Mechanical and Technical For mechanical and technical discussion

Old May 16th @ 02:44 pm   #1
2upFZ1's Avatar
From: NLV - Lone Mt & Clayton

Motorcycle: 2002 Yamaha FZ1
Do you really need "Super" Unleaded?

This is copied/pasted from

In many high-performance situation, riders clamor for higher octane fuels, thinking this will give them additional horsepower and, thus, an advantage over the competition. But this is not the case--adding higher-octane race fuel to your motorcycle may actually produce less horsepower. Here's why: Octane, an arbitrary number which is calculated as the average of the Research Octane Number (RON) and the Motor Octane Number (MON), and is only an indic ation of a fuel's sensitivity to knock, which is typically pressure-induced self-ignition. (Of these two ratings, MON is more applicable to racing fuels as it is measured under high load and high speed conditions.)

Octane, as you can see, is not a measure of how much power--or, more correctly, specific energy--is contained in a fuel. And remember that leaded high-octane race fuels burn slower than most unleaded fuels, and may reduce performance in stock or lightly modified motorcycles. A high octane rating itself, however, does not mean that the fuel is slow burning. Hence, it has no direct bearing on the power characteristics of the fuel.

The knock tendency (and hence, the Octane rating) of a fuel is a function of the amount of free radicals present in the fuel prior to ignition and can be reduced by the addition of tetra ethyl lead, aromatics and other additives.

Although some racing organizations still use maximum octane number as the discriminating factor for fuel legality, it is really not appropriate for racing purposes.

Instead one should look at the amount of energy (heat) released in the burning of a particular fuel. This is described by the specific energy of the fuel. This quantity describes the amount of power one can obtain from the fuel much more accurately. The specific energy of the fuel is the product of the lower heating value (LHV) of the fuel and molecular weight of air (MW) divided by the air-fuel ratio (AF):

Specific Energy = LHV*MW/AF

For example, for gasoline LHV= 43 MJ/kg and AF=14.6, while for methanol LHV= 21.1MJ/kg (less "heat" than gasoline) and AF=6.46 (much richer jetting than gasoline). Using the above formula we see that methanol only has a 10% higher specific energy than g asoline! This means that the power increase obtained by running methanol, with no other changes except jetting, is only 10%. Comparing the specific energy of racing and premium pump gas you can see that there is not much, if any, difference. Only alcohol s (such as methanol or ethanol) have a slightly higher specific energy than racing or pump gas.

Other oxygen-bearing fuels, besides the alcohols and nitromethanes, such as the new ELF fuel, will also produce slightly more power once the bike is rejetted. However, at $15.00 to $20.00 at gallon for the fuel the reportedly minor (1% - 2%) improvement is hardly worth the cost for the average racer.

The real advantage of racing gasolines comes from the fact that they will tolerate higher compression ratios (due to their higher octane rating) and thus indirectly will produce more power since you can now build an engine with a higher compression rati o. Also, alcohols burn cooler than gasoline, meaning even higher compression ratios are possible with them, for even more power.

The bottom line here is that, in a given engine, a fuel that doesn't knock will produce the same power as most expensive racing gasolines.

However, it sometimes happens that when you use another fuel, the engine suddenly seems to run better. The reasons for this are indirect: First, the jetting may be more closely matched to the new fuel. Secondly, the new fuel may improve the volumetric e fficiency (that is, the "breathing") of the motor. This happens as follows: Basically a fuel that quickly evaporates upon contact with the hot cylinder wall and piston crown will create additional pressure inside the cylinder, which will reduce the amount of fresh air/fuel mix taken in. This important--but often overlooked--factor is described by the amount of heat required to vaporize the fuel, described by the 'enthalpy of vaporization' (H), or 'heat of vaporization' of the fuel.

A high value of H will improve engine breathing, but the catch is that it leads to a different operating temperature within the engine. This is most important with two-strokes, which rely on the incoming fuel/air mix to do much of the cooling--even mode rn water-cooled two-strokes rely on incoming charge to cool the piston. For two-strokes a fuel that vaporizes, drawing a maximum amount of heat from the engine, is essential--the small variations in horsepower produced by different fuels is only of second ary concern.

Also important is the flame speed: Power is maximized the faster the fuel burns because the combustion pressure rises more quickly and can do more useful work on the piston. Flame speed is typically between 35 and 50 cm/sec. This is rather low compared to the speed of sound, at which pressure waves travel, or even the average piston speed. It is important to note that the flame propagation is greatly enhanced by turbulence (as in a motor with a squish band combustion chamber).

The most amazing thing about all this is that you can get the relevant information from most racing gasoline manufacturers. Then, just look at the specification sheet to see what fuel suits you best: Hot running motors and 2-strokes should use fuels wit h a value of "H" that improves their cooling, while more power (and more heat) is obtained from fuels with a high specific energy.

By the way, pump gas has specific energies which are no better or worse than most racing gasolines. The power obtained from pump gas is therefore often identical to that of racing fuels, and the only reason to run racing fuels would be detonation problems, or, since racing fuels are often more consistent than pump gas--which racers call "chemical soup"--a consistent reading of the spark plugs and exhaust pipe.

Old May 16th @ 04:44 pm   #2
Howie B
now, having read that and understanding most of it...A manufacture will recommend a higher octane fuel so the engine won't knock, Corect, Or should I just save the extra money and stick with regular unleaded? This is important now more than ever since the price of gas is INSANE..
Old May 16th @ 04:46 pm   #3
Lost Sheep's Avatar
From: Las Vegas

Motorcycle: 06 1000 rr bone stock 01 F4I
yep thats the one pat
Old May 16th @ 06:41 pm   #4
if you want the best bang for the buck get MR9 racing fuel and get re-maped

Old May 16th @ 08:58 pm   #5
jwd's Avatar
From: Vegas Knights MC

Motorcycle: 2004 GSX-R 600
Originally Posted by RDstevens View Post
if you want the best bang for the buck get MR9 racing fuel and get re-maped


sounds expensive?
Old May 17th @ 01:44 pm   #6
MR9 will mix well with pump gas,use this this fuel for track days or racing, sold in 5,15,30,55 drums Average cost $26.00 per gallon

Old March 5th @ 07:56 pm   #7
Rx Rider's Avatar
From: Vegas

Motorcycle: CBR600RR
89 in my bizzle all day
I used to run 87 in my Evo all the time haha.
Altho if I were to do some trackage I'd probably splurge on 91
Old May 29th @ 01:43 am   #8
From: Pakistan
Hi I am Dodinas Lay. I am from Pakistan. I really very happy to join this forum for disucssion. I found this forum very interesting and informative about discussed topics. I like it.
Old May 29th @ 11:56 am   #9
ridethebigone's Avatar
From: Vegas

Motorcycle: 2004 CBR600RR (track only now) '79 Yamaha XS650 (The Savage Beast!)
Who the fuck is this guy ^ ?

Modern sportbike engines have high compression ratios (10.5:1 and up) and therefore need high octane fuel (91) in order to operate as designed. Due to the knock sensor (and others), lower octane fuel (87) can be used. When you do this the ECU retards (haha) the timing so that the fuel isn't compressed as much before being ignited, therefore preventing self ignition from the high pressure/temperature, but also limiting the potential of the engine (by how much, not a lot, need a dyno to verify). I personally wouldn't use less than 91 octane in an engine with 10:1 or more compression. Plus, 91 is only $0.20 more per gallon than 87, and you're using how many gallons? Worth the extra cheese.
Old May 29th @ 12:17 pm   #10
MarvelousMatt's Avatar
From: Northwest

Motorcycle: Kawasaki Ninja 250R
That guy is someone who's been around a lot longer than either of us, and simply copy and pasted something he thought was useful while claiming no personal expertise in the area.

But people will choose to do what they choose to do, for the reasons that they themselves have. The book says 87 for me, and since I'm a commuter bike looking for cheap gas, 87 it is.

But when comparing the cogency of your arguments, I'm gonna have to go with the article on this one. It seems like they put a little more science and effort into it.

gasoline, info, octane

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