| Author | Message | ||
     Xodot |
I have seen 4 different fuel maps: one is tuber stock and another is a tuber race ECM and the other two maps have been customized by their rider. Each one of them has "twin peaks" but I can't understand why. In each case there is a peak in fuel delivery at the wider throttle opening positions at 2000 and 5000 RPM (aprox). Can anyone explain why that would be? I thought the twin peaks might be for a favourable EPA rating in the stock ECM, but I expected to see a linear progression in fuel delivery across the common used throttle position/RPM bands with a "race only" ECM. I would be interested in comparing with what you might have done and discovered. Thanks, Bob (Message edited by xodot on August 23, 2008) | ||
| Scott_in_nh |
For any engine there exists spots in the rpm range where valve timing, exhaust tuning and intake track properties (length and volume) create a situation where the reversion - air pulses that travel out of and back into the intake tract - hit the intake valve when it is open, packing more air into the cylinder. When you have more air, you need more fuel, hence the "extra" peak. | ||
| Jos51700 |
/\ || An excellent answer, and SPOT ON!!!!!!! For crystal-clear proof, look up some dyno runs for a stock XB12, and then compare to the same bike done with the exhaust valve held open. That "dip" between the peaks goes away when the valve is allowed to do it job. The fuel curve mimics the power curve during points of reversion. (Message edited by jos51700 on August 23, 2008) | ||
| Xodot |
packing more air into the cylinder I have never head of this before and am quite fascinated. It leads to another question then. If there is more air/fuel mixture than usual in the cylinder during the compression stroke, the volume of mixture must become "more" compressed (since there is more air/fuel before compression begins). Would we not expect the increase in compression to result in engine knock or pinging from pre-ignition? By this I mean the explosion of the air/fuel mixture being caused by compression instead of spark, the explosion occurring prior to the piston reaching the start of the power stroke and is in fact still on it's upward compression stroke travel. I have only a rudimentary understanding of these things so please excuse me if I am missing something obvious here. | ||
| Mmmi_grad |
Explosion is caused by two things. One good = normal spark ignition. The second BAD a hot spots as in a (lean condition)like melting a hole in a piston..... or hot carbon from build up. Remember explosion by compression in the normal matter is in a diesel engine. Pinging is usually a timing problem that can be any single thing of the motor or just a plain worn out one....and that includes a hot engine throwing air fuel mixtures out of whack... everything goes to hell. Detonation is from hot spots giving a fake spark ONLY. So now knowing all of this you can then apply the theory to a supercharged motor. What does that do? It puts more air and fuel mixture in the engine and that does not cause pinging or detonation. Now the Joker: too high compression can cause pinging technically but it aint happening with these motors, these pulses are not going to cause a premature spark. With these motors in general I never let go too long at a droning constant speed. I mean if your flying 70mph down the freeway... Im going to blip WOT the throttle sometimes and throw some gas on the fire. Its good for the motor but these days it costs us more  (Message edited by mmmi_grad on August 24, 2008) | ||
| Xodot |
too high compression can cause pinging technically but it aint happening with these motors, these pulses are not going to cause a premature spark. Is it not happening because the pulses are weak (by weak I mean not a drastic change in air/fuel mixture) or brief (by this I mean the pulsing does not last long enough to be noticed) or because of some other condition? | ||
| Scott_in_nh |
It is not happening because it is weak - it is still under vacuum - and because it is within the design parameters. That is when they test engines with varying compression ratios this condition exists - so if severe knocking is present a lower CR will be used. | ||
| Xodot |
How do the reversion required "twin peaks" of fuel flow get created in a carb? | ||
| Kalali |
"..With these motors in general I never let go too long at a droning constant speed. I mean if your flying 70mph down the freeway... Im going to blip WOT the throttle sometimes and throw some gas on the fire. Its good for the motor but these days it costs us more..."\ Sorry, not sure if I fully understand the purpose of this behavior. Does this help the motor to "cool down" intermittently? | ||
| Mmmi_grad |
OK we are talking about pulses here right. That are going to let some combustion gases back into the intake. Well hitting the intake valve hard with fuel will hopefully help keep it clean. keep other things clean too. With the old motors like panhead and shovel head this was great to do. Things just needed cleaned out. Then we got smart and started putting squish bands in the chamber to concentrate the air fuel mixture and make Efishant the process. Fast forward to 2001 and look at a Oldsmobile Aurora or caddy. Check out the 3.5 4.0 or 4.6 NORTHSTAR motor. This is a hightech motor. It was made to rule the highways and be beat on. As a matter of fact if you baby this motor carbon will build up around inside the combustion chamber. If you ever see a GM northstar car spew black smoke from a WOT its because it has been babied and just saw its first wot in maybe a couple months. No problem here its just the motor is designed to see high performance on a regular basis. We dont have that problem here but I have old habits. OK, if you really want to geek out on engine thoery there are books out that explain it. (Message edited by mmmi_grad on August 25, 2008) | ||
| Kalali |
In my old Alfa Romeos the "weekly tune-up" (recommended by my Italian mechanic) consisted of taking the car out and drive it at around 5K RPM for at least 10 minutes. I had close to 192K miles on the car when he bought it from me... |
