purple82

I'd also like to know if this was on the same dyno on the same day. Also, how many different gas suppliers did you use? Is this just a one time thing? Like I said before, the energy in the fuel will vary from manufacturer to manufacturer.

Rx7carl

Thanks for the info. I have the feeling that this isint going to really be a big benefit, but my natural curiosity gets the best of me sometimes.

Rx7carl

He'll itll vary from tank to tank. Fuel is very susecptable to temperature, age, UV rays too. Read an interesting article on racing fuel in Circle Track magazine. I inquired with VP Racing Fuels to get their opinion.

fitzwarryne

On high compression piston engines I always got better performance/running with premium fuel and with a urbo engine bumped it up a bit more to keep the tempeature down. Out of interest i tested a Suburu Outback 3 litre, 1200 km on standard and 1200 km on premium, the difference in consumption was 2-3% thus far less than the 10% price difference.

On a rotary it does not seem to matter so much and any power or consumption diferences are very marginal and within statistical error. If I was racing then for the ultimate i might stick to premium, but for street use the higher price is not really justification.

joeracer

Yes, the dyno runs were done back to back. About 10 minutes apart. The amount of time it took to pull the regular out and pour the premium in. I also let the car run for a while to give the regular time to work its way out of the lines/pump/carb float. I can't testify that these fuel samples were from the same gas company. Probably was since I wouldn't have a reason to stop at two stations, but I don't actually remember.

I'll definitely do this again if I get the chance. I'd love to hear from anyone else who's had a chance to try it.

purple82

The point is, if you ran this test again on five different brands of gas both 89 and 92 octane, you'll get a different power level every time because, lets all say it together, octane is no indication of fuel energy capacity.

joeracer

Purple82, I think we can agree that octane rating, in itself, won't change the hp of an engine. Also, given that this was a single experiment, this would hardly pass an independent review for its application of the scientific method. The point was to make it clear, with hard facts, that higher octane does not adversly affect a rotory engine, as lots of people keep spouted as if it was gospel.

Now on to my particular theory. On the other hand, could the octane be responsible for the increased hp? Higher octane prevents detonation, which quickly decreases hp. So if there were detonation taking place while using regular, that could explain why I got more hp using premium. But, everone knows that rotory motors have low compression and therefore no detonation. Well, maybe and maybe not. Here's were it gets really shaky. But just for fun follow along here. Directly applying the "common knowledge" about compression ratios and detonation that comes from piston motors to rotory motors is questionable. Where a piston motor has a circular and uniform combustion chamber, a rotory motor has a long rectangular and uneven chamber. It also known to have lots of hot spots due to the difficulty in getting cooling passages in a rotory motor. So what if, in one of the tiny, hot resesses of the rotory combustion chamber, there is a area of high pressure and heat that causes a bit of detonation? Heh, just something to think about.

mar3

iTrader: (8)

Actually, pre-igniton/detonation, the part we actually hear and does the damage, results from multiple flame fronts starting at different areas inside the combustion chamber (Other areas besides from the spark plug where it is supposed to start...) and then slamming together at the edges of their fronts.

All octane is is a number assigned to a fuel that indicates its ability to resist ignition starting somewhere besides from the ignition source. Everything else, HP, joules per molcule, vaporation tendencies are all unrelated to detonation when thinking in purchase mode.

If a fuel injected rotary gets premium fuel, the controller will advance spark until it sees pre-ignition, read:pinging, so you can't even say premium gas makes HP. It allows you to tune the ignition advance for more power, but does not create the power in and of itself. On a carb'd engine, the gains may simply be seen because the gas manufacturer will lower vaporization numbers for the pricier gas, so the vehicle will start easier. Hard starting engines when paying premium prices not being a desirable thing. Without an apples-to-apples comparison where only the octane is different, even a dyno test is not definitive because the experiment lacks proper controls to validate it as a scientific experiment with usable results.

fatboy7

Whew..... you guys have been busy. Lots of writing in this thread.

I've spoken on the subject in the past, but its an interesting subject: Why X octane gas, vs Y octane gas?

Its simple:
1) Octane is really unrelated to power
2) All engines have a required octane rating that must be met under given conditions
3) The octane requirement for NA rotaries is very low, practically eliminating detonation as a problem, and octane rating as a limit.
4) Pump gas is too variable to define which gas will make the most power at any given time. So unless you're trying to squeeze every last Hp out of your engine, buy the cheapest stuff you can. Else, try different race gases, which will give you more consistent results, and choose the best one for your engine. Or make your own.

Octane is only a measure of resistance to detonation. There is no direct correlation between octane and flame speed, released heat, and so no direct correlation with octane and power. The best fuel will likely be the one that burns at the right rate and producing the right pressure during the time that the rotor has the best mechanical advantage. Formulating a fuel is really complex... getting the right fuel is not a simple "pic X octane, it's the best." It just so happens that the fuel that worked best for Mazda in the 80's was very low octane.

Your engine has a required octane that it must have to not detonate. This required octane can vary significantly from car to car even of the same model due to variances in tune. Some cars run rich, some lean, some have the ignition set too retarded, some too advanced, some have carbon build-up, some have marginal spark plugs, some fuel pumps/regulators run at different pressures, some carbs are different, etc. etc. etc. Manufactures use statistics to determine the probability of a car knocking vs. octane, and from that determine a safe rating. Often they build the engine with a rating in mind, raising or lowering static compression to get a specific octane rating.

Rotaries are exceptionally resistant to detonation, meaning that the required octane level is lower than a piston engine of comparable compression. Rotaries' compression ratio unlike their piston counterparts' are physically limited by geometry. That’s why compression ratios have changed very little over the years. IIRC Mazda found the average octane requirement of NA rotary engines to be around mid-upper 70's. That pretty much means that octane in any currently available gasoline is more than adequate for NA rotary engines.

Joeracer found that supreme was best, but has no evidence why and his results are unlikely to be reproduced by others, because like most of us he didn't make the fuel. Gas changes from manufacture to manufacture, from batch to batch, from season to season. Because NA rotaries can practically use any commercially sold gas without worry of detonation, and there is no known gain to be had from any certain gas, buy quality gas, but the cheapest octane rating.

If your trying to get all the power you can out of your engine, try different race gases. Race gas is nice because if its worth its price, its content is much more consistent than pump gas. That means you can buy, test, and tune, then compare different fuels without worry that tomorrow the results will change. If you feel comfortable, have experience, and have access to the materials, you eventually could formulate your own race fuel that best matches your engine. Hey, and who knows you may reinvent the mix that Mazda came up with earlier.

I hope that every time I write about this it gets clearer to somebody.

Dcrasta

Well higher octane gas allows you to tune closer to the edge(higher compression more advance etc) so maybe the advantages are there but for a NA rotary you probably wont notice much difference unless you did things to take advantage of the increased octane.

joeracer

Fatboy7, very well written repy. Yes, you have helped make this subject clearer.

So, a better test would be to buy regular gas and use an octane booster on half. Dyno both and see if it makes a difference. Then do it a bunch more times to eliminate run-to-run variations. Then do that on the original fuel supply using several different brand octane boosters to make sure the octane booster isn't causing the difference. Hmmm, at $25-$50 a pull on the dyno, suddenly I'm not THAT interested in the results.

Hopefully we've convinced at least a couple of people to stop spreading the misinformation that higher octane gas hurts rotory performance. Now if I can only get people to stop writing "loose" when they actually mean "lose", I'll be happy.

-chuck-

mar3

iTrader: (8)

LMAO....that was funny...one of my sore points as well....

rotor_wizard

dis is not true, power outputs in a rotary motor does relate to octain levels,what about when u run vp racing fuel in your rx, rated at 103 oct and your car flys along like never before.....

Steve Shulz

Octane is for reducing/stopping detonation only,, nothing more nothing less. 103 Race gas is so high because most race engines are 11:1 and higher (compression equals heat which will ignite fuel - see how a diesel engine works). The higher the octane the higher compression ratio can be used without pre detonating the fuel are mixture. Race fuel is tighter controlled than what we buy at the pump so your getting what you pay for (and you will) so youll get premium power with premium ingredients at a premium price.

Steve Shulz

A lot of energy expended on this one, too bad we couldnt bottle it and put in our tanks! Wonder what the octane would be then?

excitingleopard

Oh god, not again.

I just read the whole thread, and this is my understanding...

It is agreed that octane does not affect horsepower in any considerable measure.

An illustration might be that

low octane :: short distance sprint athelete

high octane :: endurance runner

i.e., people recommend high octane fuel for racing simply because it has higher "endurance" than cheap stuff and therefore is used in boinger racers. As mentioned earlier, that's where the detonation issue comes into play.

Fatboy, that was quite informative; helped consolidate all the info everyone else mentioned.

mar3

iTrader: (8)

People, quit trying to interpret this thread. This is all you freaking need to know...

And that means you can advance spark timing w/o fear of getting ignition elsewhere in the combustion chamber. That's how you derive more power when using high octane gas.

purple82

Now that it seems you agree with my first point on the octane rating meaning, let me put the timing question to rest. If the engine has a low compression ratio, there is no advantage to advancing timing and running high octane gas. High compressionn engines can retard timing to reduce detonation, but you can't go the other way.

excitingleopard

Makes sense...

...I think it's at rest.

RippingViper

Well i use ethanol blended mohawk 90 octane fuel up here in manitoba. And right now i get about.. 450- 480 kms on 60 liters of gas.. and it seems to run alot better then with esso gas or with shell.

fatboy7

Well, gee thats the scientific proof we've all been waiting for

You point out an interesting fact: Gasoline with the same Octane, but different composition burn differently... Hmmm. Gee, maybe octane doesn't effect power directly.

RippingViper

Hmm i'm not sure, but all i know is it feels like it has alot more power when using this ethanol and it accelerates so much quicker.

cpt_gloval

iTrader: (1)

i read that in a NA rotary, you can use pretty much the lowest octane you can find... for a variety of reasons. a couple beign that 1. (with the exception of the renesis) the engines aren't incredibly high compression. 2. the dynamic nature of the rotary's combustion chamber helps to prevent detonation.

i also read that all those imsa championships were won on 80 octane fuel.

mar3

iTrader: (8)

Doesn't methanol and ethanol impart its own oxygen to the mix?

alien_rx7

Heh, I went to this school for five years. I'd say a good percentage of my friends were ME's and are also all working for the Big Three or 1st Tier suppliers now. Good school. I'd go back in a heartbeat.