Ben 84 RX-7

My father told me he read that you can get more horsepower in a rotary engine by running low octane fuel because it burns slower and the longer combustion cycle on a rotary. What do you guys think?

Travis R

I think he's wrong. :p
Higher octane fuels burn slower, and have more resistance to detonation. So you can advance your timing (or run more boost) and make more power.
If you have no way to take advantage of it's resistance to detonation then there is no need to run high octane.

cpa7man

iTrader: (2)

My guy, and he knows his **** is telling me in a n/a rotary that the lower octane produces more power. The exception is this special oxygenated race fuel that gives you 5% more hp. If your running forced induction than that changes things.

elron90sc5speed

iTrader: (4)

Never run high octane in a NA rotary engine. They are designed to run regular and ANY detonation (aka preignition) will destroy your engine. Most likley it will break an apex seal.

C. Ludwig

iTrader: (27)

Myth #1 - Higher octane fuels burn more slowly than lower octane fuels. Completely untrue.

Myth #2 - Pre-ignition and detonation are the same thing. Again completely untrue.

rusty's_silver_bullet-7

Care to elaborate for us?

Or at least point us to a reference?

Not being a wise-***, just interested in the 2 points you have raised and educating myself on these issues....

Rusty

C. Ludwig

iTrader: (27)

Octane is purely a rating of a fuel's resistance to detonation. It means nothing in terms of a fuel's ultimate ability to produce power or how quickly it burns. I attended a VP seminar years ago at the IJSBA World Finals (jetski racing) where it was discussed in detail. NHRA Pro Stock engines use very high octane fuel but it has a very fast flame front. I would guess that the fuel used in any high RPM application would have to have a fast burn attribute, think F1. An F1 engine has a relatively large piston bore and at 18,000+ RPM the fuel will have to burn very quickly to cover the squish band. High octane and "slow burning" are not mutually exclusive.

The main difference between pre-ignition and detonation is that pre-ignition always takes place before the spark event and detonation after. There is no such thing as pre-detonation, which you will hear thrown around from time to time. Pre-ignition is just that. A hot spot in the combustion chamber (carbon deposit, small flake of metal) will ignite the air/fuel before the spark plug has a chance to. Detonation is a senario where the spark plug ignites the air/fuel but instead of the mixture producing a proper flame front it explodes (for lack of a better term) and instead of a steady push on the top of the piston the cylinder pressure instantly spikes. This is akin to striking the top of the piston with a hammer. The sonic wave can hole pistons/rotors or snap rings/apex seals.

http://www.germanmotorcars.com/Detonation.htm

rusty's_silver_bullet-7

Good info - thanks.

PureSephiroth

any engine will make more power with lower octane. But if you go too low you knock. You can make more power and not knock with higher octane BUT, if you're not knocking then there's no reason to go to higher octane. If your car doesn't knock at 92 octane or 87 octane, you will make more power with 87 octane. This is true for all engines that i've seen though. not just rotaries.

Evil Aviator

The octane rating has nothing to do with power. Most pump gas generally has the same power regardless of the octane rating, but in most cases the lower octane fuel has slightly more power, although it would probably not be noticeable. I have actually seen cases where the mid-grade pump octane fuel has less power than the premium and low octane grades of the same brand. It all depends on the composition of the fuel.

Race fuel is another story, as it almost always has more power than pump gas. Race fuel also tends to have a higher octane rating, but once again, this is not a power rating, it is an anti-knock rating.

MPM

I've read a story once about some of the IMSA RX-7 race teams of the 80's. They purposely used to much premix to lower the octane rating of the fuel. I'm assuming they had a spec fuel every team was required to use. They made more power with the lower octane rated fuel. Some of the cars were black flagged as the officials thought they were dumping oil on the track because they smoked so much.

Remember, I read this story years ago and I have no proof its true or not.

Narfle

iTrader: (16)

there was a good writeup on this in one of the big car magazines(im sorry i cant quote which) but higher octanes made better power. as i recal an m3 and a mustaqng were used fo rhte test.

rusty's_silver_bullet-7

I wonder how much of that was due to the cars retarding their timing advance on the low octane gas - in response to knock.....

PureSephiroth

must have been all of it because cars don't make more horsepower because of higher octane. it just doesn't happen.

speedturn

iTrader: (1)

Ludwig said "Octane is purely a rating of a fuel's resistance to detonation. It means nothing in terms of a fuel's ultimate ability to produce power or how quickly it burns. I attended a VP seminar years ago at the IJSBA World Finals (jetski racing) where it was discussed in detail. NHRA Pro Stock engines use very high octane fuel but it has a very fast flame front. "

Yes, octane is purely a measure of a fuel's resistance to detonation. Yes, special race fuels can be formulated to burn fast and have detonation resistance, BUT for 99% of the pump gasolines out there that rotary people buy, the higher octane fuels will burn slower than the lower octane fuel, and not improve N/A rotary power. A race car is tuned (both spark timing and jetting) to burn a particular brand of race gas. If you make the slightest change to the brand and type of fuel, then the timing and jetting must be retuned again on a dyno.

For street driven N/A rotaries that will be buying pump gas, a good quality 87 octane Amoco or Chevron gas will provide maximum power.

Running more spark advance does not always make more power. Combustion before TDC will produce negative work on the face of the rotor; loosing power. Since it takes a fuel a finite amount of time to burn, then we start combustion before TDC, so that most of it will be burned at making big pressure in the combustion chamber just after TDC. This is why older Porsches will convert to 2 spark plugs per combustion chamber, so they can speed up the combustion process and reduce their spark advance and make more power. Optimum spark timing is arrived at experimentally on a dyno based on the fuel (and a few other factors) to be used. The dyno operator will advance spark timing a degree or 2 at a time until the engine starts loosing power, then they will usually retard it 2 degrees just for engine safety.

The story about the IMSA driver running large amouts of oil to lower his octane further is true. You can read about it in Jim Downing's book "How to Modify your RX-7." This highly sought after book is out of print, and usually goes for $60 to $120 when one goes thru ebay. Jim won IMSA championships using low octane fuel in his rotary, while his loosing competitors were using high octane fuel in their rotaries.

I was running a vintage race at Watkins Glen in 1998, and a then current IMSA RX-7 competitor was there to get some practice time in on his car, a 1993 tube frame, naturally aspirated 13B PP with an $8000 flat slide injection setup. He said he made 385 hp with low octane fuel in his engine, but IMSA specs that year made everyone run the same 110 octane race gas, and he said he lost power with the mandatory 110 octane race gas.

Oxygenated fuels are a whole 'nother discussion. That is something entirely different subject than this one where we are talking strictly about octane.

I run Amoco pump gas + 100:1 oil in my naturally aspirated 13B PP race motor.

Unless you can spend the dyno time to tune your motor for your experimental fuel, it is best to stick to the exact combinations that the winners use.

speedturn

iTrader: (1)

See Colemans post on "How to time a BP 12A motor"

https://www.rx7club.com/race-car-tech-103/timing-bridgeport-357892/

He says he has many years of success with low octane fuel and 25 degrees advance on his bridgeport race motors.

I have had 7 years of success with low octane fuel on my peripheral port race motors,
and 14 years of success with low octane fuel on my street port daily driver street car motor, and as far back as 1983 I ran low octane fuel in my stock port motor.

dallen

I remember what you are talking about from my old rotary/SCCA days. The mixture wasn't for a track spec or even for power. The racing carbs we were using didn't have oil injection lines. (oil is normally injected into the carb/throttle body to lubricate the rotor seals.) To insure proper lubrication, we premixed our fuel with a bit of oil (IIRC I used about 2 oz per gallon) ala 2 cycle engines. If you put a little too much oil in the mixture, you left a smoke screen on acceleration, as the gas would tend to boil out a bit on deceleration, leaving a mixture richer in oil content in the carbs. No noticable power loss, but nasty looking.
BTW: If anyone is old enough to remember the 3 cyl SAABs, they were 2 cycle motors that always smoked when using a racing blend.
We also had a backfire on deceleration problem when running the timing forward for racing that was much worse than the old street set ups. Pop your foot off the throttle to brake for a turn and it sounded like the car had blown up.(If you had a driver not familiar with rotaries behind you when it happened and you could see them back off really fast. lol) That was why the old rotaries had a small pipe that bypassed the muffler via a check valve. Mazda didn't want customers blowing out the baffles in the mufflers. That went away I believe due to some extra valving in the newer electronic engines. At least I never had the problem from my "4" on. None of my 13bs nor my 79 "7" with its 12a backfired.