Relationship Between Fuel Octane and Power

[The Critic]

In our fleet, we have a 2010 Nissan Altima 3.5SR V6. This car uses the common Nissan VQ35 engine, which is an engine that Nissan uses in a variety of applications, but each application has some slight differences power and/or tuning.

Our 2010 calls for 87 octane fuel, with no mention at all of needing to use Premium under any circumstances. The 2007-09 models called for premium fuel of 91 octane or higher. Strangely, there have not been any changes in the rated output between the 2007-09 and 2010 models. Both vehicles are SAE rated for the exact same amount of torque and horsepower. In fact, the 2010 model is supposed to earn an extra 1 mpg on the highway according to the EPA testing.

So my questions are as follows:

1) How can Nissan retune an engine for 87 octane without sacrificing power or economy?

2) How does Nissan retune an engine to accept a lower octane fuel?

3) Would it be beneficial for me to use a higher octane fuel if I desire more power, even if the manual makes absolutely no recommendation of using a higher octane fuel under any circumstances? Remember, the 2010 Altima is the only application where the VQ35 is allowed to use 87 octane. All other VQ35 applications call for Premium.

Thanks.

 

[Boomer]

I have the VQ 40 in my 2007 Frontier. It too calls for at least 91 octane but says it can run on 87. I have done some trials over the last three years and found that with 93 octane, it costs me fewer $/mile than with 87 octane! That assumes gas around $2.70 for 87 and $3.00 for 93. I can feel a major difference as on 87 the truck will downshift on hills almost immediately.

So you might want to experiment. Remeber, it is $/mile, not mpg, that I am looking at.

 

[gathermewool]

the HP and torque curves may be different. The older engine may have the same peak HP/TQ, but the curve may have a different shape, or it may make that peak at a different RPM. The manufacturer may or may not have updated the timing for certain loads/RPM to allow for 87.

 

[SteveSRT8]

Anyone who uses dynos to tune knows that it's hp 'under the curve' that is way more important than peak hp. Mfgrs will alter peak hp for purely marketing reasons.

All that is required is simple reprogramming to change octane requirements.

Modern high output motors easily detect knock and retard timing as required.

 

[gathermewool]

Originally Posted By: SteveSRT8
Anyone who uses dynos to tune knows that it's hp 'under the curve' that is way more important than peak hp. Mfgrs will alter peak hp for purely marketing reasons.

All that is required is simple reprogramming to change octane requirements.

Modern high output motors easily detect knock and retard timing as required.


Exactly. That's why any modification parts that claim +X% increase in power are useless. It's very complicated, and only a dyno will determine if the increase in peak HP at one point in the curve may be causing a dip somewhere else in the curve that can't be tuned out.

FWIW, off-the-shelf Cobb tunes for stock turbo Subies will realize overall increases over stock, but advertise using % increases because it sells more. These tunes are based on octane available in your area, so while I can reflash the ECM with the 93 octane map, some one in CA is forced to use the CA-specific 91 octane map with smaller increase over stock based on modified boost and timing changes.

The point is that Cobb was able to realize increases over stock using the same octane fuel. I'm sure, if there was a demand for it, they'd be able to tune for 87 octane with similar output as stock. This may result in lower fuel economy or increased emissions, but that's besides the point.

 

[brandini]

Originally Posted By: Boomer
I have the VQ 40 in my 2007 Frontier. It too calls for at least 91 octane but says it can run on 87. I have done some trials over the last three years and found that with 93 octane, it costs me fewer $/mile than with 87 octane! That assumes gas around $2.70 for 87 and $3.00 for 93. I can feel a major difference as on 87 the truck will downshift on hills almost immediately.

So you might want to experiment. Remeber, it is $/mile, not mpg, that I am looking at.


A million points for you sir! I did the same test over several tanks of 87 and 93 in a 1999 Chrysler 300M which had migh compression and recommended 91+ but would detune for 87. Sure enough, it was cheaper per mile to use premium. I also was sure to reset the car's computer after I made my final decision so it would relearn fuel trims on premium from scratch and I got another 1/2 to 1 mpg from that.

 

[JHZR2]

Originally Posted By: SteveSRT8
Anyone who uses dynos to tune knows that it's hp 'under the curve' that is way more important than peak hp. Mfgrs will alter peak hp for purely marketing reasons.


Thank you.

 

[mechtech2]

Cruising and full throttle ignition timing are very different.
So fuel economy may be either/ or .

Most engines can utilize a bit more ignition advance - this is what tuners, programmers, and chips do.
We used to just twist the distributor a bit! Now it costs $400!

If a car says it can ALSO use regular instead of premium, it is relying on the knock sensor and may be reprogramming as you drive. This is second best, for sure. Best overall economy and power are with the higher octane rated fuel.

Many enthusiasts unplug the Knock Sensor, and use whatever octane it then needs - you may need to go up a grade. This is what I do and works great. Knock sensors are little microphones that go off for various reasons, not just spark knock. So more power and a smoother powerband are achieved.
This won't work on all cars, but does on many.

 

[tonycarguy]

Originally Posted By: brandini
Originally Posted By: Boomer
I have the VQ 40 in my 2007 Frontier. It too calls for at least 91 octane but says it can run on 87. I have done some trials over the last three years and found that with 93 octane, it costs me fewer $/mile than with 87 octane! That assumes gas around $2.70 for 87 and $3.00 for 93. I can feel a major difference as on 87 the truck will downshift on hills almost immediately.


A million points for you sir! I did the same test over several tanks of 87 and 93 in a 1999 Chrysler 300M which had migh compression and recommended 91+ but would detune for 87. Sure enough, it was cheaper per mile to use premium. I also was sure to reset the car's computer after I made my final decision so it would relearn fuel trims on premium from scratch and I got another 1/2 to 1 mpg from that.


Have you guys compared 89 with 91/93?

 

[rjundi]

Originally Posted By: The Critic
In our fleet, we have a 2010 Nissan Altima 3.5SR V6. This car uses the common Nissan VQ35 engine, which is an engine that Nissan uses in a variety of applications, but each application has some slight differences power and/or tuning.

Our 2010 calls for 87 octane fuel, with no mention at all of needing to use Premium under any circumstances. The 2007-09 models called for premium fuel of 91 octane or higher.


I recall filling a 2008 Altima V6 with fuel(not my car) and it stated premium recommended I though on door, not recommended.

 

[rat]

My Expedition recommends 87 octane, but even with a recent full tune up, it runs its best on 89. It starts quicker, idles smoother, runs better and gets 1-2 more mpg with the 89.
It never spark knocks with 87, just feels very lazy.

 

[CaspianM]

Higher octane actually has less heat energy. Unless the engine is designed to use higher octane it is wate of money and some cases higher octane produces less mpg due to low copmpression and lower energy.
The main factor for an engine to be required to run on high octane is higher compression ratio as the low grade fuel would pre ignite under the high compression and combined with heat the engine would knock. CR of 12:1 requires premium.
Tuning an engine with high CR to use low ocaten is not easy even with ECU tweak due to high comp. And contantly advancing the timing by ecu/knock sensor feedback(due to wrong grade) is not a liable solution.
Nissan could have either changed the piston head and/or head including less aggresive valve timing for lower CR or just relying on ecu and knock sensor take care of the knock using low grade which is a cheap solution.
With gas prices and overl cost manufacturer's are under pressure to design engines to run on low grade.
If the engine is the same (same head&piston)using a high grade gas might improve. Dyno the car with different grades and find out is the sure way. Or find out the compression ratio listed spec. If it is 10:1 or lower then stick to a low grade high quality gas.

 

[Shannow]

Originally Posted By: CaspianM
Higher octane actually has less heat energy.


Really ?

of do you mean sometimes has less heat energy ?

Something that also needs to be factored in is octane creep.

As an engine ages, oil ingresses the chamber, and deposits form, increasing the compression ratio, and reducing heat transfer from the chamber.

This requires higher octane than a brand new sparkly one.

Nissan may (and I say may) have specified premium as the fuel for the life of the engine, but found in the real world that octane creep is lower than estimated, and then revised their recommendation to take advantage of that.

 

[CaspianM]

Originally Posted By: Shannow
Originally Posted By: CaspianM
Higher octane actually has less heat energy.


Really ?

of do you mean sometimes has less heat energy ?


"A high octane fuel such as liquefied petroleum gas (LPG) has a lower energy content than lower octane gasoline, resulting in an overall lower power output at the regular compression ratio an engine ran at on gasoline."

LINK

10:1 becoming 12:1 due to deposits? Let me think..That is about 20% reduction in head/piston volume. That is highly unlikely. Even if possible such engine has wrorn rings and valves seatings so not much compression to speak of.

 

[Shannow]

Comparing liquid propane at a density of 0.53 to a liquid at ambient petroleum liquid is meaningless, and not a valid comparison.

Traditionally, some of the higher octane fuels were less energy dense, but that's not necessariyl the norm now.

Where did I say 10:1 becoming 12:1 ?

I DID say that chamber volume is reduced, while at the same time deposits insulate the chamber from the coolant (also under piston deposits do similar).

It's common knowledge that a clean fresh engine needs lower octane than one with a few thousand miles on it.

Edit, the utilisation of the english language in your link should tell you a lot about the quality of the information contained there in.

 

[CaspianM]

Originally Posted By: Shannow
Comparing liquid propane at a density of 0.53 to a liquid at ambient petroleum liquid is meaningless, and not a valid comparison.

Traditionally, some of the higher octane fuels were less energy dense, but that's not necessariyl the norm now.

Where did I say 10:1 becoming 12:1 ?

I DID say that chamber volume is reduced, while at the same time deposits insulate the chamber from the coolant (also under piston deposits do similar).

It's common knowledge that a clean fresh engine needs lower octane than one with a few thousand miles on it.

Edit, the utilisation of the english language in your link should tell you a lot about the quality of the information contained there in.


If you look at the chart in the link you would see that high octane gasoline has lower BTU or KW rating.
There might be special applications fuel but that is not what we are talking about.
At the pump lower grade gas has higher enery per volume than higher grade.
AFA poorly written link I provided, I see no problem with its contents.
Do your own search and post (if you want) what you find that supports higher octane gas has more energy.
English is my second language BTW.

 

[Shannow]

I NEVER said higher octane gas has more energy.

I said that the concept of higher octane gas (note, not propane) having lower energy is not the case.

Check in your link the energy per Kg of the two, rather than the energy per gallon.

Having owned a propane/gas vehicle, you use way more litres because the fuel is less dense, not because it's high octane.

 

[CaspianM]

Aviation gasoline (high octane gasoline, not jet fuel) 112,000 BTU/US gallon
Conventional gasoline 115,400 BTU/US Gallon

I think it is talking about "liquid petroleum gas" not propane.

The only reason for more HP out of high grade gas is its compressibilty factor. Having said that the reduced enery is not really much at all but useless in low compression engines AFAIK.

 

[CaspianM]

Good read about low/high octane
LINK

 

[Shannow]

It is, especially when it says

Quote:
There is little difference in energy content of regular versus premium gasoline. They both contain about 111,400 British Thermal Units of energy per gallon.


The logic in your orginally linked article regarding propane/LPG and petrol is entirely flawed. BTU/gallon tells nothing about octane rating and power output, as engine operation is near stoichiometric, and thus the amount of heat available in the cylinder is what counts, not how many BTUs are in the tank.

Per Kg of stoichiometric air fuel gives the following
Gasoline - 2.83KJ/Kg (RON 91-99, density 0.72-0.78)
Propane - 2.73KJ/Kg (RON 112, density 0.5)
iso-octane - 2.75KJ/Kg (RON 100, density 0.692)
Toluene - 2.79KJ/Kg (RON 120, density 0.867), i.e. more energy available than propane at significantly higher octane rating.

Like I said, octane and energy available aren't correlated.

I still think part of the OP's question can be answered with Octane Creep

Quote:
"The octane requirement of an engine-vehicle combination usually increases during use, primarily due to build-up of combustion chamber deposits within the engine cylinder. While these deposits increase the engine's compression ratio modestly, their largest effect is to increase the temperatures of the outer surface of the combustion chamber. This increases the heat transfer to the fresh mixture during induction, and decreases heat transfer from the unburned charge during compression. End gas temperatures are therefore higher, thus increasing the likelihood of knock. As the combustion chamber deposits stabilise (over 15,000 to 25,000 km of driving), the engine's octane requirement typically increases by about 5 octane numbers; the increase can vary from between 1 to over 13 octane numbers"


In other words, I believe that Nissan may have made a specification based on an assumed octane creep over the engine's life, and may not be seeing that in the real world driving.

Quote and fuel information taken from
Internal Combustion Engine Fundamentals - 1988
by J B Heywood

One of my engineering texts when I was studying engine design 20 years ago. $124 back then, and more updates since then.

I'd recommend it over just about everything on the 'net.