87 octane VS 91 Concerns Please advise

[lordsaber]

I have a 04, 5 speed standard, base model vibe, a CAI and a full cat-back exhaust system at 2.25". i have been using 91 octane in my car for at least 20k and now cause gas is too uber expensive i have gone to the cheap junk, i notice that at higher speeds my engine RPM's are lower by 100-200 and my engine is slightly quiter then it was before as if its not reving as high at idle. As for the excelleration power i havent really noticed anything yet but my engine sounds much louder when i stomp on it then it used to with the 91 octane, should i go back to 91 octane, im not sure if the lower stuff is bad for my engine or if the high octane is bad for my car. I want whats best for my car and performance so please tell me whats goin on here. Thanks again n_n

[Psychobroker]

Quote, originally posted by silverawd26 »You can have my truck, it only takes $60 to fill up right now.. Stay with your 91 octane, its only $3.00 a gallon and with a 11 gallon tank or so, your still at almost 1/2 of what I pay. Unless he has a GT (which he doesn't), there's no real need to use 91 except for slightly better response/performance.

[northvibe]

Quote, originally posted by Psychobroker »Unless he has a GT (which he doesn't), there's no real need to use 91 except for slightly better response/performance. Im with psycho, the engine is made to run 87, unless your racing theres no real need to spend the money on 91. just throw some fuel injector cleaner in every 3k or at oil change like someone suggested in the fuel injector thread and it should run fine.

[engineertwin2]

Use what is recommended by the manufacturer. There will be no penalties for using 91 in your vehicle, but it has been optimized for 87 (according to both Toyota and Pontiac). Many people use 91 octane for improved performance, but unless it is accompanied with an increase in compression ratio, then there will be no realized performance gain.Remember, the octane rating measures the fuel's resistance to knock. Vehicles that require 91 octane have higher compression ratios (or are not naturally aspirated) than those that require 87 octane fuel. Knock is predetonation (the phenomenon of the fuel igniting prior to the piston being at or near top dead center), which occurs when the pressure inside the cylinder is increased such that the combustion temperature of the fuel is equivalent to the internal temperature of the cylinder (see the temperature-specific volume-specific pressure plot, aka the TPV plot, of gasoline for more).We use fuels with higher octane ratings so that we can compress more air with our fuel. The golden rule of power is more air = more power.Others may argue that higher octane fuels tend to burn cleaner, and that may be true, but you'd be better off buying 87 octane with Techron than you would buying 91 octane from Joey Joe-Joe's Gas 'N Go...Long story short, 91 won't hurt your car, but its optimized for 87. You should get better gas mileage with 87 too (thanks to the ECU programming for 87 octane).Not to sound like a dealer service center, but any difference you may perceive is most likely stuff you may not have noticed before. But what do I know, I'm just an engineer...which means I really don't know anything.

[DavidPIL]

Hi lordsaber...Here is my experience and what I would suggest... I had a Ford Ranger... Was supposed to only take 87 octane and that's what I used for the first 50,000 or so. Back in the late 90's, gas was well under $1.00 per gallon so I upped the octane from time to time to get the supposed cleaning benefits from it.What I noticed is that after 30 miles or so with the higher octane, the truck ran quieter, accelerated with much more ease. I then decided to only put in the 89 octane grade, especially since there were places that sold it only $.02-$.04 more.Consider the mid-grade, especially if it's only a few cents more. Compare (over a few tank-fulls) different brands (specifically big name brands to more local brands) see if you notice a difference. I think you will probably get the same benefits for your base Vibe with a good quality mid-grade as you noticed with your premium grade purchases. It's worth saving $.09 or $.10 per gallon, if it works.Dave

[DiveNut]

Ya...what he said. Base doesn't require anything higher that 87 octane. Probably just wasting cash on the Hi-Test stuff. However, if you go for a Unichip, it has a Mapping (B) for Hi Octane. Would really tie all the mods you installed together.

[coldmm803]

a few weeks ago i put 3 gal (all i needed at the time) of 89 in my 05 base for my trip back to campus, 120 mi. lately i've been avg'ing 28 all highway mi but that trip with the mix of 3 gal i got 33. for the heck of it on sunday when i left home to return from my 3 days of easter i put in basicly 3 gal of 89 and got 41.8 some how, the one part for bout 8 mi is a hill and this trip i watched my rpms a lil more. i'm going to start alternating or something and see what happens