opinions on fuel for 250R w/CR head gasket.

[InPiEcEs]

Just looking for some opinions on what to use in my 86 R for fuel.
It is stock, ports cleaned up, using a CR head gasket.
The pipe is a PT HI-Rev, carb is 36MM, and the reeds are Boyesen.
Stock airbox, K&N filter, 14/36 gearing, and 18" Trekkers.
I have VP C12 available, as well as Turbo Blue 110, which is out of the pump.
I don't know much about the Turbo Blue, other than it's 6 bucks a gallon, versus 10 bucks for the VP.
Just wondering what everyone likes to use, and how a guy could maybe get it.
As far as getting 100LL or anything at the airport, it was a no go, so I will be stuck geting fuel retail from someone.
Am I going to notice a worthwhile difference in power using the CR gasket.

[deathman53]

First, do a compression test on the top end, over ~190, then you need race gas. I would say to use blue 110, if it contains lead, the better. Vp c12, is overrated, its not worth the extra $4/gallon. Leaded gas really helps 2 strokes, I was told that it desists denotation and makes them run cooler. I use leaded gas in my 4 strokes also, it reduces wear on the valve seats and rocker arms. The lead kinda acts like a libricant for easier and less friction on moving parts. Some will tell you to mix 93/94 and race gas 50/50, It will only yeild around 100 octane if you do. Other factors are important also, was the head cut any, dome reshaped, advanced ingition?? If you are dealing with any of these, 100% race gas is a must, if not, you could get away with 60% race/40% 93-94 octane. I use 100% 114 leaded, as I don't want to deal with mixing the two different fuels.

[GPracer2500]

I'm pumping 235psi cold at 1300ft. I run VP110 (107 MON).

[hrc85250r]

87 pump gas, go as low as you can. the lower the octane the more power, if you hear it knocking you need to go up in octane. some people mistakenly run racve gas in motor expecting to get more power when they actually lose it. race gas has additives that make it burn at different rates usually much slower than pump gas, most "race" engines have extremely high dynamic compression, which can make an engine work like a diesel if the fuel is ignited by pressure and heat (like a diesel) before the spark plug sparks, and when the spark plug does fire, that the sound you hear, the two explosions inside the combustion chamber colliding. so, when you motor starts sounding like a diesel, move up in octane, also you want to check your plugs to make sure they are staying in a comfortable range for riding around. if you just dont care about tuning or max power just run race gas...

[GPracer2500]

87 pump gas, go as low as you can. the lower the octane the more power, if you hear it knocking you need to go up in octane. some people mistakenly run racve gas in motor expecting to get more power when they actually lose it. race gas has additives that make it burn at different rates usually much slower than pump gas, most "race" engines have extremely high dynamic compression, which can make an engine work like a diesel if the fuel is ignited by pressure and heat (like a diesel) before the spark plug sparks, and when the spark plug does fire, that the sound you hear, the two explosions inside the combustion chamber colliding. so, when you motor starts sounding like a diesel, move up in octane, also you want to check your plugs to make sure they are staying in a comfortable range for riding around. if you just dont care about tuning or max power just run race gas...

Lets stop the spread of misinformation! Sorry, but most of that is flat out wrong. At the risk of coming across as pedantic, I'll try and clear up some of these common myths.

All engines have a minimum octane requirement that will vary depending on the engine and its state of tune. But it is impossible to "over octane" and engine. You can have more octane rating then the engine requires (which can be a waste of money) but addtional octane beyond what's required has no impact on the fuel's performance. The basis for this lies in understanding what octane rating actually represents and understand the two types of combustion that can occur inside an engine. The two types are deflagration (the good kind) and detonation (the bad kind). More on that in a minute.

Octane rating is a measure of ONLY ONE THING: detonation resistance. THAT'S IT! Reading any more into octane rating is a mistake. Octane rating doesn't tell you if a fuel burns fast, slow, has more engergy, less energy, or anything else. The trouble is that octane rating is the only fuel specification that most folks are aware of. So any characteristic we may distiguish between fuels is automatically chaulked up to octane rating. THIS IS WRONG. There is a whole slew of other specifications that determine fuels' performance.

About the two types of combustion: Deflagration is a fancy name for subsonic burn and this is the good type of combustion. When everything is working as it should air/fuel mixture does not explode, it burns by way of thermal conductivity. This is an important distinction. Basically, when the spark plug fires the a/f mixture near the plug ignites. This heats up the layer of mixture surrounding that initial kernel of burning mixture. The layer heats up until it reaches its ignition point, which heats up the next layer to its ignition point, and so on until the entire mixture is consumed. Deflagration is controlled and predictable.

The other type of combustion that can occur inside our engines is detonation. Detonation is also known as auto-ignition (not to be confused with pre-ignition--that's a different animal). Detonation is an explosion. With deto multiple parts of the a/f mixture combust nearly simultaniously at supersonic speeds with no source of igniton.

Deto happens when gasoline is subjected to such high temperatures/pressures that the gas begins to chemically degrade into NEW chemicals that are so unstable they can combust all by themselves. Octane rating is a measure of how resistant a fuel is to breaking down into these new chemicals.

Lets look at the sequence of events that can lead to detonation: Deto will only happen AFTER the spark plug has already fired and the normal deflagration of the fuel has begun. Keep in mind that it takes time for deflagration to run it's course--it doesn't happen instantaiously. As the mixture deflagrates the temp/pressure inside the combustion chamber shoots up rapidly. The a/f mixture that resides around the edges of the combustion chamber (called "end gas" because it's the last part of the mixture to burn) has to sit there and wait for the advancing flame front of deflagration to get there. While it's waiting it's being subjected to ever increasing temps and pressures. It's at this point that the end gasses can begin to break down into auto-ignitable chemicals. If some of the end gasses do light off independently of the advancing flame front then you've got a detonation condition.

Deto occurs at supersonic speeds (FAST!). It creates a shock wave that spreads out so quickly the engine can't cope. The piston can't be forced down quickly enough to fully absorb the energy (like it can with the subsonic burn of deflagration). The knocking and/or pinging sound we hear when deto takes place is not caused by colliding flame fronts (even though it's often reported as being such). The sound is caused by the detonating mixture's shock wave ing into the metal surfaces of the combustion chamber. It's such a fast shock that the surfaces resonate audibly. It's like hitting the top of your piston with a ball pean .

BTW, the reason ignition timing factors into an engine's min required octane rating has to do with the time component of the above sequence. The sooner you initiate deflagration (i.e. the sooner you initiate the spark) the more time the end gases have to wait before they can be consumed normally. This is why lots of ignition advance demands a higher minimum octane rating.


So, octane rating in and of itself has no bearing on a fuel's deflagration characteristics. And deflagration is how our engines produce power. If a fuel has enough octane rating to prevent detonation in a particular engine then that's it. If the fuel has deto resistance beyond what the engine requires, that additional protection will be unrealized. It's like it's not even there. There are only minimum octane requirements, there is no such thing as a maximum octane requirement.

The only time the engine is aware of octane rating is when it doesn't have enough!

The only possible arguement I can think of that octane rating can hinder performance is if a fuel achieves its high rating by compromising some other property of the fuel. But in that case, it's not actually the octane rating that would be hurting performance. It's some other specification of the fuel that is hurting performance. Unless you're using a highly specialized fuel that was designed for a specific application (many such fuels exist, but they are not mainstream) then I can't see that happening. Most fuels that have MON ratings over 100 get there with TEL. And TEL doesn't impact deflagration characteristics.

Octane rating is ONLY A MEASURE OF DETO RESISTANCE, nothing else!

In my experience, the reason some folks experience strange or degraded performance from running a racing fuel (even in stock engines) is because they fail to re-jet for the fuel. Most racing fuels have a lower specific gravity then most pump fuels and that impacts the jetting requirements.

[deathman53]

good information, gave me some good info. I knew the higher octane=more detotation resistance, I didn't know the specs about timing and such, I knew the basics of it. How does leaded gas affect things on a 4 and 2 stroke, I was told by several that it allows you to run more compression and it lubes things in the process, reducing friction.

[InPiEcEs]

Thanks,GP..........
That was some good info.
Definitely some good stuff to keep in mind.

[hrc85250r]

sorry man, i dont really care what that says, but i go to school for automotive engineering with a minor in motorsport performance, so, unless the three textbook i have right here in front of me are completely wrong, your internet info is wrong, don't believe everything you read on the net. ok, now octane rating is the measurement of how long the hydrocarbon chains are in the fuel, now the more hydrocarbon chains there are present in the fuel at the time of burning the longer the chains take to burn, and that is a fact. now the detonation resistance is achieved by additives in the race gas which could be a number of things including lead. specifically the higher the octane of the gas( excluding additives) , the better detonation resistance it has alone because the longer the carbon chains the harder it will be for the motor to actually get the fuel burning fast enough by detonation to cause spark knock.i dont know if you guys can understand what i wrote, but that is almost exactly how the book has it just in my writing. by the way, that article really doesnt sound like its legit. i mean how could 3 of my auto engineering books be wrong? they are wrote by guys with phd's...

[cacustom]

I have just about the same exact motor as you do, I run Pemium with a 40:1 ratio of blendzall

[GPracer2500]

....ok, now octane rating is the measurement of how long the hydrocarbon chains are in the fuel...

Yes, the octane rating of individual hydrocarbons is determined by the structure of the molecule. But we're not talking about the octane rating of a particular hydrocarbon. Gasoline is made up of many many different hydrocabons (hundreds). We're talking about the octane rating assigned to a gasoline and what that is a measure of. And the answer is detonation resistance not chemical structure.

The octane rating of a fuel is measured in a laboratory on a special variable compression test engine. (sidenote: Waukesha Engine is the only supplier of these special engines AFIAK). Motor Octane Number (MON) is measured using the ASTM D2700 test protocal. Research Octane Number (RON) is measured using the ASTM D2699 test protocal. These are actual measured tests that demonstrate a gasoline's resistence to detonation regardless of which hydrocabons make up the fuel. The numbers are derived from comparisons of the test sample to known reference fuels. I believe heptane and iso-octane are the two references.

There are a number of different ways to achieve a high octane rating. You can achieve high octane ratings with a high content of aromatic hydrocabons. Or you can do it by adding oxygenates. Or you can add tetra-ethyl lead. The octane rating of a gasoline says nothing about how a fuel achieves it's rating or which particular mix of hydrocabons constitue the fuel. You can have two fuels with significantly different makeups test out with the same octane rating.

...by the way, that article really doesnt sound like its legit...

It's not an article, it's my own writing. It's based on my own research on the subject. And what about it is incorrect? Be specific. If I'm wrong about something I want to know about it.

...i mean how could 3 of my auto engineering books be wrong? they are wrote by guys with phd's...

I'd guess they are not wrong. Exuse me for saying so, but I thing you are not applying whatever it is you are reading to the right context.

[hrc85250r]

i hate these threads. im not going to argue this crap....i know what i know and you know what you know, no big deal...we dont need enemies on these friendly boards.... but, both are ideas of the facts have merit and i guess what i am trying to say is, dont waste your money on race gas because it sounds cool to say thats what you run on. run it because you have to.

[GPracer2500]

i hate these threads. im not going to argue this crap....i know what i know and you know what you know, no big deal...we dont need enemies on these friendly boards.... but, both are ideas of the facts have merit and i guess what i am trying to say is, dont waste your money on race gas because it sounds cool to say thats what you run on. run it because you have to.

No hard feelings on my end.

And I agree that unless an engine's tune requires a race gas it makes little sense for (most) people to use it.

[86 Quad R]

have ya done a compression check yet InPiEcEs?

[InPiEcEs]

Not yet, but probably soon.
The motor hasn't really been run yet, just fired for a few seconds to make sure it will start. I will probably do it before this weekend, hopefully.
Once I get the last few parts I need to get it rolling, I'll see how it does on premium.
I really don't feel like having to use race fuel from a cost standpoint.
I don't mind mixing 50/50 or something if I need to.
I will do a compression check for starters, and once I get it trail ready, see how it runs on pump gas first, and go from there.

[BigReds Forever]

Man, now i'm confused. I like the fact that you guys quit your little dispute before someone got mad, but could someone answer this for me.. I have a raptor 660. I dont get detonation no matter what octane i run, but i usually use 92 from the pump with some commercial "octane boost." It runs much better this way, there is a noticeable difference in power. Is this do to the increased octane, or some of the additives in the "octane boost." If thats the case, would i be just as well off running the boost with plain ol 87? (sorry to kinda hijack the thread, but its useful info for everyone.)