Hey Greasemonkeys. Fuel question

[ztejas 44114]

I have a 2017 Altima SR. I know I don't "need" to use gas that is above 87 octane, but I've been getting plus. 

Is this a waste of money? Should I go ahead and get premium if I'm going to pay extra? 

School me. 

[Gil Bang 26177]

You're wasting money.

[DCA_HORN 725]

Put in the regular just make sure its brand name so it has the detergents in there.

[Mother mopar 3379]

Cars are tuned to optimize fuel economy and to do that the lean out the air fuel ratio. Raising the octane rating (also known as the anti-knock index) doesn't change the energy content of a gallon of gasoline. A higher octane rating just indicates greater resistance to knock (detonation inside the cylinders). The early combustion of the fuel-air mixture that causes cylinder pressure to spike and detonate or ping.

When higher-octane fuel is used the engine controller/computer can take advantage of the elevated knock threshold and dial in more aggressive timing and higher boost pressures to improve performance....again assuming it is tuned for that. If not, you're just wasting money.

In short run the cheap shit unless your engine is built and tune for performance.

[Bevo&Pevo 5508]

Run what it is rated for.  That is how they optimized/tuned for the A/F ratio.

[TwiceHorn 31995]

40 minutes ago, Mother mopar said:

Cars are tuned to optimize fuel economy and to do that the lean out the air fuel ratio. Raising the octane rating (also known as the anti-knock index) doesn't change the energy content of a gallon of gasoline. A higher octane rating just indicates greater resistance to knock (detonation inside the cylinders). The early combustion of the fuel-air mixture that causes cylinder pressure to spike and detonate or ping.

When higher-octane fuel is used the engine controller/computer can take advantage of the elevated knock threshold and dial in more aggressive timing and higher boost pressures to improve performance....again assuming it is tuned for that. If not, you're just wasting money.

In short run the cheap shit unless your engine is built and tune for performance.

All this is true.  You may lose a little mileage, you may lose a little performance, but probably not very noticeable.

Unless your engine is turbocharged. Modern turbo engines are susceptible to knocking, moreso than normally aspirated.  You need to do everything in your power to avoid knocking with a turbo unless you like replacing pistons.

Or, if you detect knocking under normal driving conditions, With lower octane fuel than recommended, you can expect knocking at wide open throttle or heavy throttle and load (passing on a grade, for example). If those are your normal operating conditions, use the higher octane, if not, you can probably get away with the lower stuff.  A little bit may not hurt, but frequent knocking can kill your engine.

Edited October 1, 2020 by TwiceHorn

[TwiceHorn 31995]

Also, your engine appears to be a fairly low output, normally aspirated 4 banger.

Does it call for premium or higher than 87 octane?  If it doesn't, don't bother unless for some reason you hear knocking.  And if you do hear knocking, odds are decent the high octane won't fix it, so you may need to take it in before something bad happens.

The advice above relates to going lower when high octane (91 or 93) is recommended.

Edited October 1, 2020 by TwiceHorn

[markstanco 12773]

My last 2 trucks call for 93 (6.2L chevy) octane. I put in 87 unless I am hauling a trailer.

Its a compression thing with octane. Whatever the manual says do that. If it calls for 87 and you pump 93 that's dumb.

[elfenix 34661]

3 hours ago, Mother mopar said:

Raising the octane rating (also known as the anti-knock index) doesn't change the energy content of a gallon of gasoline.

iirc higher octane gas actually has slightly lower energy content

[freyguy 4366]

All is canceled out by the higher price.  You will not be driving that car for 20 years to tell yourself that you "won" in terms of a fraction of mileage increase.

[Mother mopar 3379]

iirc higher octane gas actually has slightly lower energy content

Kind of... there is only so much power in a gallon of gas. Where you can extract more power out is by increasing the efficiency of the gallon. That is where you'll get variations of power.

[TwiceHorn 31995]

9 hours ago, markstanco said:

My last 2 trucks call for 93 (6.2L chevy) octane. I put in 87 unless I am hauling a trailer.

Its a compression thing with octane. Whatever the manual says do that. If it calls for 87 and you pump 93 that's dumb.

Yeah, the knock sensors seem to be pretty effective in normally aspirated engines so it seems pretty safe to use lower-than-recommended octane.

And, with NA engines, the compression ratio is the main factor indicating higher octane gas.  The anti-knock algorithm seems to be able to deal with that successfully.

For some reason, large-displacement engines like V8s seem to be able to tolerate more knocking without engine damage than the little rascals.

With turbos, though, you have a higher-pressure, higher-temperature fuel-air mixture entering the cylinder that is more prone to ignite prematurely (detonation).  High octane fuel and synthetic oil have proven to reduce knocking and engine damage.

Edited October 1, 2020 by TwiceHorn

[elfenix 34661]

5 hours ago, Mother mopar said:

12 hours ago, elfenix said:

iirc higher octane gas actually has slightly lower energy content

Kind of... there is only so much power in a gallon of gas. Where you can extract more power out is by increasing the efficiency of the gallon. That is where you'll get variations of power.

right, the increasing efficiency from boosting the compression ratio makes up for the (rather marginal) decrease in energy content

(but again, you can only do this if the engine is designed to do so, and if your manual says use regular, then use regular)

[Alex Karev 1056]

Always buy a good brand fuel and at least once a year add some injector cleaner like Seafoam through the fuel system. 

Edited October 1, 2020 by Alex Karev
Spelling

[TwiceHorn 31995]

2 hours ago, Alex Karev said:

Always buy a good brand fuel and at least once a year add some injector cleaner like Seafoam through the fuel system. 

I am a seafoam adherent for older engines.  For newer ones, especially direct-injected, I think I would stay away, except maybe adding to fuel.  I see you specify as a fuel additive, so agreed.  It can be pretty good sucked through the intake of a port-injected engine.

Techron gets almost universal acclaim as a fuel additive and you can buy a bottle and run it through periodically.

[Mother mopar 3379]

Seafoam is good, I've also had good luck with berrymans and lucas.