I put in 48# injectors and now I cant get the injector constant right. I have done the math and it comes out way too lean. I find it odd that the stock inj. constant is .2025 for 24# injectors and the reflash is .32359 for 30# injectors. Now when I do the math I get a constant of .17977. Why is this lower than the stock constant? If you were to go by their numbers then the bigger the injector the larger the constant should be. Wrong. I have it ideling ok now by setting the constant to .2900. There was no math involved in this, Just trial and error. Now when I enter power enrichment mode my afr goes down to 10. Can anybody make any sense of this?
GM raised the injector constant for the GM S/C injectors to make them work harder. If you compare the rest of the program, you'll also see that they LOWERED the VE table (when an S/C will actually raise it). Those two things work in combination with each other to get the a/f right.
You are doing the math right, my injector constant for 42lb/hr injectors is .13500
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So what do I do with the VE tables?
Would I multiply the VE tables by .5555 like I did with the injector constant?
You could have your injector constant set a bit higher than where you would calculate it to be, that would mimic what they did, but what I would do is set the constant where it should be, and tune the VE table to bring your A/F's in line.
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I have to make an appointment at the local dyno to have it tuned. He has a guy that does all of his GM tunkng with the HPTuners and he is very good. I was quoted like 375 to do it. Does this sound worth it.
This tuning stuff is all new to me , so its probably worth it to me. Unless you have any pointers.
I'm still working on how to tune the VE, spark tuning isn't too bad. Basically, you set the high and low octane tables to be the same, go do some logging of the KR, MAP, and RPM. Do a bunch of just basic runs, then some WOT runs. Go back and look at what cells you got Knock Retard in, reduce the timing there, increase the timing in other places where you don't have retard. Then when you are done, you set the low octane table to be the same as the high again, then multiply it by .85.
I'll have to host some histograms so people can actually see what the logging is, as it is, the current histograms in the scanner don't work with 4 cylinders.
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Shifted wrote:I'm still working on how to tune the VE, spark tuning isn't too bad. Basically, you set the high and low octane tables to be the same, go do some logging of the KR, MAP, and RPM. Do a bunch of just basic runs, then some WOT runs. Go back and look at what cells you got Knock Retard in, reduce the timing there, increase the timing in other places where you don't have retard. Then when you are done, you set the low octane table to be the same as the high again, then multiply it by .85.
I'll have to host some histograms so people can actually see what the logging is, as it is, the current histograms in the scanner don't work with 4 cylinders.
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Tuning timing till you hit knock is in no way proper "spark tuning".
The only way to tune spark is on a dyno, period.
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First, the injector constant is the basic time the injector is turned on. It represents the "right" amount of fuel for the displacement of the engine. Delivered AFR values depend on this constant. Larger constants produce more delivered fuel.
If the S/C reflash ran decently with your setup on your old injectors, the easiest way to get this job done is to scale the constant for your new injectors. Your new injectors are 48 # and the reflash was designed for 30# units. Since you want the constant to be lower with the larger injectors, use 30 / 48 = .625. Now multiply the S/C reflash injector constant by the scale value to get .32359 * .625 = .20224 This is the value you start with. It may change slightly with some trial and error.
Notice how close this number is to the one you've entered. But your number is larger, meaning more fuel. The constant is the basic value used in calculating how long the injectors need to be on to deliver the desired amount of fuel. The only time the ecm can correct for a wrong constant is in closed loop. So if you tell the ecm you want 12.0:1 AFR at WOT and the ecm thinks you're running a 37# injector instead of a 48, it will blindly open the injector enough to give you 12.0:1 with a 37# injector. But since your injectors are really larger, you may end up around a 10.0:1 AFR on a wideband. If you lie to the ecm it will lie to you.
Shifted's right that the "most best right" way to do it is to use the correct injector constant and work through the VE tables. But this is also the most time consuming, and it may not provide any better result than what GM has done. In the case of GM's work, two wrongs can make a right. If it ran well before the injector change, it should run well after.
If you can get voltage corrections for those injectors, make sure to enter them. They can make a real difference when things like A/C or heater or a big amp are drawing power at idle and cruising at low speed.
Second, spark and fuel are not independent of each other. You can tune spark to the point of best safe advance for the current AFR but you cannot "set it and forget it" if you're trying to dial in the VE table. Also, the O2 (and WBO2) are positioned after the fact. They can only report on the state of exhaust as it passes the sensor. This means that in most cases, advancing timing can cause an O2 sensor to report a richer mixture, and retarding the timing can cause a reported leaner AFR. This is because more or less of the combustion reaction occurs within the cylinder depending on when the spark starts the reaction. At higher engine speeds and as exhaust temps increase, some of the reaction continues to occur as the end gasses leave the cylinder and travel down the pipe. This will affect the O2 sensor reading. Finding best timing and fuel means playing a balance game.
To make matters even worse, spark knock visible on the scantool "right now" may not be due to excess spark advance "right now." Take the case of an accelerating engine. It's often possible that knock retard seen at, say, 3500 rpm WOT is actually due to a lean condition or excess advance which happened between 2500 and 3000 rpm and heated the combustion chamber. You can decrease advance at the rpm which knock occurs, but you're only putting a band aid on the problem, not curing it. It takes review of data and careful consideration to find the real culprit.
Third, the only way to tune spark is not on a dyno. The fastest way to tune WOT spark for best power is on a dyno. I ran a chassis dyno for 3 years. To tune spark "everywhere" takes more money than most people can afford to spend. I've developed calibrations from scratch and I'll tell you that there are plenty of cases where you never need to hit the dyno to tune spark. In fact, I usually tell people to get as much correct on the street as they can then hit the dyno for final "check and tune" to save $$. I agree 100% that simply tuning spark to the point of knock then assuming it's right is bad juju. And I agree that using a combination of wide band O2, load, and datalogging on a dyno are much better. But IMO it's also possible to do the same on the street for 90% of the driving conditions which most people encounter.
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