed limit. It can not adjust beyond it's programming limitations. Once again, if your car is stock, there is zero gain from higher octane. Nada, zip, zilch, none, and any butt dyno disagreement is just that, butt dyno.

 

What, were you expecting more from a child? I am a child fyi

 

I think what LanceArt was trying to get across was that if you get worse gas mileage with a higher octane, how can it be cheaper?

With the exception of that, I agree 110% with sillypuddy. I'm running with 93 and it is CHEAPER than running on 87. But to each his own....

 

I'm not sure I follow you? What kind of math yields a lower cost for higher octane? You get lower miles/gallon and more dollars/gallon. That works out to lots more dollars/mile.
-N

 

for a second let's say you agree that higher octane means more timing advance which means better response, then you won't need to use as much gas to get up in speed, because the ecu isn't retarding your timing killing your performance.. so you use LESS gas.. which in turn means less money

-joe

 

That's a real stretch. Higher octane gas is more resistive to detonation because there is less potential energy in it for exploding. It requires more gas to yield the same explosive power as a lower octane. As I said, these are minor changes, but it definitely doesn't get better gas mileage.
-N

 

Advancing timing can yield more power out of higher octane gas. However, the fuel will never burn as purely as it would in a high compression, high temperature, high output engine. It takes more activation energy to ignite fuel with a higher octane rating. Increases in compression and heat give that energy necessary and there is clean combustion. Advanced timing from an ECU does this but only to a certain degree. The fuel does not burn as cleanly and it can leave a residue on the exhaust valves that will eventually hinder performance.

So yes, there can be performance gains from higher octane gas, but you will pay for it later. I personally use 87 in the winter and 89 in the summer because I drive hard and the south is hot.

 

my friend told me that the higher the octane the better it is there was a RX7 93 pushing about 300HP and they put in 107 octane in it and it pushes well over 600HP i dont know if it is true but who knoes?