Fuel Tips: What to Feed a Rotary Engine?

By Staff Writer
Mar 23, 2021 | Sunoco, Fuel Tips, rotary, Sponsored Content, fuel | Posted in News and Notes | From the Aug. 2020 issue | Never miss an article

Sponsored article presented by Sunoco Race Fuels


The rotary engine is a unique animal, combining low compression ratios with high combustion temperatures. It also requires a fuel that can deliver lubrication to the apex seals. Since that sounds like a tall order for the fuel, 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 pump fuel,” 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 found that.

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 higher ratio.”

For a turbocharged rotary, Prather recommends 91-octane fuel or higher. “For turbo use,” he adds, “it’s imperative to be aware of your timing and keep that lower for use with boost.”

Racing fuels in general are more refined than pump gas and will tend to burn cleaner, with less soot residue.”

What would make the ideal rotary fuel? An ethanol-enriched fuel makes more power, Prather adds, but it’s not legal under his racing regulations. A faster-burning fuel also 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 in overlap.”

Can a specialized racing fuel be a better solution? “Combustion chamber deposits/buildup can be a death sentence for rotaries,” notes Zachary J. Santner, technical specialist with Sunoco Race Fuels. “Racing fuels in general are more refined than pump gas and will tend to burn cleaner, with less soot residue. Our unleaded race fuels also contain a healthy dose of detergent to prevent deposit buildup.

Toluene is an aromatic–double-bond ring structure-hydrocarbon that burns slower than straight or branched hydrocarbons,” Santner continues. “Our best fuels fitting 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, clean fuel that can offer great performance while protecting the engine.”

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View comments on the GRM forums
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)
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...

ShawnG UltimaDork
8/26/20 2:02 p.m.



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
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!

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