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XR250 on Regular gas?

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It's a pain in the ass for me to get Premium pump gas where I live. Does anyone run their XR250 on Regular or is that a no no?

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shouldn't hurt anything, just have a little less power and start a little harder. I usually run premium, but have run reg in the past on occasion.

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if your motor is stock then regular unleaded should be just fine. premium is required if you have a higher compression ratio. basically, premium burns slower than regular.

try to listen to your motor while you ride. if you hear it "knocking" or "pinging" this is generally an indication of fuel/air pre-ignition. in this case you may need premium. if you hear no "knocking" or "pinging" then regular should be the fuel for you.

often times people think premium will increase the performance. this is generally not the case. a low compression motor will see no advantage if premium is used. a high compression motor could eventually be damaged if regular is used.

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What Mrs. Cullini said.

(Or is that Mr. Scullini?)

You will get no more power or no easier starting with higher octane ratings. A high performance engine, with higher compression, requires the higher octane to run efficiently. It does NOT derive it's higher performance or power from the higher octane gasoline.

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during thermodynamics in college, we were instructed that the energy content of gasoline is the same across the board of grades. it is the amount of anti-detonation chemicals that makes the difference. however when talking about engines and how they use that energy it comes down to timing advance and compression.

If we were speaking of regular unleaded, and if I were to advance the timing of my engine, I would begin to hear the engine detonating (igniting fuel too early before TDC). If instead of advancing timing, I were to increase compression I begin dieseling as now I am compressing the same amount of fuel into a smaller volume, increasing the heat created, and essentially igniting the fuel with the heat of compression. Again this happens before TDC, and if detonation is occuring it is too early before TDC.

Correct me if I am wrong. That is just something I took away from that one class 7 years ago.

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it is mr. scullini.

mhr, the info i took away from my internal combustion engines class seem to coincide with your info.

i believe the issue with timing advance is that on higher revving engines you want the timing advanced so you ignite the fuel earlier. this needs to happen because the pistons are moving faster. by igniting the fuel earlier, there is more time to have complete combustion. i believe this is the case.

you have the issue of dieseling. the reason higher compression motors require higher octane is to prevent dieseling. higer octane=slower burn

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yes, I didn't want to equate higher anti-detonation chems with slower burn, but I do believe that is the case. Also early ignition (advanced timing) creates higher pressures because the fuel is beginning the burn earlier before TDC, causing more pressure before TDC, more pressure during the power stroke. With that much more pressure you will create that much more heat, causing a more complete burn up to a point.

Up to a point...in that vaporized fuel burns at the speed of sound. Over an inch or so of headspace this happens very quickly, but if you create a cylinder so large with a piston movement so much faster, you will not completely burn all of the fuel distributed by the carburetor. Which is why differing C.I.D. engines use different jets.

It all comes down to efficiency.

BTW, if memory serves me correctly, internal combustion engines are somewhere around 30% efficient because of friction and friction due to heat. Imagine if someone were to improve the efficiency to 75%. 80mpg from a ~350C.I.D. engine with amazing HP and torque.

Then there is the topic of super/turbo chargers. We'll discuss that elsewhere.

Sorry, I get all gitty when talking mechanical philosophy.

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Sorry I got a bit OT.

Regular compression (9.3:1) + regular timing (no aftermarket ignition box) = sludge for fuel. 87 octane is normal regular unleaded for most everywhere. Here in CO 85 octane is the "regular", and even newer fuel injected vehicles that are "supposed" to run med. or high grade fuel, can in most cases get away with regular as their variable timing takes care of detonation.

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I wish. The mechanical engineering of machines is where I wanted/want to be. Went to college for three years at Colorado School of Mines, got married, got sick of profs, got sick of throwing money away, so I went out got a job as a programmer, and made more money starting that, than my masters degree metallurgist brother was making at the time.

My heart still lies in machines though. I learn as much as I can into the philosophy/science of anything I can tear apart (oh and put back together). Engines intrigue me.

I do wish I had picked up/retained a lot more math in college.

Anyone want to talk turbo/superchargers? 🙂

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Also depends on your elevation. The higher you are, the less need for premium grade fuel.

Less need for higher levels of anti-detonation chems.

Agreed.

Peeps here in CO dont complain about paying the same for 85 as others pay for 87. They dont know any better.

Me...I HATE that I pay the same, but it is still less for 87 here than it is in many other places, so I cant bit&h too loud.

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