I know that we have discussed this before but I can't seem to find the thread (search for octane and you'll find millions of posts about high boost). I was trying to find the reason why NA rotaries run better (more power/torque) or lower octane fuel. Can anyone help answer this for me?

 

The fact that they don't need high octane fuel is a good thing! Some cars require higher octanes to prevent pre-detonation or pinging.

 

The higher the octane, the more it takes to make it explode. Why it needs to be ran in a turbo car, is with all the boost, it creates more heat, and with a lower octane gas it can ignite early and really damage the engine. I believe why our cars run better with low octane is it ignites and explodes at the right time, and creates more power. You could probably run a higher octane, but I think you would have to advance the timing some to compensate for the gas igniting a little later.

If I'm completely wrong on this, please tell me. It's just what I remember from the old threads discussing this.

-Matt

 

The octane is the resistence to knock. The regular unleaded with the lowest sold octane fuel is best for rotaries not getting boost. Boost requires higher octane because they get higher pre-ignited chamber pressures that will self ignite before the spark can do it at the proper time. As a result boost with low octane causes bad things.

Also on the max chamber pressures before the mixture ignites is way too low on a n/a rotary to nessessate anything higher then 87octane. However some n/a piston engines DO require higher octane because of the higher compression some performance engines have thus producing the critical cylinder pressures near the threshold of knock.

 

High-compression engines need gas with a higher octane rating because it burns faster. Octane is a measure of how fast gasoline burns.. and high-comp engines need a gasoline that burns real fast!

I don't know the recommended octane for the NA engine of any given year.. it may vary from year to year, for all I know. Some people say to run the lowest octane you can find, but I'd suggest reading the owner's literature and trying different brands of gas (before trying different octanes) to see what works best.

 

I thought it was the resistance to detination? Either pre detination, or actual detination when its supposed to happen...Or something.

~T.J.

 

i read that 80 octane was ideal. i stick with 87.

 

~T.J.

 

I always use the lowest-octane fuel available. The lower, the better, cause its a rotary.

To add to wackyrotary's point, not only does the chamber pressure in a rotary not reach a critical level, but due to the high turbulence of the gasses in the chamber (due to the inherent design of the engine), a high resistance to preignition is not needed, because no hot spots/high pressure zones are formed.

 

That's odd...this thread didn't move to the top when I posted.

EDIT: Well, it seems to be doing it now...strange

 

Information I can use.

 

Here's a stumper:

Older NA rotaries with a 9.4:1 and 9.7:1 compression ratios could pretty much use the lowest octane you can find...

The info that has come out on the 10.0:1 compression renesis so far indicates that it needs "premium" gas...

Why?

 

Where did you see that? Show us? Up till now, I've seen many articles indicating its a regular unleaded engine. So I don't believe that.

You'd have to have higher then 10:1 to need premium in a rotary with no boost. I doubt the rotor faces can be made so shallow that it would ever get compression pressure so high before ignition that it would need premium, possibly mid-grade at best.... possibly.

 

Hi WackyRotary,

I understand your skepticism as I don't understand it either. The below link on the RX8forum has a scan from the RX-8 Specification Deck which has the date of Jan 7, 2003.

http://www.rx8forum.com/showthread.p...1041#post21041

I have found a few things in there that are incorrect and am hoping that the recommended fuel is also a mistake.

This is the real deal from Mazda and magazines have also been reporting that the recommended fuel is supposed to be premium.

It still doesn't make sense to me though. Any clues here?

Thanks, Brian

 

You're way off. Octane is the measure of how resistant a particular fuel is to detonation. High octane fuels are more resistant and need more heat to detonate which means they're going to burn slower if anything.

High-comp engines typically need higher octane fuel to keep from detonating due to the increased combustion chamber pressures.

 

...aaaand. as a bit of a retarded history lesson, lead makes an excellent octane enhancer. but the downside is that if you run a nation full of leaded-gas powered cars, you'll eventually develop a thin layer of lead over everything exposed to air. and lead is bad mmmkay.

 

high octane burns SLOWER not faster.

that is how it resists detonation. It is harder to ignite.

 

resistance to detonation which causes knock (at least in piston motors). a rotary is actually fairly resistant to detonation due to there being few "hot spots" in the relatively smooth chamber. thus the reason it is able to run lower octane. however, when detonation does occur, it easily snaps the apex seals. the imsa and many of the factory race cars supposedly used 78-80 octane fuel due to this natural ability. so anyone here running premium gas on an na, you are throwing out money, power, and gas mileage. exceptions being the higher compression renesis.

 

Wow there's a lot of arguing arguments here..

I think in the end it's safe to say that the fuel with more octane burns slower and needs more spark to ignite...
The rotary needs faster burning fuel so the lower octane is the way to go!
this right?

 

any engine will like faster burning fuel. easier on ignition and more of the combustion energy is in the combustion part of the cycle instead of trailing off into the exhaust part. so with any motor, running the lowest, SAFE octane is the best way to go.

 

In my understand, the higher the octane....the harder it is to ignite it and harder to burn. On my 91 Infiniti G20 MT for example, it requires only regular out from the factory. I have experienced a better gas mileage out of regular fuel than running on Premium. It is based at the gas mileage in the long period when I used regular and compare to the long period when I used Premium, and on the same route thru out.

My theory behind it, it based on what I stated earlier. Higher the octane rate, harder for it to burn. So it is more efficient on running on regular if that is the factory recommendation, and if the car is nearly stock meaning the timing isn't being advanced and etc.

Correct me if I am wrong. I am no thermodynamic specialist but it is based on what I read, and what I studied in part of my Civil Engineering study.

Nelson

 

I've heard that too, If i could get 80 here I would. Friggin $1.59 for 87 here, gross...

 

In general stick with the manufacturer recommended octane level.

meaning you should be running 87 Octane.

I occasionally will use Ultra94 but only because I get better mileage.

I personally wouldn't go to a lower octane fuel (85 or less) simply because I've found such a fuel typically has a higher sulphur content and doesn't burn very clean. i.e. cheap dirty fuel which will probably run great but foul your plugs & O2 sensor, and help clog your cats along with it.

So stick with 87 and find your formula preference. Mine runs great on Shell 87.

F-

 

I use 98 octane in Australia and it gives my about 5% more mileage than the standard which I believe is 92-94.

 

With gas here now at $1.75 a gallon Im glad that my 7 runs best on it as opposed to the higher octane grade.