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Old 02-28-2011, 10:34 AM   #31
supershaft
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Quote:
Originally Posted by Weaverman View Post
It depends on which parameter you're tuning. When developing an engine, the first variable you start analyzing is MBT, since it impacts losses directly. Fuel mapping comes later.
When I run out of gas, that impacts my losses directly.

Sorry but you are going to have to start from the beginning.

For instance, why isn't every torque reading mean best torque since the reading is a combination of numerous variables? Not to mention the dyno run itself!

Personally, I am talking about what works on my own personal bike and what I have seen work on numerous other BMW airheads in the dyno room and then later tested with track time and or raod miles. Let's get back to bikes. What A/F works best in your bike? My A/f is around 13.25. It has been for about 50,000 miles now with nothing to show for it but better mileage and more power.

supershaft screwed with this post 02-28-2011 at 10:42 AM
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Old 02-28-2011, 10:52 AM   #32
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Lornce - Kinda sorta. I was a spark plug/ignition/combustion system engineer at AC for about 25 years. AC was a GM subsidiary (later Delphi, now a zero). I was responsible for all GM V8s. It doesn't matter though, all engine development programs follows the same path. I've worked on programs for Ford, Lotus, McLaren, Mercury Marine, OMC, Stihl, Suzuki, a whole bunch of NASCAR teams, all kinds of independent race teams from outboards and snowmobiles to top fuel, Bonneville cars, etc. I've also raced bikes, boats, cars, & karts over the last 40 years or so. I'm still building and wrenching every day in my shop, and consulting.
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Old 02-28-2011, 10:58 AM   #33
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I love this place!
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Old 02-28-2011, 11:08 AM   #34
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It is a great place but lets get back to jetting our bikes. I think I figured out how to bottom line it. There is a reason why the dyno operators we have seen lately on this forum set their A/F target line at 13.5 or 13.25. It's because that is what they have seen work! That is what I have seen work. The shop that I had my bike dynoed at has the target line at 13.5 and they by and large dyno mostly Harleys. Aircooled Harleys with a hot rear cylinder and all.
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Old 02-28-2011, 11:12 AM   #35
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There's only one "best"!

Consider a "simple" application; one without adjustable timing. Let's say you're tuning a Sprint Cup car or NHRA Pro Stock car (or a Grand Prix hydroplane, or a kart, etc.). No electronics allowed, and mechanical advances won't survive, being reduced to fragments inside your distributor in fairly short order. You've got to select a spark advance number...over a fairly wide range or engine operation. That's why it's called MBT. There's no spark "mapping" here because the timing is fixed. But you better believe the builder knows exactly how much torque a given advance setting nets him at every RPM increment, and he'll know how much time he spends at each point, and will select timing accordingly. Even though it's called "Mean", for average, the builder will integrate the area under the curve and try to pick the best number for the day and track. After that he'll jet to the edge of detonation. If you watched this year's Daytona 500, and think about this, you know there were a few tuners there who were wishing they had dropped a degree or two of timing since it was warmer on race day than it had been all week. They most likely would not add a jet since it impacts fuel economy more directly. Several engines were burned down that day. MBT timing can't always be run if you're not willing to add fuel!

Modern EFI engines select timing from a map based on a variety of input parameters, the selected timing may or may not be MBT timing since sometimes (at some conditions) the engine is knock limited at MBT, particularly with the fuel specified (regular in most cases). Even then, the spark map can't comprehend everything, like injectors that are clogging, etc. That's when the knock sensor steps in and pulls some of the spark out.

LBT is, in most cases, is somewhere in the range of 12.8 - 13.3 or so to 1 for "neat" gasoline (no ethanol). Add some ethanol (at every pump in who-the-hell-knows what percentage) and that number drops. In many case timing changes closer to MBT make more power, but it takes a lot of runs to find MBT across the range to set the curve correctly. That's why most engines don't run at LBT, a little additional fuel staves off the detonation that comes with the timing increase, and yields more power. Also, it depends on how long you're planning to run. Most of our bikes never see the track, so does an extra 1 or 2 HP really matter? Do we want to tear it down because the valves are recessed just because we wanted an extra HP or two? Drag racers can run much leaner than cars running at Lemans (or Bonneville) for example because their time in the spotlight is a lot shorter. If you think about it, a top fuel car only fires each cylinder about 275 times during a run, and even at that at least two of the cylinders are auto-igniting (glow engines, basically) by the end of the run, and burned down. When the track will hold it, they're running MBT/LBT every run. But they rebuild the engine after every run, too. The price of power.

Weaverman screwed with this post 02-28-2011 at 11:38 AM
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Old 02-28-2011, 11:44 AM   #36
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For starters, by the time a dyno starts meaningfully reading the torque and A/F, most all ignition curves are fixed. If they aren't, they soon will be.

Best torque can't always be run? You are splitting hairs that most of us are never going to grow. MBT can always be run for a particular situation. You can jet our bikes above 13 and have plenty of safety zone. I have seen it with my own eyes countless times. Can our bikes melt at that A/F? Sure they can! It all depends. You have got to jet YOUR setup. If you could only get best power from reading about it on the net!
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Old 02-28-2011, 12:31 PM   #37
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Hahaa! Yup...just follow the 'net!

The best thing on topic here, since the original question had to do with jetting, is to think about fuel. You never know what you're going to get at the pump these days, unless you plan to run race gas or some research grade. The GS in question was jetted by BMW to fit the US market 20 years ago. Ethanol in the fuel was unheard of (although there were some oxidizers around then, in some fuel markets). We're about to move to 15%...thanks, EPA. Stoich on "neat" gasoline is ~ 14.7:1. 100% Ethanol stoich is a LOT lower ratio...not sure exactly, but somewhere around 8:1 (without doing a lot of math). That means we need BIGGER jets to run this 10 - 15% ethanol shit. Don't touch a thing. In a year or so you'll probably have to put 140s in it.

Here's a decent read... http://www.amtonline.com/publication...ubId=1&id=1171

MBT can't always be run...sorry. Seen it a thousand times. At least, not without race gas. The bigger the bore, the bigger the problem. An air cooled 500cc cylinder is a perfect example of something that probably can't tolerate all the spark advance it takes to make peak torque; particularly at elevated operating temps. Of course, the GS has a low CR, which helps. So consider an old R100RS.
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Old 02-28-2011, 12:36 PM   #38
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Supershaft, if what you're thinking about MBT timing always being attainable, why do they make premium fuel? For rich people?
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Old 02-28-2011, 12:36 PM   #39
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Lots of turbulence and dual plugs.


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Old 02-28-2011, 01:11 PM   #40
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On a side note.. hows that ethanol going to affect our fiberglass tanks ... just kidding

As for now.. plans are to get rid of the air inj. system, check the air filter and replace if necessary, and maybe down the road..do something with the exhaust.
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Old 02-28-2011, 01:37 PM   #41
supershaft
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Quote:
Originally Posted by Weaverman View Post
Hahaa! Yup...just follow the 'net!

The best thing on topic here, since the original question had to do with jetting, is to think about fuel. You never know what you're going to get at the pump these days, unless you plan to run race gas or some research grade. The GS in question was jetted by BMW to fit the US market 20 years ago. Ethanol in the fuel was unheard of (although there were some oxidizers around then, in some fuel markets). We're about to move to 15%...thanks, EPA. Stoich on "neat" gasoline is ~ 14.7:1. 100% Ethanol stoich is a LOT lower ratio...not sure exactly, but somewhere around 8:1 (without doing a lot of math). That means we need BIGGER jets to run this 10 - 15% ethanol shit. Don't touch a thing. In a year or so you'll probably have to put 140s in it.

Here's a decent read... http://www.amtonline.com/publication...ubId=1&id=1171

MBT can't always be run...sorry. Seen it a thousand times. At least, not without race gas. The bigger the bore, the bigger the problem. An air cooled 500cc cylinder is a perfect example of something that probably can't tolerate all the spark advance it takes to make peak torque; particularly at elevated operating temps. Of course, the GS has a low CR, which helps. So consider an old R100RS.
Who would call best torque a torque reading that can not be sustained under normal operating conditions? Perhaps to see how quick you can lunch an engine? Why even consider a torque that isn't doable? It seems pointless to me other than see just what will destroy the engine. I have no interest in that. My point is that best torque can be found for any particular situation. Regular OR premium. My above 13 A/F ratio has plenty enough room to cover both because, even above 13, it is still not close enough to rule one out. That's my main point.

The fact that our bikes run just fine with 10% ethanol demonstrates that our jetting is indeed a bit fat. If it wasn't already, the stuff would be melting engines right and left which it isn't. Maybe 15% will get our bike leaned out about right without rejetting?

Thanks EPA? I think you mean the corporate farming lobby. Talk about corporate welfare. Ethanol mixed gas is a double whammy!
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Old 02-28-2011, 01:52 PM   #42
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I love this site!
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Old 02-28-2011, 01:58 PM   #43
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I have one & I'll probably never sell it, but what a waste of 1,000ccs!

Talk about understressed.......with that HP to displacement ratio they should run forever without a rebuild.
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Old 02-28-2011, 08:07 PM   #44
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I love this site!
Yeah me too, even if I dont understand a thing and just like the way the words sound

I would be curious to get mine on a dyno too, but I assume it would be pretty anticlimactic.
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Old 02-28-2011, 09:04 PM   #45
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It's going to take me an hour of research on the internet to even understand what ya'll are talking about but I'm sure it'll be worth it! And I thought that short piston/long rod thread was enlightening.
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