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Some pondering lead to some ?'s

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So alot of people having the "popping on deceleration" issue got me thinking. Usually not a good thing, but thought I would get some opinions anyway. We all seem to be of concensus that this is a lean condition. Either pilot jet/ fuel screw or air coming in the exhaust joints.

Now follow me here. Lower octane fuel such as pump gas doesn't provide the most complete burn compared to say a race fuel. Is it possible this popping is actually unburned fuel? I only ? this because I am running a race fuel blend and don't seem to experience alot of the jetting/ backfiring issues I see from other forum members.

To put you rocket scientists out there to the test. Could someone please explain to me how lower octane fuel makes more power in nothing except single cylinder 4 strokes? Some jetting is necessary, but I have yet to see a 2-stroke snowmobile or dirtbike that didn't benefit from higher octane fuel. Why don't dirt track stock cars just fill 87 at the pump? My Husky rocks on 98 octane, but yet it is supposed to run better on 92 (which it doesn't)? This goes against everything I have learned since I was a teenager. Technically, I should be able to add more timing advance and make even more power with high octane. Not to mention the slower burn allowing you to give the motor more fuel.

Just looking to get some thoughts from those of you more knowledgable types out there. Hopefully George can throw some facts our way.

Happy 4th of July everyone!:thumbsup:

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I run VP 100 octane with a TI-4 Pro circuit and have none of the popping issues and my bike runs very clean.

Happy 4th! Taking my kids to the local parade today!

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I only use 98RON gasoline just like the manual says (that's our best pump gasoline...we also have here 95RON)...no backfire in the aggressive ignition curve...only some popping when using the torque curve if i pull too high the RPM and cut the throttle suddenly but that's very rare because i only use that curve in low to middle RPM...

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Monte-

What are you blending?

I've thought about trying something higher than the 92 I find at the pump, just to see how she does.

paul

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Currently I'm using 60% 92 octane pump with 40% vp Red (I think 100 octane). Upon doing the math again I think it comes out to be about 96 not 98 like I thought. Pretty cost effective to mix and I commute alot so I wouldn't want to spend too much on fuel. I have some vp c10 I will be using next probably mixing about the same. The c10 is unleaded (the environmentalist that I am) and non oxygenated with roughly the same octane as the red. Apparently the more oxygenated fuels shouldn't be allowed to sit as they can gum up carbs rather quickly.

The # that AJSB is referring to is the research octane which is usually the highest #durring testing. I really don't know enough about how they come up with these #'s. I wouldn't be surprised that he may have access to better fuels. In the states we use the R+M/2 (you may see this on the pumps). This is the research octane plus the motor octane divided in half. All very confusing. This final # can vary from different fuel producers and types of fuel. Check out VP's site to be completely confused. http://www.vpracingfuels.com/index2.html Scroll through and check out some of the differences.

So, from the replies so far......high octane and no popping:thinking:

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Currently I'm using 60% 92 octane pump with 40% vp Red (I think 100 octane). Upon doing the math again I think it comes out to be about 96 not 98 like I thought. Pretty cost effective to mix and I commute alot so I wouldn't want to spend too much on fuel. I have some vp c10 I will be using next probably mixing about the same. The c10 is unleaded (the environmentalist that I am) and non oxygenated with roughly the same octane as the red. Apparently the more oxygenated fuels shouldn't be allowed to sit as they can gum up carbs rather quickly.

The # that AJSB is referring to is the research octane which is usually the highest #durring testing. I really don't know enough about how they come up with these #'s. I wouldn't be surprised that he may have access to better fuels. In the states we use the R+M/2 (you may see this on the pumps). This is the research octane plus the motor octane divided in half. All very confusing. This final # can vary from different fuel producers and types of fuel. Check out VP's site to be completely confused. http://www.vpracingfuels.com/index2.html Scroll through and check out some of the differences.

So, from the replies so far......high octane and no popping:thinking:

We are lucky enough in SoCal to have local stations at least in Corona/Norco/Temecula that have VP on tap.... my local 76 station pumps 100 Vp out of one of their 12 pumps... I had the popping until I rejetted, switched to the Husky TI race silencer and then ultimately to the Pro Circuit Race silencer ( spark arrested with a 96 db tip available) and then started using the 100...... I never have it now... $5.99 gallon

I did play with the T handle fuel screw for about 1/2 hour to find the right setting after rejetting and since then have not touched it... 1 1/4 turns out using the yamaha needle in clip pos #3 60 leak 45 pilot stock main

I'm not using it as a commuter though......

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...To put you rocket scientists out there to the test. Could someone please explain to me how lower octane fuel makes more power in nothing except single cylinder 4 strokes?...

I happened to see this in the new posts column and I was intrigued by your questions. I didn't copy the whole quote, but I'll do my best to answer all of it. :busted:

The deceleration popping is from a lean condition, but it's perhaps not what you think. During decel, without an ACV to enrichen the mixture, the a/f ratio becomes to low for combustion to occur and unburt fuel collects in the exhaust pipe. A similar effect occurs with an exhaust leak. After a short time enough raw fuel has collected to ignite and the exhaust "pops". Running the pilot circuit rich tends to keep the a/f ratio within the combustion range longer, with less observable popping.

I disagree with you regarding low octane fuel not burning as completely as race fuel. Lower octane fuel burns faster than higher octane fuel, and depending upon engine RPM, may burn more completely than a higher octane fuel. Although race fuel has a different composition than pump gas, the principle is the same.

Since higher octane fuel burns longer, the net effect (without rejetting) is that the engine "sees" a richer fuel mixture throughout the combustion cycle. As noted above, a richer fuel mixture = less decel popping, so switching from pump gas to race fuel or fuel blend should result in less popping, and observably it does. :thumbsup:

Properly jetting for race fuel includes leaning the fuel circuits so that the engine "sees" an a/f ratio that is the best compromise between power and a/f ratio linearity over the engine rpm range. Properly tuning a carb for race fuel will still result in noticeable decel popping.

But - if the octane rating is too high, the fuel doesn't burn completely enough during the power stroke and less engine power is produced per combustion cycle. This means that some of the "push" that would have been created by the a/f mixture is wasted into the exhaust as heat giving both a reduction in power and an increase in fuel consumption (since a wider throttle opening is required for a similar power output). You can see this in dragsters where a huge amount of unburt fuel is wasted and long jets of exhaust flame are the norm. (In that particular case, the energy of the fuel burned within the engine vastly outweighs the concern for wasted fuel).

Perhaps the primary reason for using a high-octane fuel is to allow a higher compression ratio, which creates more power for a given volume of a/f mixture (with the same a/f ratio as a lower compression ratio) without detonation occurring. In this case, a "balanced" octane rating of the fuel is chosen to allow operation at the higher compression ratio and also to ensure that the fuel burns fast enough so that as little "push" is wasted out the exhaust as possible so as to provide for a net power increase.

For example: NASCAR engines run a relatively low octane rating* for their compression ratio (14:1) with pre-1997 engines running around 18:1. But they spin the engines around 8,000 rpm so that the relatively rich** a/f mixture doesn't detonate (create excessive pressure before TDC) and as little "push" as possible is wasted out the pipe.

*100 octane Sunoco 260GT through 2007, 98 octane Sunoco 260GTX in '08

** compared to what we run in our bikes

You can add advance to some degree to offset the slower burning of higher octane fuels with an improvement in overall power. However, if combustion occurs too soon on the compression stroke, the mixture will cause too high of a cylinder pressure with subsequent excessive piston/pin/rod/main bearing loading on the compression-side of the stroke and the engine will mechanically fail. :thumbsup:

(This timing effect would occur no matter what octane fuel is used, it's just a different degree BTDC for a given fuel)

Blending a race fuel with pump gas can help with decel popping and possibly add power if the pump gas is burning too rapidly and the engine could benefit from a longer "push". Up to a 50-50 blend can often be used without requiring rejetting, although a bike should be tuned for whatever gas/fuel/fuel-blend is being used.

There really isn't a performance issue with light decel popping, other than it's annoying - especially in the woods riding behind someone with a really lean bike... :cheers:

Hope this helps! :bonk:

(yeah, I get a little crazy with the smilies...)

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Thanks David that was awsome! :thumbsup: You did of course lead me to another question. I always thought that the slower burning of a high octane fuel made the engine think it was lean? I'm probably over simplifying, but I have always had to run bigger jets in my 2 strokes when running a race fuel. In fact, the plug on my husky looked way too rich when I was just putting pump gas through it and without changing jetting and just mixing the fuel it looks picture perfect. From what you are saying it sounds like I just have it backwards. :thumbsup: I remember seeing a combustion chamber video years ago comparing high and low octane fuels. The lower was basically a red flash explosion, where as the higher octane was actually a intense blue burn? I can see how that explosion could produce more power, but us average guys don't get new motors every weekend like Ricky Carmichael. I would think that motor takes quite a hammering. :busted: Dang, who thought we would learn anything on our day off? Thanks again David your insight is much appreciated.

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I always thought that the slower burning of a high octane fuel made the engine think it was lean? I'm probably over simplifying, but I have always had to run bigger jets in my 2 strokes when running a race fuel. In fact, the plug on my husky looked way too rich when I was just putting pump gas through it and without changing jetting and just mixing the fuel it looks picture perfect...

You're right, it's not that simple. :thumbsup: It highly depends on the fuel/fuel-blend used and the combustion physics of a particular engine.

"Ballpark" Lean vs. Rich matrix:

Pump Gas vs. Race Fuel = Result

non-oxygenated vs. non-oxygenated = Slightly Rich

non-oxygenated vs. oxygenated = Lean

oxygenated vs. non-oxygenated = Rich

oxygenated vs. oxygenated = Slightly Rich

Here's a TT link with Eddie that touches on the subject:

http://www.thumpertalk.com/forum/showthread.php?t=505346&highlight=race+gas+jetting

(and thanks for the nod guys!) :thumbsup:

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Well guys as I see it there is only one thing left to do. David will have to sell his yamaha and buy a husky and start hanging out over here more often. :thumbsup:

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Well guys as I see it there is only one thing left to do. David will have to sell his yamaha and buy a husky and start hanging out over here more often. :thumbsup:

Hehe, yeah. Someone needs to convice Bert's in Azusa to start selling them and stocking parts (unlike the KTMs that they sell and "minimally" stock parts for)...

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I happened to see this in the new posts column and I was intrigued by your questions. I didn't copy the whole quote, but I'll do my best to answer all of it. :busted:

The deceleration popping is from a lean condition, but it's perhaps not what you think. During decel, without an ACV to enrichen the mixture, the a/f ratio becomes to low for combustion to occur and unburt fuel collects in the exhaust pipe. A similar effect occurs with an exhaust leak. After a short time enough raw fuel has collected to ignite and the exhaust "pops". Running the pilot circuit rich tends to keep the a/f ratio within the combustion range longer, with less observable popping.

I disagree with you regarding low octane fuel not burning as completely as race fuel. Lower octane fuel burns faster than higher octane fuel, and depending upon engine RPM, may burn more completely than a higher octane fuel. Although race fuel has a different composition than pump gas, the principle is the same.

Since higher octane fuel burns longer, the net effect (without rejetting) is that the engine "sees" a richer fuel mixture throughout the combustion cycle. As noted above, a richer fuel mixture = less decel popping, so switching from pump gas to race fuel or fuel blend should result in less popping, and observably it does. :thumbsup:

Properly jetting for race fuel includes leaning the fuel circuits so that the engine "sees" an a/f ratio that is the best compromise between power and a/f ratio linearity over the engine rpm range. Properly tuning a carb for race fuel will still result in noticeable decel popping.

But - if the octane rating is too high, the fuel doesn't burn completely enough during the power stroke and less engine power is produced per combustion cycle. This means that some of the "push" that would have been created by the a/f mixture is wasted into the exhaust as heat giving both a reduction in power and an increase in fuel consumption (since a wider throttle opening is required for a similar power output). You can see this in dragsters where a huge amount of unburt fuel is wasted and long jets of exhaust flame are the norm. (In that particular case, the energy of the fuel burned within the engine vastly outweighs the concern for wasted fuel).

Perhaps the primary reason for using a high-octane fuel is to allow a higher compression ratio, which creates more power for a given volume of a/f mixture (with the same a/f ratio as a lower compression ratio) without detonation occurring. In this case, a "balanced" octane rating of the fuel is chosen to allow operation at the higher compression ratio and also to ensure that the fuel burns fast enough so that as little "push" is wasted out the exhaust as possible so as to provide for a net power increase.

For example: NASCAR engines run a relatively low octane rating* for their compression ratio (14:1) with pre-1997 engines running around 18:1. But they spin the engines around 8,000 rpm so that the relatively rich** a/f mixture doesn't detonate (create excessive pressure before TDC) and as little "push" as possible is wasted out the pipe.

*100 octane Sunoco 260GT through 2007, 98 octane Sunoco 260GTX in '08

** compared to what we run in our bikes

You can add advance to some degree to offset the slower burning of higher octane fuels with an improvement in overall power. However, if combustion occurs too soon on the compression stroke, the mixture will cause too high of a cylinder pressure with subsequent excessive piston/pin/rod/main bearing loading on the compression-side of the stroke and the engine will mechanically fail. :thumbsup:

(This timing effect would occur no matter what octane fuel is used, it's just a different degree BTDC for a given fuel)

Blending a race fuel with pump gas can help with decel popping and possibly add power if the pump gas is burning too rapidly and the engine could benefit from a longer "push". Up to a 50-50 blend can often be used without requiring rejetting, although a bike should be tuned for whatever gas/fuel/fuel-blend is being used.

There really isn't a performance issue with light decel popping, other than it's annoying - especially in the woods riding behind someone with a really lean bike... :cheers:

Hope this helps! :bonk:

(yeah, I get a little crazy with the smilies...)

David,

Does this mean that I have just got lucky with my special and or unique combination of using an open race pipe combined with the higher octane and dialed in jetting? I truly feel that my bike runs cleaner and pulls harder with the higher octane yet from your post above , I would gather that the higher octane may rob power unless the rejetting and or combination of open pipe, jetting, clip position and octane have all blended to make the bike run cleaner and harder...?

Curious To know your thoughts.... I am a novice in a field of professionals here when it comes to jetting....

Tim

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So for little ole me and my 93 octane from the pump running 450, I am experiencing a lean condition that is mostly annoying?

I called JD and spoke with Dee and she recommended leaving the pilot (42) yet bumping up the main one size to a 185 and dropping the clip one position to the 6th to fatten it up a touch.

Does this sound okay? I thought it was lean in the range of the pilot?

I just want it to stop. waaahhh.

Do the pipes really leak bad enough to attempt to seal them? What stuff out there will actually work to seal the junction of the mid-pipe to the header and exhaust?

Thank you gents?

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Hey Tim, I think you and I stumbled onto sorta the same thing. The only difference is I didn't go with a open silencer and Yamaha needle. The part that I believe applies to us husky riders is that our motors are probably benefitting from the "extra push". The compression is 13:1! Not sure if most of the four strokes are that high. I'm still pretty new to thumpers.

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I thought it was lean in the range of the pilot?

The pilot jet & fuel mixture determine the amount of fuel going to the engine when the throttle is closed - almost exclusively.

There are some people around here that have Huskys that I have met. They re-jetted & needed 45 pilots for summer conditions near sea level. Initially the TE510 had a 42 in it and with the throttle closed rolling down a gentle hill it sounded really bad. To the point of other people turning their heads.

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David,

Does this mean that I have just got lucky with my special and or unique combination of using an open race pipe combined with the higher octane and dialed in jetting? I truly feel that my bike runs cleaner and pulls harder with the higher octane yet from your post above , I would gather that the higher octane may rob power unless the rejetting and or combination of open pipe, jetting, clip position and octane have all blended to make the bike run cleaner and harder...?

Hi Tim,

You're probably not "just lucky". The chemical engineers formulate fuels to help us with stuff like this, and dialing-in your jetting maximizes your bike's performance for whatever mechanical and fuel combination you're using. VP100 isn't too dissimilar to pump gas like VP's CMP or DRT would be.

Here's a great link regarding octane ratings and fuel formulation:

http://www.dmperf.com/fuel_brands_and_octane_ratings.htm

Here's a link for VP fuels. Note that some of the MX racing applications like the U and M formulas use low octane ratings to maximize power - MR9 (87 octane) makes more power** than MRX01 (98 octane). Go figure... :bonk:

http://www.vpracingfuels.com/vp_01_fuels.html

**what works well in one bike may not work the same in another bike or with another riding style

What you're feeling is likely not due to an increase in octane, but an improvement in fuel formulation that allows you to use more fuel per combustion cycle, releases the fuel's energy in a way that is more efficient for your engine and mechanical configuration, or both.

-Dave

I had some free time at work, so I kept typing. :busted:

Here's some more fuel formulation stuff that may help clear up some things, or make your head hurt, not sure which: :lol:

The octane rating of a fuel is simply the resistance the fuel has to detonation in a research engine. RON is determined at one load setting, MON is at a higher load. The big difference that most people feel in their bike is not an improvement in performance simply due to an increase in octane, but due to an improved fuel formulation that releases energy in a manner that's more efficient for their engine, allows an increase in timing that improves rpm capability or when the energy is delivered, allows a richer mixture, or a combination of the above. Unfortunately, picking a fuel isn't really a one-size-fits-all solution.

"Street Blaze 100" (which is the VP100 that I believe you're using - the orange stuff) is an ethanol-oxygenated fuel, so if you were already running a tuned bike with oxygenated pump gas, you may not have had to make much of a jet change. However, since it is oxygenated, you may have been able to run the bike richer which winds up having something of a "nitrous" effect.

Since air going into the engine is constant for a given RPM, and we bump up the main jet with oxygenated race gas to get the fuel portion of the a/f ratio correct to make "best power", additional oxydizer from the ethanol component of the fuel is also added to the cylinder charge. This means that there is more oxygen available for combustion than we'd have with a non-oxygenated fuel at the same a/f ratio. So, we can add more fuel (the petroleum component of the fuel) to the cylinder charge with a resulting higher cylinder pressure from combustion and that translates into more HP. :thumbsup:

(this is what some street racers used to do by adding propylene oxide to pump gas and tuning for best power with that blend - not that it's legal to use that stuff anymore... hehe)

Something we can try "on the cheap":

We can oxygenate our own pump gas at home simply by adding ethanol to non-oxygenated pump gas at a much lower cost than buying $6/gal VP100, but our results may vary. Because of the pump gas formulation, jetting richer to add fuel to the cylinder charge might not make up for the energy deficit that ethanol has vs. gas, even with the oxygenation and additional fuel. If you do this, use fresh ethanol and don't store the blend in the bike because it pulls water from the air.

Why we pay $6/gal for VP:

That brings us back to the fuel formulation of the race gas. In order to get a performance increase, we have to have higher cylinder pressures and/or a longer duration of useable energy transfer to the piston (in pretty weak terms: more integral (total energy transferred) => more HP, more peak cylinder pressure => more torque). The chemical engineers that design the race fuels blend specific fuel molecules so that the "fuel" component of the race fuel burns with a desired amount of energy within a desired amount of time, and potentially at varying rates within that period of time. This allows "custom blends" to ignite with more total advance, but provide the bulk of the energy after TDC and maximize energy transfer to the piston before the piston reached BDC. Some of the stuff is pure genius! :thumbsup:

Why jetting and tuning for fuel is so hard:

VP100 must have a fuel formulation that contains more energy in its fuel component and/or burn in a more efficiently timed way than ethanol-oxygenated pump gas in order to make more power in an engine. But there's more to tuning for a fuel than that.

Cylinder pressure is a relative term and primarily depends on altitude (and atmospheric pressure), temperature, and humidity. However engine designers use "absolute cylinder pressure" (a 15 psi tire at sea level is about 15 + 14.8 = 29.8 absolute psi) when designing an engine in order to determine how much strength they need to build into the engine components, and for sizing the cooling and oiling systems, etc., and is a single constant value. This means that since each of us is located differently geographically and probably not at sea level on a perfect "standard temperature and pressure" day, our cylinder pressures are probably lower than we could have, and we need to use higher compression pistons, higher energy fuels, and intake/exhaust tuning to get our bikes to run cylinder pressures that are closer to the design absolute cylinder pressure limit.

Of course, since everyone's engine/intake/exhaust configuration is slightly different - including how much exhaust packing has been burnt out, header dings, etc. - even for two exact same stock bikes, picking the best fuel, intake, exhaust, cams, and jetting combination for a given day's atmospheric conditions is mind boggling. Even the gearing that we choose changes how fast the engine revs under load and can affect intake charge density and exhaust extraction efficiency - which directly affects cylinder pressure. Some of these things are not practical to change, so we live with some compromises. We simply get our tuning close or wrong most of the time, have a boggy bike at the track that ran fine the day before, and Eddie gets another PM... :cheers:

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...Here's some more fuel formulation stuff that may help clear up some things, or make your head hurt, not sure which: :thumbsup:

...

Think I understood that - explains much regarding the variety of jetting with the same model/make of bike - including the summer & winter fuel formulations.

Believe it also confirms that the MPG on our cars went down when oxygenated fuels were introduced in CA. Of course that's another topic...

.

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THANKS DAVID:thumbsup:

And after reading that I definitely feel that I have been lucky or have been blessed or both that my combination is working great...where I live and with the pipe and jetting I have.

The VP 100 that I use is indeed almost a bright orange in color

Hell, I was happy just to get through that read with a haedache....:thumbsup: you know your fuels!:thumbsup:

And I am running the 45 pilot and typically ride 0 to 4000 ft

Thanks, Tim

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Thanks again David! :thumbsup: I'm feeling alittle selfish having all this info located in the Husky section only. This is some of the best reading (and rereading) I've done in awhile.

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