vipernicus42

iTrader: (4)

I see in the forum sometimes that lower octane is better for your rex than higher... is this true? a sports car that actually WANTS cheaper gas? with gas prices hitting 84.9c/L this morning I'm saying my halliluyas(sp?)

Apparently 94 octane fuel adds to carbon buildup... I just wanna know if this is true, and if it's absolutely fine to run 87 (the lowest I can get) in my rex w/o killing performance.

Narcisse91

The best thing to do with any car is to use what the manual recommends. If you go lower, you can damage the engine, if you go higher, the car may run less efficiently and carbon will build up. It'll also probably smell worse.

That changes if your RX-7 is modified. You may need higher octane, depending what you've done, particularly with forced induction.

mar3

iTrader: (8)

Actually, the lower octane is absolutely fine. Performance gains or losses are not what octane is all about. The octane number just represents that fuel's ability to resist pre-ignition in a combustion chamber. Higher octane means it has higher resistance to pre-ignition and detonation. In a new car with a computer onboard that can change timing, you will notice a performance difference because the ECU will retard timing if it senses spark knock. A car designed for 93 octane has an ignition map in its ECU that optimizes engine performance assuming you will put that in every time. That's when it will run its rated power. Go lower octane and the possibility is now open that you will get pre-ignition. The ECU compensates for that and you feel a loss of power. This is not true for 1stgens that are carbureted, so you shouldn't notice anything except that your wallet doesn't get as light as fast...With a combustion chamber that is not fixed, but sweeping past the plugs, igniting the flame front becomes difficult. Anything you can do to start that igniton easier makes the 12A rotary more efficient in terms of fuel consumption. Lower octane (87) does that for you. I don't know the bottom limit before you get spark knock in a rotary, but it doesn't matter since most gas stations go only as low as 87, which is completely safe...

I'll shut up now...

fatboy7

Go with the 87... if everything else is in working order, and the gas itself isn't just a pile of crap (read some 87 is better quality than some 94), it will perform as well or better than the expensive stuff.

MarkPerez

thats right. high octane no good for rotory. no pistons,
no valve train, no valve seats, no cams, no octane,,

onepointone

i did a poll a while back because i was wondering the same thing. someone mentioned that the 787B and alot of the IMSA cars had to bring their own "race gas" to the track because the track didnt have such low number octane (87 octane).

pretty much what i got out of it, is that the lower the octane, the longer it takes to burn, which pushes on the apex of the rotor longer, giving you a better combustion in the housing. but if you are running any form of boost, you need high octane. correct me if im wrong, but thast what i got out of the poll i started a while back.

SoRRoW

NOT true..
N/A rotaries work better on lower octane 87,It burns better

Boosted rotaries need higher octane 93

When you going to be ready for my crap Mark?

BTW vipernicus42 use the search next time...

pillage6

My car gets crap mileage when I use high octane.

I think that is true boosted or not boosted.

Narcisse91

Yeah, but turbocharged engines are more likely to detonate, and the extra help avoiding that with higher octane makes it worth it. With a turbocharged engine, you get more performance, since you can run more boost and more advance without detonation, or you can run the same boost and same advance more reliably.

WackyRotary

Just remember when someone says you will get more power with high octane, let them be. And, you will know this:

gasoline 87octane = 114,000-118,000btu's/gallon
gasoline 90octane = 114,000-118,000btu's/gallon
gasoline 93octane = 114,000-118,000btu's/gallon

The variation in btu content will be the same in a particular area, with slightly more or less of the ingredents that effect the overall octane rating.

As for what we n/a rotaries need, 87 is fine. If you produce higher pre-ignition compression pressures, like with forced induction on a rotary, then the octane requirement increases. Piston engines are different, with greater variation in compression ratio's. The higher the ratio, typically needs higher octane to resist pre-ignition.

fatboy7

true, very true. Just remember also, its not the energy in the gas/gal, but how it is used to move the car. Alcohol has a btu/gal about 1/2 that of gas, but their can be plenty of power made in the engine with the right mixture and spark. Mar3 in this respect said it well, our cars won't make the most use of higher octane fuel, because they aren't built and tuned to use it. Also, there are more burn characteristics than just a knock number. How it burns in your engine is what detirmines the best gas. It just turns out that mazda's propriatary race-fuel had a low (83 if I remember hearing correctly) octane number.

jeremy

iTrader: (1)

good od this comes up often.

octane = detonation resistance

causes of detonation: boost, high compression, advance timing, a sharp metal bur etc that wouls hold high heat and become an ignitor.

octane does not equal performance. unless you are running mad timing you don't need high octane gas. it will hurt performance and gas mileage. it is harder to ignite and keep lit.

a rotary is rather detonation resistant due to its flat open chamber. i believe the mazda rx7 race teams ran 80 octane.

when in doubt about octane compatibility, atf it. then check your blinker fluid. adjust flux capacitor to 1.21 jigawatts.

WackyRotary

Alcohol has ~61,000btu's/gallon. And your right the a/f ratio varies when you go to different fuels. Here in minnesota, all gasoline sold(except select stations pumps have race gas-pure gasoline at 119,000btu's/gallon) otherwise all have a 10% alcohol added. Our btu/gallon averages about 112,000-114,000btu's/gallon in MN depending whether its winter or summer. Minnesota and some adjcent states sell E-85(85% alcohol, 15%gas) that is used in FFV's(flex fuel vehicles), a free option offered on some Ford Torus, caravan, some hondas, and several others. ITs btu content is 81,000btu's/gallon. Optimal fuel ratio of ~9.2:1 range. Gas would be the 12.5:1 for power, and 14.7:1 stocometric.
E85 has a octane rating of 110octane about, so higher compresion is best to get the most efficient use out of the fuel.

The reason the fuel is made is because of all the ethanol being subsidied here, and runs cleaner, and sort of reduces dependence on foreign oil in theory-it takes energy to produce the fuel, but slightly less then what you get out.