The reference period to calculate duty cycle is the amount of time the injector can physically stay open in relation to its current opening duration. You can run the same pulsewidth on an injector, say 5 msec and the duty cycle will rise with RPM. Increase the pulsewidth and you increase fuel delivery, but fuel delivery also increases with rpm as the duty cycle increases. I will make two specific examples here to use. Keep in mind that TBI injectors fire alternately at each reference pulse, 2x per crank rotation each. There is an injector firing every reference pulse, but the alternate back and forth between left and right. You also want the injectors to run no more than 80-85% DC. With that knowledge we are first going to look at what is happening at 600 rpm, again at 1,800, 3,600, and finally 6,000 rpm. I am assuming an engine with a hot-rod nature and peak torque around 3,600 rpm and a 6K redline. It is easiest to look at it in terms of multiples of 6.
600rpm/60sec = 10 r.p.s. 1sec = 1000 msec. 10 r.p.s. is 1 revolution every 100 msec. During that 100 msec time period, you have to squeeze in two injector firings. That means there is 50 msec worth of time to fire the injector each time. At the engines normal idle speed/load you will see around 2 msec pulsewidth. At idle you are at roughly 4% duty cycle and 2 msec pulsewidth @ 600 rpm.
1,800rpm / 60sec = 30 rps (revolutions per second) or 60 fps (firings per second) = 1000/30 = 16.67 msec per firing, max. Your commanded pulsewidth will be around 3 msec at normal load. 3/16.67x100 = 17.99 or about 18% DC for 3 msec @ 1,800rpm
3,600rpm/60sec = 60 rps or 120 fps. 1000/120= 8.33 msec MAX. Your commanded fuel will likely be around 7 msec around peak torque 7/8.33 = 84% DC for 7 msec @ 3,600 rpm.
6,000 rpm/60 sec = 100 rps or 200fps 1000/200 = 5 msec MAX. Your pulsewidth will put you very static here, without a strong fuel delivery system. With the TBI firing stategy you either need to run large injectors or go asynch at high load. Asynch mode fires the injectors 85 times per second on BOTH injectors. This allows you to dump in A LOT more fuel from the same injectors. I am going to go ahead and run the math though. You will very likely need 5+ msec of pulsewidth here. Lets say 5.25 msec. 5.25/5 = 1.05*100 = 105% DC. Obviously once you have reached about 85-90% DC your injectors are static. To stay under 85% DC, you need to keep you pulsewidth under 4.25 msec @ 6,000 rpm.
Now finally here is why to go asynch @ WOT. Your injectors are firing at a fixed 85 hz or 85 times per second which is once every 12.5 msec. If you are firing once per 12.5 msec you are able to run 10.625 msec before you cross 85% DC. You are able to flow ALOT more fuel with one 10.625 msec firing than you are with two 4.25 msec firings totaling 8.5 msec! You are able to flow something around 20% more fuel through the same injectors in asynch mode.
Last edited by Fast305; 10-20-2008 at 01:04 PM.
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