Truth about running higher octane than require

[severach]

Originally Posted By: mechtech2
No engine code can occur from yanking the knock sensor. It doesn't know. 2003 Ford.

How could it not know with a piezoelectric microphone attached to the engine? A running engine cannot be silent. It's as easy as you telling whether the engine is running or not by putting your ear on it.

If it doesn't set P0325 then it was meant to be disconnected.

 

[Dieseldoctor]

My audi tt if u used 87 octane it would misfire and set a engine code off when it was cold.

 

[Durango]

Try mid-grade (89 octane) gas and see what happens?. In my newlt bought 08' Impala SS the book say to use 91 octane but the sales man told me I can use 87 but I won't try it as it's under warranty.

Durango

 

[PT1]

Originally Posted By: lexus114
Originally Posted By: Char Baby
My Lexus RX-300 OM says the same thing... "87 is fine, and higher octane for improved vehicle performance".


I can most definitely tell the difference between reg.and prem.in my RX.


You can barely tell the difference between regular and premium in the ES350. If you run 89 there is no noticible difference versus 91

 

[lexus114]

Well in a heavy vehicle like mine, I can tell the difference. Your power to weight ratio is better due to your lighter weight. So maybe you cant tell as much.

 

[ziggy]

Ive read about his so many times over on the neons forum. I drive a 95 neon. Carbook says to use 87 as higher octane will give no benefits* IVe used 89 octan, and it dosnt run so good. Ive used 93 octane, and it runs a tad better than 89, but 87 it runs no problems.
What ive read on the forums is, octane is requiremnt to burn the feul. Using lets say 93 octane, will ause backwashing of the fuel over time, down the cyclinder walls and into the oil. plus winter time 93 dosnt start as fast as 87.

 

[GMBoy]

It's so easy guys - use the octane rating spec'd by the automaker. It's how they tuned the car.

If you happen to have high miles and carbon or some issues that cause knock than you can go up a level until you fix the root cause.

 

[Jaymus]

On paper, lower octane should work better.

In life, it depends on the car. Wife's Eclipse 2.4 4 cyl SOHC runs rich. I don't like the factory tune. Anyways, if I run 93 in it, it actually feels slower. 87 feels best, and it still smells rich and runs rich. Little black out the tail pipe on WOT for a tad here and there, and the oil wreaks of gasoline all the time. That's with little/no idling time, 20-25 minute trips, and RTS 5W-40.

My 1986 F-150 5.0 EFI has a noticeable power increase when I run 93. Maybe the timing is bumped up, I don't know. I've also read that these old computers tend to run lean. Still...

All I know is to try it for yourself. Check your MPG, check your in the seat performance, and then check your wallet. Make your own equation up and choose which octane/brand/pump # you want to use every time and have fun.

 

[jhenle]

My 2007 Jeep Liberty's owner manual says the recommended grade for the 3.7L engine is 87 octane.
Then it goes about saying "The use of premium gasoline is not recommended. The use of
premium gasoline will provide no benefit over high
quality regular gasoline, and in some circumstances may
result in poorer performance."

 

[mikered30]

Originally Posted By: jhenle
My 2007 Jeep Liberty's owner manual says the recommended grade for the 3.7L engine is 87 octane.
Then it goes about saying "The use of premium gasoline is not recommended. The use of
premium gasoline will provide no benefit over high
quality regular gasoline, and in some circumstances may
result in poorer performance."


Is the poorer performance from carbon build up?

 

[Kaboomba]

In most cars that only spec regular, running mid-grade or premium will almost always offer two "fringe" benifits:

1. Guarantee that ignition will occur the same way in each cylinder when ignited by the spark plug ONLY--the idea here is that we want the most tightly controlled burn event possible
2. Better quality fuel with better additive package keeps deposits and carbon at bay

The degree of benefit in a particular car will vary. As others have said, you've got to go by ear, butt dyno, and wallet weight!

 

[elwaylite]

Im not understanding why 87 and 93 vary with number 1?

 

[Gary Allan]

Originally Posted By: elwaylite
Im not understanding why 87 and 93 vary with number 1?

 

[elwaylite]

Kaboomba stated a point in number one. I do not understand why 93 does this better than 87 in a car that asks for 87.

 

[Ursae_Majoris]

Originally Posted By: elwaylite
Kaboomba stated a point in number one. I do not understand why 93 does this better than 87 in a car that asks for 87.

Maybe because 93 resists detonation better than 87, so it can be compressed in all cylinders before spark ignites it? Just speculation at this point.

 

[lexus114]

Originally Posted By: Gary Allan
Originally Posted By: elwaylite
Im not understanding why 87 and 93 vary with number 1?

Yeah....

 

[Kaboomba]

Originally Posted By: Ursae_Majoris
Originally Posted By: elwaylite
Kaboomba stated a point in number one. I do not understand why 93 does this better than 87 in a car that asks for 87.

Maybe because 93 resists detonation better than 87, so it can be compressed in all cylinders before spark ignites it? Just speculation at this point.


EXACTLY. This will vary among engines.

IDEAL: ALL CYLINDERS TO HAVE IDENTICAL TIMING AND IDENTICAL BURN AS THEY "GO UP N DOWN" THROUGH THE POWER STROKE.

REALITY: Some cylinders run hotter than others, have different fuel/air charge, have different upper cylinder condition (carbon buildup, etc). This causes each cylinder to have a slightly different threshold for pre-ignition or pinging, and possibly different flame-front/burn and different overall power curve/output relative to other cylinders. On most cars, the knock sensor works to mitigate any irregular ignition by retarding timing FOR ALL CYLINDERS to avoid abnnormal stresses and vibration, even though only one cylinder is whacky. Sometimes, using higher octane fuel such as 93 will eliminate this "mis-timing" by "requiring" the hot spark in order to ignite and resisting "post-ignition" on the pressure front after spark.

High compression, forced induction, and otherwise stressful operation exacerbate the above conditions (you hear the most pinging on hot days at low RPM's while accelerating).

Clear as mud?

Higher octane fuel is like the Roman bo'sun on a galley that comes along and whips the galley slave to get him back on tempo with his rowing. When it's hot, the slaves have trouble keeping time!

 

[elwaylite]

Very interesting, thanks.

So, use shell v-power for example. The 93 octane offers the benefits above, plus 5 times the detergent, which are all good (maybe not worth the xtra money, but they are pros). Are there any cons to running 93 vs 87? I know some see slightly worse mpg's on 93 over 87.

 

[lexus114]

I would just use the 89 octane. thats what I always use in a vehicle that only requires 87 octane. It has always worked well for me.

 

[elwaylite]

Thats what ive come to think. Many new cars Ive bought were pingers out of the box on 87, and I ended up running 89. Running 89 in a 87 car, if it does not ping offers no cons, but gives you a little head room if there is some ping. No need to spend the 93 octane dough unless you want the detergent.