The Truth About Gas! Does Premium Fuel Perform Better?

"Bozi Tatarevic" (hoonable)
12/20/2013 at 15:06 • Filed to: None

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There are many stories and anecdotes and what fuel octane is best and what effects it has on a vehicle. One of the most common discussions is whether higher octane will give you a benefit. There are many myths about fuel octane and power and gas mileage and I will try to break them down using real world data.

Octane and Knock

The octane of a specific fuel is a number that states how likely the gas is to ignite under pressure. The scale works so that the higher the number, the lower the chance of igniting under pressure and causing detonation inside an engine. The marketers that sell gas would love to lead you to believe the higher numbers are cleaner or more efficient but that is simply not the case unless the higher octane is mixed with special detergents or other additives (Thanks for the tip !!!error: Indecipherable SUB-paragraph formatting!!! ).

. Higher octane may be slightly more efficient when combined with modern engine computers but it does not save money overall.

The main question people ask in regards to this explanation of octane is "Why wouldn't I want the fuel to ignite in an easier fashion? "The answer for that is that, higher octane provides for less of a chance of pre-ignition of fuel which causes knock. In a normal scenario, fuel is sprayed into an engine and only ignites when the spark plug lights up and therefore causes proper movement for the engine. In a pre-ignition or knock condition, the fuel ignites before a spark plug lights up and causes the engine to run less efficiently or to get damaged.

Premium Fuel Required

In most modern engines, the combination of low compression ratio and advanced engine management prevents knock from happening. The compression ratio is the pressure that air and fuel is put under inside a cylinder. Engine management works by monitoring knock sensors to see if there are pings and adjusts fuel and timing accordingly to manage it. In this case, if the vehicle requires 87 Octane then the engine computer has been tuned for that specific octane and will run efficiently with it.

Many of the modern high compression engines or engine that use forced induction require premium fuel at 91 octane or higher. The reason for this is as the compression ratio of an engine increases the chance of knock increases as well so higher octane prevents the knock from happening. In this case, putting a lower grade of gas will cause knock to become apparent and the engine computer will make changes to prevent the level of knock.In testing, we have seen lower octane to cause efficiency to go down as much as 20%.

Cheaping Out but Losing Money

To put our theory into real world terms let's say "Car A" usually averages 25 mpg, requires premium gas and has a 10 gallon tank. The price for 93 Octane is $3.50 and the price for 87 Octane is $3.10 for our example. A full tank of 93 Octane will cost $35.00 while a tank of 87 Octane will cost $31.00.Driving on the 93 Octane yields 25 mpg so the driver is able to pass 250 miles for $35.00 dollars and the cost per mile will $0.14. With the 87 Octane gas in the tank the engine computer scales back and the car only averages 20 mpg. Driving on the 87 Octane yields 20 mpg so the driver is able to pass 200 miles for $31.00 dollars and the cost per mile will be $0.16.

The example above is simplified but shows that even though the entry cost is lower and in the end, the gas will end up costing more. What's even more important is that the detonation caused by running lower octane will reduce the health of an engine over time and if strong enough can cause permanent engine failure at once.

The Placebo Effect

Many people live by the fact that even though their car requires 87 Octane, they choose to fill up with 91 or 93. There are a few reasons for this action and one of them is people stating that it keeps their engine clean. This is a myth as all of the fuels from a single source will run just as clean in any engine. The biggest statement that is put out is that running higher Octane will cause the car to run more efficiently. We did some testing and found that in certain situations, the engine computer will adjust and run in a more aggressive manner but we never saw increases in fuel efficiency of more than 3%.

Using an example similar to the one above, we will show the economics of these changes. Let's say "Car B" usually averages 30 mpg, requires 87 Octane and has a 10 gallon tank. The price for 93 Octane is $3.50 and the price for 87 Octane is $3.10 for our example. A full tank of 93 Octane will cost $35.00 while a tank of 87 Octane will cost $31.00. Driving on the 87 Octane yields 30 mpg so the driver is able to pass 300 miles for $31.00 dollars and the cost per mile will $0.103. Driving on the 93 Octane yields 31 mpg so the driver is able to pass 310 miles for $35.00 dollars and the cost per mile will $0.113.

The example above is simplified but shows that running higher octane in the best possible engine management scenario still yields a higher cost than running the lower octane. On the economics side of this argument, running the higher octane is a waste of money. On the engine health side, if a car has been designed and tuned for 87 Octane, it should not have any issues if it is properly maintained.

In the end, the best advice is to run what you manufacturer recommends.

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Bozi is the founder of !!!error: Indecipherable SUB-paragraph formatting!!! and creates articles on everything from engine swaps to late model car restorations. You can follow him on !!!error: Indecipherable SUB-paragraph formatting!!! and !!!error: Indecipherable SUB-paragraph formatting!!! or drop him a line at !!!error: Indecipherable SUB-paragraph formatting!!!

DISCUSSION (90)

Straightsix9904 > Bozi Tatarevic
12/20/2013 at 15:10

I don't know about better fuel, But the nice thing about winter is the air is colder and I get about 5 more HP, I've tested it with my butt dyno.

HammerheadFistpunch > Bozi Tatarevic
12/20/2013 at 15:10

Here is the rule for maximum economy and performance: put in your vehicle what is recommended. If it requires premium, put in premium. If it suggests premium, put in premium. Otherwise use regular.

themanwithsauce - has as many vehicles as job titles > Bozi Tatarevic
12/20/2013 at 15:13

Random question as I am not a fuel expert - When I autocross my not at all performance oriented cars, I typically put in a few gallons of premium when I splash some gas in before the event even though they are simpler engines meant for regular. My logic is that since the engine is going to be going up and down the rev range very rapidly, it is worth the extra buck or two for the safety that my engine won't have any issues due to fuel.

I'm 100% sure it doesn't hurt the engine to do that, but I'm curious if there is any risk at all to just running regular in more extreme conditions? Am I really mitigating any dangers by doing this?

ddavidn > Bozi Tatarevic
12/20/2013 at 15:13

As far as engine cleaning goes, I'm not saying run 92 octane for this reason, but Shell does put more additives in their premium fuel than the lower grades, as far as I understand it. So the cleaning thing is partially true.

Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW. > Bozi Tatarevic
12/20/2013 at 15:14

Well said. Really good writes.

But how about the myth that "Old Cars can't run more than 91 Octane". You know, in my country that's a quite common believes, so those guys who drive old mercs generally put 87 Octane on their tank.

Yowen - not necessarily not spaghetti and meatballs > Bozi Tatarevic
12/20/2013 at 15:16

My car (SHO) can be run on regular, ford doesn't even recommend premium. But they do state you get 10 extra horsepower and I think it'll also beat your 3% efficiency gain. I was in the process of testing this, but then winter blend hit and my numbers were no good, then to screw with them even more I cut my commute in half, so I have no idea what my switch to premium did for me. But I'm going to be running a tune soon, so I'll have to run premium anyway. Also I do not regularly eat socks or sweet potatoes.

Sorry that was the most disjointed story ever.

deekster_caddy > Bozi Tatarevic
12/20/2013 at 15:21

Here's one problem I have with your results - driving on 87 Octane on a car designed for 91 Octane IF DRIVING EFFICIENTLY will probably not reveal any loss in MPG. It may actually show a slight increase, because "87 ignites easier than 93"... However, if you are driving under any kind of medium-heavy load, you will see the engine start to yank timing etc to compensate for detonation and lose MPG. So hypermilers may claim better gas milage on 87 octane, even though their "performance" potential may go down.

Another issue that some real world testing may resolve - some cars "designed" for 87 octane may have knock detection and performance loss running on 87 that goes away on 89. Many cars are tuned from the factory to rely heavily on the knock sensor, so they will run on 87 even though it's costing some performance.

edit- aftermarket tuning can benefit any engine! The factory is very conservative in their tune and claims.

webmonkees > Bozi Tatarevic
12/20/2013 at 15:22

Mine is factory tuned for 89 octane, so do I get equal amounts of 87 and 91? Because I know the cheap stuff makes it ping, so I generally go for mid-grade, except on Fridays when some stations have premium at mid price.

Satoshi "Zipang" Katsura > Bozi Tatarevic
12/20/2013 at 15:23

My father could care less of what it needs - hence why whenever I give him the keys to whatever I own, I told him "If I find a single drop of unleaded regular, I will kick you out of the house for a week per drop."

TwoFortified > ddavidn
12/20/2013 at 15:28

Was gonna say the same thing. So does Chevron (they plop some Techron in there...which, to be fair, can be purchased pure at AutoZone or Oriela;sldkfjaasdf). Technically, these people are correct (assuming that Techron actually has a cleaning effect).

Fun side note: Firefox does not recognize the word "Techron". It's 1 and only recommended word? "Chronometer". What the fuck?

Party-vi > Bozi Tatarevic
12/20/2013 at 15:29

Compared to their older counterparts, modern car engines actually have higher compression ratios, but this is more to do with the better refinement of gas and more advanced engine machining techniques and tolerances allowing us to raise CRs.

Also, there is little chance of damaging a modern engine from running a lower octane gas than required, as (like you mentioned) the ECU will detect knock and adjust (retard) timing accordingly. Vehicles have had the ability to do this since the early or mid 1980s. This was a larger issue when engines used carburetors as there was usually not a way to automatically adjust timing (anyone remember twisting their distributor caps to adjust the timing? Get off my lawn!). Case in point, the Mazda 6 SkyActiv engine has 13.0:1 compression but uses regular (87) gas without knocking.

Excellent article!

desertdog5051 > themanwithsauce - has as many vehicles as job titles
12/20/2013 at 15:31

Well put. Also, you are building up higher internal temperatures than normal driving.

desertdog5051 > Bozi Tatarevic
12/20/2013 at 15:35

I agree with what you are saying. And that is what I tell people. The funny thing is that years ago when I had a 73 mile each way commute. I found that Premium fuel made my gas mileage increase by 3 mpg.

Party-vi > themanwithsauce - has as many vehicles as job titles
12/20/2013 at 15:43

If the engine is fuel injected you shouldn't have an issue.

When the engine detects knock it retards timing, which leaves the intake valve open a bit longer as the piston reaches TDC, thereby reducing the dynamic compression ratio in the cylinder and ensuring the more easily ignitable lower-octane gas does not pre-ignite. This does decrease the amount of force from ignition of the fuel/air mixture on the power stroke which decreases performance. Chances are if your car tells you to use regular gas it isn't going to knock during a performance run.

Bozi Tatarevic > Party-vi
12/20/2013 at 15:47

I am in full agreement with you comment and have seen 98% of cars survive while running lower octane but every now and then I see piston ringlands blown off on completely stock cars that are running a lower octane. This is most commonly seen on turbo or supercharged cars that run lower octane, combined with heat soak and other elements that cause enough detonation that the ECU can't catch up.

Bozi Tatarevic > ddavidn
12/20/2013 at 15:47

Good call, almost missed that. Updated article and credit given

Bozi Tatarevic > webmonkees
12/20/2013 at 15:48

Haha, did you not get your 89 Octane mixer when you bought the vehicle?

Bozi Tatarevic > Yowen - not necessarily not spaghetti and meatballs
12/20/2013 at 15:50

I see you argument and my argument is that the car was built for the premium fuel but de-tuned to run on 87 so it can be marketed as such. This is a special case where a vehicle has advanced enough mapping to detect lower know and make significant changes

Bozi Tatarevic > desertdog5051
12/20/2013 at 15:50

That very well may be the case but I would take all of the commute and the cost and calculate if the additional cost for the premium gas resulted in any savings with a 3mpg increase

Party-vi > Bozi Tatarevic
12/20/2013 at 15:51

Ah well yes forced induction cars should always get higher octane gas, as the ECU can only compensate for so much.

Bozi Tatarevic > Party-vi
12/20/2013 at 15:53

Completely agree

RamblinRover Luxury-Yacht > Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW.
12/20/2013 at 15:54

That's a very odd belief indeed. It's probably based around a myth of higher octane producing more power, and thus an older engine not being designed to absorb it. If that's the case, it's unfounded - unless a low grade gas is more adulterated (as sometimes happens in the US with ethanol), there shouldn't be a power difference. It's also the case that fairly primitive cracking and petroleum refinement could produce a higher heat content fuel at a given octane (closer to, say, diesel grade with increased octane), but as it burns more slowly at a higher octane, that usually doesn't increase engine temp or combustion chamber pressures considerably. That's mostly something I'd have expected to see many decades ago before modern blending techniques and refinement.

I can tell you right away that some older engines if going back all the way to the 50s/60s run better than modern engines do on sub-standard gasoline (i.e. below 87 octane) as many countries did not have access to good gasoline in the decade plus after WWII. These engines often have *very* low compression, reverse-flow heads, and sometimes don't even have overhead valves - many particularly British models being designed expressly for bad gas. That doesn't mean, however, that good gas will harm them - far from it. It does mean that many of those older engines have not a single benefit from higher octane, however.

I'm unaware of any quality present in high grades of gas today that will damage an engine at all - unless it is customary in the Pacific for higher grades to have detergents not present in lower grades, in which case it's possible that certain rubber seals or other components would degrade in their presence.

Yowen - not necessarily not spaghetti and meatballs > Bozi Tatarevic
12/20/2013 at 15:55

yeah, it certainly falls outside the norm. I certainly agree with you. My cousin consistently puts 93 octane in his 5.4 F-150 because he wants to give it a treat right now and there is no convincing him that it doesn't matter AT ALL.

desertdog5051 > Bozi Tatarevic
12/20/2013 at 15:58

Maybe that would. The boss was paying for the gas, so I did not care.

deekster_caddy > Party-vi
12/20/2013 at 16:00

What's wrong with twisting distributor caps? I can simply change springs if I'm pulling a heavy load. I can hear detonation, knock sensors are for wimps! ;)

Party-vi > deekster_caddy
12/20/2013 at 16:03

I'm lazy. Vacuum advance FTW!

Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW. > RamblinRover Luxury-Yacht
12/20/2013 at 16:07

Well, i remember my grandpa, he owns a 220SE Mercedes from 1950's if i remember, and he keeps constantly telling everybody that do not put anything more than 87 octane on his merc or it'll blow up due high compression. Same goes with his 190SL and 190E. Very weird belief, but yet i'm still won't argue with his believe because well, it's impolite in my place to start a debate with the older guy.

deekster_caddy > Party-vi
12/20/2013 at 16:09

Umm. My understanding is that when the engine detects knock it retards ignition timing, not cam timing. The valves are not affected. (unless you are talking about a VVT engine, I'm not sure how that is handled)

RamblinRover Luxury-Yacht > Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW.
12/20/2013 at 16:13

Well, at least the 50s engine is demonstrably designed for 87 octane and has no trouble with it, for the reasons I mention. Given that there's no benefit in a step up, that one at least will not be hurt by following his belief.

Party-vi > deekster_caddy
12/20/2013 at 16:20

You are correct - I was assuming some sort of VVT engine.

Without VVT (as you stated), the ignition timing is retarded, pushing the ignition point closer to when the piston is at TDC which is an attempt to get the peak of the fuel/air mixture's explosive force to occur just after TDC to get the most force possible on the downward stroke of the piston.

themanwithsauce - has as many vehicles as job titles > Party-vi
12/20/2013 at 16:21

Hmmmm, so basically it won't hurt itself if it starts to knock but I could lose horsepower. So while I wouldn't be gaining performance by running premium, it might help keep performance on hotter days?

deekster_caddy > HammerheadFistpunch
12/20/2013 at 16:22

What if the vehicle calls for 87 but you know there is knock retard/performance reduction happening when you use regular and it goes away when you use 89? This does happen on many cars.

Party-vi > themanwithsauce - has as many vehicles as job titles
12/20/2013 at 16:22

As deekster_caddy pointed out, if you have an engine without variable valve timing the ECU will still retard your ignition timing if it detects knock, in an attempt to prevent pre-ignition. You should be ok.

Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW. > RamblinRover Luxury-Yacht
12/20/2013 at 16:23

Well, that's true.

And oh, he also have another believe. Everytime he fills up his cars he'll be shae it by pushing the car left and right so the fuel mixes with oil, and lead to more power and more efficient engine, he said..

shpuker > Bozi Tatarevic
12/20/2013 at 16:26

Better yet. Throw an exhaust and intake on that bitch, tune it up to optimize 93 octane, and have a lot more fun.

deekster_caddy > Party-vi
12/20/2013 at 16:26

pfft vacuum advance? We used to have to twist a lever to adjust the timing!

Kinja'd!!!

RamblinRover Luxury-Yacht > Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW.
12/20/2013 at 16:27

Actually required with a two-stroke Trabant.

HammerheadFistpunch > deekster_caddy
12/20/2013 at 16:27

Then it will probably say so and premium is actually the recommended fuel. I.e. the 1gr-fe in the Tacoma, 4runner, fj cruiser, etc. recommends premium because you get more power and torque but is certified to run on regular just fine.

philipilihp > Bozi Tatarevic
12/20/2013 at 16:37

This is why I love you guys. You teach me stuff!

Also, my E90 requires at least 91, why are there so few gas stations that actually sell 91? Or is that just in my area (Northern VA)?

Aya, Almost Has A Cosmo With Toyota Engine Owned by a BMW. > RamblinRover Luxury-Yacht
12/20/2013 at 16:37

Really?
That's cool!

We don't get trabants here in my place (I live in Indonesia), but i always wonder how it feels to drive one.

Battery Tender Unnecessary > Bozi Tatarevic
12/20/2013 at 16:57

There was a article in one of the major car magazines a few years back where they dyno'd a V8 Mustang with 87 and 93 octane. It lost a significant chunk of power running premium, basically just use what the automaker tells you to.

Party-vi > themanwithsauce - has as many vehicles as job titles
12/20/2013 at 17:09

Yep. You would be better off using premium for maximum potential from your engine.

Party-vi > deekster_caddy
12/20/2013 at 17:12

Kinja'd!!!

Funn you mention that - they still make those in one form or another.

It's a "Porch-uh" > Bozi Tatarevic
12/20/2013 at 17:13

Just to be clear (and I don't think this is what you're saying, just want to clarify), higher octane rating DOES NOT equate to easier ignition. It's the opposite. So if you have a lower-compression motor without forced induction, running a higher octane fuel will actually be less effective because (as the author stated) it's not tuned for it.

You want your fuel to ignite as easily as possible right until the point (but not when) it will pre-detonate.

My Spec944 ran 87 octane all the time until I shaved the head (as much as I could within the rules) and increased the compression. Then I ran 91 octane for safety (reduce the chance of pre-detonation).

GreenN_Gold > Bozi Tatarevic
12/20/2013 at 17:16

My old Neon ran on regular (of course) and the owner's manual basically said not to use premium. It said you can use it with no harm, but don't bother, because the engine is designed for regular and there is no benefit and you're basically throwing your money away.

BlazinAce - Doctor of Internal Combustion > Bozi Tatarevic
12/20/2013 at 17:19

I wish someone would do this kind of research with Brazilian fuels... IIRC, my Tipo was factory tuned for premium gas, but we did a slight head rework in 2006, and then the government started adding varying amounts of alcohol in all fuels sold in Brazil. Nowadays I've no idea what I'm supposed to use anymore, only that my engine will knock with all regular fuels and sometimes with Petrobrás Podium (supposedly 91 octane) as well. The only fuel I've had success so far is Shell's V-Power R, which is no longer available at the places I used to fill up, but apparently, the government is about to regulate an even higher amount of ethanol in the gas sold in Brazil and I've no idea how that will affect antidetonation capabilities...

hike > Bozi Tatarevic
12/20/2013 at 17:43

Orange Exige > Bozi Tatarevic
12/20/2013 at 22:42

Anybody else with an NA Miata?
(Rhetorical question - I know the answer ;)

What do you run?
I just use regular though it does say "Premium Recommended" inside the gas lid.

Bozi Tatarevic > Orange Exige
12/20/2013 at 22:51

NB Miata V8 running premium ;)

Orange Exige > Bozi Tatarevic
12/21/2013 at 00:09

-_-
lol close enough

Dirty Harry Callahan > BlazinAce - Doctor of Internal Combustion
12/21/2013 at 08:01

Alcohol generally has a higher tolerance for compression and is helpful to reduce knock but it has less chemical energy available per liter compared to gasoline/petrol. Will help with knock, but not with efficiency.

wagnerrp > Party-vi
12/22/2013 at 22:14

Case in point, the Mazda 6 SkyActiv engine has 13.0:1 compression but uses regular (87) gas without knocking.

It accomplishes this by "tainting" the mixture going into the engine. It takes your exhaust gas and cycles it back through the engine, reducing the combustibility of the mixture, and its tendency to auto-ignite. While it does allow higher compression, and thus higher efficiency, it's displacing a significant volume that could otherwise be used for fuel. For a given engine displacement and RPM, it will produce considerably less power than a more traditional engine. If you're using this to drive a generator, which drives electric motors, great. If you're using this to drive the car itself, it's going to be anemic.

Party-vi > wagnerrp
12/22/2013 at 23:06

Right - it functions as an EGR valve like every other engine with emissions controls. Also FIAT was doing the same with their engines but I don't quite remember what it was called.

Party-vi > wagnerrp
12/22/2013 at 23:06

Right - it functions as an EGR valve like every other engine with emissions controls. Also FIAT was doing the same with their engines but I don't quite remember what it was called.

Camshaft190 > Bozi Tatarevic
12/23/2013 at 17:14

http://www.toptiergas.com/

This is a somewhat useful site for detergent info irrespective of octane rating.

deekster_caddy > HammerheadFistpunch
12/24/2013 at 13:15

I know several vehicles (late 90's and early '00 GMs for example) that all say 87 is what you need, but I know from live scanning that they have knock retard happening while driving, sometimes significant amounts. 89 makes it go away (usually). Does it affect gas milage? Probably not enough to justify the cost of 89 for most people. I don't like 'bouncing off the knock sensor' as I call it. When I retune a street car (for economy) that _should_ be fine on 87 I make sure they are running 87 before I start, and usually end up dialing back the OEM timing map a good amount in certain areas so it doesn't have any active reduction. Other areas of the default timing maps leave a few degrees on the table for improvements. One thing I've found for sure is that every car is different, even if built the same. Nothing beats a custom tune.

suss6052 > BlazinAce - Doctor of Internal Combustion
12/28/2013 at 12:58

Ethanol is a great anti-knock additive, but it's a poor fuel as it has significantly lower energy content than Gasoline. Ethanol has a higher latent heat of evaporation and can absorb more energy before reaching the auto-ignition point.

Korea Miéville > Bozi Tatarevic
12/29/2013 at 11:36

What confuses me is, in my area gas comes in 86/89/91. Recommended octane for my car however is 87. Am I doing my car a disservice by using 86?

Manwich - now Keto-Friendly > Party-vi
12/29/2013 at 21:03

"If the engine is fuel injected you shouldn't have an issue."

Noooo... if the engine has a knock sensor combined with electronic ignition that is capable of retarding the ignition, THEN you shouldn't have a problem. With no knock sensor, the ECU won't detect anything, which means it can and will knock under the right circumstances.

I've heard many fuel injected engines knocking under load. Often those were ones with cheap, single-point fuel injection.

BUT... even a car with a knock sensor can knock because apparently it can only adjust things within a certain range.

For example... the Saab 9-3 turbo... the 185HP base model I had. On a hot summer day in stop and go traffic, under load, I would occassionally hear it knock under load even though that engine has an advanced coil on plug ignition system, knock sensor as well as an "APC" valve that is supposed to make the ECU adjust for lower octane fuel.

Thus I would put in 89 or 91 octane in during the summer even though it didn't technically need it according to the owners manual.

And one other factor, if I chipped the engine, I could easily get 240HP. But that would mean 91 octane would be mandatory because once the turbo spooled up to the higher level of boost, it most certainly will knock without the higher octane.

GTRZILLAR32-Now saving for Godzilla and a condo > Bozi Tatarevic
08/16/2014 at 22:12

All I know is that if I ever pay under $4 a gallon for 93 octane I'll probably pee on myself out of excitement.

ray > Bozi Tatarevic
08/17/2014 at 21:29

Where are you driving where premium is 40cents more expensive then regular.

If you take the oddball fact that premium has somehow remained stable at 15-20cents surcharge even though the price of gas steadily increases, it's % cost has decreased as price increases.
So now in california with $3.99 regular and $4.15 premium, it's about close to the magical 3% efficiency number.

If anything you should have promoted that the "myth" now is approaching plausibility

ray > ddavidn
08/17/2014 at 21:31

this shouldn't be true, otherwise they fail the toptiergas requirements and would be stripped of that certification.

http://www.toptiergas.com/faqs.html
No. TOP TIER fuel marketers use the same detergency treat rate for all octane grades of gasoline sold at their stations.

ddavidn > ray
08/17/2014 at 23:42

Since it's been a year since I posted that comment... yes. I understand better now.

Sir_Stig: and toxic masculinity ruins the party again. > Battery Tender Unnecessary
08/18/2014 at 10:05

How far back?

Mike_Smith > deekster_caddy
08/18/2014 at 16:20

You wanna talk about detonation? Ran out of gas once in a '75 Monte 350, less than a mile from a gas station. Had just got back from a camping trip, so I had some Coleman fuel in the trunk. We're not talking about some pinging here, but loud, full-blown knock on every cylinder on every rev. I thanked it for getting me to the gas station without blowing up, by giving it a nice full tank of Sunoco 94. ;-)

Destroyer > hike
08/18/2014 at 23:05

I have a 2011 Honda Accord that i recently bought with 50000 miles on it and i started the 1st tank with regular then on my 2nd tank i filled it up with 89 and 91 gas from only Bp and shell (My favorite stations of course) and to me it felt like it was more responsive and had more hp, but when i told my friend about it he told me that i was really throwing money away cause it does nothing, but when i switched back to regular my engine rpm gauge started to bounce Up and down like if it was about to stall out like a misfire symptom, but when switching back to 91 the idle returns back to normal. So why was this happening if my engine only needs regular gas?

Destroyer > hike
08/18/2014 at 23:05

Boiled Frog > Bozi Tatarevic
09/03/2014 at 13:26

My old Subie's owner's manual said 87, regular is 85. I live at altitude where I was told knocking happens with lower octane at sea level but not so much as at altitude so I drove it 189,000 miles on regular gas with no ill effects.

Kevin Barrett > Party-vi
03/21/2015 at 09:59

How will retarding ignition compensate for detonation? If the fuel air mixture is igniting before the spark plug fires, then firing it later won't achieve anything. Retarded ignition is usually just for starting an engine to avoid kickback.

SidewaysOnDirt still misses Bowie > Party-vi
03/21/2015 at 10:13

Even in VVT engines, it's not going to adjust your cam timing for knock. The cam timing is usually locked in to change at a certain RPM and won't really deviate from that.

desertdog5051 > Bozi Tatarevic
03/21/2015 at 11:29

Elementary stuff, Oh wise leader. How about an article on cetane for the diesel folks.

Party-vi > Kevin Barrett
03/21/2015 at 14:12

By retarding ignition timing you move the spark closer to the top of the compression stroke, and reduce the maximum cylinder pressures and ignition temperatures which lead to detonation.

Nerd-Vol > Orange Exige
03/23/2015 at 07:13

that's strange that your NA calls for premium. Mine simply calls for regular.

Jake - Has Bad Luck So You Don't Have To > deekster_caddy
03/23/2015 at 08:13

Exactly what I was going to mention. My 2000 Buick LeSabre may only "require" 83 octane but after 240,000 miles if I put anything less than 89 in there I get a consistent 20 degrees of knock retard and a very noticeable drop in performance.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Party-vi
03/23/2015 at 08:19

ignition timing is not the only thing that gets adjusted. The fuel/air mixture will also be changed when knock is detected. This is what leads to the most dramatic changes in power and efficiency. A richer mixture, with a higher ratio of fuel to air will cool the combustion chamber, reducing the occurrence of hot spots that can ignite preignition conditions and will resist preignition by requiring a higher temperature to ignite than a fuel-air mixture with more oxygen (leaner). The richer mixture will burn cooler and slower, reducing power output from that of a "perfect" mixture and the later spark timing will increase pumping losses and shorten the rotational angle-length of the power stroke. I believe the only mass-market engine capable of the valve-timing based compression control you posited is the Fiat Multi-air line. But turbo engines with modern boost controllers will reduce manifold boost pressure if knock is detected, leading to effectively lower dynamic compression ratios.

deekster_caddy > Jake - Has Bad Luck So You Don't Have To
03/23/2015 at 11:48

Ack, I sure hope some of your numbers are off! It should require 87 and hopefully you aren't seeing 20 degrees of KR anywhere, that's exploding engines... or false readings. But many 3800s have 3 to 4 degrees of KR on 87 that goes away on 89. Many of the supercharged 3800 motors have as much as 5 even when running on the required minimum 91 octane (less with 93 if you can get it). Anything 5 and over is getting dangerous.

Jake - Has Bad Luck So You Don't Have To > deekster_caddy
03/23/2015 at 12:35

nah, those are accurate numbers. Was using some stupidly sophisticated tuning software on a laptop. Even on 87 I see 4 or 5 degrees of KR, but that isn't much to be worried about.Goes away completely on premium, which is nice :D

Coty > Bozi Tatarevic
03/23/2015 at 12:52

Focus STs blow ringlands bad, usually when you try to shove too much boost down its throat though.

DailyTurismo > Bozi Tatarevic
03/23/2015 at 12:54

The problem you are missing is that you won't see different fuel economy with 91 vs 87 octane if you are just putting around town. The retarded timing from running low octane only happens when you are near autoignition limits, which is going to be at high loads (open throttle positions). If you drive like a grandma, the two fuels should perform the same AND technically the BTU/gallon for lower octane fuel is slightly higher.

But don't trust my BSME with an emphasis in combustion, as this subject has been BEATEN to death all over the internet.

http://www.csmonitor.com/Business/In-Ge…

Scorpio GTX1 > webmonkees
03/23/2015 at 13:18

Most stations will have a mid-grade 89 option, but it's just 87 mixed with 91, so doing the mixing yourself won't change anything.

deekster_caddy > Jake - Has Bad Luck So You Don't Have To
03/23/2015 at 13:35

I don't think it can pull 20 degrees of timing! Can you buy 83 octane?

GhostZ > deekster_caddy
03/23/2015 at 14:06

In this case, those cars should recommend 89 in the manual but allow 87.

Snuze: Needs another Swede > Bozi Tatarevic
03/23/2015 at 14:07

I'll throw some real world numbers at you, really just more reaffirmation for the information in your post:

My wife was convinced for a long time that her Lexus RX330 should get premium fuel, when it does not call for it. I convinced her to switch to regular. Guess what... no change in fuel mileage.

I used to have a 2009 Cobalt LS. It had the 155HP Ecotec and a 5spd manual. Only thing I did to the car was a K&N drop in filter. Average fuel economy jumped from 36 to 37 mpg. I figured I'd try running premium in it, so I ran 2 tanks back to back. I don't remember any change, if it did improve, it wasn't enough to warrant the extra fuel cost. I switched back to regular.

I now have a 2012 Cruze LT with the 1.4 Turbo and 6spd manual. This one will really highlight the nuances of computer tuning and turbo vs. non turbo engines. Bone stock it would pull 34 mpg on 87 Octane. Its 9.5:1 compression + boost, and while it can run safely on 87, it pulls out so much timing, especially in the summer, as to render driveability awful. Switching to premium fuel yielded around 35.5 mpg and a much happier running car. I then flashed a Trifecta dual mode tune to the car. Performance mode bumps power from 138HP to about 180HP with a combination of boost and tweaking fuel and spark. Premium fuel is absolutely required but I'm seeing a constant 37 mpg average. The second mode is a an Economy tune. This mode makes about 140HP but with trimmed fuel curves, and slightly less boost but more aggressive ignition timing. I rarely use this mode except on highway trips so it's hard to give real world averages. My greatest triumph, though, was 41 mpg in a car loaded with luggage and 4 adults - it enabled me to go from College Park, Maryland to Brooklyn, Michigan (Michigan International Speedway) without having to stop for gas.

deekster_caddy > GhostZ
03/23/2015 at 16:55

I agree, but they want to make sure the car is the cheapest to operate, so they rely heavily on the knock sensor. In my opinion, too heavily. But I'm a cautious kind of operator. What happened, did this article get linked to from the FP? Suddenly lots of activity on a year+old article!

Socraticsilence > Bozi Tatarevic
06/03/2015 at 11:32

I don’t know if anyone will check this at this point, but I’m working on a long roadtrip post about driving from Portland to Anchorage in an 04 WRX wagon (was going to be done diary style every night but hotel wifi skoshed that plan on Night 1)— anyway we’ve reached the one terrifying part of the trip— I detoured to run an epic mountain highway through the Canadian Rockies (the Stewart-Cassiar) but it appears I’m going to be stuck running 87 for a long, long portion of it— leaving in 30 minutes, not going to lie—kind of terrified.

Socraticsilence > Bozi Tatarevic
06/03/2015 at 11:32

jdrgoat - Ponticrack? > Bozi Tatarevic
04/15/2016 at 21:45

Ok, I read a lot of replies before commenting to see if anyone else mentioned this. I didn’t get through them all, but I saw no mention.

The compression ratio is the pressure that air and fuel is put under inside a cylinder.

What!? No! For one, you’re saying that a unitless metric (ratios are by definition unitless) is the same thing as a metric measured in a variety of units depending on case. Let’s say psi because it’s a common one. A ratio cannot be measured in lb/in^2!!!

I get what you’re trying to imply, but it is incredibly oversimplified. A static compression ratio (not even going to get into dynamic compression ratio and the near impossible variability that would entail, let’s just say you need to know the cam timing to start calculating that) can be used to simply demonstrate a difference between two engines when other things are equal between them.

A SCR is simply the ratio you get by comparing two volumes. A) The volume in the cylinder when the piston is at bottom dead center (BDC), and B) the volume in the cylinder when the piston is at top dead center (TDC). This includes the volume in the combustion chamber, as well as any amount that the piston is still in the cylinder bore. SCR = 1:(A/B)

Typically the higher the SCR, the more likely you are to have an engine that runs at higher cylinder pressures. But there are far, far too many variables to say that they are directly related. Let alone the same .

Jake - Has Bad Luck So You Don't Have To > Bozi Tatarevic
04/29/2016 at 18:53

2000 Buick LeSabre. After 220k miles the engine has built up enough carbon deposits or other hot spots in the cylinder that the engine knocks like a Jehova’s Witness unless mid-grade is run in the engine which brought the Knock Retard down from 45 degrees to 4-6 degrees.