Does it matter what fuel is used to reach an octane level?

[TalkToTheFiST]

So for example, tuning e30 is so we can get away with higher boost, we need the octane of the ethanol. But too much ethanol and we lose power even with more boost, bc it has less yeild and we have limited fuel volume. So, how does race fuel fair? If an e30 mix puts us at maybe 98 octane, and you had some "race" fuel at 98 octane but with greater power yeild, would that be a bad idea? Can we swap in fuels (not methanol) solely based on octane rating?

 

[Dialcaliper]

Octane is just a measure of knock resistance - there are many other properties that are different between fuels - energy density, heat of vaporization, specific heat and “oxygenation”, etc.

The “pump gas” you get actually varies a lot in terms of the hydrocarbon content, blends, seasons, etc, but because of tight regulations on certain properties (not just octane) your engine will usually operate safely but things like gas mileage (energy content) will vary.

But once you deviate from gasoline to high ethanol blends, methanol, nitromethane, etc, the fuels are as different as diesel and gasoline, maybe moreso. Air fuel ratios are completely different because the above fuels all contain some of their own oxygen, so less air is required to burn. Or alternately, since airflow is the limiting factor, you can run more fuel and make more power off the same amount of air. You also kind of have to run more fuel in order to stay in the range of flammability.

The tuning approaches of “race gas” and ethanol are also slightly different - high octane race gas just lets you gain power by advancing timing closer to the “ideal” point where peak cylinder pressure can be reached without running into detonation. Ethanol also lets you run more timing by virtue of high octane rating, but because it’s oxygenated, you can dump more fuel in. Even though ethanol is less energy dense (and so results in lower gas mileage), the additional fuel means more total energy release, and more power.

In other words, E40, which is ~98 octane will, if properly tuned with the extra fuel flow required be capable of higher power output than similar 100 octane race gas, because there’s more total hydrogen, carbon and oxygen flowing through the engine when given the same inlet airflow, in exactly the same way that a nitrous fogger with additional fuel would

This is taken to an extreme in top fuel and nitromethane burning dragster/funny car engines, where instead of the 14.7:1 air-fuel ratio of gasoline, because of the extra free oxygen, you get AFRs:

Ethanol: 9.0:1
Methanol: 6.5:1
Nitromethane: 1.7:1

Other words, nitromethane is a large part of the way to dumping massive quantities of rocket fuel containing its own oxidizer into your engine, supplemented by additional air.

As long as you are just dealing with gasoline equivalent blends of various 6-8 chain hydrocarbons, octane rating is the primary limiting factor in approaching ideal peak cylinder pressure, but once you throw oxygenated fuels into the mix, or fuels that require a drastically different AFR (like propane or hydrogen), you are no longer looking at the same thing.

Ultimately, your engine power is limited by how much air you can stuff into it, and then by the quantity of fuel you can burn with that air, unless you run into a volumetric limitation on fueling, a knock threshold that prevents reaching peak cylinder pressure or a mechanical strength limitation because you bend a con-rod. Which is how 80’s formula one cars managed as high as 1400hp out of the 1.5 liter displacement - more air (by running 50-80 psi boost) and more fuel

 

[the duke]

Octane is just a measure of knock resistance - there are many other properties that are different between fuels - energy density, heat of vaporization, specific heat and “oxygenation”.

The “pump gas” you get actually varies a lot in terms of the hydrocarbon content, blends, seasons, etc, but because of tight regulations on certain properties (not just octane) your engine will usually operate safely but things like gas mileage (energy content) will vary.

But once you deviate from gasoline to high ethanol blends, methanol, nitromethane, etc, the fuels are as different as diesel and gasoline, maybe moreso. Air fuel ratios are completely different because the above fuels all contain some of their own oxygen, so less air is required to burn. Or alternately, since airflow is the limiting factor, you can run more fuel and make more power off the same amount of air. You also kind of have to run more fuel in order to stay in the range of flammability.

The tuning approaches of “race gas” and ethanol are also slightly different - high octane race gas just lets you gain power by advancing timing closer to the “ideal” point where peak cylinder pressure can be reached without running into detonation. Ethanol also lets you run more timing by virtue of high octane rating, but because it’s oxygenated, you can dump more fuel in. Even though ethanol is less energy dense (and so results in lower gas mileage), the additional fuel means more total energy release, and more power.

In other words, E40, which is ~98 octane will, if properly tuned with the extra fuel flow required be capable of higher power output than similar 100 octane race gas, because there’s more total hydrogen, carbon and oxygen flowing through the engine when given the same inlet airflow, in exactly the same way that a nitrous fogger with additional fuel would

This is taken to an extreme in top fuel and nitromethane burning dragster/funny car engines, where instead of the 14.7:1 air-fuel ratio of gasoline, because of the extra free oxygen, you get AFRs:

Ethanol: 9.0:1
Methanol: 6.5:1
Nitromethane: 1.7:1

Other words, nitromethane is a large part of the way to dumping massive quantities of rocket fuel containing its own oxidizer into your engine, supplemented by additional air.

As long as you are just dealing with gasoline equivalent blends of various 6-8 chain hydrocarbons, octane rating is the primary limiting factor in approaching ideal peak cylinder pressure, but once you throw oxygenated fuels into the mix, or fuels that require a drastically different AFR (like propane or hydrogen), you are no longer looking at the same thing.

Ultimately, your engine power is limited by how much air you can stuff into it, and then by the quantity of fuel you can burn with that air, unless you run into a volumetric limitation on fueling, a knock threshold that prevents reaching peak cylinder pressure or a mechanical strength limitation because you bend a con-rod. Which is how 80’s formula one cars managed as high as 1400hp out of the 1.5 liter displacement - more air (by running 50-80 psi boost) and more fuel

 

[SteveS]

The simple way to look at this is if you can deliver all the fuel necessary to get the right air/fuel ratio, and can tune to the max cylinder pressure available with the given octane without knocking, you would get the same power, but with less energy dense fuels you would get worse fuel mileage.

Obviously there are constraints to the amount of fuel you can pump into the cylinder and how well you can mix it, so varying degrees of oxygenation or other characteristices of the fuels will have more effects than that in most cases.

 

[Dialcaliper]

The simple way to look at this is if you can deliver all the fuel necessary to get the right air/fuel ratio, and can tune to the max cylinder pressure available with the given octane without knocking, you would get the same power, but with less energy dense fuels you would get worse fuel mileage.

Obviously there are constraints to the amount of fuel you can pump into the cylinder and how well you can mix it, so varying degrees of oxygenation or other characteristices of the fuels will have more effects than that in most cases.

One of my points was that this isn’t quite true for oxygenated fuels. The primary limitation of an engine is airflow. Fuel flow is secondary, much simpler to modify and usually has some overhead compared to airflow. Given the same engine with a fixed displacement, oxy fuels are capable of significantly higher power output explicitly because of the altered AFR compared with pure hydrocarbon/gasoline of the same octane rating.

Running ethanol fuel has an inherent effect inside the cylinder that’s similar to increasing boost pressure or adding nitrous/oxidizer, and don’t forget that air is 80% useless nitrogen. The extra oxygen literally changes the chemistry equation

A more true statement is that for the same amount of Carbon, Hydrogen and Oxygen you flow through an engine, any way you combine those with the same octane rating is capable of a similar peak pressure and power output but even then, effects like charge temperature and in-cylinder cooling will change that as well.

 

[TalkToTheFiST]

Ok TYVM for the info so far, it actually makes sense and i like knowing the little details. With that said lets add another wrinkle to the question, which is in fact why the question arose to begin with. I was looking up different tuners last night to see what's out there as i have bosch injectors in the mail as we speak. Once my current e30 Dizzy tune is finished, i'll be looking at getting a base tune ready for the injectors, then i can schedule installation and go from there with the tuning process. Upon looking through Tune+'s site, they have a link for "race-gas" dot com and list it as supported fuels. https://race-gas.com/

When you go to that website, they list 3 fuel additives that supposedly increase octane levels, up to as high as 112 (starting with 93 octane). It's something like 3 cans per tank of a FiST, which means a full tank of "112 octane" would run me like $150 (3 bottles of booster plus 12 gallons of 93). Cost aside, how would this compare to an e30 or e50 tune? Per volume since it starts as normal gasoline it SHOULD have more explosive yeild over ethanol right? And higher octane means more boost as well.

If nothing it's all out of curiosity but i also wonder if there's any legitimate practical way to do it using octane boosters vs an ethanol blend.

 

[Dialcaliper]

Ok TYVM for the info so far, it actually makes sense and i like knowing the little details. With that said lets add another wrinkle to the question, which is in fact why the question arose to begin with. I was looking up different tuners last night to see what's out there as i have bosch injectors in the mail as we speak. Once my current e30 Dizzy tune is finished, i'll be looking at getting a base tune ready for the injectors, then i can schedule installation and go from there with the tuning process. Upon looking through Tune+'s site, they have a link for "race-gas" dot com and list it as supported fuels. https://race-gas.com/

When you go to that website, they list 3 fuel additives that supposedly increase octane levels, up to as high as 112 (starting with 93 octane). It's something like 3 cans per tank of a FiST, which means a full tank of "112 octane" would run me like $150 (3 bottles of booster plus 12 gallons of 93). Cost aside, how would this compare to an e30 or e50 tune? Per volume since it starts as normal gasoline it SHOULD have more explosive yeild over ethanol right? And higher octane means more boost as well.

If nothing it's all out of curiosity but i also wonder if there's any legitimate practical way to do it using octane boosters vs an ethanol blend.

That sounds pretty cool - from what I can find it’s a blend of 3 antiknock additives (or 2+ toluene) that function similar to Tetrethyl Lead or MTBE to suppress knocking.

Definitely a viable option, however don’t waste your money blending all the way to 112 octane - for a street engine, unless you plan to run a massive laggy turbo at high boost and Lee the charge cool, you’ll probably hit timing for max MBEP (cylinder pressure) well before you suffer knock. To really take advantage of high octane, at some point you have to alter geometry (either low CR/high boost/high RPM or similar boost with higher CR).

Just a total SWAG but 98-100 octane will be more than enough for any kind of bolt-ons, hybrid/big turbo setup unless you tear down and build the engine, change the compression ratio and/or run a bigger hairdryer.

 

[TalkToTheFiST]

That sounds pretty cool - from what I can find it’s a blend of 3 antiknock additives (or 2+ toluene) that function similar to Tetrethyl Lead or MTBE to suppress knocking.

Definitely a viable option, however don’t waste your money blending all the way to 112 octane - for a street engine, unless you plan to run a massive laggy turbo at high boost and Lee the charge cool, you’ll probably hit timing for max MBEP (cylinder pressure) well before you suffer knock. To really take advantage of high octane, at some point you have to alter geometry (either low CR/high boost/high RPM or similar boost with higher CR).

Just a total SWAG but 98-100 octane will be more than enough for any kind of bolt-ons, hybrid/big turbo setup unless you tear down and build the engine, change the compression ratio and/or run a bigger hairdryer.

Ok yea i kinda figured the 112 was overkill. Im already touching 28psi on e30 so i can't imagine we have even 10psi more, i've heard of low 30s but then as you say, pressure. Is the falloff linear you think? For example 93 to 112 is 19 octane, so using a single can would get to say 99, two cans up to 105?

 

[Dialcaliper]

Ok yea i kinda figured the 112 was overkill. Im already touching 28psi on e30 so i can't imagine we have even 10psi more, i've heard of low 30s but then as you say, pressure. Is the falloff linear you think? For example 93 to 112 is 19 octane, so using a single can would get to say 99, two cans up to 105?

It all depends on the turbocharger. If you’re talking the stock turbo, it’s already being pushed way past its efficiency island on aftermarket tunes, and falls off a cliff at higher pressure ratios because the input power required increases because of inefficiency and output gets too far too hot because of all that excess energy going straight to heat instead of increasing pressure.

If you swap out for an appropriately sized turbo that’s efficient at higher pressures and flow rates, then you can create ludicrous boost levels and airflow until your pistons, rods and crank fail.