Octane | GTAMotorcycle.com

Octane

MacDoc

MacDoc

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This is a good read for anyone interested in the early days of aviation. Very well written and rocks along. I've read accounts of Lindbergh's flight but this was engaging, "you are there" feel and yet brief and I learned new things. Doolittle and Rickenbacker were known but not in any detail so really enjoying it.
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Something that stood out tho ....

While he was in England, Jimmy had made a determined pitch to the Royal Air Force, pointing out the extra power boost its planes—especially the defensive fighters—would get by converting to 100 octane gas. He showed British engineers that a 1,000-horsepower-rated Merlin fighter engine would produce 1,700 horsepower when fueled with 100 octane gasoline. This gave the RAF an enormous edge during the Battle of Britain, when its Hurricanes and Spitfires could develop much higher manifold pressure and outclimb and outrace their German counterparts, which used only 87 octane fuel.8
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Because Doolittle’s words did not fall on deaf ears, by the middle of 1940 all RAF fighters had begun to use 100 octane fuel, and after the war the British petroleum secretary said of the conversion: “This octane was thirteen points higher than the fuel used by German aircraft. Those extra thirteen points ended the threat of any Nazi invasion of England.”9
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Engineers made an amazing discovery: using 100 octane fuel would increase power even in existing engines up to 30 percent, and that with high-compression engines the higher-grade fuel would get up to 15 percent in fuel savings.
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Does this still apply to modern engines ?? I notice BP100 is still advertised as available and seems to be a "thing" in racing.
🍿
 
100 octane race gas in a Toyota Echo that normally makes 108hp = a Toyota Echo that makes marginally less than 108hp due to less energy in fuel.

Regular 87 in the average GTAM baller track bike tuned to use race gas = a bike that runs like crap and eventually becomes permanently sad due to predetonation.

You can slightly advance ignition timing if you use premium fuel in an engine that was not originally designed for it, and gain a little extra power. You can more than slightly retard ignition timing in a high compression engine to use regular fuel instead of premium, but the engine will be less efficient and/or powerful than if it had been designed to run on regular in the first place. All other things being equal, the benefit of premium fuel is being able to design an engine with a higher compression ratio than you could otherwise, and you get more power for the same displacement.
 
10% ethanol = 3% reduction in fuel range.
Unless a motor has specific changes to design requirements going to a higher octane fuel is a waste of money.
 
Thanks
So is there a fundamental difference between the modern engines and WWII aircraft engines? :unsure:
 
MacDoc said:
Thanks
So is there a fundamental difference between the modern engines and WWII aircraft engines? :unsure:
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Octane is simply a measure of resistance to detonation, so higher octane fuel doesn't inherently have more energy. What it does do is allow for the use of higher states of tune (compression, ignition timing, etc.) in engines without blowing up, which can then produce more power than a lower octane equivalent.

I suspect much of the above is leaving out changes in piston design and ignition timing to take full advantage of the higher octane fuel. A 70% jump is a huge amount.

That said, Spitfires and other Merlin powered aircraft offered a lot more control over fuel and engine management than a typical motorcycle, allowing short bursts of high power that would blow the engine if used too long. If I remember correctly from flight simulators, aside from rad flow, rpm/prop pitch and throttle, you can also adjust both the fuel mixture and supercharger boost from the cockpit, which I think could push an engine into detonation later with a higher octane gas. Add that the above controls were also there to accommodate for altitude differences (higher altitude needs more supercharger boost), and the comparisons to motorcycles gets even more distant...
 
100 octane low lead is available at almost every airport in the country. Some small planes are certified to run 87 octane as well. They are down a bit on power compared to an engine designed to run 100 only but cheaper to fill up. Fwiw, aircraft engines are relatively huge displacement, low rpm, low specific power which is the opposite of modern road engines. 540 cu in aircraft engine is ~250-350 hp @~2700 rpm. On a road engine of similar displacement, I'd expect ~double the rpm and double the hp.

Many planes are still quite manual (manual control of mixture, watch cht's, watch manifold pressure, some planes have adjustable supercharger as Priller said, I don't think any have manually adjustable ignition timing as it would be too easy to screw yourself).
 
Somewhat related info - Gen7 Fuel in Roseneath now has Marine-Premium 91 non-eth and it's 20c cheaper than 91 off the Res. Also have 93 with eth. I do see the mileage improvement when using the non-eth version.
 
Without changing anything, as others noted, more octane does not mean more power if the motor is not set up for it.

Higher octane rating does many things.... It allows for more timing and more importantly more compression. The Merlin engines ran superchargers, more boost is just like more compression and means way more power. Higher octane meant they could increase "manifold" pressure (increase boost) which increased power. The Germans running 87 octane just could not run as much boost...

If your bike (or car) is boosted (super or turbo) you can increase boost via hardware and/or software (many refer to people that do this as tuners) and you can increase the power like in the book going up to 100 octane avgas. But the downside will be fuel cost and possibly the inability to run pump gas. In the olden hot rod days we ran higher octane NA with higher top pistons and/or smaller combustion chambers to increase compression, no boost.

Another caveat, some pump brands increase octane by increasing ethanol content which has less energy per volume...
 
Good stuff guys . (y) ..I was startled by the claim but makes more sense now with the ability to make changes from the cockpit and the super charger aspect.
 
No
Octane controls pre ignition or knock
The low octane crappy heavy fuel they WERE using was prone to pre ignition. At the time the Brits had "PETROL", while the Americans had "GAS" (which was better/cleaner than petrol) and high test "ETHYL" (Which was 100 octane AND higher in ethyl... the burny stuff)
Using higher octane cleaner fuel, they could optimize ignition timing and JAM more fuel in... and take more power out... then increase the compression... then add another super charger. They ended up tripling the power output of that motor.
There is a finite amount of power you can take out of a motor and modern electronic controlled motors are state of the art (built to a price). Most modern motors have at least one "knock" or pre ignition sensor, and the ECU will crank the timing up till it knocks, then back it off a couple of degrees. That is how we get "multi" or flex fuel motors.
A modern motor will run on kerosene if you can get it started (Kerosene is very low octane).
Pre ignition is caused by compression in a well designed motor. There are lots of motors that are known for pre ignition (and require "premium" fuel) because of bad head or piston design
To a point, you NEED compression to pull power: more compression=better burn = more power
... so this story is kinda disingenuous. They didn't get any REAL power increase by pouring different fuel in the plane.
MORE higher octane fuel + less timing = a little more power
+ compression (new pistons, shave the head or block) = lots more power
+ a second super charger (which is the same as more compression) = TONS more
All stuff they couldn't do with the fuel they were using, but they could using the Yank's HighTest Ethyl

Fuel only allows you to realize the motor design potential
So to answer your question: YES... if you redesign a modern motor to the point where it it NEEDED 100 octane fuel, it would make more power... but NOT MUCH more. Modern designers aren't leaving much, if any, on the table.
There are more than a couple modern 5 - 7 litre car motors that put out more power (on pump gas) than a 12 cylinder, 27 litre, supercharged Merlin

... I see "TIMING" being mentioned repeatedly... on modern (non ethanol) fuels you want to run about 32 degrees BTDC on a hemi and 34 on anything else. If you're not making HP with those timing numbers, you're doing something wrong. Cranking the timing up went out with domed pistons in the '80s. Flat top pistons and a swirl combustion chamber is where it's at in 2022.
... and octane doesn't SLOW the burn, it reduces pre-ignition. A fuel that has a higher specific gravity will burn slower. The Yank fuel had a lower specific gravity, so would burn faster, reaching maximum cylinder pressure faster. (What you're aiming for is maximum cylinder pressure at 11 degrees after TDC for torque and 18 degrees after for HP... and with modern fuel 34 degrees of advance SHOULD get you in the middle of those numbers)
... and if someone is telling you they HAVE to run leaded fuel: they're an idiot. We figured out how to make power on un-leaded 35 years ago. There are lots of planes still using leaded fuel... because to use a fuel in aviation it has to be certified for EACH engine/plane, and it's cheaper to fly with leaded fuel, making children MORE stupid, than to get the engine certified for un-leaded... and there is less "power" in 100 octane avgas than 101 octane C12 (more ethyl in the C12, less ethyl, more emulsifiers in avgas)
 
Have a mate back home that builds supercharged bmw engines. He uses the lowest octane fuel he can without the engine pinging (knocking). Usually 91 Ron with as much ethanol as possible, which is about 83 octane in north America.

Octane rating is not the be all and end all for fuel either. Most track bikes I've had, have made a lot more power on MR12 87 octane then on U4.4 103 octane with engines tuned/built for the specific fuel.
 
If you want to up the ante (and shorten the fuse) dump something like toluene (dangerous) or nitromethane (very dangerous) in the tank.
They make their own oxygen, so compensate your jetting accordingly.
The drag racers call it 'tipping the can.'
 
TK4 said:
If you want to up the ante (and shorten the fuse) dump something like toluene (dangerous) or nitromethane (very dangerous) in the tank.
They make their own oxygen, so compensate your jetting accordingly.
The drag racers call it 'tipping the can.'
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A buddy and I tuned his sled to run a Toluene blend. Pretty fun project and increased top speed by a decent amount (>30 km/h). Pain in the ass only being able to fill at home though. Had to pay close attention to EGT's but we ran it that way for a winter and didn't blow it up. Tuned for pump gas again as it is more than fast enough already and toluene is expensive.
 
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Gas turbines and diesel motors are also based on ICE technology. These motors are not as sensitive to octane ratings as spark-ignition motors.
Please can anyone share their insight on this?
TY!
 
Chris-CJ said:
Gas turbines and diesel motors are also based on ICE technology. These motors are not as sensitive to octane ratings as spark-ignition motors.
Please can anyone share their insight on this?
TY!
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Diesel cares about cetane not octane. The short version of why diesel and turbines don't care about octane is the fuel isn't added until it is time to burn. You can't have detonation if there is no fuel.
 
and a diesel is only ignited by detonation ...not a spark...but maybe the that clatter when a diesel starts is early denotation :unsure:
But maybe the glow plug is the way it gets running until compression ignition kicks in... 🤪
 
Doing a little vehicle window shopping. Mazda 2.5 turbo motor 250 hp on 93 oct. 227 hp on 87 oct

”Connected and engaging driving dynamics mean road trips are no longer just for special occasions. Each experience in the 2021 Mazda3 Turbo feels more pronounced and spirited. The Skyactiv-G 2.5 T engine will deliver an impressive 250 horsepower and 320 lb-ft of torque with premium (93 octane) fuel or a solid 227 horsepower and 310 lb-ft of torque with regular (87 octane) fuel. This engine is specifically calibrated for the Mazda3 to deliver the unique driving dynamics expected by our most passionate drivers. The evolution of Mazda’s turbo engine brings a sense of refinement befitting the brand’s recent updates to the i-Activ all-wheel drive (AWD) system. The high torque characteristic of the Skyactiv-G 2.5 T paired with the predictive i-Activ AWD create harmony with the driver’s intentions – delivering greater responsiveness and confidence”
 
GVH said:
Doing a little vehicle window shopping. Mazda 2.5 turbo motor 250 hp on 93 oct. 227 hp on 87 oct

”Connected and engaging driving dynamics mean road trips are no longer just for special occasions. Each experience in the 2021 Mazda3 Turbo feels more pronounced and spirited. The Skyactiv-G 2.5 T engine will deliver an impressive 250 horsepower and 320 lb-ft of torque with premium (93 octane) fuel or a solid 227 horsepower and 310 lb-ft of torque with regular (87 octane) fuel. This engine is specifically calibrated for the Mazda3 to deliver the unique driving dynamics expected by our most passionate drivers. The evolution of Mazda’s turbo engine brings a sense of refinement befitting the brand’s recent updates to the i-Activ all-wheel drive (AWD) system. The high torque characteristic of the Skyactiv-G 2.5 T paired with the predictive i-Activ AWD create harmony with the driver’s intentions – delivering greater responsiveness and confidence”
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I was curious about this on my 2.5 NA, it's still a relatively high compression engine, I'm wondering if the engine can take advantage of the higher octane fuel. A few places seem to hotly debate (as is tradition) the benefits of higher octane, and I'm thinking of just trying it out and logging my results. A bit of a tangent away from motorcycling but still interesting.

Thread on an external forum that dives into this topic: bobistheoilguy.com
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