I've been hearing as of late running 91 octane fuel in a stock motor isn't good. I was always under the impression was alright, but I'm starting to second guess that. What are the pro's (if any) and cons of running 91?

 

There is actually more pro's than cons. Higher cylinder temps, more effiecent, better fuel economy. I avg 2mpg better when running 93 than 87. The engine can the most possible amount timing according to the ford pcm. I have seen a stock truck qith no programmer get a maximum of 34* of advance at wide open throttle, versus 24-27 with 87. My grandfather runs 93 in his 03 Fx4 with 3.73's. He avg 17.8/20.5 respectively. But when he runs 87 its 14.9/18.7

 

But you need to increase the compression or cylinder temps by increasing the timing right? because our stock motors do not burn hot enough to burn all the fuel that's why they recommed 87.

 

higher Octaneis betterthan lover, but much more octane does not.
because the Compression Ratio,
A higher compression Ratio NEEDS higher Octane.
but lower compression Ratio engines like to swallow hieroctane also.

if ur ENG is Made fot 87 Oct, then its no Problem tu use 91 or 95 Octane like in Europe it isnormal.
but 98 or 100 or 101 Octan like some Special fuel from European Shell or OMV or ESSO Gasstations, is waste of money.

better for the Spark Plugs is higher octane.

example.
My old Audi 2.8 Quattro has 178 HP with 98 Octane.
and 170 with 95 octane.
and 91 OCT is only for emergency, when higher not awailable

 

The octane level in gas is related to the pressure required to ignite the fuel without an ignition source. The higher the octane, the higher the compression it can withstand without igniting. Running high octane in a motor designed for low octane is a waste of money and can lead to the acceleration of carbon deposits due to the fuel not being fully burned. Running a low octane fuel in a motor designed for high octane can lead to predetination and pinging. Most later model vehicles can adjust timing for the different octane levels automatically through digital ignition systems. If you have high compression pistons, forced induction, etc. then use a higher octane. If not, use what Ford designed the motor to run.

 

If the engine is designd for 87 octaine and burns it without detonation[spark knock] then run it you will never recoup the added cost of premium fuel in performancev or mpg gains As to higher cylinder pressures and temps 87 burns faster and cleaner than 91 so if you dont have the advantage of higher compression or more spark advance and the need for it use the lower octaine I would however stay away from the 85 octaine sold at some stations at higher elevations. [not refering to E85]

 

Well I can tell you that even a stock 5.4/4.6 2v can benefit from higher octane fuels. These engines are relatively inefficient when running low timing. Accelerated carbon deposits are a mute point in a vehicle that use a Waste spark ignition systems. My truck is not stock, so 87 is not a option any longer. But even when it still had the stock engine it ran a lot better on 93 that 87.

 

IIRC, the compression ratio on the 5.4 is roughly 9:1. I'm not really sure how a motor with a ratio that low would benefit from a higher octane fuel without forced induction or high comp pistons.

 

this is no problem.
Self igniting is more suppressed by higher octane.

Because the igniter plugs, it is irrelevant which Octane Number the Fuel has.

a difference is only feel able at lover octane as recommended

rule:
u can use higher octane with no modification
u must use higher octane with compression ratio increasing

if the Engine is build for leaded fuel, u should use higher octane plus lead-replacement

 

I have yet to try my 93 octane tune on my SCT. Think I'll try it soon; see if my mpg increases over the 87 tune.