The Effect of Fuel Octane on MPG?

[The Critic]

On a 2010 Maxima, the VQ35DE engine has a 10.6:1 compression ratio.

Due to a revised intake system, revised exhaust system and different fuel mapping, the engine now puts out 290HP. Of course, premium fuel is "recommended for maximum performance."

The Nissan Product Manager said in the Maxima press conference that there will be a significant loss in performance when using 87-- possibly a 15 to 20% decrease in horsepower. However, my question is: will there be a similar decline in fuel economy?

Over on Maxima.org, there are numerous threads saying that octane and fuel economy are independent of each other. However, someone told me that whenever timing is retarded due in situations like this, fuel economy also declines.

What do you guys think?

Thanks.

 

[Kaboomba]

in general fuel economy will drop off, but not as much as power--so, 5-10%--but I'm guessing it is not hard-and-fast thanks to clever engine management.

The engine has a 10+:1 compression ratio, and so requires premium fuel to run as designed. When regular fuel is used, all engine management controls work to accommodate by changing ignition and/or valve timing and/or mixture.

K

 

[HTSS_TR]

I used 87 instead of 91 recommended in 1994 Lexus LS400 for several years, the power lost was obvious when needed to climb up hill at Death Valley in summer, but I did not see much fuel economy lost for normal city driving or highway.

 

[rshunter]

Originally Posted By: Kaboomba
in general fuel economy will drop off, but not as much as power--so, 5-10%--but I'm guessing it is not hard-and-fast thanks to clever engine management.

The engine has a 10+:1 compression ratio, and so requires premium fuel to run as designed. When regular fuel is used, all engine management controls work to accommodate by changing ignition and/or valve timing and/or mixture.

K

A high compression ratio does not necessarily mean that premium fuel is a prerequisite. There is far more to it than that. Current high performance motorcycles are a perfect example as many (if not most) sport bikes only require regular fuel, despite their high compression and high specific output engine designs.

 

[Nederlander75]

That has been my experience, especially with my Nissans. I even notice a difference between brands that is measurable.

 

[Quattro Pete]

Originally Posted By: The Critic
Due to a revised intake system, revised exhaust system and different fuel mapping, the engine now puts out 290HP. Of course, premium fuel is "recommended for maximum performance."

In addition, the manual states that

Quote:

If unleaded premium gasoline is not available
unleaded regular gasoline with an octane rating
of 87 AKI (Anti-Knock Index) number (Research
octane number 91) can be used, but only under
the following precautions:
* Have the fuel tank filled only partially with
unleaded regular gasoline, and fill up with
unleaded premium as soon as possible.
* Avoid full throttle driving, and abrupt acceleration.


I guess they're worried that computer may not be able to retard timing sufficiently to prevent pinging?

 

[Kaboomba]

I left a lot unsaid for the sake of simplicity.

Yes, there are bikes that mfr spec fuel is 87 or "regular", and the engines are high compression (2009 Kawi Ninja, others), but others (Aprilia, HD, others) that spec premium 91+.

Note that bikes that are high-hp/L and high compression, that use regular fuel, are generally "small" displacement (NInja is 125cc/cylinder) and are high-revving. The 2010 Maxima is 583cc/cylinder and lives at less than half the revs, and must deliver power/torque starting at "low" RPM's.

Without going into boring detail, displacement, geometry, RPM's, power curves, and timing are where the issue is.

Nearly every car engine that uses 10+:1 compression must also use premium fuel for top performance and MPG.

That and premium fuels are cleaner and have better additive packages, and so are better for fuel systems. I happen to think that my Buick's engine oil is cleaner when I'm using premium as well.

 

[Kaboomba]

Originally Posted By: Quattro Pete
Originally Posted By: The Critic
Due to a revised intake system, revised exhaust system and different fuel mapping, the engine now puts out 290HP. Of course, premium fuel is "recommended for maximum performance."

In addition, the manual states that

Quote:

If unleaded premium gasoline is not available
unleaded regular gasoline with an octane rating
of 87 AKI (Anti-Knock Index) number (Research
octane number 91) can be used, but only under
the following precautions:
* Have the fuel tank filled only partially with
unleaded regular gasoline, and fill up with
unleaded premium as soon as possible.
* Avoid full throttle driving, and abrupt acceleration.


I guess they're worried that computer may not be able to retard timing sufficiently to prevent pinging?


I think you're right. Timing or no, 10.9:1 is pretty high, so they want you to keep the throttle partially closed all the time to lower the compression ratio, in a sense, to prevent pinging.

 

[rshunter]

I just wanted to make sure that someone didn't get the idea that there was some direct link. People read these forums and draw conclusions. Far more than they should, IMHO.

 

[Kaboomba]

I hear ya

There IS a direct link between compression ratio and required fuel octane rating, but that link varies across different engine sizes, designs, and applications. I agree, it's not simple!

The safest/best thing for a owner to do is follow mfr specs and if you have to err, go with premium/higher octane.

 

[ctc]

I had a 2008 Altima with the VQ. 91 was recommended for maximum performance. I would use 87 from time to time and saw no drop in MPG.

 

[Kaboomba]

Originally Posted By: Quattro Pete
Originally Posted By: The Critic
Due to a revised intake system, revised exhaust system and different fuel mapping, the engine now puts out 290HP. Of course, premium fuel is "recommended for maximum performance."

In addition, the manual states that

Quote:

If unleaded premium gasoline is not available
unleaded regular gasoline with an octane rating
of 87 AKI (Anti-Knock Index) number (Research
octane number 91) can be used, but only under
the following precautions:
* Have the fuel tank filled only partially with
unleaded regular gasoline, and fill up with
unleaded premium as soon as possible.
* Avoid full throttle driving, and abrupt acceleration.


I guess they're worried that computer may not be able to retard timing sufficiently to prevent pinging?


If that was my owner's manual, I would keep 1-2 bottles of octane booster in the trunk just in case!

 

[Gary Allan]

Originally Posted By: ctc
I had a 2008 Altima with the VQ. 91 was recommended for maximum performance. I would use 87 from time to time and saw no drop in MPG.


The energy content is identical so I'd figure that as long as you weren't in constant combat with fractured flame fronts, you really shouldn't see much difference. I don't think I'd be doing any mountain climbing or semi-higher speed sustained bouts using it though.

 

[gathermewool]

Originally Posted By: Kaboomba
I hear ya

There IS a direct link between compression ratio and required fuel octane rating, but that link varies across different engine sizes, designs, and applications. I agree, it's not simple!

The safest/best thing for a owner to do is follow mfr specs and if you have to err, go with premium/higher octane.


I agree with everything you've said through out this thread; however, if you drive mostly highway and never drive WOT, balls-to-the-wall-style you won't see any difference at all.

If the manufacturer has designed the motor and mapped the ECU to benefit from higher octane fuel I would use higher octane fuel. I'm sure the ECU will retard timing just fine if it senses knock, but that's not where you really want to be, and that's not where you'll see the most efficient operation.

You want the ECU to be as happy as possible, which means staying with in prescribed fueling and timing look up tables, not having to change short term fueling and pulling timing every time you need to aggressively get out into traffic. Doing this will lose you mileage in the short term, and possibly reduce longevity due to infrequent pinging and knocking.

I know people rarely want to follow manufacturers recommendations, but this is one I would follow. We want the most HP and MPG for our money, and this is how Nissan, and a lot of other manufacturers choose to do it, short of forced induction.

 

[The Critic]

Originally Posted By: Quattro Pete
In addition, the manual states that

Quote:

If unleaded premium gasoline is not available
unleaded regular gasoline with an octane rating
of 87 AKI (Anti-Knock Index) number (Research
octane number 91) can be used, but only under
the following precautions:
* Have the fuel tank filled only partially with
unleaded regular gasoline, and fill up with
unleaded premium as soon as possible.
* Avoid full throttle driving, and abrupt acceleration.


I guess they're worried that computer may not be able to retard timing sufficiently to prevent pinging?


That message must be from the 2009 owner's manual. This is what the 2010 owner's manual states:

From Section 9-3:
Quote:
FUEL RECOMMENDATION
NISSAN recommends the use of premium unleaded
gasoline with an octane rating of at least
91 AKI (Anti-Knock Index) number (Research
octane number 96). If unleaded premium gasoline
is not available, you may use unleaded regular
gasoline with an octane rating of at least 87 AKI
number (Research octane number 91), but you
may notice a decrease in performance.
However, for maximum vehicle performance,
the use of unleaded premium gasoline
is recommended.


Originally Posted By: ctc
I had a 2008 Altima with the VQ. 91 was recommended for maximum performance. I would use 87 from time to time and saw no drop in MPG.


Nice to know, thanks! On the 2010 Altima, they retuned the engine for 87 octane though.

 

[The_Jeff]

I suspect they EPA certified the things on 91 octane and need a dire-sounding warning to keep people from using 87 to keep the gov't happy. Just like they have to warn you not to run out of gas or push start your car to keep the catco safe.

Knock sensors are becoming a regular feedback part of engine management; they aren't just the "oh no! pull timing!" safety valve first found on 1980's turbo cars. Some other makes, Dodge comes to mind, advertise "200 peak hp" on some econo car, marque escapes me, that's intended for 87 (and ergo epa certified on 87) but gains added benefit from 91. Confused?

In short though I would not get the Altima if your Dad is just going to put junk fuel in.

 

[a2ssei]

In 2007, Car and Driver tested the Nissan Altima with the 3.5 engine. In their long term review, they said the following:

"Through both the sun and snow, the Altima was surprisingly frugal with fuel, returning 24 mpg over its 14 months with us, 5 mpg more than its updated 2008 EPA city rating. Plus, a few long hauls on freeways resulted in better numbers than the EPA’s 27-mpg highway estimate for 2008. Only fire-sale city driving or an accidental fill-up with lower-octane gas returned fuel economy below 20 mpg. The mileage drop from using less than premium gas occurs when the engine retards the spark to reduce the chance of knock, which also reduces power and efficiency, and the Altima seemed especially sensitive to fuel quality."

See: http://www.caranddriver.com/reviews/car/08q3/2007_nissan_altima_3.5se-long-term_road_test

Admittedly, the Car and Driver testers are not your average owners. If you want to use regular gas, buy a car that uses regular...

 

[NHHEMI]

Originally Posted By: rshunter

A high compression ratio does not necessarily mean that premium fuel is a prerequisite. There is far more to it than that. Current high performance motorcycles are a perfect example as many (if not most) sport bikes only require regular fuel, despite their high compression and high specific output engine designs.


Correct. On today's computer controlled vehicles how the PCM programming is setup is far more important than compression. Many vehicles will actually get WORSE MPG with premium( 91+ )than they will regular( 87 )or mid grade( 89 ). A prime example is the Dodge 5.7L HEMI. Mfg states that optimum is 89 acceptable is 87. Run premium and you will see a decrease in performance and MPG.

 

[ctc]

Originally Posted By: Gary Allan
Originally Posted By: ctc
I had a 2008 Altima with the VQ. 91 was recommended for maximum performance. I would use 87 from time to time and saw no drop in MPG.


The energy content is identical so I'd figure that as long as you weren't in constant combat with fractured flame fronts, you really shouldn't see much difference. I don't think I'd be doing any mountain climbing or semi-higher speed sustained bouts using it though.


I work on a mountain and had no issues climbing it on 87.

 

[rshunter]

Originally Posted By: ctc
Originally Posted By: Gary Allan
Originally Posted By: ctc
I had a 2008 Altima with the VQ. 91 was recommended for maximum performance. I would use 87 from time to time and saw no drop in MPG.


The energy content is identical so I'd figure that as long as you weren't in constant combat with fractured flame fronts, you really shouldn't see much difference. I don't think I'd be doing any mountain climbing or semi-higher speed sustained bouts using it though.


I work on a mountain and had no issues climbing it on 87.

Required octane level drops as altitude increases. That's why they get 85-octane regular gasoline in high-altitude areas.