Fuel

[Rev Head]

Do many people here know about a category called Formula Vee?

Well im from Australia and we run 1600cc engines.

The driver who currently leads the championship ran Elfin fuel in his engine at $7 a litre, with a higher octane reading. However his lap times were not as good as the last time he went to the track and he did not dominate. My understanding is that its better to have a lower octane fuel in engines which are old, supercharged, turbocharged, or engines which need a lot of boost power.

The higher the octane the less energy in the fuel, lower octane fuel burns more explosively and quickly right? With this in mind running a higher octane fuel actually reduces performance to a slight degree rather than increase performance. And in a sport where 1hp makes a big difference it doesn't seem logical to me to run high octane.

Can anyone help me with the differences between low/high octane and their density readings. As isn't higher octane usually a more dense fuel increasing the weight to a certain degree?

Also running a too low octane fuel can cause your engine start pinging right?

[manchild]

Higher compression ratio - greater risk of self ignition of fuel mixture

Higher compression ratio – need for fuel with higher octane number

Higher octane number - greater resistance of fuel to self ignition under high pressures and temperatures

[riff_raff]

Rev Head:

The "octane" rating of gasoline is a relative value of it's resistance to detonation when run in an SI engine. There are two common methods of establishing octane rating with gasoline: the "research" method (RON) and the "motoring" method (MON). The research method involves igniting the fuel in a combustion bomb, under lab conditions. The motor method involves running the fuel sample thru a variable compression engine, and noting where it begins to detonate.

The rating scale is based on an octane fuel sample as being 100, and a heptane fuel sample as being 0.

The amount of power an engine produces from a particular fuel is a result of the theoretical LHV (Lower Heating Value) of the fuel and the BTE (Brake Thermal Efficiency) of the engine. An SI (spark ignited) engine only needs a fuel with an octane rating high enough to prevent detonation, under all operating conditions. Using a fuel with a higher octane rating does not produce additional power.

Regards,
Terry

[DaveKillens]

It's a common assumption that running a higher octane results in higher power. Actually, it's just incorrect logic. Most high performance spark ignition reciprocating engines run an elevated compression ratio, and thus, usually require a higher octane fuel.
So usually, having a fuel with a higher octane rating than required is unnecessary, while running a lower octane fuel almost always results in physical engine damage. And as stated in previous posts, a higher rating may actually have a slight negative impact on power levels.
I can think of only one reason to run a higher level than required, and that is if detonation is suspected, or expected. It's much safer to run the higher rating, you almost guarantee no detonation.
I can think of one historic example of where a high performance racing engine ran a low octane fuel. It was back in the 60's, and companies like Honda were exploring multi-cylinder, high revving engines in motorcycle road racing. Honda had a 6 cylinder 250 cc DOHC engine that ran at very high revolutions. Because the engine spun so fast, with the available high octane fuels present, they could not attain a complete burn during the ignition cycle. So the engineers used a lower octane fuel, one that would burn quickly enough to get that complete burn each combustion cycle.
It has to be mentioned that the great Mike Hailwood was the rider.

[Guest]

Interesting point Dave. I wonder how they managed to provide sparks and burn the fuel with the 50cc twin cylinder engine, that ran to 21,000rpm or higher! BTW, I saw 250cc 6 engine, at the Mallory Park post TT event. An indescribable sound from the 250 while being warmed up.

[DaveKillens]

It had to be one of the major engineering challenges to deliver the optimum fuel mix, and ignite it. Back then, they started the engines with hot spark plugs, and after a few installation laps, they changed to colder plugs. That was just one standard practice to get those tempermental beasts to run properly. It's also possible other tricks were eventually used, such as exotic fuels.
I'm not familiar with the history of ignitions, but I'm sure that great advances were made in those days, with those challenges. Maybe that was one driving force to such components as CDI ignitions. I recall in the mid 70's, we were very happy to see CDI on our motocrossers.

[Spencifer_Murphy]

Hey, i dont know too much about fuels, im mopre of an aero guy......but if Higher Octane doesn't produce more power....why is it that Petrol Companies sell higher octane fuel at the pump for an increased cost, claiming more power or more fuel efficiency (eg. BP Ultimate, Shell Optimax)?

What are the advantages of using these fuels in your road car?

[DaveKillens]

Generally, if you increase compression ratio, you increase power. But with increased compression ratio, a fuel that can tolerate this without knocking or pre-ignition is required. So the common assumption is that high octane goes with high power. But higher octane doesn't really give more power by itself, it just allows the engine to operate with a higher compression ratio, which usually goes hand in hand with more power.
To me, it's not logic. But hey, it makes good advertising.

[Spencifer_Murphy]

lol...so its all a money making scam.

Well I did a bit of research...I only run a 1.1 Ford Fiesta...so ANY increase in power is quite apparent. I filled up with a tank of Shell Optimax and...to be honest its not much better...

...ALTHOUGH....I did notice ONE BIG improvement...the trottle response is much better...you can pull out onto a roundabout or out of side turning with much more confidence. And so far a half tank of fuel has provided me with 40miles more...so in theory I am getting 80miles more for a tank of this Optimax stuff than I am with normal fuel...thts quite a bit of money saving (even when you consider the fuel is more expensive in the first place!)

So extra power...NO....better fuel efficiency...somehow yes. Oh and the engine seems to idle better....maybe it really does clean the engine better (as they say it does?)

[RH1300S]

I don't know the science of this.....as far as I understand, what Dave says is right. If your motor ain't knocking you don't need more octane.

BUT............. when I raced MX, adding octane boosters improved smoothness & throttle response (it worked in my R5 tow car too ).

[manchild]

... (it worked in my R5 tow car too ).

You (ab)use R5 for towing? [-X [-(

[DaveKillens]

Well, don't forget that fuels have to meet a lot of needs. I was just generalizing in previous posts, but it has to be remembered that usually, people who have vehicles that require higher octane (premium fuel) also expect more. Some dude driving his BMW would have problems if his car delivered nothing less than stellar acceleration in all modes. And in the opposite end of the spectrum, a person who drives a car not expected to show performance doesn't really expect perfect engine performance. For instance, I presently own an Oldsmobile, a car designed for sedate, middle class families. It doesn't pretend to have any high performance, it has a weak V-6, and it doesn't require anything more than the cheapest gas. My opinion is that that the demographics for the owner of such a vehicle doesn't usually drive aggressively, or place any real performance demands on the car. (I guess I don't fit in that demographic, lol)
So what's the point in my ramble?
Well, fuels are a complex brew, they have all kinds of weird chemicals in them, depending on the requirements. (For instance, up here in Canada they have to take into account the much lower temperatures of winter, compared to California). So, fuels are much more than differences in octane, the fuel engineers mix in all kinds of paraffins, other stuff to deliver a fuel that not only tolerates compression ratios, but allow the engine to perform as designed. Fuel injections need fuel different than carburetors.
So if the car appears to perform better with more expensive gas, maybe it's not because it can handle the higher compression, but that it has additives that just plain make it work better.

[riff_raff]

Spencifer_Murphy-

It's not a scam on the part of auto manufacturers or fuel refiners. Just try reading your owners manual. You don't need to use a fuel with a higher octane than recommended in the manual. It's a waste of money.

All cars now-days have knock sensors. Even if you use a gasoline with a lower octane than recommended (ie: 87 octane instead of 90 octane), your engine's ECU will recieve a signal at the onset of detonation (knock), and will immediately (within less than 1 millisecond) retard the ignition timing and richen the fuel mixture, to eliminate the detonation condition. Your engine's power output will temporarily be reduced, but your engine will also be saved from damage. Your engine's ECU is much smarter than you and its reflexes are infinitely faster.

If you really want to be impressed by the innate intelligence within modern engine control systems, take a look at GM's Flex Fuel cars. They can tell the difference between ethanol and gasoline, or any combination of the two.

[rob82]

Please take note:

The reason fuel companies provide higher octane rated fuel is to increase horsepower. The RON rating for an SI system as stated above is the fuels ability to resist detonation. The lower the octane the more prone to detonation.

Within the standard PCM's there are usually two ignition MAPS high and low. The PCM determines which map to use based on the amount of detonation seen over a certain time, rpm, MAF value, ECT and numerous other operating conditions using the KNOCK SENSOR. This is how putting higher octane fuel into your car will increase horsepower as it will use the higher octane MAP/MAP's therefore using higher ignition advance. A good test is to drive a 00 onwards WRX as they use an active ignition system that is very sensitive. Drive RON91 compared to RON98 and see if you notice the differnece.

Back to the first post. The reason there was no horsepower gain in using the higher octane fuel is that it was not tuned for higher ignition or AFR. Usually when tuning a race fuel(higher RON) a tuner (myself anyway) will lean mixtures off as to promote a slower burn rate which makes it easier to advance the timing. However in doing so EGTs must also be monitored.

[doodzed]

I run a US-spec 2000 Civic Si which uses the old 1.8l motor out of the delsol. It is a pretty trick piece made in japan that gets NA 180hp at 8000rpm. For a high end motor like that at least 92octane is required. When I picked it up at the dealer it was obviously at least partially filled with the cheapo stuff. If your engine really needs preimum you will know it.

One of the TV shows took a recent 911 and dynoed it on both regular(87) and premium(92) and found that it made a few more hp with the premium gas. in the order of 7-9 if I remember right.