# does the CPU sense premium gas



## =WD=big-X (Jan 30, 2006)

i hear the CPU in my frontier will sense premium fuel, adjust itself
accordingly and make more power.(06 4.0L)

anyone else heard this?


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## Misery-Tom (Oct 16, 2005)

if its setup anything like the titan, the ECM will actually retard timing when higher octane fuel is used


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## =WD=big-X (Jan 30, 2006)

your Titan looks VERY nice.


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## Misery-Tom (Oct 16, 2005)

thanks....


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## canuuu (Dec 9, 2005)

interesting. I like to 'splurge' and use premium in my 95 hardbody 2.4, but am I screwing up?.... I have a CEL on most of the time......havent run codes but it runs fine.....LOL.....I think. Any thoughts on this subject? thanks


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## Misery-Tom (Oct 16, 2005)

probly not messin anything up, the CEL is probly just a sensor


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## GTIMANiac (Jan 14, 2006)

Misery-Tom said:


> if its setup anything like the titan, the ECM will actually retard timing when higher octane fuel is used



Retard or Advance?
I thought as you increased octane you would advance timing.


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## Misery-Tom (Oct 16, 2005)

no, the ECM's retard it, ( at least on the titan ) i can only assume the fronty would be similar


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## mainerunr (Jul 30, 2005)

The Fronty has been dyno'd on premuim. No gain.

http://www.reverendbiker.com/frontier/frontier.htm


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## GTIMANiac (Jan 14, 2006)

Misery-Tom said:


> no, the ECM's retard it, ( at least on the titan ) i can only assume the fronty would be similar


Why on earth would it retard timing with higher octane? Performance is gained by advanced timing.


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## Misery-Tom (Oct 16, 2005)

thats because of the retard by the ECM, thats why most cars/trucks now a days recommend 87 oct fuel


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## GTIMANiac (Jan 14, 2006)

Misery-Tom said:


> thats because of the retard by the ECM, thats why most cars/trucks now a days recommend 87 oct fuel


Well it the Frontier I have only owned German autos, Porsche, VW and Audi. I guess that is where my mind set comes from the German ECU tuning advances timing and always reccomends at least 91 octane.


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## jeffro (Jan 6, 2004)

I am pretty sure that it retards it also, it is supposed to be about the same no matter what fuel you use (within reason). Unlike my old hardbody that had to have mid-grade in order not to ping all the way down the road.

Jeff


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## Reverendbiker (Jan 29, 2005)

=WD=big-X said:


> i hear the CPU in my frontier will sense premium fuel, adjust itself
> accordingly and make more power.(06 4.0L)
> 
> anyone else heard this?


The VQ40 Frontier's owners manual states that 87 octane can be used but higher octane can be used for increased performance. That would mean that the ecm is capable of ADVANCING the ignition timing to create more power. Some--but very few--vehicles have ecm's that are capable of this, so I ran dyno tests and fuel economy tests, as carefully as I could, to determine if the VQ40 engine did the same. Dyno tests showed no significant horsepower or torque increase when using 93 octane instead of 87 octane. I got a very small increase in fuel economy when using 93-octane, but it was so small that it didn't justify the additional cost. See the tests here:
Dyno tests--93 vs 87 octane 
Fuel Economy--93 vs 87 octane 
My truck runs great on 87-octane fuel and delivers nice gas mileage; I see absolutely no reason to spend my hard-earned cash on the expensive stuff.


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## GTIMANiac (Jan 14, 2006)

I have a tank of 87 in right now. I will log data on my way to and from work and post the xls files tomorrow. Then in a week or so I will run 91 through it and post that data as well. My equipment will only log at about 3hz but it is better than nothing.


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## GTIMANiac (Jan 14, 2006)

I logged data on my way to work this morning. About a 26 mile drive. 87 Octane.
I logged the following;
RPM
Speed
Cyl #1 Timing
Calculated Engine Load

The Raw CSV is here 

The XLS is here


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## MrFancypants (Nov 18, 2005)

That just doesnt make sense... if you retarded timing when you increased octane, you would make less power. All other things being equal, higher octane gas has a less powerful burn. The only reason to run it would be to advance timing. If the ECM does retard the timing... well thats just... retarded. More reason to run regular old 87 every day.


- Greg -


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## abmobil (Nov 22, 2004)

The titan or any vehicle definently does not retard timing with the use of premium. 
The titan can probably take more advantage of premium than the frontier.
The qx56 supposedly uses the same ecu and engine hardware as the titan but is rated 10hp more and it calls for premium while the titan calls for regular.

It may just me mental but a few people at titan talk say they can feel a diference after a couple of tanks of premium but I havent seen a dyno of both fuels.


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## Misery-Tom (Oct 16, 2005)

sorry guys, i was thinking backwards lol, if the timing changes at all it would advance it, i was thinking of the knock sensor causing a retard


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## avenger (Oct 7, 2005)

Misery-Tom said:


> no, the ECM's retard it, ( at least on the titan ) i can only assume the fronty would be similar


proof of this please, because its against what a knock sensor does... no car senses what gas it has until it senses a knock, and then it retards... regular will knock long before premium


edit: i see you corrected with the last post...


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## B13boy (Dec 16, 2004)

There is a little camera inside your fuel filler door that takes a picture of the pump and tells the ecu what grade of fuel you put in and changes the ecu program based upon that...  Not exactly! It cannot tell what octane you have. There are a combination of events that happen. The knock sensor, the o2sensor, and the ecu all talk to each other based upon voltages. The ecu changes timing and other things based upon the sensor output. It cannot tell what OCTANE you have. It can only react to what it is burning. It will adjust accordingly.


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## James (Apr 29, 2002)

avenger said:


> proof of this please, because its against what a knock sensor does... no car senses what gas it has until it senses a knock, and then it retards... regular will knock long before premium
> 
> 
> edit: i see you corrected with the last post...


since the engine has continuous variable valve timing I would think it would advance or retard the timing more aggressively everywhere throughout the powerband that would optimize power (though it may not make that much of a difference).


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## GTIMANiac (Jan 14, 2006)

James said:


> since the engine has continuous variable valve timing I would think it would advance or retard the timing more aggressively everywhere throughout the powerband that would optimize power (though it may not make that much of a difference).



Valve timing and ignition timing are very different animals though. I am not sure I can tap into the valve timing aspects for any logging with the current software that I have but I can do a little digging and see what I can come up with.


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## James (Apr 29, 2002)

GTIMANiac said:


> Valve timing and ignition timing are very different animals though. I am not sure I can tap into the valve timing aspects for any logging with the current software that I have but I can do a little digging and see what I can come up with.


well the ignition timing is all controlled by the ecu now as well through the coil packs so i think the ecu has become a dictator in terms of what makes power.


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## CT_Nismo (Dec 25, 2005)

*Interesting Data*



GTIMANiac said:


> I logged data on my way to work this morning. About a 26 mile drive. 87 Octane.
> I logged the following;
> RPM
> Speed
> ...



While in Denver, I have noticed that gasoline octanes are lower than for sea level states. I infer from this that the higher altitude reduces the likelihood of pinging or knocking. It would be interesting to see your data stream on a trip from Phoenix to Scottsdale which is at a higher elevation. I would think that spark advance would be higher at higher altitudes.


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## jeffro (Jan 6, 2004)

I too was thinking backwards yesterday. I must have been running a little sloooow. 

Today, advanced timing makes all the sense in the world. That data is neat, is that something you just have or is that something you do for a living?

Jeff


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## GTIMANiac (Jan 14, 2006)

CT_Nismo said:


> While in Denver, I have noticed that gasoline octanes are lower than for sea level states. I infer from this that the higher altitude reduces the likelihood of pinging or knocking. It would be interesting to see your data stream on a trip from Phoenix to Scottsdale which is at a higher elevation. I would think that spark advance would be higher at higher altitudes.


It almost was. It was from the 101&10 in the West Valley to the 101&17 in North PHX.


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## GTIMANiac (Jan 14, 2006)

I actually think a better indication for elevations would be a trip from Phx to Flag. I am heading to Mormon Lake next weekend for some Pheasant hunting, I will log that trip as well and post data.


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## scuba91ta (Aug 19, 2005)

From the tuning experiance i have with aftermarket ecu's, elevation makes no difference, there is merely a fuel table correction for it. The MAP sensor records barametric pressure, which of course is less at higher altitudes... because the air is less dense, the computer actually has to cut fuel to keep the a.f ratios's proper. 

Timing i think would have very little to do with altitudes unless they make some attempt at bringing back some power, by adding advance, to counter act the power loss from higher altitudes...


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## MrFancypants (Nov 18, 2005)

scuba91ta said:


> From the tuning experiance i have with aftermarket ecu's, elevation makes no difference, there is merely a fuel table correction for it. The MAP sensor records barametric pressure, which of course is less at higher altitudes... because the air is less dense, the computer actually has to cut fuel to keep the a.f ratios's proper.


Actually it's even better than that. Nissan uses MAF sensors that measure mass air flow rather than pressure. That means they dont need to account for pressure because they know the mass of air entering the motor (altitude becomes meaningless).


- Greg -


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## CT_Nismo (Dec 25, 2005)

*I am torn between the two*



scuba91ta said:


> From the tuning experiance i have with aftermarket ecu's, elevation makes no difference, there is merely a fuel table correction for it. The MAP sensor records barametric pressure, which of course is less at higher altitudes... because the air is less dense, the computer actually has to cut fuel to keep the a.f ratios's proper.
> 
> Timing i think would have very little to do with altitudes unless they make some attempt at bringing back some power, by adding advance, to counter act the power loss from higher altitudes...


I am totally upside down on this subject. All you have said makes sense. Another poster said Nissan measure air flow as *mass*. This is even better. So it should make no difference at Pike's Peak or Death Valley on the operation of the engine with respect to engine timing. Then why do they use lower octane fuels in Denver? I'm sure you have all seen the postings on the pump. It must be that engines in Denver have lower propensity to knock. There must be a reason and it's not because John Elway has numerous dealerships in Denver and he dislikes the number 87 as it was the uniform number of a defensive lineman who tormented him.


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## inyourface1650 (Oct 6, 2005)

well, temperature has to do with it too.....In arizona, engines have a higher tendancy to knock while on 87 than most places. 

Or maybe its a law? Maybe the other states have a minimum ocatane set to 85? Just throwing out ideas...


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## James (Apr 29, 2002)

mass of the air ingested can change with altitude and temperature because the density of the air will change and the amount of Oxygen in that air. I think that is why most mass air flow vehicles have a MAP sensor anyways to give a better resolution on the density of the air.


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## jurassicjet (Jan 17, 2006)

> mass of the air ingested can change with altitude and temperature because the density of the air will change and the *amount* of Oxygen in that air.


Acutually, the amount of Oxygen never changes for a given mass of air, unless you artificially alter it, like breathing through an oxygen mask. The amount of oxygen is ALWAYS 20.9%.

The density of air will change with altitude and temperature and to a much lesser extent, humidity. 
The higher the temp = less dense
The higer the altitude = less dense
The higher humidity = less dense (i know, this one seems wierd).

How all of this affects our vehicles, i dont know. I do know the above facts though. 

I really dont see how timing would be affected by air density. It should just boil down to the fuel/air mixture. And that fuel/air ratio is, to the best of my knowledge, measured by mass.


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## scuba91ta (Aug 19, 2005)

MrFancypants said:


> Actually it's even better than that. Nissan uses MAF sensors that measure mass air flow rather than pressure. That means they dont need to account for pressure because they know the mass of air entering the motor (altitude becomes meaningless).
> 
> 
> - Greg -



Now, i haven't actually run a scan on the truck, but i would bet that there IS a MAP sensor... most mass air ECM's do - i know the ford 4.6 and 5.4 motors do, and the LS* series motors are also MAF/MAP...

I can only speak for the LS1 ecm here, but there is a comparitive table in those ecms that compares the maf and map readings and uses it to ultimately determine the air density - the maf CAN read air mass, but it doesn't read air density as well with out comparing numbers to a set chart, and in this case, instead of a chart, it uses the MAP sensor...


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## scuba91ta (Aug 19, 2005)

> Then why do they use lower octane fuels in Denver?


Well, i'm glad you asked, because i know the answer  Lesser octane fuels can be used in denver because the engines aren't able to use the higher octane... higher octane fuel requiers to a certain extent, more air, and because more octane is more difficult to burn, and combined with less dense air, higher octane fuel in high elevation actually causes more pollution from unburned fuel. 



> The higher humidity = less dense (i know, this one seems wierd).


Yes - but it's not wierd - the air is less dense because there is more moisture in the same volume of air taking up space


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## MrFancypants (Nov 18, 2005)

If youve already got a MAF sensor to tell you the mass of air entering the motor, you dont need a MAP sensor. I suppose you could add one as a sanity check, but my experience with Nissans has been that they only use air pressure sensors for emissions (controlling the EVAP system).


- Greg -


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## scuba91ta (Aug 19, 2005)

well, as i mentioned before, i don't have any experiance with nissans setup, but i can speak from other experiances...


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## GTIMANiac (Jan 14, 2006)

scuba91ta said:


> well, as i mentioned before, i don't have any experiance with nissans setup, but i can speak from other experiances...


Who uses a MAF and a MAP then.
Because all the tuning exp I have is with VAG(Volkswagen Audi Group) and Porsche. And they all use only a MAF and have been Drive By Wire for years.


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## scuba91ta (Aug 19, 2005)

GTIMANiac said:


> Who uses a MAF and a MAP then.
> Because all the tuning exp I have is with VAG(Volkswagen Audi Group) and Porsche. And they all use only a MAF and have been Drive By Wire for years.



LS1, FAST, LT1

LS1's still use a map sensor


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## GTIMANiac (Jan 14, 2006)

scuba91ta said:


> LS1, FAST, LT1
> 
> LS1's still use a map sensor


A MAF and a MAP?


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## scuba91ta (Aug 19, 2005)

yup... sure do


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## AZdriver (Sep 8, 2005)

*Burning Hi-Test*

I've tried 91 octane on several runs between PHX->TUC, and found absolutely no improvement in either fuel economy or enhanced performance. IMO, it only lightens your wallet.


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## scuba91ta (Aug 19, 2005)

i usually only use 87 - i'm running a tank of 93 this week and see if it improves it at all... i highly doubt it, but running through a better fuel once in awhile can scrub a lot of crap out of the fuel system...


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## GTIMANiac (Jan 14, 2006)

scuba91ta said:


> i usually only use 87 - i'm running a tank of 93 this week and see if it improves it at all... i highly doubt it, but running through a better fuel once in awhile can scrub a lot of crap out of the fuel system...


Actually I have always heard the opposite as lower octane has a higher flash point and burns hotter. True or not, I do not know.


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## scuba91ta (Aug 19, 2005)

i dont know about burning hotter - i would think the harder to combust 93 would burn a little hotter... in reality - i would bet that the difference is minimal... premium gas does however have a higher amount of detergents - helps clear some of the crud out of the system... just filled up today - picked up an extra mile to the gallon, and i would say i did a little more around town driving, include some 4x4ing in the snow... 

all that aside - 16.7mpg..


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## canuuu (Dec 9, 2005)

Hi, from what I remember about octanes is that high octane gas ignites slower and burns longer than lower octanes. This does two things, the cylinder runs cooler and the longer burn provides more (total) power. The 'explosion' from premium is slower and of longer duration hince providing a lower shock to the piston wrist pin and rod bearing....extending the life of these parts. Sorta like using a dead blow hammer as opposed to a metal hammer. I dunno how gas makers change the octanes. Either thru additives to inhibit the combustion rate of 87 octane or using longer chained hydrocarbons to make premium. Any ideas? Ahaha, what ever they do it costs more.


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## CT_Nismo (Dec 25, 2005)

canuuu said:


> Hi, from what I remember about octanes is that high octane gas ignites slower and burns longer than lower octanes. This does two things, the cylinder runs cooler and the longer burn provides more (total) power. The 'explosion' from premium is slower and of longer duration hince providing a lower shock to the piston wrist pin and rod bearing....extending the life of these parts. Sorta like using a dead blow hammer as opposed to a metal hammer. I dunno how gas makers change the octanes. Either thru additives to inhibit the combustion rate of 87 octane or using longer chained hydrocarbons to make premium. Any ideas? Ahaha, what ever they do it costs more.


To create a gasoline with higher octane, refiners use branched hydrocarbons. Imagine a tree limb with many branches. If you light off one end, the flame advances along the limb and out into the branches. This takes time which leads to the slow burn you mentioned. Ironically, octane the molecule is a linear hydrocarbon and is lousy for premium gas. It is only used as a knock standard.


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## canuuu (Dec 9, 2005)

Thanks CT............The high altitude lower octanes makes sense. At altitude there is less Oxygen for a given volume than at lower altitudes. The cylinder is a set volume and with the correct fuel/air mix provided by the ECU, the cylinder will have less O2. So apparently, at altitudes there isnt enough O2 to fully combust higher octanes. Perhaps the ECU further cuts back the fuel induction to compensate for this. My gut feeling is that there is a threshold of leaness for premium that just doesnt produce more power or mpg for lack of available O2 and nothing the ECU can do will fully combust the premium or 87 for that matter. >pats head, rubs belly, and wiggles toes<


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