I was reading some of the posts in the pits about Dino's car recently blowing up on a dyno and it made me question my own fuel setup.
From my own experience, it seems that our cars will either run rich from the start or start lean and transfer to rich depending upon where the car is in closed loop during the car's transition into boost.
For example. Suppose you are cruising at 20 mph and you depress the accelerator when the ECU is correcting for a lean condition (bringing it back to rich). During this time, your car will run rich from the start of boost preesure to the end. However, if you are cruising at 20 mph and you depress the accelerator when the ECU is correcting for a rich condition (bringing it back to lean), the car will momentarily run lean (because of the ECU correction) until the fuel system catches back up (Vortech SFMU sees boost, closes off the return, increases fuel pressure, and more fuel is supplied).
It is the second situation that has me worried as you could run lean during boost for a 1,000 or so rpms. Does anyone else experience this?
Fuel Setup Walbro Intank HP Fuel Pump Accel Inline Fuel Pump Vortech SFMU 2.72 Disk and Hard Spring 390 cc/min Injectors Built Motor @ 10 psi
#65044, "RE: Fuel Question - Delayed Response" In response to Reply # 0
Yeah I experience that too with my low boost setup. Check out ner987 post below. He is trying (or he has successfully completed it) to make this circuit thing that prevents that.
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#65050, "RE: Fuel Question - Delayed Response" In response to Reply # 0
Originally posted by KTECLIPSE For example. Suppose you are cruising at 20 mph and you depress the accelerator when the ECU is correcting for a lean condition (bringing it back to rich). During this time, your car will run rich from the start of boost preesure to the end. However, if you are cruising at 20 mph and you depress the accelerator when the ECU is correcting for a rich condition (bringing it back to lean), the car will momentarily run lean (because of the ECU correction) until the fuel system catches back up (Vortech SFMU sees boost, closes off the return, increases fuel pressure, and more fuel is supplied). It is the second situation that has me worried as you could run lean during boost for a 1,000 or so rpms.
You're vastly over simplifying the situation. Plus what was said about Nate's circuit is off. First off, what does "depress" mean 20% throttle, 50% throttle, 100% throttle? You make no mention of being in closed or open loop. That about your S-FMU isn't THAT slow and it's not the only thing controlling your fuel. You can have a crap load of FP but with the incorrect injector pulse width you can still run lean. Like I said you vastly over simplified.
Ok, let me first talk about Nate's circuit. What Nate has designed is a circuit that will "trick" the ECU into going to OPEN loop when ever boost pressure is seen. If I'm not mistaken Nate's design just sent a higher voltage on the TPS wire (make it seem like you are WOT) to fool the ECU into doing that.
Now, back to the question at hand. There is no way that you would be lean for 1000rpms because you were at the lean side of the cycle shown by your NARROW BAND o2 sensor. Once you get on it the o2 sensor should jump right back to the rich side. Also this isn't on a wide band so it's not all that accurate. Also the efficiency to raise the fuel pressure quickly is not related to your S-FMU (it has a small role) but the amount of pressure that your fuel pump can push and how quickly. The higher the flow of the pump the quicker it can acheive the high fuel pressure (now I'm over simplifying but you get the idea).
Anyways, the only time you would be lean during boost if you have it in good tune for WOT is that you are in closed loop so the ECU is trimming fuel off the o2 signal. If you're in open loop and you're lean you have other problems you need to fix (FP, injector size, tuning).
#65057, "RE: Fuel Question - Delayed Response" In response to Reply # 2 Feb-10-04 08:50 AM by KTECLIPSE
I know I over simplified the situation but I was just trying to think of possible causes. To be honest, I was just trying to generalize my situation and didn't even think of the all possible causes (ex: flow rate of the pump).
When I mentioned depress the pedal, I know that the car is still in closed loop until the pedal is fully depressed (WOT). When I floor my car (WOT), sometimes it temporarily goes lean and then bounces right back to dead rich. It seems as though it is only lean during the time it takes to build full boost (a couple of seconds). I have datalogged my car and under full boost, the o2 sensor is reading over 1.0. Here are two runs under the same setup. Notice that on the first one the o2 initially dips (1.0 reading on the o2 sensor is not achieved until 3800 rpms) and on the second the o2 stays much richer in the beginning (2800 rpms) of the boosted run.
Just for some more info, I have also datalogged my injectors to see how the car runs (STFT and LTFT) and have adjusted to get them in a decent range.
#65145, "RE: Fuel Question - Delayed Response" In response to Reply # 3
Hey Kurt,
I was just wondering, you know that you can have the pocketlogger plot all curves on one graph right? Personally I think it makes it easier to read so you don't have to scroll down. to see the graphs that are hidden.
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