yamatokh

Seems like people are saying if for NA, it's better to run with lower octane......

Is this true?

I know for the wallet it's better, but how about everything else? Performance? Reliability?

I always thought that the higher the octane the better the gas....does this rule apply to a rotery?

My engine is stock by the way.

thanks

Mykl

Has something to do with flame travel I think. I read something on it a while ago, but I don't remember any of the specifics.

ItsNiceToBurnRice

no turbo, no lifters, no valves=no use for high octane

Mykl

I don't understand what lifters or valves have to do with fuel octane. Care to explain?

yamatokh

my machnic friend told me that with lower octane, the fuel will not burn as clean, therefore will have more carbin deposit, which is bad for an engine.

wouldn't this be true piston or rotary?

ItsNiceToBurnRice

In piston engines higher octane burns cleaner and therefore these parts of a piston engine dont work as hard kinda, thats why some cars will ping and knock without the use of high octane.

ItsNiceToBurnRice

true for a regular combustion engine my friend, turbo's need high octane for turbo reasons, Im not a turbo guy. I know that rotary motors will build up carbon if they are ran at low RPM's constantly because carbon will have a chance to build up. If you run it like its suppose to at High R's then carbon dosent have a chance to build up, maybe this is why low octane would not matter-it just gets blown right out.

relvinnian

http://www.howstuffworks.com/question90.htm

Think about it... it's not that hard

MAZMAN

a higher octane has a higher burning point,so if you have a high boost turbo engine,the temp will be high and the higher octane will be able to ignite easy.
with a NA motor,it will be harder(then a lower octane) to ignite and thats why it wont run as good.
The higher the burning point the less chance there is of a pre-ignition/pinging of the motor.
Basically the higher the boost you wanna run,the higher the fuel octane you should run as well.

No7Yet

AHHHHH!!!! THE MISINFORMATION IS SUFFOCATING ME!!!! AHHHH!!!!!

First, my diatribe on octane:

Octane is NOT a measure of gas quality. Due to gub'ment regulations, all gas is of similar quality (i.e. 93 octance doesn't burn "cleaner" than 87). The difference lies in it's resistance to preignition. Higher octane fuels consist of more uniform hydrocarbon chains, which makes them "harder" to ignite. It doesn't mean that they ignite at a higher temp, it's just harder to catch them on fire

Now, preignition (which, if you'll recall, higher octane gas is less likely to encounter) is caused by a number of things. Unlike ItsNiceToBurnRice said, it has nothing to do with how many parts an engine has and everything to do (for the purposes of this discussion) with dynamic compression ratios. The higher the compression ratio, the more heat gets "added" to the air/fuel mixture, and the more likely the chance of spontaneous combustion.

Turbos raise effective compression ratios, by cramming more air/fuel into a combustion chamber. That effectively raises the compression ratio, and thus the likelyhood of preignition, and thus requires higher octane fuel to avoid damage. SO RUN 93 OCTANE IN YOUR TURBOS!

N/A engines (we're talking strictly rotaries here) have a static compression ratio of about 9.5:1, and their dynamic compression doesn't change much (it's a function of VE). Due to both the turbulent nature of the intake charge of a rotary, and the fact that said charge is "swept" across a relatively "cool" housing (which, in turn cools the charge off, and thus makes it harder to ignite), the extra "help" that lower octane gives the spark plugs actually makes the car run better.

So, run the lowest octane you can find in your N/A. It helps your wallet, your fuel mileage, and your engine's performance.

Brandon

(P.S. I know I've left a lot out. I'm beginning to develop carpal tunnel and my hands hurt today. The advice is kosher. Use it.)

GTUsGUY

No7Yet, you hit it right in the middle.

Also, a read somewhere that you can run safely 67 octane (if available) in your N/A.

easy p-cheesy

I was just wondering, is it true that since 92 is more resistant to combustion, that it would burn slower?

C. Ludwig

iTrader: (27)

THANK YOU NO 7!!!!! Couldn't of said it better.


Chris

No7Yet

Well, no, not really. Once gas gets ingited, it releases enough heat to ignite the gas around it. Higher octane is more resistant to that initial ignition, but burns just as fast once it is actually burning.

Good question, though.

Brandon

crossbar17

its simple in a piston engine higher octane burns longer...it burns on the way down and on the return stroke more than say 87 or 85 octane would, however lower octane tends to run better in colder weather and high octane runs better when its warm atleast in a piston engine, any questions??




crossbar

easy p-cheesy

thanks. wasn't quite sure.

No7Yet

Yeah, where can I get some of what you're smoking?

There is no difference in differing-octane gasoline other than its resistance to preignition. NOTHING. Not "burn times" or how well it burns in whatever weather you're having (that's a function of tuning - have you tuned your carb lately?)

With all respect, crossbar, I'd not speak of that which you do not know.

Brandon

RarestRX

Yo,


So sayeth Felix:

What's the best fuel for my rotary car?

It depends. If your rotary Mazda is equipped with a turbocharger, all the normal rules about octane apply. Use the highest available octane premium fuel for best power and best protection against the ravages of detonation. You may find slighly better fuel mileage using lower octane, but you need to be very careful about using the available power on lower octane. If you are good at exercising restraint, you can save a little money on a long trip using regular, but it's probably best to stick with premium for normal use.
WIth the normally aspirated (NA) rotary, the highest octane you should use is US pump (AKI - anti-knock index) 87, typically RON 91 outside the US, no matter how heavily your engine is modified. Octane in excess of any engine's actual requirement is always wasted. The issues of purity and additives in more expensive fuels are entirely separate issues. There's no reason not to want either in the NA rotary.

The rotary engine's high turbulence combustion chamber provides a very high resistance to detonation. Its duration of combustion is also longer, remembering that the rotors turn at 1/3 of the tachometer reading, and the slow burn* of high octane is undesirable in it. Pump 80 octane is more than sufficient for most of them. Best power and mileage is usually produced with the lowest available octane.

Many serious rotary racers bring their own low octane gasoline to tracks that supply only racing gasoline. From "How to Modify Your Mazda RX-7", by Dave Emanuel and Jim Downing, HP Books, 1987, p 47-8: ". . . the best results are obtained with conservative spark-lead calibrations provided the engine is fed a diet of low-octane fuel. The fact that both 1985 and 1986 IMSA Camel Lights championships were won with low-octane fuel is a rather definitive statement . . . ."

So if you want best performance from your NA rotary, you want lowest octane. The lower cost of it is a nice bonus.

* Note - the time allowed for combustion at high RPM is measured in ten-thousandths of a second. Some literature ascribes lower volatility rather than a slower burn as the characteristic of a higher octane value. In contrast, consider the following: From "14-to-1 compression", By David Green, NASCAR Winston Cup Scene: "One problem that has developed in the 9.5-to-1 engine is high exhaust temperatures, due to a less-efficient burning of 108-octane gasoline in the lower-compression combustion chamber." (emphasis supplied)


From the best rotary FAQ:

http://mrmazda.members.atlantic.net/...html#RENESISHP


KS
1989 GTUs "I use Havoline 'cause of Felix."

crossbar17

oh really master No7Yet


what makes you think higher octane doesn burn longer??? ask someone who knows before voicing BS flaming


crossbar

Shamrock1054

Hey No7YET,

So that's why my friend has to run AV-gas in his 13:1 rat motor!

Gregg

MasteRX

Contrary to previous posts, you are correct. I had always understood that higher octane fuels burn slower than lower octane fuels and when NoRX7Yet started making his posts, I decided to investigate. After a simple web search, it is revealed that that higher octane indeed burns slower and is a well documented fact. Now on to why lower octane is better for N/A cars. There are a couple of reasons. First of all is the fact that lower octane is a faster explosion it is better for the fast turning rotary. Now how much actual power gains you would get would be minimal, but why pay for higher octane when it gives you no benefits. The only benefit to higher octane fuel only has a higher pre-ignition resistance, a benefit only realized in high-compression or in instances of advanced timing. High octane fuel doesnt have any other additives or anything to make it burn cleaner or more powerfully or anything, so paying for it is a waste of money. So save your money and buy the lower octane.

crossbar17

the higher the octane the longer it burns through the stroke.....its harder and believe me it is harder to start a car with high octane in the winter

No7Yet

Really? Can you quote some sources here? How about this:

From the rec.autos.tech NG found at rtfm.mit.edu:
(emphasis added).

Sources cited:
21. The Chemical Kinetics of Engine Knock.
C.K.Westbrook, W.J. Pitz.
Energy and Technology Review, Feb/Mar 1991. p.1-13.

22. The Chemistry Behind Engine Knock.
C.K.Westbrook.
Chemistry & Industry (UK), 3 August 1992. p.562-566.

Rebuttal?

Brandon

Bambam7

Another, more pedestrian aspect to the "Premium fuel" debate is that higher octane fuels generally have cleaners mixed with them. Running a tank of premium fuel now and then is equivalent to tossing in a can of injector cleaner now and then.
This is where the misconception about carbon buildup stemmed from.
The combustion of the premium fuel still deposited just as much carbon, (possibly even more due to additives) but solvents added to the fuel maintains a cleaner internal environment by continually dissolving them.

And I'll glady enter this other raging debate to try and cool things off a bit.

Higher octane gas "lights up" slower given the same compression compared with lower octane gas. Flame speed IS inherently slower (but isn't a major factor at all) with high octane gas due to the combustion inhibiters, but cumulative energy potential is the same. (Same initial mass/final expansion ratio)
It's actually the compression that determines the stability of the fuel.
At atmospheric pressure the two fuels will ignite almost identically, but under compresion, the differences increase more and more depending on pressure.
The higher octane fuels are designed molecularly to exhibit it's principals under pressure only.
This is why it's a waste on lower compression engines.

Or, think of it like a match head- Those "waterproof" ones with a bit of a coating on them need a little harder strike to ignite, but then produce the same energy.

flying taco

ok, say I'm using 87 octane gas. That means that the gas is 87% octance and 13% other hydrocarbons right? Now, each hydrocarbon is going to have a different enthalpy right? So, say octance has the highest enthalpy of the hydrocarbons in the mixture, if I had 93% octance, wouldn't I have a higher power output? However, I REALLY don't believe the length of the hydrocarbon chains (8 for octane) is going to significantly increase or decrease the burning time of the fuel.

Aaron