I got Shane's Big Bored cylinder back, as soon as I'm done porting the head I'm hoping to get it fired up this weekend. The instructions with the kit say I must run 100 octane or higher which I've never used. I wasn't pleased with the tuning instructions for Shanes bb kit, all it said was go up 2 sizes on my jets. Would that be accurate?
My quad is right on the edge of premium/racing fuel so figure as long as I'm switching Shanes to racing fuel, might as well change mine too. Should I just go with larger jets? I'm currently running an Edelbrock oval bored carb but about to switch it to a 39mm FCR Carb and I have a jetkit for stage2, not sure if it'll be big enough. I've seen stage 3 jet kits available, should I get one or wait and see what the stage 2 jetting does? Remembering that the FCR carb is off a YFZ450 and mine is only a 400cc, that may make the difference. My atv is pretty modded too.
I'll have to check to make sure but I believe the racing fuel close to me is 104octane. I've heard of people mixing racing with regular fuel too, is that an option or should I just run full on racing fuel? Probably a stupid question since I'll find out in the next month anyways, but is there a noticable difference just switching to racing fuel?
Ahhh the sleds are put away and the quads are getting prepped of the drag strip.
"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"
Originally Posted by Rubi
Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
you are going to high octane because of compression. you might want to advance timing a little bit as well? if you had a sled with 400 mains on 92 octane pump gas, you could drop several sizes very easily by going to 92 octane race gas. by switching to a higher octane race gas you are jetting up right there.
for safety it is always good to check the plug for detonation. it will get grey speckles on it if there is det. always jet fat and work your way down.
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It's probably going to be different on Quads due to ambient temps, but I noticed on UBR if I mix too heavy on the 110 and it's a cold morning, it is a little reluctant to fire off. It'll fire, just takes a couple extra pulls on it. Without the 110 it fires off fine when cold, just tends to det. when at operating temp.
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^^Higher octane burn slower which is why its harder to start when cold.
Its almost like comparing gasoline to diesel, the diesel burns slower which means you can raise the compression more on a diesel engine than gas, compressing the lower octane fuel too high will cause it to combust early(before TDC)
100ll airplane fuel isn't worth a shit except maybe in a airplane. I tried it last weekend as a cheaper alternative to high octane race fuel. $3 vs $8. it made my bike run sluggish and it hot. I don't have a temp gauge to back that up but it sure smelled hot. I don't know if the shit is oxygenated or what but my plug was a nice tan color so I don't think it was lean. I suppose I'll have to go back to the wallet draining 110. ah the price of compression.
AVGas: The Truth
Posted by Rich Rohrich
The simple answer is:
100LL (Blue) Avgas seems to be the most readily available version so I'm assuming that's what we are talking about. 100LL Avgas USUALLY isn’t the best choice but it won't hurt anything.
** For those of you in a hurry, or just sick of me rambling on about this crap skip down to the bottom of the thread to the >>>>>> for a summation.
For those of you still with me, here are some details.
Contrary to popular belief this isn't 100-octane fuel. Aviation fuels are rated on an ASTM Lean/Rich performance number system. 100LL is rated at 91/96 By comparison; Unocal Leaded race gas that is used in lots of spec fuel racing classes has performance number of 112/160. 100LL is closer to 91 octane (MON); by comparison VP C12 is rated at 108 (MON).
For our purposes Avgas has a couple of problems:
1) The 90% boiling point for 100LL Blue Avgas is set at 275 degrees F, which in an engine that turns over 7000 rpm will likely make less power than a fuel that has it's 90% point lower. Pump gas has similar problems, but most good race gas will have 90% Point MUCH lower. As an example Phillips B32 has a 90% boiling point around 235 degrees F and VP C12 has a 90% boiling point around 220 degrees F.
2) Depending on the refiner 100LL can have fairly high aromatic hydrocarbon content, in the 30% by weight range. This level of aromatics will tend to make the throttle response mushy and flat in applications that see big throttle opening transitions on a regular basis. It's similar to what happens when you dump a lot of Toluene based octane booster in your fuel. Throttle response becomes a distant memory.
3) The vapor pressure and distillation curve of Avgas just doesn't seem right for our purposes. The distillation curve or Volatility curve of a fuel determines to a large degree the warm-up, transitional (on & off) throttle response, and acceleration characteristics of an engine.
Here's the simplified version:
A fuels distillation curve designates the maximum temperatures at which various points between 10% and 90% of the fuel will be evaporated as well as the maximum end point temperature. So for any Engine/Air Temperature combination there is a minimum volatility that is required for proper running. As you probably know gasoline is made up of different hydrocarbons, with different boiling points. By combining these Hydrocarbons together you get a Distillation/Volatility curve. Some hydrocarbons (light ends) boil off at low temps some at much higher temps. Depending on the intended application, a petrochemist will blend hydrocarbons to get a curve that matches the rpm range, temp, altitude, and acceleration characteristics for the application. The problem with avgas as a race fuel is the fact it is blended for an application where Acceleration and throttle response is not a high priority. If you think about the average light airplane application, you're talking about a fairly low compression engine that runs in a fairly narrow rpm band, and is rarely called on to provide the type of transitional throttle response that a high rpm, acceleration critical application like motocross does. What's more important to the Avgas designer is controlling mixture strength by eliminating the possibility of vapor lock and icing while making sure that light end hydrocarbon fractions don't boil off too early. The lowered rpm ranges used in these engines allow them to push the boiling point up on the upper end as well. As you can see, by using straight Avgas or by mixing various types of fuel together you are modifying a number of important fuel design parameters. You may hit on a combination that works well, but more likely you'll have an engine that doesn't detonate, but doesn't accelerate very well either. So Avgas is SAFE, but not a very good choice. The high paraffinic hydrocarbon content of 100LL makes a very good base stock if you want to play back yard petrochemist, and I believe this is how some of the smaller race fuel blenders start out. I can tell you from experience that it's a ***** to document and test various changes unless you have a lot of time and patience, so trying to come up with your own Super Fuel is probably more trouble than it is worth.
So it sounds like Avgas is really bad for our purposes, and for the most part it is, but given the sorry state of pump fuel today, Avgas is looking better all the time.
Here's my short course take on things based on my experience and personal biases, (keep in mind this is pretty generalized)
- In almost every case 100ll Avgas is a better choice than alcohol pump fuels
- If you don't need the additional octane that 100LL provides, then MTBE based pump premium (especially Amoco) will tend to provide better throttle than Avgas assuming you have any jetting skill. If you can't jet you're just wasting your time worrying about any of this stuff on a stock bike.
- Mixing 100LL Avgas with a good race gas designed for your application and rpm range is a reasonable way to save some money.
- Mixing alcohol based pump fuels with ANYTHING in an attempt to make it BETTER is just a chemical circle jerk, and if you're that cheap or that ignorant you deserve the crummy performance and the insurmountable jetting problems that you will invariably be blessed with.
- Milspec Avgas is a different animal entirely, but isn't readily available so we won't worry about it.
- The correct race fuel for your application will outperform ANY of the above, regardless of whether the engine is stock or modified. The more demon tweaks hiding in your engine, the more you have to gain.