The rotary engine is a unique animal, combining low compression ratios with high combustion temperatures. It also requires afuelthat can deliver lubrication to the apex seals. Since that sounds like a tall order for thefuel, we asked for product recommendations from someone who knows championship Mazda race cars: Jesse Prather of Jesse Prather Motorsports. “I will use either 89- or 91-octane pumpfuel,” he explains, citing the rotary’s low compression ratio. “Many builders will claim to see more power on even lower octane levels, but I have not foundthat.
“When you port them for more power,” he continues, “the main issue is keeping the combustion chamber oiled properly. The problem with this is that [standard] engine oil does not burn efficiently and leaves a carbon residue that can cause all sorts of problems. This is why we use racing two-stroke oil at a higherratio.”
For a turbocharged rotary, Prather recommends 91-octanefuelor higher. “For turbo use,” he adds, “it’s imperative to be aware of your timing and keep that lower for use withboost.”
“Racingfuelsin general are more refined than pump gas and will tend to burn cleaner, with less sootresidue.”
What would make the ideal rotaryfuel? An ethanol-enrichedfuelmakes more power, Prather adds, but it’s not legal under his racing regulations. A faster-burningfuelalso helps increase power. “You have to wrap your head around the fact that the combustion chamber is always moving, and the port timing makes the biggest difference inoverlap.”
Can a specialized racingfuelbe a better solution? “Combustion chamber deposits/buildup can be a death sentence for rotaries,” notes Zachary J. Santner, senior specialist of quality atSunoco. “Racingfuelsin general are more refined than pump gas and will tend to burn cleaner, with less soot residue. Our unleaded racefuelsalso contain a healthy dose of detergent to prevent depositbuildup.
“Toluene is an aromatic–double-bond ring structure-hydrocarbon that burns slower than straight or branched hydrocarbons,” Santner continues. “Our bestfuelsfitting these criteria would be Sunoco Optima, 95 octane, for non-ethanol use and Sunoco 260 GT, 100 octane, for street-legal use, where 10% ethanol helps performance and should be used if rules allow. I can’t stress enough that I didn’t pick these because of high octane: It’s all the other properties that will make a fast-burning, quick-atomization, cleanfuelthat can offer great performance while protecting theengine.”
More Fuel Tips from Grassroots Motorsports:
• What separates a race fuel from a street fuel?
• The science of high-octane fuel
• What happens to gasoline as it ages?
• Ethanol or not for your gasoline?
• Does all E85 contain 85% ethanol?
• Why does E85 run cooler and make more power?
• More oxygen for more power?
• Why not methanol?
• What's lead got to do with it?
• Picking the proper gas regarding lead and oxygen
• Narrowing the fuel field
• 91 vs. 93 octane
• Why top tier street fuels matter
• Fueling your boosted engine
• What fuel to feed a rotary engine?
• Lawn and garden equipment needs the proper fuel, too
• What is vapor pressure of gasoline and why should you care?
• What is specific gravity of gasoline and why should you care?
• Adding additives to your fuel. | Yes or no?
• What's really in that fuel additive?
• Antioxidants, fuel stabilizers, and you
• Winter fuels for wintertime
• Fuels for a post-apocalyptic world
• Need help selecting a fuel? Call the experts.
• Gas station basics that aren’t that basic
• How dirty fuel can lead to a dirty engine
• Is all premium gas the same?
• How to buy and store race fuel
• Toluene or Xylene: Legit low-buck octane boosters?
• How to make your own ethanol blend
• Do fuel additives actually work?
• Where does gasoline comes from?
• How to stretch the gas budget?
• How fresh is that fuel?
• How to handle gasoline safely and remain fire-free
• Expert tips on finding and transporting race fuel
More Fuel Facts from Classic Motorsports:
• What is race gas?
• Which race fuel is right for you?
• Buying and storing race fuel
• The right fuel in the fall might help your engine start in the spring
• Fuels for storage: good, better, best
• How much octane is enough for my classic?
• 93-octane not enough? Solutions exist.
• Can you blend fuels to get the right mix?
• Tuning for today's oxygenated fuels
• Picking the proper gas
• How to choose a quality gas station
• Why is there ethanol in our gasoline?
• Understanding additives
• Why not avgas in your car?
• Water is great for your plants, not so much for your fuel system
• Gasoline makes a terrific degreaser, yet this practice can kill you
• Are high-octane fuels more stable?
• Ideal fuels for wintertime storage
• What ever happened to leaded gasoline?
• Homebrewed, non-ethanol fuel: A potentially deadly DIY
• Was yesterday's gasoline better?
• Cars that sit need clean fuel, too
• Is all that alcohol good or bad?
• Does the octane number in your manual match the one on the pump?
• What's the right fuel for forced-induction cars?
• What's in aftermarket additives anyway?
• The perils of running rich
• Should you fill up your classic before parking for the winter?
• Do all gas stations offer the same quality of fuel?
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Comments
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Ranger50 UltimaDork
8/26/20 10:59 a.m.
What you mean it's not unicorn blood and whale blubber?
I kid...
Carbon (Forum Supporter) UltraDork
8/26/20 11:16 a.m.
Can you use castor oil premix on these?That smell might make the brappy experience irresistible to me.
MadScientistMatt PowerDork
8/26/20 1:45 p.m.
How would one do on methanol? That ought to lower combustion temperatures...
Ranger50 UltimaDork
8/26/20 2:45 p.m.
MadScientistMatt said:How would one do on methanol? That ought to lower combustion temperatures...
Only if you have double the fuel capacity to inject and port volume to accommodate....
David S. Wallens Editorial Director
8/26/20 9:19 p.m.
And I'll admit, this was one of the more challenging ones to do.
FSP33Hotruck New Reader
8/26/20 11:42 p.m.
In reply to Carbon (Forum Supporter) :
I used to build Open Mod snowcross engines for snowmobile racing, the only two premix oils that we never had crankshaft problems while running were the Ski-Doo XPS full synthetic injection oil, and Redline Two Stroke Race Oil. Personally, I think the Redline smells even better than castor oil, and burns super clean. So you can have your great smell AND clean seals at the same time!
fidelity101 (Forum Supporter) UltraDork
12/8/21 10:16 a.m.
this is why i run 89 pump, its always E10 and for non turbo it doesn't matter, hell i should even do 87
Pete. (l33t FS) GRM+ Memberand MegaDork
12/8/21 11:48 a.m.
Carbon (Forum Supporter) said:Can you use castor oil premix on these?That smell might make the brappy experience irresistible to me.
You can, but they leave disgusting thick tarry deposits that cause issues.
Mr. Peabody UltimaDork
12/8/21 1:21 p.m.
Pete. (l33t FS) said:Carbon (Forum Supporter) said:Can you use castor oil premix on these?That smell might make the brappy experience irresistible to me.
You can, but they leave disgusting thick tarry deposits that cause issues.
In that case run 927 which is a blend of synthetic and de-gummed castor.
No goo, no deposits, and none of the bad things associated with straight castor
Pete. (l33t FS) GRM+ Memberand MegaDork
12/8/21 5:39 p.m.
MadScientistMatt said:How would one do on methanol? That ought to lower combustion temperatures...
Apparently there are people making 450hp from a two rotor on methanol. Naturally aspirated.
Nitpick. Rotaries havelowcombustion temperatures. That us why they are naturally low in NOx production. They have high EGTs because of the way the exhaust ports open, and how they can make a lot of power at relatively lean fuel ratios. Some racers would run lean of stoich at WOT!
Raszaron New Reader
12/28/22 11:07 a.m.
After going to a standalone using a EFR 7176 twin scroll turbo, I upgraded everything with all purposes towards managing heat (upgraded entire fuel system, oil system, electriconics, cooling system, etc.)!
So I also had the car flex-fuel tuned. And I did my own testing of pre-mixes with the E-85 I was running (mix suspension test, burn test, residue test) and found that only one pre-mix worked absolutely awesome with Ethanol based fuel (in my case - never below E-78). Redline makes a pre-mix for alcohol fuels, and it works. I still, 2 years later, have a sealed jar in my garage with E-85 and the Redline Alcohol pre-mix suspended in it. Everything else separated and/or left a horrible residue after burning: (AMS Oil, Idemitsu (sp?), cheap 2-cycle from Lowes, Pettit Racing, and a couple Castor based that I don't have anymore so I don't remember the brands). I ran 2oz of premix per gallon, and I had zero issues and the car was injector limited on power produced.
New owner loves the car and upgraded injectors (took the 1700 secondaries to primary, and added 2600 in secondariesand had it tuned by someone else, car is putting down nearly 500whp now flawlessly (my setup was 360whp.)
Rotary_Pilot New Reader
12/28/22 7:31 p.m.
In reply to Raszaron :
At least with regards to the solubility of E85 pre-mix, I found that Redline 2-Cycle Alcohol Oil, KlotzOriginal TechniPlate (KL-200), and MotulMicro2T all remained in solution:
Spic Racer GT40R - Page 15 - RX7Club.com - Mazda RX7 Forum
That said, would love to hear how you residual, and burn tested the sample oils.
Pete. (l33t FS) GRM+ Memberand MegaDork
12/28/22 9:29 p.m.
In reply to Rotary_Pilot :
An interesting thing I read is that anything castor-based needs to be used right away, or the castor oil will degrade. "Right away" meaning within a day of mixing. When it degrades, lubricity goes away fast, and it also leaves thick tarry deposits everywhere... sounds familiar?
Not something you premix and then forget about.
To be honest, I'd think alcohol fueled engines would be a great reason to run a MOP with an external tank so it feeds 2 stroke oil, not crankcase oil. Maybe modify one/double them up somehow so that you can increase the amount of oil injected, as Mazda really did not inject enough oil for best power even at stock power levels.
Racing Beat found that in a turbocharged engine in the power levels we are talking about here, they found best results in the 6oz/gallon range, on gasoline. That is roughly 20:1 fuel:oil. So about 15-10:1 on alcohol, unless I did my math wrong. (narrator: he did. he meant roughly 30-40:1 on alcohol)
My 270-ish horsepower bridge port 13B gets fed 50-60:1 premix, and I never see any deposits. I stop there because more oil than that makes the exhaust visible 
I also have a stock port 12A with immaculate rotor housings, a rarity nowadays, and it has a working MOP with the adjuster arm maxed out, AND I premix at about 120:1. With the working MOP, I use Motorcraft oil made for Diesels with particulate filters, because I figure it has really low ash content. And it's cheap.
j_tso Dork
12/28/22 10:54 p.m.
Pete. (l33t FS) said:Apparently there are people making 450hp from a two rotor on methanol. Naturally aspirated.
Was it this car?
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As a rotary engine enthusiast with a deep understanding of the complexities involved, it's clear that the article delves into the intricate world of fuel choices for rotary engines. The insights provided by Jesse Prather of Jesse Prather Motorsports and Zachary J. Santner, senior specialist of quality at Sunoco, showcase a practical understanding of the challenges faced by rotary engines and the need for specialized fuels.
Jesse Prather emphasizes the importance of using 89- or 91-octane pump fuel for rotary engines due to their low compression ratios. He also highlights the necessity of using racing two-stroke oil at a higher ratio when porting the engines for more power. This addresses the challenge of keeping the combustion chamber properly oiled, as standard engine oil may not burn efficiently and can leave carbon residue.
For turbocharged rotary engines, Prather recommends 91-octane fuel or higher, stressing the significance of being aware of timing and keeping it lower for use with boost. The article further discusses the advantages of racing fuels, noting that they are more refined than pump gas and tend to burn cleaner, with less soot residue.
Zachary J. Santner from Sunoco adds valuable insights, pointing out that combustion chamber deposits can be detrimental to rotaries. He recommends racing fuels for their refinement and cleanliness, specifically mentioning Sunoco Optima (95 octane) for non-ethanol use and Sunoco 260 GT (100 octane) for street-legal use with 10% ethanol.
The article explores the properties of toluene, an aromatic hydrocarbon, and its role in creating a fast-burning, quick-atomization, and clean fuel. Santner suggests that these properties contribute to great performance while protecting the engine.
Additionally, the article touches upon the use of ethanol-enriched fuel for more power, but notes that it may not be legal under certain racing regulations. The concept of a faster-burning fuel is also discussed in relation to the dynamic nature of the rotary combustion chamber.
In summary, the article provides a comprehensive guide to selecting the ideal fuel for rotary engines, considering factors such as octane rating, lubrication, combustion efficiency, and the unique challenges posed by the rotary engine design. The insights shared by experts in the field contribute to a nuanced understanding of fuel choices for rotary enthusiasts and racers alike.
