rx7fcfc

iTrader: (1)

i been using 93 octane on my car and it seem that it runs better and get better mpgs has anyone else notice the same?

my car is 79 with 5sp and with electronic dist (fb)

BrGreenSA

iTrader: (1)

i never noticed a difference in my car after switching to 87 from 91, i could be wrong but i think they say high octane won't make a difference unless you are boosted in a rotary....

DreamInRotary

iTrader: (49)

There is a difference when you are boosted. On lower octane fuels and ethanol a rotary with a turbo, or the few with a supercharger, run better and achieve more power. As to why that happens I cannot tell you - I have no experience with it.

As far as the octane thing, I have a 12a and when I've run higher octane I tend to get worse gas mileage - but that could be just the way I drive it since it does run better and spools faster

rx7fcfc

iTrader: (1)

maybe my timing might need to be adjusted better

RotaryEvolution

iTrader: (30)

higher octane fuel will get you better fuel mileage and keep the engine internals cleaner but the power increase is just in your head.

assuming you mean higher octane, not lower. higher octane allows more boost before hitting engine knock. more boost = more power. you can run lower octane fuels in turbo engines but you are limited in boost, generally to the point of being below the real efficiency range of most any turbo.

elwood

iTrader: (1)

[quote=RotaryEvolution;11323673]higher octane fuel will get you better fuel mileage and keep the engine internals cleaner but the power increase is just in your head.
. . . /quote]

I have no direct knowledge of the effect on rotary engines, but here's the story for piston engines:

Higher octane fuel is more resistant to detonation, but it generally burns slower and has slightly less energy. In some OEM setups that are tuned for higher octane (whether NA or turbo), the ECU is tuned with advanced timing to achieve better power and economy when using higher octane fuel. In those setups, it will retard timing if it senses knock with lower octane fuel, which will hurt power and ecomony slightly.

In a system that doesn't have those capabilities, lower octane fuel usually yields better power and economy.

If you notice the ads for high octance gasoline, they are now forced to caveat the performance claims by adding " . . . for those vehicles that benefit from higher octane fuel." Vehicles tuned for high octane fuel will specify this in the owners manual and on the fuel fill cap by saying "XX Octane Fuel Recommended." If your car doesn't say this, you're generally best running the lower octane stuff.

peejay

I run 91-92 in my N/A. I don't have electronic ignition control, so I'm limited in what I can do with ignition timing. I have 25 degrees total advance, all in by something like 2000rpm. I can cruise just fine on 87 but under full throttle it doesn't feel very happy. Not like detonation, but more like detonation onset. Engine is much much happier with the high octane fuel, and it overall feels better than running less timing and lower octane fuel.

YMMV of course. I used to run way more timing with a stockport 12A on 87 octane, but that was also an engine that made about half the power and maybe 70% of the torque, so chamber filling (and therefore pressures) was a lot lower.

j9fd3s

iTrader: (3)

i run 91 in my P port, just for the added detonation margin, it ran the same on 87 and 89. my timing is locked too, @18 degrees and it does ping around 1800-2000rpm, just like the Rx8

chamber filling indeed!

ive been running 87 in the Rx8... but the engine is tired

aa35199

iTrader: (1)

In America and canada premium and regular gasolines and brands have different amounts of ethanol blended into them, despite the octane boost provided by ethanol you will typically find less or sometimes none in premium gasolines. Less ethanol and more gas equals more MPG, premium gas is blended with a higher detergent content, and our cars were designed to run 100% gasoline, but the jury's still out on whether or not ethanol blends makes a stock rx7 run different. Ethanol also attracts moisture from the atmosphere, this is why E85 conversions are not too popular with rarely driven garage queens since you have to be concerned with water contamination. At the moment my 9.5:1 compression edelbrock fed cast iron v8 doesn't seem to care, I run 36 degrees of mechanical advance, about 50 degrees of vacuum adv. at idle, no knock or any other problems with 87, so thats what I run, because I'm a cheapo. Every rotary car I've owned was NA and got premix 87 as well, BTW did you guys know 2 stroke oil lowers octane rating? not by much with the ratio's we run but a 50:1 mix lowers octane by 2 points according to Rotax. The folks at Rotax also mention that alcohol reduces lubricating abilities of gasoline and 2 stroke oil so if your regular gas contains more ethanol it could possibly accelerate apex seal and housing wear by a small degree. Lastly there is the fact that ethanol doesn't vaporize as easily as gasoline, Brazil's alky cars typically have a a reserve tank of gasoline or some type of intake heater for cold starts to get the engine to operating temp before the alcohol can take over, granted 5-15% shouldn't make a noticable difference, but you might notice a slightly easier start with a lower blended premium gas.

Link the the fuel and premix info on Rotax's website
http://www.rotaxservice.com/rotax_tips/rotax_feed2.htm

peejay

Ohio's been on 10% since at LEAST the mid 80s, so all of my RX-7s have lived on a diet of nothing-but during my time of ownership. Never noticed an issue.

The premium here is also 10%. At least, my EFI runs open-loop all of the time and I don't notice a lambda change when going from one fuel to another.

I CAN attest that some states just have shitty fuel composition. When driving to Oklahoma this summer, I had to stop for fuel in Illinois, which is in a different fuel blend zone than Ohio. Wow did my car not like that fuel. Light load misfire to the max. I put up with it until the tank was almost completely empty, then filled up in Missouri. Ran great again. The car liked Oklahoma fuel too, but I used my spare can of Ohio fuel during competition just in case.

Interestingly, a friend of mine drove his E85-fed WRX from Fort Wayne, and he noted poor drivability and frequent knock after filling up in Illinois that went away after emptying that tank. Coincidence?

elwood

iTrader: (1)

Good point about the ethanol content in lower octane fuel.

I know a calibrator for a major OEM, and I asked him a while ago which was the cheaper route to go in a vehicle calibrated for premium fuel -- pay for the higher octane fuel and get the improved economy or save on the price of fuel and suffer a fuel economy loss.

His answer was that in most cases in the US (based on our fuel quality and price), it was more economical to run the lower octane stuff. The minimal economy loss was more than offset by the cheaper fuel.

But that was about 10 years ago, and a lot has changed since then.

. . . and this wasn't the question asked by the OP anyway . . .

OneRotor

iTrader: (3)

Octane is resistance to detonation, and that's it. If you're using the same brand of fuel, 87 vs. 93 won't make any difference in a low-comp. motor, rotary or piston. A rotary should make slightly more power on 87 than 93, because 87 burns slightly faster (faster moving flame front). This is why the IMSA GTU RX-7's ran a custom blended fuel that was (IIRC) 70 octane.

peejay

That's highly dependent on the fuel price split and the particular car.

I experimented with my VW Golf. 1.8 8v engine, 10:1 compression, EFI with knock sensing ignition control. On 87 octane it would get 35mpg. On 92 octane it would get over 40mpg. The breakpoint was something like $2.50 - if gasoline was under $2.50 then it was cheaper to run 87.

At the time, gasoline was $4.50 for 87, $4.80 for 92. Neglible difference in price for a dramatic gain. Golf lived on 92 or 93 if I could get it. The Sentra I had before it (1.6l 12-valve, unsure if knock sensing) was completely insensitive to octane so it lived on the cheap swill.

But as they say, "your mileage may vary". When people ask me what fuel to use in their car, I tell them that if their car says premium only, then use premium, but if not, try it anyway and log fuel economy.

The Ford Ecoboost-powered F150s will run happily on 87 octane, but if heavily loaded, they will get better fuel economy on 91-92. Numbers thrown about were 17-18mpg on 87 and 22+ on premium.

peejay

Not always. Race fuels will often burn faster than pump fuels. If it burns more quickly, it has less time to detonate. (Engine wrecking detonation only happens at low RPM, after all - at high RPM it doesn't have time to occur)

The story I heard about the IMSA teams is that they were running high octane race fuels "cut" with kerosene. Sounds like a clever bit of cheating, myself Kerosene is very hard to ignite yet high in energy and detonation prone, while race fuels' higher aromatics make it super easy to ignite.

In the end, any given engine under any given load and a given fuel's burn rate has an optimal ignition timing. More or less timing than that leads to less power. The trick is to design the engine (and fuel ) so that its optimal timing is as retarded as possible, since that reduces negative work. (This is irrespective of detonation - I'm assuming that the fuel used is sufficient to allow optimal timing and that the engine is not detonation-limited)

RotaryEvolution

iTrader: (30)

it also depends on the pricing, it's fairly rare nowadays to find premium gas to be only 20 cents more expensive. only a few years ago it was standard for fuel to go up 10 cents per 2 octane points.

don't get me started on diesel, it makes no sense or reasoning why it is more expensive than gasoline since it is easier to produce.

ethanol and methanol also used to be less than a dollar per gallon when i was getting into the rotary game 10 years ago, now they are about $5 a gallon after the flex fuel vehicle boom. gas back then was about $2.50 per gallon/avg, basically more than tripling it's value including inflation.

all of the taxis here in las vegas have to be a hybrid of sort. gas/hybrid, propane and E85. makes it nice though, since there are E85 filling stations all over BUT i would say the ethanol content is actually quite low because of the demand here.