I have the dellorto DHLA48 carb.. i heard it's better to use lower octane..


Max

 

Correct

 

why?

 

oil mixture raises the octane and it makes it easier to mix. but a tank of high octane every month will keep the carbon away. it's great for cleaning the motor. thats all that gas treatment and stp does, raies the octane rating

 

This is what I've heard... High octane fuel will burn longer than low octane fuel. The extra time the high octane fuel burns will actually keep the rotor from spinning completely freely, because it is still burning when it is being pushed out of the exhaust manifold, thus kind of pushin the rotor backwards.

What's weird is that N/A guys use low octane and turbo guys use high octane... So maybe my explaination isn't correct...

Now that I've tried to answer this question, I've created another question... Someone help me!

FS

 

octane does 2 things. makes burns last longer and also requires a lot higher compression to cause detnation. for those of you who don't know waht that is, it's when you compress a liquid(in this case gas) untill it's super heats itself and causes it to ignite. desil engines run on this property, they run a compression around 20:1. so people who run turbos and high compression engines run a hgiher octane and also retard their timing to keep it from detonating. why is detonating bad on a gasoline engine? very simply put that the rotor/piston doesn't get threw compessing the gas and when it exploids, it cause the rotor/piston to move backwards. needless to say it's bad.

on the other side. if you have a normal compression engine like a stock 12a or 13b, and run high octane, 2 things happen. a and b put together) the burn doesn't completely happen in the camber, this means that there is less energy being created to push the rotor b/c the gas is still being burnt in the exhasut pipe. so you run a lower octane that will all be burnt in the chamber meaning that the full amount of energy is being used to push the rotor. if you run that higher octane, you will stillhave the same amount of gas tring to burn but it won't burn fast enough. so you end up loosing power. but the diffadvantage of having all the burning in the chamber is that carbon will build up(happens in piston and rotory engines). to fix this, about once every 2 months run a 1/2 tank of higher octane(the premium stuff). the burn will last jsut long enough to get to the exhasut manifold, that way all the carbon is pushed out of the engine. it's the same as going and buying STP or any other fuel treatment. al they do is raise the octane level.

hope this answeres some questions

 

thanks for the right explenation 13B4Port

 

Yeah thx guys... i was running 92 ever since i got the engine... didn't know it could harm anything or i could actually spend money to lose power.

Zachstylez

 

Yeah,

just supplemental to 13B4Port's awesome write-up:

Compression in a rotary engine is lower than that in a piston engine AND the surface area of the rotor is larger as well. What does this mean? It means that under normal circumstances, flame propagation takes longer than it does in a piston engine. Octane retards (slows) flame propagation, so the fuel takes longer to burn/is harder to ignite. This is the exact *opposite* of what you want in a rotary engine.

When Mazda used to race these, they would bring special 83 octane "race gas" with them because the stuff that was available at the race tracks was insanely high octane (which was required to prevent detonation in those high-compression pissed-on engines).

I'd assume that turbo guys use higher octane because with the extra pressure of forced-induction (turbo) they have to have higher octane to prevent detonation... You gotta think... IF they're forcing 10psi of air/fuel mix into a *non-compressed* chamber, by the time it gets to the spark plugs, it's got enough compression to pre-detonate if you were using low octane.

Jon

 

Just a little tidbit for ya'll: the Mazda IMSA team's pits used to smell strangely like Kerosene....

 

In Belgium we don't even have anything less then 95ron. 98 is the next step, with some stations offering 99+. IF high octane was bad, we wouldn't be able to run very long, would we??? However, it is important that your ignition timing is correct for the fuel you're using.
Other then that, a rotary can run on pretty much anything. Octane levels aren't as important as with piston engines. So really, you don't have to worry about running 92 in a Dellorto NA.

 

13B4port, vipernicus42,

The octane rating of gasoline is a comparison to the compression ignition properties of a pure isooctane/heptane mixture. It has NOTHING to do directly with the rate of reaction of the gasoline mixture.

The only thing that can be said about the octane you should buy is to use the highest rating you need to to keep your engine from knocking. More than that is not necessary.

The quality of gasoline you buy is a totally different matter. Gasolines have many different mixtures and therefore burn rate properties. Buy from the more reputable gasoline suppliers and you will most likely get a better quality gasoline.

 

On the subject of gas I was wondering if anyone could answwer this question?


quote:
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Originally posted by 813KR$
I would Pre Mix! That is what I do Caster 4 oz to a gallon, RB suggests 3.5 oz a gallon.
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Someone posted not to long ago to run 1oz per gallon. thats what I did. I havn't run the engine much since the mixture but could that have caused any damage. I have run it a total of about 30 mins since the mix change over. I was told to use a 100:1 mix,

I'm using Castrol super outboard oil.

1.3oz per gallon = 100:1 ratio
3.2oz per gallon = 40:1 ratio
4.0oz per gallon = 32:1 ratio

 

wow, man. if there wasn't a difference in octane other than "quality" then they won't offer a lower quality gas for jsut 5 cents less now would they. octane refers to the size of the moecule. the higher the octane rating, the lager the moecule. notice the little equation next to the octane rating at the pump? thats the equation they used to determine the rating. this explains why high compression/turboed engines need higher octane, because it can stand to be compressed more without the gas/air mixture heating up as much. that also explains why the burn will last longer b/c the moelcule is larger, it takes more energy to make a complete burn of it. if you have any doubts try this. run a digital temp meter on your motor and run a low octane. write the temp down. then go and run a high octane in your motor and write the temp down. thats running temp. you will notice that the high octane will run slightly cooler by a few degrees.

as far as running high octane all the time, well its not a huge noticeable loose in power, something like 1 or 2 hp. but if you got up into 110 octane, you'd notice a huge difference.

if you have anyother questions, i'll be happy to educate you

 

Ha ha, unless you've got more than six years in mechanical engineering and something more than a masters degree doing engine research (MSME, UW-Madison), I'd like to know what you're going to teach me.

Read my post again, what I'm saying is the only sure difference in octane rating is the gasoline's resistance to compression-ignition. You can't make any other assumptions about the gasoline's behavior base on the octane number.

Octane can be raised by "cracking" which is changing the molecular structure of the hydrocarbons in the fuel, but there are also many additives that can be added to do the same thing. There are different ways for gasoline to get to the octane rating that it has.

There could be formulations that burn faster at lower octanes, but there could also be formulations that burn at the same rate or slower.

As far as a mixture heating up more or less on compression, this is a matter of the number of moles (molecules) in the mixture. PV=nRT. It is only very loosley related to the combustion rate of the mixture and is again dependent on the other ingredients that have been added to the gas.

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

 

Sorry, that sounds a little too cocky now that I read that over. I'm just saying that I know a little about combustion and fuel formulization through my graduate education.

 

i work for a guy named kevin where all we do is build rx7's mainly second gens. and they get used in autox/drag or w/e the customer wants. you are right that you can assume on octane alone. but you can assume octane with compression. and thats what i did. thanks for clarifying that

 

Whew... I've never know this much about gasoline
I feel much better now! Man... I absolutely love this forum!

FS