Need Help 04 Accord Ex coupe, what octane level?

[Deuxmasta]

hey, i just bought a used 04 honda accord coupe EX with 53k milage on it, i haven't recieved the manual for it yet and just filled half the tank with Super at the gas station i was wonder if it mattered? everyone has their own opinion on it but i don't want to putting in Super (supposedly better) if the car was made for regular, i'm not sure. what do you guys think?

[jeffcoslacker]

It's a low compression engine, doesn't require anything over 87 octane. Premium fuel in a low comp motor is a waste, it can't do anything with the additional octane.

[StealthSonic]

Does higher octane matters if you have headers and highend exhaust? Isn't it higher octane has higher combustion; therefore, burns cleaner?

[jeffcoslacker]

Originally Posted by StealthSonic

Does higher octane matters if you have headers and highend exhaust? Isn't it higher octane has higher combustion; therefore, burns cleaner?

There's a lot of misconceptions about octane.

Octane is simply a fuel's resistance to detonation. The higher the octane level, the less likely it is to ignite from a non-spark source, like compression heating or a hot spot in the combustion chamber...higher compression engines squeeze the mix much closer to the threshold of spontaneous combustion before the spark plug lights it off, so they can detonate before spark ignition under certain conditions, which causes knock and pinging that are destructive to the engine and lower performance...

Higher octane fuel does not burn any hotter or faster than lower octane...and produces the same amount of energy per unit of fuel in combustion as low octane gas...the difference is with a high-comp engine you are squeezing the mix tighter than a low-comp engine, so the expansion on ignition of the mix is stronger and applies more power to the crank on each firing event...more potential energy + smaller area = more powerful reaction when that energy is released...

The only reason a low-comp engine can benefit from from high octane fuel would be if:

It had combustion chamber deposits so bad that it had detonation due to hot spots or reduced cylinder volume, doesn't happen much anymore, fuels burn very clean now, or

Ignition timing was over advanced or mixture was too lean causing detonation, in which case repair is needed, higher octane is merely a band-aid fix, or

The motor boosted by supercharging or turbo...both of which are simply a way of increasing compression artificially by stuffing more fuel/air mixture into the cylinders than what natural aspiration can deliver, which raises cylinder temps and pressures and creates an environment where spontaneous detonation is more likely, just as in a high compression engine...

So in a low-comp, N/A motor designed to run on 87 octane fuel, if there is some substantial increase in performance or mileage from higher octane fuel, there is some issue that needs to be adressed, the higher octane is just reducing abormal combustion that is taking power away from the engine.

Also all fuel brands I can think of contain the same amounts of detergents in all grades, so premium fuel does not keep injectors and combustion chambers cleaner than the other grades...they used to do that as a marketing gimmick in the 70's and such, enticing you to buy the more expensive fuel with promises of better performance and cleaner fuel systems...cars that were still running high-comp engines could definitely benefit from the higher octane and cleaner combustion chambers, but 99% of drivers don't have a clue what compression ratio their motor is, so the fuel vendors took advantange of the ignorance to try to market high octane fuel to everyone...it still continues today...there are about 10 car models I can think of built in the last 15 years that actually have compression ratios or boost high enough that require 93 octane fuel, the vast majority are designed to run on 87 octane and will never require any more than that.

And anything built in the last 10 years or more has a knock sensor incorporated into the engine control system, even if they were to run lower than reccomended octane level, the ECM hears the beginnings of unwanted detonation and adjusts the timing slightly to control it, so the motor may have slightly compromised performance under load, but will not see any destructive effect from it...

There's a gas station not far from here, right near the area where kids still cruise and drag race, they sell 110 octane racing fuel for about $6/gallon and I see young guys pumping it into their N/A and mildly modded Civics and stuff and I just about pee my pants trying to keep from laughing out loud....