*sigh*

[&:]

It has nothing to do with motor size, mingstah. Here, let's try this:

The short answer is:

If you don't have the gear to measure knock in your specific motor/setup, then either you trust the manufacturer and run what they suggest in the owners manual, or you distrust them and run a higher octane rating because you honestly believe they are not accounting for knock that you truly believe your motor is developing. Without tuning gear to measure the knock on a specific motor/setup, it's all a guessing game no matter what anyone says.

If you do have the gear to measure, you already probably understand all this, and you are tailoring your fuel, fuel curves, and perhaps more than that (valves/timing, exhaust, etc) while tuning your motor system, and you shouldn't even need to read any of this!

Sorry, I know this all started to get more complicated as I continued to explain it more.

E.

 

I like the short answer. So basically you really won't know what is good or bad unless you have the proper tuning to figure it out? Good stuff E.

 

Eggs'zakaly.



And that's why it basically falls back on whether you trust the manufacturer and their published specs, or you don't. That boils down to a personal choice that each and every rider has to make on their own; not something that anyone can ever force upon another rider.

Stay safe!

E.

 

So to sum it all up on simple layman's terms. Reccomend running the octane rating that the manufacturer rated the engine for.

Running a lower octane might give you a negligible increase in power/economy, but at the risk of needing very expensive engine rerpairs if you start getting detonation and it goes unnoticed.

Running a higher octane rating will never hurt your engine, but you'll get marginally less power (with no other changes) for a more expensive gasoline.

Normally I run the highest octane I can find, even though I know I might be wasting money on the fuel price. My '07 is rated for 91 Octane, so is my RX-8

My wife's Matrix is rated for 87 octane, but I'll normally put 91 in it as insurance since she'll buy the cheapest swill she can find when she puts gas in it and some of the gas stations she fills up at look pretty shoddy.

 

the quote best sums up this whole 86 vs 93 octane battle ... imo

i normally get badgered from my responses to the fuel issue...my 05 recommends 86 octane ...however ...i have been runnin 92 or 93 w/e they have when i need gas...i notice no difference in mileage or power.

 

Elmo you are one knowledgeable mothertrucker. I appreciate the feedback, as you have brought up a lot of good points. I do have one interesting question for you. The higher the octane, the higher compression the fuel can handle, higher flash-point, etc... Based on this statement, how come diesel fuel is like 40 octane, but is run in very high compression applications? This seems to be the opposite of the already determined rule?

Personally I like to run shell 93. Its more then likely much more then I need, however it is the cleanest. From what I understand the higher octane fuels come off the top of the petrol after it is refined, thus making it cleaner. Less carbon buildup on your valves, etc will make for a cleaner running, and thus more efficient engine.

 

The reason Diesel is used in such high compression even when it has such a low octane rating is because of the way a diesel engine ignites the fuel mixture.

Diesel engines don't use spark plugs like regular gasoline engines.
they are actually designed to self ignite the mixture just by the fact that it is compressed on a hot cilinder. (to initially start the engine Diesel's have "glow plugs", bassically the same thing used on a car's cigarrete lighter, or on the nitromethane R/C cars, until the cilinder is hot enough to ignite the fuel on it's own)

Diesel engines are designed to handle the stress of detonation, (that's why they sound like theyre falling appart, and why they only rev to about 3000 RPM)

 

I know how a diesel works... But wouldnt you need a fuel with a higher detonation resistance to withstand 20+:1 compression?

 

Nope, it's perfectly INLINE with the determined truth.

Keep in mind that the goals and idea of Diesel technology was to SIMPLIFY the I.C. engine. Diesel is a 'finished product' earlier in the refinement process. (ie, it takes less effort to make) With a higher yield of MPG, along with this easier refinement process, it was supposed to be the first step towards solving the worlds fuel problems. Also, the goal for Diesel was to have an ignition-less system. (modern demands for convenience dictate that one be installed though for faster easier 'cold weather' starts to make standard Diesel passenger vehicles more user friendly) The big problem with Diesel was that the eco-people saw the NASTY polution that first generation diesel's created, and stifled it's growth for decades; that which you didn't refine out beforehand, passed straight through and out the exhaust as unburnt gunk. I'm sure every reader of this thread has seen diesel cars/trucks that were covered in sooty deposits near their exhaust.

Therefore, raw diesel is dirty, but it has much more potential yield for power. And with such a low octane, it's inherently unstable once compressed to a certain point; which is exactly what engineers DO want, since they WANT to guarantee that ALL the diesel/air mixture ignites on EVERY compression stroke. Remember, a diesel typically fires using compression alone; the fancy ignition assist systems I mentioned before typically are only used to get the motor going and up to temp... once it's there, it relies mostly on the compression alone to fire every cylinder, for every cycle.

For you diesel mechanics out there, yes, I know, I'm grossly simplifying the workings, but were trying to keep this in 'laymen terms' territory right now. =)

You are correct in that premium fuels usually result in less deposits and gunk; not so much because of the refining process, but because the fuel is filtered and post-processed more from what I've been told. (this is circumstantial though, as I've never worked for a fuel refinery, and most companies are pretty tight lipped about their final refinement processes) Post processing involves copious filtering, then the addition of certain additives like cleansers, stabilizers, and nowadays, oxygenators. (Ethanol or the like; not so much MTBE since that was found to be a dangerous chemical that was leaching into the ground water) The price difference we all pay for premium fuel compared to 'regular' is only partly comprised of the 'extended refining', it's moreso the post-processing. (once again, from what I've been told)

From a 'clean' standpoint, yes, your running of premium will prevent buildup and whatnot. This will extend the life of your motor, if you were to never do any kind of maintenance on the fuel system. Simply dumping a bottle of 'fuel injector/fuel system cleaner' in once a year would probably do the same though, for considerably less cost. (even if you were to do it every six months, it'd still be alot less) There is of course no harm in doing what you're doing; as long as you're willing to sacrifice a marginal increase

 

Think about it for another second...

If you had a fuel that could withstand 20:1, and it survived to TDC and past, and there was no spark plug to ignite it, what would happen?



You WANT to guarantee that when that compression level spikes in the last few mm of piston travel towards TDC, that fuel/air mixture ignites.... Because there isn't (typically) going to be anything there to help ignite it!

E.