Northman Posted May 4, 2005 Share Posted May 4, 2005 The one thing you're missing is that, yes, the throttle is more open at lower rpm to make the same power, BUT the engine is also asking for less air, which means that it still is pulling in the same amount of air, and same amount of fuel... BUT the throttle is more open, which means less intake manifold vaccum. I've read that statement 6 times now, and I still can't make any sense of it at all... :scratchhead: Quote Link to comment Share on other sites More sharing options...
demon Posted May 4, 2005 Share Posted May 4, 2005 Me either Chris. I've worked on cars all my life professionally and as a hobby and this just doesn't make any since to me. Usually speaking, as a result of RPM, the higher the RPM the more power potential is available. Horsepower is a result of torque and RPM. You don't have to open the throttle at a higher RPM to get the same amount of power at a lower RPM. The potential for more power is there but not needed, thus a throttle to limit the amount of air/fuel in. Do you ever wonder why a truck that's set up for pulling heavy loads is geared lower? It puts the engine in a better RPM for torque output and is more efficient with it's power output. The same load that a lower geared truck (4.11) pulls with ease is harder on a high geared truck (2.73) due to lower RPM and more throttle opening resulting in less efficiency and ultimately less mileage. Now talking about carbs, the lower the intake vacuum, the less efficient the carbs will work at a low RPM. Carbs don't work off of vacuum, they work off of air velocity which is a result of the difference in ambient air pressure and engine vacuum. You ever tried a carb on a car that's too big and it performs like shit? The air velocity is what pulls the fuel out of the bowls through the venturi, not vacuum. The throttle plates are below where all the fuel is introduced to the air stream. The bigger difference in vacuum and outside air pressure increases air velocity through the carb. On modern carbed motorcycles, the carbs are of the constant velocity type. A slide in the carb varies the opening as the engine needs more air. When vacuum from the intake air velocity is applied to this slide, the slide opens to keep things in check. Yada Yada Yada. Basically I agree with Northman on this one. Quote Link to comment Share on other sites More sharing options...
EVLXX Posted May 4, 2005 Share Posted May 4, 2005 You guys are killing me! But keep going.... it's interesting to watch, you guys argue over symantics. Oh and... I'm still with Chris on this one. You can't drive 2 identical vehicles down the same road under the same load with one at 10% throttle and the other at 40%..... and expect to get better gas milage out of the 40%'er.... You know what you're going to get.... Acceleration -or- Decceleration. Quote Link to comment Share on other sites More sharing options...
bartonmd Posted May 4, 2005 Share Posted May 4, 2005 The one thing you're missing is that, yes, the throttle is more open at lower rpm to make the same power, BUT the engine is also asking for less air, which means that it still is pulling in the same amount of air, and same amount of fuel... BUT the throttle is more open, which means less intake manifold vaccum. I've read that statement 6 times now, and I still can't make any sense of it at all... :scratchhead: phraised wrong I guess... rephraised: the throttle is more open at lower rpm to make the same power, but, at low rpm, the engine is asking for less air. This means that, at low rpm/high throttle, the engine is still pulling the same amount of air and the same amount of fuel as a higher rpm/lower throttle position situation. The only thing that makes the higher rpm, lower throttle position less effecient is the frictional losses (minimal) and the pumping losses (higher intake vaccum). I'm disagreeing with the theory that less vacuum is equated with better efficiency. Â Now, don't confuse volumetric efficiency with mileage, either, which is what you're referring to with the "engines are most efficient at WOT theory volumetric effeciency and fuel mileage are related... Less vaccum DOES mean greater effeciency... this is the main reason that diesels are more effecient than gas engines... because they are effectively at WOT all the time, and way lean most of the time. Effeciency vs. vaccum is a pumping loss thing... try this... (on a fuel injected vehicle ONLY, because it shuts off the fuel when the ignition is off) go down the road at 60mph or so in top gear (helps to be a manual tranny)... turn the engine off and go WOT... notice the deceloration... now go closed throttle... notice how much faster you slow down? Same deal as with the engine running, except you can FEEL the power being used to turn the engine with the closed throttle because it's taking the kenetic energy from the vehicle and using it to create the intake manifold vaccum. FI engines go into open loop above 90% throttle, and they do run richer at WOT than they do during closed operation. OK, I had my % wrong... although I thought it was somewhere around 85%... depends on the vehicle and year I guess... the "richer at WOT" was covered when I said "this is how much fuel I need, +20%" Don't the hybrid guys use full throttle to gain full benefit of the electric boost? That's one case where it makes sense. yes, covered by "It also has to do with how much of the hybrid drive is being used on those cars" Those cars are also closed loop at WOT, so they don't ever say "+20%" Mike Quote Link to comment Share on other sites More sharing options...
bartonmd Posted May 4, 2005 Share Posted May 4, 2005 counter-intuitive, I know... I didn't believe it until the PhD guys I worked with/for showed me on the engine on the dyno... These are guys who were working on their PhD's in engine control... I trust them and what I saw. Mike Quote Link to comment Share on other sites More sharing options...
bartonmd Posted May 4, 2005 Share Posted May 4, 2005 BTW, I did get across that I'm talking about lower rpm/higher throttle vs. higher rpm/lower throttle right?? not talking about just having less vaccum at a given engine speed... Mike Quote Link to comment Share on other sites More sharing options...
bartonmd Posted May 4, 2005 Share Posted May 4, 2005 I'll add that the concept I'm talking about it the EXACT same thing as a diesel sitting and idling on almost no fuel while a gas engine is BY FAR the least efficient at idle... like to the tune of running through a half tank of gas idling overnight vs about a half gallon of diesel... Mike Quote Link to comment Share on other sites More sharing options...
Wario Posted May 4, 2005 Share Posted May 4, 2005 My bird get ~35mpg with a k&n and punched mufflers. It gets marginally better mileage if I run between 4k and 7k rpm's. My issue is that as soon as I go past 7k rpm's everything gets blurry... I was wondering if anyone else has this same problem?! Oh, by the way, you're having a milage problem because your sparkplugs are cupped... Quote Link to comment Share on other sites More sharing options...
bartonmd Posted May 4, 2005 Share Posted May 4, 2005 My bird get ~35mpg with a k&n and punched mufflers. It gets marginally better mileage if I run between 4k and 7k rpm's. My issue is that as soon as I go past 7k rpm's everything gets blurry... I was wondering if anyone else has this same problem?!Oh, by the way, you're having a milage problem because your sparkplugs are cupped... I get that too... WTF man!! :wink: Quote Link to comment Share on other sites More sharing options...
FiXXation Posted May 4, 2005 Share Posted May 4, 2005 For a first person, real world example (not motorcycle related): I have a 1989 Ford HD F250 4x4, 460ci FI gas V8, 5sp manual transmission, pulling a 5th wheel camper with a Jeep in tandem behind it (most every summer). Rear end is 3.50:1. If I maintain 65 mph on US 160 across Colorado in OD 5th gear, even with the throttle completely on the floor, I will get about 14 mpg. If I drop down to 4th gear to run the same 65 mph, running less than full throttle (can't tell you what %), my mileage will consistently drop to 9 mpg or less. This will happen every time. Quote Link to comment Share on other sites More sharing options...
bartonmd Posted May 4, 2005 Share Posted May 4, 2005 exactly! towing race trailer... identical trucks... same load, same camper shell, etc... one with 350 at WOT all the time gets 8-9mpg... 454 with plenty of power to spare, not very high throttle position gets 5-7mpg consistantly. Mike Quote Link to comment Share on other sites More sharing options...
XXblitzkrieg Posted June 7, 2005 Author Share Posted June 7, 2005 Just filled up again down to 20 mpg....this tank didn't even see 100 mph once. My crank case oil is smelling very gassy now. something is very wrong. Brett Quote Link to comment Share on other sites More sharing options...
Pete in PA Posted June 7, 2005 Share Posted June 7, 2005 OK all this Fn... theorizing is great but BACK to his problem. Unless I missed it I still don't know if he's carbed or FI. I got 44 mpg on my Sunday ride and I frequently ride in high gear unless engine is lugging. (carbed 98) Quote Link to comment Share on other sites More sharing options...
XXblitzkrieg Posted June 7, 2005 Author Share Posted June 7, 2005 2002 Florida (no emissions)......I'm thinking maybe a sensor has shit the bed. A coolant temp sensor preventing closed loop ?? Quote Link to comment Share on other sites More sharing options...
bartonmd Posted June 7, 2005 Share Posted June 7, 2005 In cars, that'll kick a code, and, therefore an FI light... You can take out the coolant temp sensor and put the end in ice water and mesure the resistance, and then put it in boiling water and mesure the resistance... If it doesn't chance (or is open all the time or shorted all the time) that'll tell you for sure... Mike Quote Link to comment Share on other sites More sharing options...
Steve Posted June 7, 2005 Share Posted June 7, 2005 I got around 35 mpg when slower fun riding, 25-28 mpg when burning the local canyons. My only suggestion would be to get pitch the K&N (hated mine, I know I'll take some heat on this but it killed my gas mileage AND I noticed lots of dirt blow by inside the air cleaner) and make sure teh right plugs are in there. Good luck. Quote Link to comment Share on other sites More sharing options...
XXBIRD Posted June 7, 2005 Share Posted June 7, 2005 Given an ideal ratio of 14.7:1 air:fuel, that would mean less air = less fuel. What is the "optimum" air/fuel ratio for the bird if there is such a thing? I had mine remapped recently. Tech pulled it down to 13 from roughly 14 stating that 13 is the "best suggested". Max hp now is 133.55 @10,000rpm up from 130 according to his dyno. Previous one was 147 peak. Max torque is now 77 @7500rpm up from 74. There were two dips at 5300 and 8200 rpms in the old map. Quote Link to comment Share on other sites More sharing options...
Northman Posted June 7, 2005 Share Posted June 7, 2005 13.2-13.5:1 is ideal for power, but as far as emissions goes, 14.7 is ideal. Basically, that is when there is a perfect amount of oxygen:fuel to ensure a complete burn, theoretically. Quote Link to comment Share on other sites More sharing options...
rgillies Posted June 7, 2005 Share Posted June 7, 2005 my bet is the K&N filter. I had the exact same problem, low mileage black pipes. I put the stock filter back in and it's back to normal. I would try putting the stock filter back in and see what happens. Quote Link to comment Share on other sites More sharing options...
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