Why premium fuel?

[Hai Tong]

Just a question for those in the know.

 

What determines whether an engine needs premium vs. regular fuel?

 

I realize that the compression ratio of a modern motor is usually around 10:1; and that the higher the compression ratio the greater the difficulties to control proper ignition timing and flame front travel. Premium or higher octane fuel is more stable, less likely to auto-ignite and therefore less likely to cause pinging or detonation.

Are there othe engine factor for needing higher octane?

 

As the price of fuel increases; I'm quite sure buyer will be more sensitive to car/motos needing regular vs. premium fuel. Which make me wonder how do Europeans buy their BMW, Mercedes, Audi, etc??? Do they European manufacturers offer Diesel as the only economical option or do they have regular/lower octane gas vehicles?

 

Just asking as I would lover to own a used Miata or TT roadster or even a Z3/4 but all need premium and thus negating their smaller cc economics. I realize they're "performance" vehicles; but not what I would consider sport car performance needing higher octane.

 

I would hope manufactures become more sensitive to these operational cost as fuel cost rises.

[Gregori]

FWIW - I've had a couple of Miatas, and they only needed 'regular' grade...

 

OTOH, I had a Ford van with the 351, and it preferred Super. It also got 15% better fuel economy on the high-test, so it was actually cheaper to run it on the most expensive fuel...

 

[Hai Tong]

Hi Gregori,

that's what I also thought of the new Miata but

http://www.mazdausa.com/MusaWeb/displayPage.action?pageParameter=modelsSpecs&vehicleCode=MX5

 

look under specification/engine/fuel system.

 

[Paul Mihalka]

My plan is to use the lowest grade on which it does not ping.

[Bob Palin]

My plan is to use the lowest grade on which it does not ping.

Doesn't your 1200 have a knock detector? That means you can use the lowest grade but at the cost of performance, which may or may not matter of course.

[Hai Tong]

Is there a notable performance difference?

[Bob Palin]

Is there a notable performance difference?

I don't know, I have an 1150 which doesn't have a sensor and pings quite badly on hot days if I don't use premium and sometimes even when I do.

[Ken H.]

Is there a notable performance difference?

I can't tell any difference on our R1200GS. I gave up on spending the $$ for higher octane fuel long ago.

 

 

[Rinkydink]

I run mid-grade in my 04 RT and have never had a pinging problem. A couple of stations around my neighborhood sell premium for mid-grade price on Tuesdays and I can't tell the difference so me no buy it.

[YesMan]

Timely topic.

 

I tried 87 (for the first time) in the 12RT. According to the computer, (which has always been pretty close) the gas milage has dropped down to 44mpg. It usually sits between 49 and 52. I actually haven't refilled yet, so I dont know how accurate it is, but I figured I get an extra 25 miles, for $1 (based on 5 gallons) with premium (I know, it was a slow day ).

[velomoto]

+1 on what others have said... My 12RT runs fine with any of the grades but fuel economy seems to drop with the lower grades. Seeing how the price difference between grades hasn't changed as fuel prices skyrocketed the percentage difference in price is now quite small - about 5% which amounts to $1.00 for a five gallon fillup.

[Francois_Dumas]

I too use 95 Octane (regular) whenever I can. At $ 9,15 a gallon that beats the 98 (premium) that now costs $ 9,57.

 

Runs fine in our flatlands.

 

When riding in the mountains or in hot weather, I need to put in the premium grade to stop it from pinging at low revs and heavy pulling.

 

It IS noticeable in those circumstances.

[hANNAbONE]

Is there a notable performance difference?

I don't know, I have an 1150 which doesn't have a sensor and pings quite badly on hot days if I don't use premium and sometimes even when I do.

 

mY '03 KRS pinged all the time once it was warmed up (read "hot") under heavy accel...I used premium...even the 93 stuff and with additives sometimes - didn't matter...pinged anyway - I traded that rig for the non-pinging Triumph ST/ABS

[Joe Frickin' Friday]

What determines whether an engine needs premium vs. regular fuel?

 

I realize that the compression ratio of a modern motor is usually around 10:1; and that the higher the compression ratio the greater the difficulties to control proper ignition timing and flame front travel. Premium or higher octane fuel is more stable, less likely to auto-ignite and therefore less likely to cause pinging or detonation.

Are there othe engine factor for needing higher octane?

 

Most late-model gasoline-engine vehicles (including BMW hexhead bikes) are equipped with a knock sensor; when the computer detects knock, it retards spark timing to eliminate it. That means the engine will tolerate the use of low-octane fuel without destroying itself, but fuel economy and peak power will be reduced due to the lower efficiency conferred by retarded spark timing.

 

OTOH, an engine tuned for low-octane fuel (typically just a lower compression ratio) won't perform any better if you run it on high-octane gasoline. In fact, it may get worse fuel economy. A lot of high-octane fuels are rendered high octane by the addition of alcohol, which contains less energy per unit volume. The low-octane engine isn't less efficient when running this fuel, it's just that the alcohol-modified high-octane fuel has less energy per gallon, so you burn more of it to go the same distance.

 

The upshot is that if your vehicle says "premium fuel recommended," your MPG and peak power will probably be worse with low-octane fuel, and if you're running your vehicle on a higher-than-recommended octane, you may also end up with worse fuel economy there, too.

 

Just asking as I would lover to own a used Miata or TT roadster or even a Z3/4 but all need premium and thus negating their smaller cc economics. I realize they're "performance" vehicles; but not what I would consider sport car performance needing higher octane.

 

Unless you buy a beater car that's been in a rollover accident, your biggest operating cost is likely to be depreciation on the vehicle itself. You might consider laying all of this out on a spreadsheet, as was done here for a few vehicles.

 

At a typical price difference of 20 cents per gallon between premium and crap-grade fuel (call it $3.75 versus $3.95 per gallon), for a vehicle that's getting 30 MPG, you're talking about a per-mile fuel cost of 12.5 cents (crap-grade) or 13.2 cents (premium grade). Over the course of 50,000 miles, that difference adds up to a whopping $333. The random variability in depreciation (rock dings, parking lot swipes, beer stains on the dashboard) over that period of time will make $333 look like nothing.

 

Seriously, lay it all out a spreadsheet - tires, oil, insurance, maintenance, depreciation, fuel - and put all these costs in writing so you can do a by-the-numbers comparison. You may find that the car you really want isn't so bad after all.

[Bullett]

FWIW, I've been running mid-grade in my '03 Miata over the winter. It wants premium during the hot summer months for sure. Mileage averaged about 26 mpg. on a couple of highway runs for business. I do think I was getting closer to 30 mpg on premium.

 

Mitch--thanks for the primer on engines and octane.

 

Sharon

[Francois_Dumas]

Just wondering (I don't think it was mentioned anywhere).... in Holland we only have 95 and 98 Octane ('normal' and 'premium'). Ist that the same as in the US and the 95 is referred to as 'crappy' ??

 

(I saw 91 Octane in Germany the other day).

[Joe Frickin' Friday]

Just wondering (I don't think it was mentioned anywhere).... in Holland we only have 95 and 98 Octane ('normal' and 'premium'). Ist that the same as in the US and the 95 is referred to as 'crappy' ??

 

(I saw 91 Octane in Germany the other day).

 

Pretty much. Back off about 4 or 5 points from the European number, and you get the US number.

 

Lowest available octane over here is typically 89 (85 at high altitude).

[rob1100r]

My plan is to use the lowest grade on which it does not ping.

 

Sounds like my plan always.. My 4.0 Nissan Frontier manual says it 'can' run on 87 Octane, but it is a bit flat on acceleration, and in hotter weather, especially with any load in the bed and/or running the air it pings a bit. Even though it isn't throwing an engine light, I won't run it. Manual says optimal performance on 'at least' 91 octane, but I have been running the mid/89 Octane and it runs just as good as with the high/93 Octane, with or without extra 'load'.

 

But do the math!!!! If you are filling a 5 gallon bike tank, It really isn't THAT much more to run better stuff (my R1100R runs fine on 89, no better with super, no pinging, so I run it). With a 21 gallon tank, it would only cost about $4.00 more to run the Super over the Regular. Bottom line is, you are getting screwed deeply at the pump on either Regular or Premium, just run the minimum octane needed for proper engine performance..

[VinnyR11]

+1 on what others have said... My 12RT runs fine with any of the grades but fuel economy seems to drop with the lower grades. Seeing how the price difference between grades hasn't changed as fuel prices skyrocketed the percentage difference in price is now quite small - about 5% which amounts to $1.00 for a five gallon fillup.

 

Anyone know why this is? I thought the different octane ratings still have the same energy potential, and that the higher octanes are purely for knocking issues primarily due to higher compressions. i.e. no knocks and you're OK. I can't figure why it would show up with lower MPG. Does it have to do with the newer electronics that can detune the engine in cases of low octane?

[Bob Palin]

+1 on what others have said... My 12RT runs fine with any of the grades but fuel economy seems to drop with the lower grades.

Anyone know why this is?

Go back in this thread and read Mitch's post - he already explained it there.

[finallyabeemer]

Engineer chiming in:

There are many factors that affect octane requirements. Probably the most significant design factors are compression ratio, combustion chamber design, and cam timing.

The latter two are why you see some modern engines running on pump gas with 13:1 while some older engines with only 8.5:1 ping wildly.

 

And of course, how much cylinder pressure you are generating based on rpm and throttle setting. You can get away with much lower octane at minimal throttle settings and higher rpm.

 

As others have alluded to, with the advent of knock sensors you can no longer use your ears to see how low of an octane you can get away with. If you run lower than optimal octane, The ECU will retard timing to prevent you from damaging the engine. And most knock sensors are sensitive enough to tell the ECU to back off long before you can hear rattling. So no audible pinging, but the retarded timing will affect performance and fuel economy. And as some have already proven, the mpg impact can more than offset the fuel cost savings. With gas at $4.00 per gallon, an mpg loss of even 5% will offset the typical $.20 savings for running 87 rather than 91.

 

The only ways to determince what is the minimum you can get away with on modern engine is to monitor mpg or install a device to display the knock sensor signal or the actual engine timing output from the ecu. Otherwise, you are probably best off following the recommendations of those that designed and tested the machine. And maybe cheat by a couple octane points if you're just droning on the flats....

 

 

[tonyla]

I only use shell or Mobil in all my vehicles. I always used premium gas in my rt until I read here that I was wasting my money. For the last month I have been putting in regular. After 4 tank fulls I can say that the bike performs better with premium. Yesterday while traveling at 70mph I needed to accelerate away from a Tractor trailer so I punched the throttle and I thought there was a chain raddiling under me. So Back to Premium for me.

[Francois_Dumas]

Gas crossed the Euro 1.70 mark yesterday...... so.....