I've got gas!

[Jimmy P]

In times of old I'd heard that if you mix different octane rated fuels (say 89 and 93) you'd end up with a higher octane mixture then the higher of the two octane ratings. This logic made no sense to me so I panned it off as myth. But, here's my question......gas/petrol prices in my area of Florida go up and down like a yo yo so......what would be the benefit, if any, of mixing 93 octane rated fuel with racing fuel rated at 100+. Say 7 gallons of 93 and 3 of 100+ (which is about $8-$9.00 / gallon) in my 10 gallon tanks.

1) is there any benefit and will it add to my overall performance?
2) would this benefit the engine or damage the engine in any way?
3) would this have any effect on my Webers?

Some of you race your cars so does anyone do this as a cost saving measure?

One last question.....and forgive my ignorance but can someone advise me on running colder or hotter plugs for winter or summer. Any reasons provided is appreciated.

I'll thank all of yous in advance.

 

[roaldin]

Mixing higher octane unleaded with a lower octane leaded fuel was commonplace in Australia for a while. The extra lead gave a net gain supposedly.
I have no info to prove either way but sounds reasonable I guess.

Tim.

 

[Cliff Beer]

The octane rating question is a threshold question, meaning, you need enough octane to prevent detonation (typically at WOT and full ignition advance), but more octane does not add more hp/tq. Highly tuned engines (such as a race engine) are hungry for higher octane fuels because they typically use a higher CR (for more power) which, in itself, requires a higher octane to avoid detonation. In other words, it ain't the octane that's making the power, it's the higher compression ratio (which in turn requires higher octane). If you're not running higher CR and you don't have a detonation problem, then higher octane isn't going to do anything for your hp/tq and you're wasting your hard-earned dollars.

 

[Seymour Snerd]

Cliff's exactly right. So to turn that into direct answers to your questions:

1) is there any benefit and will it add to my overall performance? If your runs OK on pump gas of a given octane, you will get no benefit by raising the octane further, regardless of how you do it.
2) would this benefit the engine or damage the engine in any way? No.
3) would this have any effect on my Webers? No.

4) advise me on running colder or hotter plugs for winter or summer. Your engine does not know what season it is, and an effective cooling system pretty much hides from your engine the outside temperature, with the exception of the temperature of the intake air. If the engine is set up properly in the first place, a single plug heat range should work fine.

 

[roaldin]

In these days of EFI, computers monitor knock and will back off timing if knocking is detected.
Some systems are better than others. The original factory computer in one car I own changed timing in 5 degree increments...

For those of us with a penchant for turbo's, octane rating has been a headache in Australia for a while. IMO there is wide variation within the various standards, ie: 98, 95 etc (Ron).

Tim

 

[Jimmy P]

Thanks for the replies Gents. Thats what I was looking for. This forum is fantastic.

 

[Scott Calabro]

Hi Jimmy,

Regardless of your engines actual octane requirements, any time you mix race gas or Avgas in with pump fuel you will increase power output, simply because you are increasing the BTU content per pound of fuel.

Current pump gas has oxygenates (ethanol) in it.

I don't know the exact BTU difference... but I remeber reading on an aviation forum thread about automotive fuel use in low powered piston aircraft, that pump gas has only 96% the BTU content per pound of 100% gasoline.

Empty one of your tanks and put 5 gallons of race/avgas in, then you can simply switch tanks back and forth to see for yourself.

Cheers,
Scott

 

[Eric]

The ocatne number is perhaps one of the most misunderstood ratings by most people. In very layman's terms, it is simply a relative measure of the ignition temperature of the fuel; higher octane = higher ignition temperature. (But note, 97 octane does *not* mean the ignition temperature is 97 deg F or C.) In a nutshell, that's why higher compression ratios require higher octane; higher compression ratio means greater heat of compression, therefore you need a fuel that ignites at a higher relative temperature. That's it. It has nothing to do with the amount of energy in the fuel. The octane rating exists simply as a measure of the fuel's antiknock capability.

The octane number you see advertised in the US is calcualted by the (R+M)/2 method, where R = research octane number and M = motor octane number. (Look at a gas pump and you will see this formula indicated.) In other words, it is an average of the research octane rating and the motor octane rating. As you might have guessed, the two ratings vary slightly.

FYI, if you run a higher octane, your engine will run hotter. Not necessarily out of range, but hotter than if you run a lower octane. I always get arguments on what I am about to say, but I always tell people to run the lowest octane gas that your engine can live with. Otherwise, you're really just throwing money away.

As I alluded to, this is an explanation for laymen. Don't hold my feet to the fire for non-engineering-precision details.

BTW, Scott is correct about alcohol-blended fuels vs. pure gasoline. Alcohol has less energy per "unit" than pure gasoline. For example, E-85 contains only about 71% of the energy as the same volume of pure gasoline. (Ironically, though, E-85 is awesome for forced-induction applications because the alcohol is fantastic at removig latent heat from the compressed air charge; in effect an intercooler without an intercooler.)

Eric

 

[Eric]

Oh, and by the way, it is impossible to mix two different octane ratings and end up with a HIGHER octane than the highest rating you started with. That's a shadetree mechanic's myth. The resulting octane number will be higher than the lowest octane number you started with, but lower than the highest octane number you started with.

Eric

 

[roaldin]

The idea with the mixing of the two fuels here was supposedly the effect of the lead in the leaded fuel on the more highly refined unleaded fuel.

Enter fuel chemist please. One should turn up - This topic has been discussed on the Internet for as long as I've been using it, and eventually one turns up.

As for lowest octane you can get away with, just keep in mind that modern electronically controlled engines attempt to compensate if they detect knock. If you think the car seems to have lost performance after using a lower octane fuel, reset the ecu, tip in some higher octane and see what happens. The performance difference can be substantial, especially in a turbocharged vehicle.

Although e85 doesn't contain as much energy per unit a lot more units are used - ie "richer", something like 30 percent more fuel for the same event. Running e85 can mean bigger injectors etc.


Tim.

 

[roaldin]

BTW there is some useful information on this subject on the Elf fuels site.

Also BTW after talking with someone thar swore by it, I had it the wrong way around in my first post. The mix was 50% 95 (possibly also 91) RON unleaded with 50% 96 RON leaded.
The rest of the theory I related correctly though:-
The important part of the theory is - more highly refined unleaded/lead from the leaded.
Make of it what you will. Not my theory.

Tim.

 

[Eric]

...Although e85 doesn't contain as much energy per unit a lot more units are used - ie "richer", something like 30 percent more fuel for the same event. Running e85 can mean bigger injectors etc...

Exactly. That's where the fuel composition sensor comes in (for EFI). It tells the ECU what percentage of the fuel is gas and what percentage is alcohol, then the ECU adjusts the injector pulse width as appropriate; i.e., longer pulse width for E-85. When you increase the performance of an EFI engine, say through adding a more aggressive cam, you may end up needing larger injectors if the current injectors at 100% pulse width will not supply a quantity of fuel great enough to contain the energy needed by the engine. For example, you may need to go from 30 lb/hr injectors to 40 lb/hr injectors (or 48 lb/hr, etc.).

Relating this back to Scott's post, you will only see a difference in performance for a carburated vehicle or an EFI vehicle that does not have a fuel composition sensor (with a caveat on the EFI system: the ECU will try to adjust the conditions it sees to its programmed settings, so there will be some adjustments made and, hence, the power difference will not be as great as for a carburated vehicle...which of course cannot adjust to the conditions; there is even some debate about whether an EFI system even needs the fuel composition sensor to run E-85 or will be able to adjust based on the other sensor inputs). An EFI vehicle with a fuel composition sensor will show no difference in performance between E-85 and pure gasoline (in theory) because it is adjusting injector pulse width to match the energy content of the fuel passing the sensor at that given moment.

Bringing this back to the discussion of octane, lead (tetra-ethyl lead) was added to gasoline because of its anti-knock properties (it also has some lubricating properties, which is a completely different discussion, but the reason older cylinder heads need to have hardened valve seats inserted when they are rebuilt). Unleaded fuel (at least in the US) has MTBE added in place of lead to improve the anti-knock properties of the fuel; in other words, to increase the octane rating of the fuel. MTBE is methyl tertiary butyl ether.

So, if you take pure gasoline of octane rating XYZ and add lead (or MTBE) to it, you will end up with an octane rating greater than XYZ. However, there is no difference in the level of refinement between unleaded and leaded gasoline. The two are taken from the same point in the cracking tower at the refinery, just different additives have been included down the processing line. One is no more or less refined than the other, and contains no more or no less energy than the other (prior to inclusion of the additives). So, the theory that adding leaded fuel to unleaded fuel will result in an octane rating that is higher than either of the two blends you started with is incorrect. As I stated before, it is impossible to blend two fuels and end up with a higher octane than either of the base fuels (without including more additives). It's quite simple, you are spreading the additives contained in the higher-octane fuel (that result in the higher octane rating) over a larger volume of fuel without increasing the volume of additives.

By the way, "octane" is an actual molecule, but what we are talking about is the "octane number." Octane the molecule is C8H18 (but there are many isomers). The "octane number" compares the anti-knock properties of a fuel to those of octane the molecule (specifically, its isomer 2,2,4-trimethylpentane, commonly known as isooctane). Octane the molecue is a hydrocarbon (because it consists of C and H) and is naturally present in refined gasoline.

And a final note, what defines the "advertised" octane rating varies from country to country. As I stated in a previous post, in the US, the advertised octane rating is an average of research octane number and motor octane number: (R+M)/2. In other countries, the advertised octane number may be the research octane number, the motor octane number, or the average. (There's probably at least one country that does something completely different.)

Hopefully this has helped clarified "octane" a bit.

Eric

 

[Jimmy P]

Bloody Hell there's some smart buggers on this site! 'Preciate all the input.