I normally just fill up with regular gas (I know, i'm a cheap bastard), but with recent gas shortages most of the gas stations around my house are running out and offering premium at the same price.

Will switching to premium from regular have any negative effects? And if i were to switch and then eventually go back to regular would that have any effects?

 

No, no and no.

It might take the ECU a tank or two to adjust to the higher octane fuel.

You should run a minimum of 89 octane.

Do you have Husky or Mohawk stations where you live?

In Greater Vancouver, we can get 90 octane 10% ethanol for the price of 87.

 

yeah i do believe Sunoco gas contains 10% ethanol, i like PETRO their gas is amazing...

Hello guys i ALSO live in canada, close to BARRIE (i am in North of Toronto, Maple near Wonderland.. Zeppelin340 you should know where that is)

Anyways i like Petro 91 because it contains an additive called TACTROL (check their website and read up) it cleans injectors and the rest of the fuel system. The main difference between gas stations is their additive package. Right now during the winter times Petro also switches to Winter gas, which contains stuff that doesnt let it freeze and burns better at startup at like -30 C. i checked the other web sites for Esso, Sunoco and could not find a damn thing about whats in their gas.

Many 4th gens have a common cold start problem, where by upon the starting the engine is starved of gas and the rpms drop and wabble or most often stalls. Petro gas saved my A$$ and got me home in some -25 C weather the last 2 months, so i rather stick with it, Esso gas did nothing for me , where sunoco was pretty close to Petro.

The moral of the story and back to your original post, use AT LEAST 89 octane, but stick to 91 as much as possible, thats the reason Japanese engineers put the Knock sensor into the VQ30DE.

 

The ECM has no idea what the octane rating is of the gasoline you put in. The only things it will adjust for is a knock or oxygenated fuel.

 

Wouldn't the engine be more prone to knocking with a low octane fuel?

 

yes, but its more just a caution, not a rule of thumb.

 

The reason the engineers put a knock sensor in your engine is so that you can burn either premium or regular and get appropriate performance from either. I burn 85 octane here in Colorado and it performs just fine in an engine with a 10.3 to 1 compression ratio for two reasons: The knock sensor is working properly and because I drive where the elevation is about 6 K feet. (For each 1 K feet increase in elevation, the octane requirement for an engine decreases about 1 octane number. Following this general rule, 85 octane at 6 K feet elevation should perform like 91 octane at sea level).

My experience is with the 6 Gen Max, and with it you can switch back and forth between grades with no problems. I did that in the early days with this car trying to determine if there was any improvement in mileage burning premium. I did not find any, but the results were not difinitive due to other factors that appeared to have more impact on mileage than octane.

I'm told by an engine expert on this site that it takes from 200 to 500 miles for the fuel system to completely adjust to a change in gasoline octane -- but my experience is that when you reduce from premium to regular the knock sensor keeps this octane reduction from knocking except under extreme driving circumstances.

 

Actually it's to detect preignition before the spark or detonation by detecting the sound frequency of a knock. Unfortunately if it's detected, that means you have already inflicted harm to your piston. Yes, the timing will be retarded until no knock is detected, but the ECM will keep trying to advance it back again until another knock is detected creating a vicious circle

Corrected

 

Wrong, Wrong, Wrong.

I quote the Nissan Engineers in the 2004 Maxima Owners' Manual (page 9-4):

"Octane rating tips:
In most parts of North America, you should use unleaded gasoline with an octane rating of at least 87 AKI (Anti-Knock Index) number. . . However, now and then you may notice light spark knock for a short time while accelerating or driving up hills. This is not a cause for concern, because you get the greatest fuel benefit when there is light spark knock for a short time under heavy engine load."

I take "greatest fuel benefit" to mean "best fuel economy" -- which is consistent with Mechanical Engineering's Engine Lab 101 experience with gasoline. Light spark knock is more likely to happen burning regular than burning premium.

There is no discussion in the Manual about "you have already inflicted harm to your piston" or "creating a vicious cycle." This is the way the VQ engine was designed. Don't obsess about octane. Drive your Max and enjoy doing so. Burn the grade of gasoline that is consistent with your driving style -- if you frequently do a WOT (wide open throttle) burn premium. If you are a more conservative driver, burn regular. In between, burn mid-grade. But enjoy your Max and don't obsess about it or the gasoline you burn in it.

Finally, I just noticed another mistake in your post: "predetonation or pre-ignition" does not occur "before the piston down-stroke" but as the piston is coming up to the top of the cylinder, just before the spark fires -- and it fires just before the piston reaches the top of the cylinder. So pre-detonation would be before the top of the piston stroke is reached -- before the spark.

 

Very interesting. SilverMax_04 you make a very good case for me to try to switch to another octane level and see what happens, the thing is that i bought the car with 203.000 KM and I am pretty sure the previous owner, for whatever the length of time he/she had her, burned 87 octane gas. I think once the cold weather passes i will try to run 89 octane for one month (thats 4 gas tanks x 550KM each tank = 2200 KM total). This should give me enough time to see a dignificant difference.

My driving style is one of economy, i sometimes will enjoy the growl of the engine, but most of the time i try to be as an efficient driver as possible. I live in Toronto so there is no elevation here at all. From what you said I think running 89 would give me the best money for performace ratio possible, as well as great gas mileage.

Thank you for your thorough response.

 

Thanks for your comments, Andrei.

You are implying by your post that you are slightly worried that you car may be too used to burning regular and not like a change in octane. That issue is hard to address. Every car and every engine is slightly different. That is why I advocate doing what suits your situation the best -- your individual car and your driving style. Based on the little bit of information you provided, I would recommend continuing to burn regular in your Max. Only if you get more than occasional light engine knock would you need to consider going to 89 or even 91-3 octane (premium).

Only if you plan to change your driving style where you get on the gas harder and more frequently would you need a higher octane.

If you really want a thorough answer, this thread has a more detailed discussion of Octane and the factors that influence an engine's need for higher octane.
http://forums.maxima.org/showthread.php?t=396716

The bottom line is that there are 3 factors that affect an engine's need for octane: Temperature, Pressure and Spark Advance. All other factors (such as engine deposits) influence one or more of these 3 major factors.

Earlier I told you that I burn 85 octane in my Max. I do get some slight light engine knock burning this fuel when the outside temperature is 80 degrees or warmer and I'm climbing the hill to my house in 3rd gear. In this situation, the heat (impacted by increased engine load) is enough to cause light engine knock. I simply downshift to the next lower gear and the knock disappears.

 

Be aware that the recommended fuel spec. for the Gen. 5 and I think the Gen. 5.5 is premium gas with an octane rating of AKI 91, if premium is not available, use at least AKI 87. The Gen. 6 appears to have been changed. Light intermittent pinging (probably seen more on manual tranny cars) is OK, but........ there is a caveat in the Gen. 5 & Gen. 5.5 FSMs. Basically it states that the knock sensor/ECM circuitry was not designed for the continuous control/prevention of detonation.

 

The Nissan recommended fuel for the 6th Gen Max is also 91 octane premium. That fuel gets the best performance from your VQ engine when you frequently call for WOT. But 87 octane will perform in an acceptable manner.

Andrei suspects that the pre-owned Max he bought only saw regular gasoline for fuel from the previous owner. So perhaps that warning is overstated ? I do know that before the 6th Gen, the compression ratio on the VQ engines in Maximas was only 10 to 1 and not the 10.3 to 1 for the 6th Gen (this impacts the pressure seen in the engine just before the spark).

Don't know enough about the exact setup of the 5th Gen to comment further.

I suspect that you are correct about manual trannies being more susceptible to light engine knock. The engine is "hard wired" to the drive wheels when in gear, while the auto trannies have slip in the torque converter. I also get light spark knock in my Dodge truck with a manual trannie in the same driving circumstances as with my Max -- and Dodge says to only use regular in this truck.

I also don't doubt that the computer control for the fuel system on the 6 Gen is more sophisticated than in the earlier Gens.

 

Do they have Husky or Mohawk gas stations in Toronto, Andrei3333?

In Greater Vancouver, I can buy 90 octane 10% ethanol gas at 87 octane prices and my Maxima runs like a top on that fuel.

It is a savings of 6 cents a litre over 89 octane and 10 or 11 cents a litre over 91/92.

I know US .orgers will knock 10% ethanol gas, but I believe the Canadian gas is wheat-based rather than corn based. My performance and fuel economy has not been adversely affected one iota as a result of using it.

 

No we don't have Mohawk and Husky stations in Toronto.

 

I quote from the 2000 Maxima Nissan Factory Service Manual, page EC-31:

The knock sensor retard system is designed only for emergencies. The basic ignition timing is programmed within the anti-knocking zone, if recommended fuel is used under dry conditions. The retard system does not operate under normal driving conditions. If engine knocking occurs, the knock sensor monitors the condition. The signal is transmitted to the ECM. The ECM retards the ignition timing to eliminate the knocking condition.

__________________________________________________ _____

There are dozens of articles on the Internet on knock sensor function. I picked one at random. http://www.wellsmfgcorp.com/pdf/counterp_v4_i4_2000.pdf. This is a fairly long article, so here are a couple pertinent paragraphs. It states:

Most knock sensors send a base or “no
knocking” reference signal to the PCM.
If knocking occurs, the sensor detects the
increased vibration and increases its signal
to the PCM. The PCM then slightly retards
timing until the sensor signal returns to the
reference level.

If a temporary condition caused the knock,
the PCM will set spark timing to the
programmed advance. If the knock returns,
the PCM will cycle spark timing, advancing
timing until knock is encountered and then
retarding timing until the knock is gone.

_____________________________________________

I agree that pre-ignition is before the spark.
Also the correct term is vicious circle, not vicious cycle, but everyone gets that wrong.

 

It appears that there is a difference between 2000 and 2004 VQ fuel control systems. I am not surprised as the technology continues to improve on these systems over time.

I will continue to burn regular in my Max because Nissan says it will perform adequately -- and for me it does. I rarely ever do a WOT and don't need premium gasoline performance.

Those of you with 2000 models (and probably earlier) should follow the words from the Nissan Factory Service Manual as reported in the post above.

 

The 2002 FSM has the same description of knock sensor function for the VQ35DE on page EC-35.

Edit:

I just checked 2004 FSM and it too has the same description on page EC-33.

However in all cases, knock sensor data is also one of the many sensors that determines injector open duration, but then again there has to be some knocking present before any adjustment is made.

 

this is an interesting thread.

I'm a bit surprised by the language in the 2k4 owner's manual. I've always considered knock of any kind/level to be bad. "light knock" may not cause immediate serious damage, I suppose. but if you consider what's happening inside the engine it's hard for me to see it as "safe". just my opinion.

as for the KS operation -- it is important to understand that there are just 2 timing maps (each with a spectrum of set points based on various operating parameters). a "normal" (e.g. default) timing map, and a "safe" timing map. the default map is designed not to cause knock if >91 gas is used, while the "safe" map will not cause knock if <91 gas is used (under "normal" conditions). normally the ECU would like to be in the "default" map for optimum performance, but if the KS senses knock for any reason it will revert to the "safe" map. now here's the point -- the ECU is not constantly advancing ignition, in real time, until it senses knock, and then backing off a bit. in other words, the KS is not an "active" performance management system - that is not its function. its function is to protect the engine.

The KS is indeed intended for emergency situations. but it will kick in *if* low octane gas is causing knock. now, depending on your perspective, you could take this to mean the KS is there to allow for the use of low octane gas. that is a legitimate interpretation, and the marketing people that write sales pamphlets and owner's manuals certainly see things that way. but here's the problem with that -- eventually the ECU will try to get back to the "default" map, and when it does the engine *might* knock if 87 is in the tank... the "vicious circle" comment.

but how "vicious" is it?? there's a few things to consider...

1) how often does the ECU attempt to switch back to the "default" map?? I have no idea. I believe resetting the ECU forces it to try.

2) how different are the 2 maps? not very. during the live on-road testing I did with both a good and bad KS, the only difference I saw was under very hard acceleration at high RPM. the "safe" map kept advance to single digits <10deg, while the "default" map allowed advance to go up to 18 or 20deg. that's it. all other conditions were essentially the same.

3) what effect does momentary knock have on the engine?? I guess that's debatable (given the 2k4 owner's manual text).

4) what effect does knock have on the KS itself?? Now, here's where I chime in... I spent several years in my early engineering career performing failure analysis on piezoelectronic devices like the KS. based on my experience, I can tell you these things are VERY fragile and will tend to fail if they are constantly exposed to the shock/vibration of knock (to which they are tuned). Furthermore, they will also fail from "transients" (e.g. a sudden burst of knock after a long period of inactivity) b/c they can be weakened by temperature extremes.

So what does this mean?? In my opinion:
a) if you run 87 octane, you can expect your KS to fail more often if you drive hard (heavy acceleration @ high RPM).
b) even if you usually run >91, and just once run 87 you risk breaking your KS due to transient stress.
c) the best way to prevent KS failure is to run >91 octane always, and never deviate.
d) if the KS fails, the ECU will stay in "safe" mode until the KS is fixed and the code cleared. Then, at some point, it will go back to the "default" map.

incidentally, I always run 93 octane, but just once put in 87 b/c I had to buy gas downtown. 1 week later I threw the KS code. I don't believe in coincidences, especially when there is a scientific explanation.

now bring on the flood of people who will tell me they ran 87 for over 100k miles and never had any KS problems, etc.... well, I too shipped out thousands of cell phones with weak piezoelectric RF filters, and most of them never had problems either, but some of them did, and there was always something that caused them to fail (nothing just breaks on its own, there is ALWAYS a root cause, even if it's just a manufacturing defect).

 

Good and informative post, Sky. Thanks for explaining in more detail the workings of the KS.

What you say does not coincide exactly with what another engine expert on this site says about the KS. SteVTEC maintains that it takes 200 to 400 miles of driving with a different octane for the KS to completely adjust to that new octane. Perhaps your statement that it takes extreme operating conditions like "under very hard acceleration at high RPMs" to get the KS to reset is the equivalent of driving these miles?

The only time I get light spark knock in my Max (and also in my 87-octane-burning truck) is at lower RPMs (below about 2 K in the Max and 1.2 K in the truck) climbing a hill when the outside temperature is above about 80 degrees. A simple downshift eliminates the light knock due to reducing both the load on the engine and the engine temp by increasing the RPMs.

SteVe also opined that he thought that burning regular for normal highway driving would result in better gas mileage than burning premium. (In this case it was based on interstate driving and not needing to do high-RPM passing on single-lane roads.) My limited tests early in the life of my Max seem to confirm this opinion -- but there were many other uncontrollable factors beside octane in my tests that probably render these tests less than scientific.

While I have a BS in Mechanical Engineering, I don't consider myself an expert on engines with "modern engine technology" because I got my degree in the very early 60s. Back then the word was that occasional light engine knock for short periods (not defined but understood to be seconds and not minutes) would not damage an engine unless it happened too frequently. Because I worked for an oil company for 35 years, I consider myself to be somewhat of an expert on octane and all of the issues around octane. I'm not an engine expert, and don't purport to be one.

Having said all of that, I intend to continue to burn regular gasoline in my Max (85 octane here in Colorado and 87 octane elsewhere -- although I buy 89 octane in Iowa and Nebraska because it is cheaper there than regular because it contains 10% ethanol). You may well be correct that my KS will eventually fail because of this practice. But because I rarely call for WOT in my Max, I tend to doubt that will happen. I believe that my KS is running in the default mode here in CO (because 85 octane at 6 K feet elevation is like 91 octane at sea level). When I drive away from the mountains, I believe it continues to operate at this setting unless the KS senses knock. Since I don't go to WOT (which would be more likely to produce knock with regular), it continues to operate at that setting burning 87 octane at lower elevations. Doubt if this will cause the KS to fail prematurely.

If I were racing my Max I would stick to only premium (outside of the Rockies).

Final point after reading your post again. The difference between 10 degrees (or less) BTDC (before top dead center) and up to 18-20 degrees BTDC is a real big difference in when the spark fires in an engine. So the safe mode becomes substantially safer than the default mode, which is understandable.

 

stevetec is referring to the switch (or attempted switch) back to the default map from the safe map. like I said, I have no idea how long that takes to happen automatically. it may very well be 200-400 miles. I noticed it after a manual reset of the ECU using an OBDII scanner, but certainly the ECU will automatically change on its own at some point. but if it senses knock it will go right back to safe mode.

the switch from the default map to the safe map happens only when the KS senses knock (regardless of octane) and it is almost instantaneous. it is quite possible, however, to run 87 octane without knock and stay on the default map (high altitude, low compression, etc). likewise, under the right circumstances (excessive CC deposits) you could have knock with 93 octane and be running in safe mode. so it is a bit of a misnomer to say the ECU adjusts to octane - rather, it responds to engine knock. it really doesn't care what octane you are running. all things being "normal", the timing maps are indeed optimized for different octane ratings, but that doesn't necessarily mean the ECU will switch maps when a different octane is used. it all depends on the presence or absence of knock.

the point on hard acceleration is not related to resetting the ECU to default mode. rather, if you look at the difference in timing maps, it is quite the opposite. hard acceleration could induce knock with 87 octane, and that would trigger the ECU to switch to safe mode. this would happen instantaneously, and it would stay there for, well, 200-400 miles according to stevetec. I have no reason to doubt that. but putting 93 octane in and stepping on it will not force the ECU back to default mode - that I can tell you.

as for the fuel economy -- does premium gas have more ethanol than regular? to get the higher AKI? that could explain your fuel economy observations. also, you have a manual right? there could very well be differences between the manual and auto timing maps, I don't know. mine is an auto.

 

about this --. obviously that is still enough to eliminate knock, but is it really that big a difference?? I don't really know. but at low RPM highway cruising the advance gets up to 48deg, so there is a pretty wide range in timing set points.

I can tell you that I noticed absolutely no difference whatsoever in power or acceleration between the safe mode and default mode. I know many people here claim otherwise. placebo? maybe. or maybe with a 5sp you'll feel it more than with a slush box in the drivetrain? I don't know. but my power and fuel economy didn't change a bit that I could tell.

 

The Feds don't allow more than 10% ethanol in gasoline (unless you go to E 85), so that is not the explanation. Also, when I did my tests, ethanol was not used very much outside of the Midwest and Mountain West, so I don't think that is part of the explanation.

I do have a 6-speed manual. There could be a difference in maps between the manual and the auto trannies.

Again, thanks for your more detailed explanations. They help me to better understand what I'm dealing with in my VQ by always burning regular gasoline.

 

So this is the appropriate time for me to jump back in with a question.

So for a 4th gen with a good KS (i think) and not caring about HP but caring about fuel economy and having no engine knock: what octane should i use ?

 

Based on Sky's info, he would say stick to premium. I used to be emphatic that you should burn regular. But now that Nissan engineers have spoken (see Sky's posts above), I would not be so emphatic about regular use for earlier generations. It is actually your call. I know that in my 6th Gen Max it will continue to be regular gasoline.

 

I would run no less than 89 octane. You don't need to use 91 octane.

I would be inclined to use a higher octane gas in the summer time, as opposed to the winter time.

 

What about 89 octane ? best of both worlds or a complete waste ?

I have read a user on this site saying the 4th gen actually runs as well on 89 as it does on 91, but one should not go lover than 89.

For the money 89 seems like an attractive option, what do you guys think ? (im not rich; money is expensive HAHA!)


PS: haha i posted this 4 minutes after Bobo posted his post on 89 octane, i must have psychic abilities hidden within me ;-)

 

Given that engine temperature is one of the 3 primary factors that influence a vehicle's need for octane, this is probably a good idea. But again I'll say that I will continue to burn regular year-round in my 6th Gen.

 

if your operating conditions are "normal" (all engine parameters in spec, low altitude, etc) then 89 is a waste of money. it will still tend to cause knock on the default timing map, so you'll be stuck in safe mode where 87 will work just fine... remember, there is not an infinite continuum of timing set points for all possible octane ratings - you are either in default mode or safe mode. so either go 87 or 91+. in my experience, there is no difference in MPG. this is b/c the only real difference between the timing maps is under heavy acceleration, which sucks gas anyway and represents a small fraction of your total driving time. and if I understand SilverMax correctly, the energy content of premium and regular are the same, so should MPG be the same everywhere else on the timing map, which agrees with my observations.

so, if your primary concern is fuel cost, run 87. if your KS breaks it is no big deal -- you'll just be stuck in safe mode all the time, you'll save money on fuel, your car will run just fine, and you'll notice no difference in power or fuel economy (in my experience). this is why the KS code does not set the CEL. it really isn't a problem at all.

but, if you want to minimize the risk of KS failure, stick with 91+. only you can decide what your priorities are. I'm now wondering why I bothered to replace the thing.

 

Very good post. I would like to slightly clarify the point below.
Each bacth of gasoline is blended in the refinery to meet certain criteria including, but not limited to, octane. Some of the high-octane blending components have more energy than some of the lower-octane blending components used in regular. So if either grade is likely to have more energy than the other, it would be the premium -- but you can't count on this always being the case.

Bottom line, it's hard to generalize about differences in gasoline energy content between premium and regular.

Sky,
I have enjoyed exchanging these ideas with you -- and I learned something in the process. Too bad that others on this site can't always have a friendly debate without having to resort to name-calling and other insults.

 

In the case of the Nissan ECM, there is only has one baseline "map." Instead of "map switching," the Nissan ECM has a different way of performing mixture and timing adjustment, short-term and long-term learning. Both modes operate exactly as their names imply.

The long-term learning mode makes changes over a longer period of time to compensate for engine and environmental conditions such as the continued use of lower octane rated gasoline that is causing repeated problems.

The short-term learning mode handles "emergencies" that include something like knock or temporary injector clog.

Regardless, Nissan ECM doesn't switch settings to preset "maps" like less sophisticated ECMs and that the Nissan ECM will always try adjust the engine to optimum operation.

When the ECM is reset, any learning adjustments are erased and operations reverts back to the baseline settings or "map" until there's been enough driving time for the ECM to "learn" the engine and environmental conditions.

 

Thanks for your clarifications on the Nissan ECM system. Does this description apply to VQ engines built before those used in the 6th Gen Max?

This better explains why SteVTEC has said that it takes between 200 and 400 miles of driving for the system to fully recognize a change in the octane in your tank.

Your description also seems to say that there could be an advantage for burning 89 octane mid-grade -- given that this system should be able to adjust to that mid-range octane.

 

^^^I believe that Nissan has always used this methodology ever since they introduced OBD II, but I know that it has been at least used since 2000.

Also, ECM "learning" will continually try to adjust the engine to what the ECM has been programmed to consider "optimum," and that programming assumes the use of gasoline with a 91 octane rating, so it will bump into issues caused by a lower rated fuel if used and adjust to it, but I can't see that as major issue.

 

very very interesting, please do continue guys...

i am still waiting for the gasoline gods to come down from the petrolium heaven and tell me i can use a lower grade gasoline without any side effects. There was an accident at the main refinery in Ontario 3 weeks ago and now there is a shortage of gasoline, some stations have closed down, others are rationing, not allowing anyone to get more than 75 Litres.

Today i saw a price of 102.5 Cents (CAN) / per Litre for 87 ( that includes a ridiculous amount of taxes and all ) PLEASSSEEE tell me to use lower then 91!!!!

 

^^^ There was a fire in a Texas panhandle refinery a few weeks ago, and our gasoline prices started to rise. Even with that fire and the higher prices, our pump prices here remain below the guide I use to see if gasoline prices are reasonable.

Gasoline Price Guide: Take the NYMEX closing price for regular gasoline (now referred to as R-BOB, I believe) and add 50 cents per gallon for US and state taxes and transportation costs. For premium just add 70 cents. I get this closing price from the PBS-TV program "Nightly Business Report" but I'm certain that the Wall Street Journal also publishes it.

As to whether it is OK for a 1997 Max to burn 87 octane regular, that answer is beyond my knowledge -- sorry. You might try 89 octane mid-grade to see how well it tolorates a lower octane.

Yesterday the closing exchange rate was $1.1775 CAN = $1.00 US. Also, 1 gallon = 3.785 liters. So per gallon you were paying $3.879 CAN or $ 3.295 US. I bought gas yesterday here in CO and the premium (91 oct) gas price (I bought regular) was $2.569 US. This price was 3 cents per gallon cheaper than most of the stations were charging ($2.599/G). So you are paying $1.28 US ($1.50 CAN) more per gallon that was the retail price in Colorado on Friday. Surprised that in addition to the higher price they are also limiting purchases to just under 20 gallons per customer. High prices should be enough to ration a limited supply -- unless the government has restricted how high the prices are allowed to rise.

 

Here's a dyno chart that's been posted elsewhere on this site comparing 89 with higher octane rated gasoline.

http://www.dynospotracing.com/octane.htm

 

a few comments...

for a 4th gen, the learning function of the ECU applies to the closed-loop control of fuel injector pulse duration, not ignition timing. the ECU uses feedback from the O2 sensors to compensate for variation in injector performance (among other things) so as to keep A/F optimal. when the ECU is reset it goes off the default fuel map, and over time makes adjustments and stores them in the fuel map. feedback is required if the ECU is to learn, and functional O2 sensors are critical for this process to work -- so bad O2 sensors will set the CEL and your car will not pass inspection.

however, the FSM clearly states the KS is not used in normal operation, it is only for emergencies, the timing map is pre-programmed, and the KS code will not set the CEL. if the ECU "learned" the best timing map, it would have to use the KS for continual feedback, but by design it does not (despite what many here believe). and if the KS were bad, the entire learning function would be corrupted, which would certainly be cause to set the CEL -- but we know that doesn't happen. conclusion - the ECU does not "learn" ignition timing.

to be clear -- this does not mean ignition timing is static. indeed it is constantly changing in response to engine speed, injector pulse width, throttle position, and a host of other variables. but the ECU uses a pre-programmed map to determine the proper timing set point to correspond to these variables. it is not constantly "pushing the boundaries" of knock to figure out the best possible set point, and then "learning" what those boundaries are.

this is verified by my own live testing -- I spent the better part of 6 months playing around with my OBDII scanner plugged in while driving (like a kid playing with a new toy). with both a good and bad KS. I reset the ECU many times, and observed timing at every conceivable operating condition. as I stated earlier, the only time I saw a noticeable difference in timing was under heavy acceleration with the KS code present - and even then the difference appeared to be constant over time. in all other scenarios -- short term or long, there was no difference in timing that I could discern -- hence there was no "learning". Furthermore, the change back to default timing was immediate after resetting the ECU, and stayed there -- no learning required. for A/F and injector pulse width, yes it learns - but not for timing.

now, the one thing I did not test was <91 octane with a good KS and no code -- I didn't want to risk breaking the thing again. I would now be interested to see exactly what would happen. would it "buck" back and forth as it senses knock, retards, and immediately resets? or would it sense knock and stay in safe mode for 200-400 miles? I don't know. but I can tell you with the KS code present it does not oscillate between normal timing and safe timing - it stays safe.

the above dyno graphs are interesting, but they are for 2 different VWs. not maximas, and they're not even the same car -- so you can't infer too much from them. with a 4th gen I'd expect a slight decrease in dyno results with 89 octane above 3k rpm, but as I said before, I doubt many people could tell the difference in normal driving. I personally could not, but I don't race my car so perhaps I'm not "in tune" with slight performance differences. plus I have a slush box in the drivetrain.

 

Good catch, Sky. You need good eyes and have to look carefully to see that the cars are not the same. Why not run the same car with two different grades of gasoline? And VWs are not Maximas.

Good call. Yesterday I had reason to "get on it" and noticed that my Max seemed to "jump" when the engine RPMs hit about 3500. Ran it up to about 5500 RPMs and it was really moving. This allowed me to safely merge into traffic on the highway. I don't do this enough to know whether running 85 octane caused any problems with this maneuver, but my Max performed to my satisfaction. Like you implied (and I have posted many times), if you race stick to premium gasoline.

 

Today i was finally down to about 15 liters of gas and i had to fill up after work. With the recent refinery fire the prices are so high that i cant afford premium anymore. I switched to 89. I assume it will take up to 3 full tanks of 89 to see any difference if there is one. If a hear knock I will be forced to go back to 91, but if not, I will stay at 89 as long as I can afford that. My friend used to have a 95 max and ran 87 for about 5 years that he had the car and tells me there was nothing wrong with it.

I do have a question for you guys; When I check the CEL codes on my 97 (its NOT lit up but i wanna see if i have the ghost KS code 0304) using the turning screw method, after chcking I would have to reset the ECU would that make it "learn" whatever it does learn all over again ?

 

do you have an auto trans? if so, you should not hear knock at all b/c the ECU will respond so quickly that you will not know it happened. you will only hear knock it if the ECU is already on the safe map but the knock is still present. this can happen if you are "lugging" the engine in the wrong gear with a 5sp, or if there are other issues at play (excessive deposits, overheating, etc).

if you find the KS code, reset the ECU and then recheck sometime later. it would have been good to check it before you switched to 89. you don't have to reset the ECU for injector learning, it is always happening and will adjust as necessary to any differences in the oxygen content of the new fuel.