91 octane

[thegawd]

is there any harm in running 91 octane (top tier shell) fuel in a stock 5.7 vortec motor? I only buy ethanol free fuel for my small engines, and lately that seems to only be available in 91 octane. the small engines love it and so I put 5 gallons of it in my farm truck and it loved it. friggen tonnes of power. I understand the benefits of top tier gas and how it cleans the fuel system but is there any drawback other than the price? my burb has a 42 gallon tank, i dont think I would want to pay the premium price to fill that sucker up!

Al

https://www.shell.ca/en_ca/motorists/my-fuels/shell-v-power-nitro-plus-premium-gasoline.html

 

[df2x4]

It won't hurt anything, but it won't help anything either. The factory tune in the PCM is expecting 87 octane, putting in anything higher is a waste of money unless you're tuned for it. Every time I've run 89 or higher on a stock tune in these trucks my mileage has actually been worse than normal.

 

[kennythewelder]

X2, it wont hurt, but wont help ether unless your tuned for it.

 

[thinger2]

Yep. It wont help.
A fuel octane rating is basically its resistance to igniting.
The higher the number, the longer it takes before the burn starts.
Higher octane is used in high compression engines to give a fraction of a second of delay before the fuel ignites.
This allows the cylinder to reach full compression before ignition.
That is why running low octane in a high comp engine causes "pinging"
The low octane fuel ignites before the cylinder is at full compression and before it reaches the correct point in the timing sequence.
The opposite happens when you run high octane in a low comp engine.
The high octane ignites after the cylinder has reached full compression and that ignition happens late in the timing sequence.
And of course these engines were not designed with ethylated fuels in mind so that adds another little kink into the theory

 

[thegawd]

thanks guys, I appreciate the input. one day ima get the 0411 ecm and tune this truck. theres no doubt in my mind it was running much better than yesterday with regular gas. this truck basically hauls firewood from January until thaw. iv only put 80 Kms on it in 3 years. all the while iv been working on it when funds and time and weather allow. I basically have all the parts to finish it but its work time. hopefully it's on the road this summer. since it sits for extended periods of time I dont want any ethanol in the system at all, I used to be able to get ethanol free regular gas from shell but it's gone, I'm not guna search far and wide but my research says its no longer readily available.

thanks guys

so would I be correct to believe the top tier high octane fuel iv used has probably cleaned the fuel system, making it run better and thats what I was feeling in my "seat of the pants dyno", some restored power. it was also pretty damn cold out.

Al

 

[kennythewelder]

Yes, a clean engine will run better. More so, clean injectors are a plus.

 

[kenh]

FWIW.... I ride a 2002 Kawasaki ZRX 1200. About 115 HP and 65 ft# of torque. I normally burn 87 octane ethanol bearing fuel. Stock jets and exhaust except the muffler has been gutted into basically a glass pack style. Didn't do anything for HP as all the restriction is in the header.

OK, to the point. I thought I would try some 91 octane fuel as this is a fairly high performance engine. Can't hurt, right... Well it did hurt fuel mileage. Not a lot. Only a couple MPG. Asking around it is confirmed with this engine/bike.

As it was explained to me the 91 fuel burns slower with less peak pressure as the engine is not set up with enough compression to warrant that fuel. So in reality a little less HP which demands a larger throttle opening which used more fuel ETC. Maybe a bit over simplified explication but it gets the point across.

Of course not every engine will respond in the same way. YMMV

Ken

 

[thegawd]

Just like your signature.... "Just because you can dosent mean you should." Haha thanks Ken!

 

[thegawd]

in the summer when I was talking about how well my 1962 Johnson outboard motor was running on the same fuel a buddy of mine said every single 2 stroke engine he ever blew was running the best it ever did right before it died a catastrophic death. he kinda scared me a lil. LOL. He was a speed freak doing high speed catwalks on crotch rockets. he has a Harley now tho.

 

[Godholio]

Top Tier fuel is cleaner, but octane level has nothing to do with that. There have been a number of tests over the past 25 years or more that show a slight performance REDUCTION (probably not enough to notice day to day) because your engine may not be burning all the fuel. Which, if you live in an emissions state, might hurt in other ways. Then there's 60c/gal price difference.