What I do based on no science or measurements

I was gonna ask this on the car specific forum for my particular vehicle (I'm talking about my Infiniti FX35 daily commuter), but figured they'd give me an answer of if you wanna protect your baby, use the best gas kinda answer, so here goes- after running 50+ miles per day roundtrip to work each day for over a month, I recently discovered that although the manufacturer "recommends" 91 grade fuel (premium) for best performance, the SUV technically only needs 87 grade fuel (finally read the friggen manual). So, with my calculations, that amounts to a savings of about $300 a year if I run regular gas now.

So, my question- should I just run the 87 from now on? I don't need the extra power/performance from 91 as I'm just using the thing for commuting, and an extra $300 a year is a nice dinner or Boxster mod , but I don't want the hassle of having engine problems down the road if running 87 will cause engine deposits/knocking issues over the long haul. What do you guys think? Continue running 91, or switch back down to 87?

Current: 07 Carrera S Cab - Midnight Blue/Sand Beige
Previous: 01 Boxster - (formerly boxtaboy), 86 944, Instagram: @Carreralicious

... [www.pedrosgarage.com]

Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

Thanks Pedro. The manual states that one should use at least unleaded regular gas with 87AKI (91RON), but they recommend premium 91AKI(96RON), for improved performance. I read that as 87 will suffice, but 91 if you want to extract the most power out of it. Unless I'm reading it wrong, your site is saying to just use what grade your vehicle is made to run.. eg., running 91 on an engine made for 87 is no real benefit.

... optimized to a particular gasoline octane rating depending on their compression.
Modern cars have very accurate knock sensors that can alter the engine's timing to retard ignition so you really can't hurt the engine by using lower octane ratings.
What happens is that by retarding ignition you loose power because the volume of mixture won't be used to its fullest.
Even though you're "saving" money because you're purchasing cheaper fuel, in the long run you're using more fuel (because there's less power) than what you're saving.
The best combination of engine power/mileage will be when using the recommended octane rating for that engine.
Remember that the manufacturers have to adhere to the CAFE standards, so their numbers are optimized by using the recommended octane ratings.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

If I'm on a long trip, on level land, in mild temperatures, in my wife's Acura...I use an every other tank of mid-grade. If the conditions aren't optimum, it is the higher grade. And the Porsche gets only best octane premium.

Quote
boxtaboy
I was gonna ask this on the car specific forum for my particular vehicle (I'm talking about my Infiniti FX35 daily commuter), but figured they'd give me an answer of if you wanna protect your baby, use the best gas kinda answer, so here goes- after running 50+ miles per day roundtrip to work each day for over a month, I recently discovered that although the manufacturer "recommends" 91 grade fuel (premium) for best performance, the SUV technically only needs 87 grade fuel (finally read the friggen manual). So, with my calculations, that amounts to a savings of about $300 a year if I run regular gas now.

So, my question- should I just run the 87 from now on? I don't need the extra power/performance from 91 as I'm just using the thing for commuting, and an extra $300 a year is a nice dinner or Boxster mod , but I don't want the hassle of having engine problems down the road if running 87 will cause engine deposits/knocking issues over the long haul. What do you guys think? Continue running 91, or switch back down to 87?

to use premium, almost certainly because it has a high compression ratio or has a low (9:1 or thereabouts compression ratio) and is super-charged.

But to soften the blow of higher car prices, higher fuel prices, that are especially onerous for SUV drivers, the automaker has indicated 87 octane gas is ok.

Generally if an engine is desgined to use 87 or 89/90 octane gas it will not benefit from using a higher octane of gas. (This ignores that as an engine accumulates miles its octane requirements can go up.) The engine's controller does not support advancing the ignition timing ahead of what the ignition maps allow to derive any benefit from the higher octane gasoline.

If an engine is designed to use 91 (or higher) octane gas it can get by on a lower octane of gas (though maybe not 87 octane -- I believe my Boxster's owners manual states nothing lower than medium octane grade allowed) because the engine controller with the input from the knock sensors can retard ignition timing and prevent engine damage due to knock/detonation.

Anyhow, in your car's case, if the engine is really intended to use premium gasoline while you will recognize an upfront savings from purchasing a cheaper (lower) octane of gas the engine will not derive as much from burning this gas. There is not more or less energy in the gasolines, the real benefit of using a premium grade of gas in an engine intended to use it is the engine controller can leave the ignition timing as advanced as its basic maps allow and start the air fuel mixture burning much sooner in the combustion chamber. The result is more pressure is developed and at the most optimum time in the piston's travel to produce more mechanical energy from the burning of the air and fuel mixture.

When a lower octane grade of gas is used, the engine controller retards the spark and this lowers the pressure created in the combustion chamber. It also delays when maximum pressure is reached and this delay results in less mechanical energy being derived from the burning of the air fuel mixture.

Thus while you pay less at the time of fill up the lower octane gas results in less distance traveled per unit of gas burned. In other words, gas mileage gets worse. You then end up buying more of the lower octane gas then you would of the higher octane gas.

Also, the retarded ignition timing delays burning of the air fuel mixture and because of this the exhaust gases have a higher temperature. This subjects all components exposed to the exhaust gases to more heat and more wear and tear.

You really are better off in the long run to run the proper grade of gasoline. If the owners manual states the engine requires premium run it. In a pinch you can run a lower grade of octane though of course consult the owners manual to see just how low you can go.

Sincerely,

MarcW.

The engine in my SUV is the same 280hp 3.5L V6 used in the 350Z, so I was surprised that the manual stated all it needs is 87 octane. It did say that you would get better performance with 91, however. I'm going to fill up today with 87 and see how it goes, and if mpg suffers. Thanks Marc and Pedro for the input.

getting any knock, you can see if the results are significant enough to justify a change to lower octane.

As gas prices go up, which is certainly a currently accelerating trend, it may change your equation.

Regards, Maurice.

Just filled with 87, and it was $2.99 vs $3.39 per gallon, so $0.40 difference per gallon. So far can't feel a difference in power, but not really pushin it cause it's snowing out!

of a chassis dyno.

EVO mag (UK published car mag) did a gas (petrol) test a few years ago and using a VW GTI (with a 1.8l turbo-charged engine) as a test vehicle found differences of up to 40ftlbs of torque between the best and worst gasolines. I can't remember the HP differences.

Professional driver on a track noticed big differences in the way the engine felt.

Where the high octane really comes into play is in the every day driving we do where the engine rpms are fairly low, the engine load is rather high. Engine combustion chamber filling is good under these conditions and with the proper grade of octane gas the engine controller can advance the timing (36 degs or thereabouts) and really wring the last bit of energy from the air/fuel mixture's burning.

WOT no so much. Under these conditions fuel mixture is richened up -- which lessens the detonation odds -- and timing retarded a bit. I've seen it drop to under 25 degs or even less under hard acceleration.

Sincerely,

MarcW.

Shell Optimax: Max power 209hp @ 5500 rpm; max torque 242 lb feet @ 2500 to 2900 rpms.
Shell 95: max power 205hp @ 5600 rpms; max torque 234 lb ft @ 2650 rpms.
BP 95: Max power 204hp @ 5600 rpms; max torque 242 lb ft @ 2300 to 3100 rpms.
Tesco 99: Max power 212hp @ 5500 rpms; max torque 242 lb ft @ 2400 to 3500 rpms.
BP Ultimate: 212hp @ 5300 rpms; max torque 252 lb ft @ 2400 to 2500 rpms.
Sunco Racing 105 octane fuel: Max power 218hp @ 5200 rpms; max torque 248 lb ft @ 2500 to 2800 rpms.

Shell Optimax was a poor dyno performer equal to the 95 RON stuff but was the pro driver's choice. He said the engine felt the crispest and the GTI was the most fun to drive using this gasoline.

This is not a recommendation by me to run Shell. Just so it is clear the above are UK brands of fuels. I'm just trying to make a point there may not be a corelation between dyno performance and how the engine feels.

Except for a racing fuel: Sunco race fuel. Very expensive: 352.2p/litre vs. 89.9p/litre to 100.9/litre for the other fuels. Runs even with BP Ultimate initially but beyond 3500 rpms is way above pump fuels.

One of the conclusions from the test was that freshness was very important with the higher octane fuels.

Supermarket fuels can be quite good but the quality varies cause in the UK the various outlets can use gasolines from different suppliers. I suspect the same is true here in the USA and that is gas stations with no brand of their own gas (unlike say Shell, Chevron, etc.) use gasoline from various suppliers.

Sincerely,

MarcW.

octane in a pinch. I would use what they're recommending, i.e. 91 octane or higher for the reasons Pedro & Marc W. highlight.

JB/SE SoDak
1997 986 - Wolfi
Yankton, SD

"Wisdom is the most perfect knowledge of the most important truths in the right order of emphasis, accompanied by a total, permanent disposition to live accordingly.

-- Br. Francis Maluf, M.I.C.M.

"(School is to be a factory) in which raw products, children, are to be shaped and formed into finished products . . .(m)anufactured like nails, and the specifications for manufacturing will come from government and industry."

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can use whatever crud you want without even triggering a different timing map.

Its all about avoiding knock.

Grant

You might not hear subtle pre-ignition at all, or perhaps it'll be a trickling noise. Also, I believe octane requirement drops with increasing altitude.



Edited 1 time(s). Last edit at 12/29/2010 11:34AM by Laz. (view changes)