Jump to content
Sign in to follow this  

race gas

Recommended Posts

hey guys, i have a new '05 yz 250f, and i was lookin at compression numbers and at 12.5:1 wouldnt u want to run some race fuel in this thing? i drag race and to me 12.5=at least a mix. if this thing gets warm i dont want it to ping itself to death

Share this post


Link to post
Share on other sites

If you mean to ask whether the engine needs more octane than pump premium offers, the answer is no. Not even with 13.5:1 compression.

VP U4, a fuel especially formulated for modern MX 4-strokes is 92 octane. The advantage it offers is more power production because of a higher energy content in the fuel, not higher octane.

Two-stokes need 98-100 octane (sometimes more) at such compression levels, four-strokes don't.

Share this post


Link to post
Share on other sites

i run vp c 12 in my crf 250, but i dont know about the yami

Share this post


Link to post
Share on other sites

premium unleaded with an octane rating of 98 is fine anything lower from the pump and the bike will ping.

Share this post


Link to post
Share on other sites

So is it alright to run 98 octane in my 05 YZ250F? I don't notice any difference between 98 and 92. Except 98 lightens up the wallet faster.

Share this post


Link to post
Share on other sites
So is it alright to run 98 octane in my 05 YZ250F? I don't notice any difference between 98 and 92. Except 98 lightens up the wallet faster.
Running an excessively high octane fuel won't hurt anything, but it will not provide any benefit at all, either. Actually, if you can't find a fuel that exactly meets the engine's requirements, it's better too high than too low. But you don't need more than 91-92 octane.

First, you need to understand that the octane number of a fuel tells you about only one characteristic of the fuel's behavior, that being its resistance to pressure induced detonation. Because octane can be raised or lowered in a number of different ways, users may notice that a particular high or low octane fuel runs better or worse than the fuel they normally use, but this is not because of the octane number itself. A fairly common complaint about high octane fuels in engines that don't need it is difficulty starting or in keeping the engine running while cold.

Briefly, start by understanding that in order to work in a way that is useful, gasoline must burn from the point of ignition outward in a wave-like manner. Detonation is when the entire fuel mass reaches the point of ignition simultaneously and explodes. This is about as bad for your engine as putting rocks in your combustion chamber and making the engine break them up.

When the fuel is ignited, the flame front spreads outward, igniting the fuel it comes in contact with as it goes, something like dropping a match on a tabletop covered with match heads. As the flame spreads, the unburned fuel/air mix is subjected to tremendous pressure and heat, which, if it gets too high, can cause the remaining fuel to detonate spontaneously. If this happens late in the combustion cycle, it sounds lie the light "pinging" many of us are familiar with. Earlier in the cycle, and it can be devastating.

Once a fuel has enough resistance to this pressure detonation so that it doesn't happen, adding more is simply not useful, and can sometimes even be counterproductive.

For more on the subject, read: Gasoline FAQs - Octane

Share this post


Link to post
Share on other sites

i used to drag race a '72 chevelle and now a 1991 eagle talon tsi. 12.5:1 has always meant race fuel to me

Share this post


Link to post
Share on other sites

Thankyou very much for your answer Grayracer. I'll keep using the 92.

Share this post


Link to post
Share on other sites
premium unleaded with an octane rating of 98 is fine anything lower from the pump and the bike will ping.

Octane is rated differently in Australia (and Europe) than in the US. In the US, 98 octane would be a race gas (can't buy it at a normal pump). Just for reference, can't really compare Aussie/US octane ratings for same motor set ups. In the US, premium unleaded (pump) is 92-94 octane, with most being 93. I think our 93 would be the same as 98 in Oz.

Share this post


Link to post
Share on other sites
I think our 93 would be the same as 98 in Oz.

Not by any method I'm aware of. Octane is rated in Research Octane Numbers, (RON); Motor Octane Numbers, (MON); and Pump Octane Numbers (R+M/2). Pump Octane Numbers are what you see on the yellow sticker at U.S. gas stations, and it's the average of the fuel's MON and RON. The MON method more accurately simulates racing conditions. The conditions under which fuels are tested using the RON method are not as demanding, so the number is normally higher than the MON rating. Fuel companies like to rate their fuels using the RON to make them appear more resistant to detonation. Neither method will produce a result that varies by 6 points from the other, and Yamaha is most likely calling for an octane of 92 RON in their manual, since that's how it's typically sold in Japan. scheffaa is simply wrong in stating that a YZF will ping on less than 98 octane. He may have had the experience of having a fuel which was supposed to have been 92 ping in his engine, but that either means there is a problem with his bike, or the fuel simply was not what it was represented to be.

Share this post


Link to post
Share on other sites

Unfortunately in Southern CA, we're down to 91 every where I go....Amoco's used to 93, but haven't seen that for awhile....anybody know of higher octane pump gas in San Diego??

Share this post


Link to post
Share on other sites

I have put in 87 octane a couple of times in my 2002 yz250f. But the rest of the time I put in pump premium 91 octane. The bike has ran just fine and dandy!

Share this post


Link to post
Share on other sites

A Pump octane number of 91 is high enough. It's going to be a lower number than the research number of the fuel, as I said, and meets the standard of 92 RON. Besides that, it works.

Share this post


Link to post
Share on other sites

Grayracer has provided a lot of good info regarding octane levels. What race fuels are on the market for MX that offer "energy boosters", and not simply a higher octane? I ran CAM2 110 octane before and all it did was kill the performance of my YZF (the fuel was TOO resitant to burning in my opinion), so my bike ran like crap. I think it was more of a high octane race fuel, and not a "energy boosting" race fuel. VP U4 was mentioned as having a higher energy content but still only a 92 octane.

When people ask (in 5-7 threads a day) if they need to run race gas, I guess they should be asking if they need to run a high octane race gas. Is it correct to say a 92 octane VPU4 type gas has higher energy, but wouldn't help anything in a high compression engine?

Share this post


Link to post
Share on other sites
i used to drag race a '72 chevelle and now a 1991 eagle talon tsi. 12.5:1 has always meant race fuel to me

Sorry, I forgot about this post, I would have replied earlier. You answered exactly how I hoped that you would. As Grayracer stated, combustion occurs at a fixed speed. The farther the burn has to travel, the more opportunity there is for detonation. Humongous bores ('72 Chevelles), and old-fashioned combustion chamber designs ('72 Chevelles), are very prone to detonation with even mild compression increases, and the milder the cam, the more apt they are to detonate and need race gas when compression is raised. The cam specs for a 250F would make any street-based V8 cam seem mild by comparison, even drag race cams. This decreases the engine's low speed efficiency, making the higher combustion pressures occur at higher engine speeds (remember, combustion occurs at a fixed rate), where there is less time for detonation to be allowed to start. Our Yamahas might have the best combustion chamber design out there, good meaning fast and complete combustion, fast meaning less chance for detonation, and better tolerance of lower octane. And the bore is so small compared to a car's engine that there just isn't enough time for detonation under normal circumstances. If somebody made 4 or 5 valve, modern combustion chamber heads for a first generation Chevy V8, I would bet money that you could run it on pump gas at 12.5:1. I hope this didn't put you to sleep. P.S.- I left out the part that improved combustion chamber designs require less ignition advance for best power, and blah blah blah, blah blah blah blah.

Share this post


Link to post
Share on other sites
What race fuels are on the market for MX that offer "energy boosters", and not simply a higher octane?
Yes. That's the primary function of VP's U4 ands U4e fuels. They are around 92 octane, but will add about one full horse power or more to your 250F if properly jetted.
I ran CAM2 110 octane before and all it did was kill the performance of my YZF (the fuel was TOO resitant to burning in my opinion), so my bike ran like crap.
There are a lot of things that go into "designing" a fuel for a specific engine. VP makes a great line of fuels, but they are formulated for different kinds of engines. VP Red 110 was apparently made for carbureted automotive V8's with 12" or so of intake manifold between the carb and the combustion chamber. The rate of vaporization is slower, so that it arrives at the chamber in a proper state for good combustion after having been exposed to all that heat along the way. That same fuel used in the very short intakes of a YZF would be in a much too liquid state on arrival in the cylinder, and burn poorly. It would also be less than ideal for a port fuel injected engine for the same reason.
When people ask (in 5-7 threads a day) if they need to run race gas, I guess they should be asking if they need to run a high octane race gas. Is it correct to say a 92 octane VPU4 type gas has higher energy, but wouldn't help anything in a high compression engine?
The question is usually asked because people are equating their experience with other engine types to their small (compared to cars) four-strokes. Car engines and two-strokes each behave differently than small 4-strokes, and U4. or any other 92 octane fuel has enough octane for an YZF up to 13.5:1

Share this post


Link to post
Share on other sites

grayracer, when you talk about the high energy content, is it just oxygen? what else gives it more energy? i know oxygenated gas does wonders for 4-strokes, what else is in it?

Share this post


Link to post
Share on other sites

Oxygenation by itself means that there is more oxygen in the fuel which is released into the combustion process. It does not indicate that there is more heat energy stored in the chemicals that make up the fuel.

In California, the term "oxygenated fuel" usually either makes our skin crawl, or makes us puke reflexively, because to us, it means the crap that the Clean Air Resources Board (CARB, oddly enough) crams down our throats for part of the year to make the air cleaner. It was oxygenated by adding MBTE, a variant of alcohol. This lowers the energy content of the fuel, and gives it an affinity for absorbing water, as all alcohols do. So it doesn't run as good.

Gasoline is stored heat in liquid form. The purpose of your engine is to extract that heat and convert it to rotating force, so the more heat you can get from it, the better. There are chemicals other than alcohol that can be added that will add to this energy content, and some of them increase the bulk amount of oxygen in the fuel. For more on the specific components, you can read the complete Gasoline FAQs It's comprehensive, but readable. Or, go to VP Fuels and dig around in their data sheets.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...