Questionable/faulty info on how to set LTFTs
02-09-2006, 08:29 PM
#1
Melting Slicks
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Questionable/faulty info on how to set LTFTs
I still have not gotten any real answers to some of the questions I posed before and to what I see in my own tuning. I've seen others quote some LTFT adjustment methods that just don't make sense. For example, quoting from RWTD's site:
This has never made any sense to me and in fact is counterintuitive and also doesn't match by own findings. Let's take the above example. The fuel mixture you are currently getting is dictated by the injector pulsewidth which is calculated from the base+LTFT+STFT. +6 LTFTs indicate that at part throttle, the base injector pulsewidth is calculated, 6% more fuel is added to that base, and then whatever value appears in the STFT's is added on top of that. At full throttle, the PE is calculated from the base table plus WOT LTFT so in the +6 case, PE would be base+6%.
Now, when you change injector slope, you are changing the base value. Decreasing slope by 6% will cause 6% more fuel to be used as the base calculation. Now LTFTs are zero because the base table already gives you the proper amount of fuel. At WOT, the base table also is giving you the right amount of fuel because the WOT fuel mixture is calculated from a point on the base table. So the fact that LTFTs are zero at WOT doesn't mean you are running leaner or richer. Technically, you should not have to adjust anything in the PE.
Example 1 (LTFTs are +6):
Part throttle (closed loop) mixture: 25ms+6%=26.5ms
WOT (open loop) mixture: 105ms+6%=111.3ms
Example 2 (after zeroing the +6 LTFT):
Part throttle (closed loop) mixture: 26.5ms+0%=26.5ms
WOT (open loop) mixture: 111.3ms+0%=111.3ms
So zeroing LTFTs simply shifts the 6% from the LTFT to the base table. Base+LTFT+STFT will still give you the same mixture as before. All you've done is move the 6% from the LTFT to the base. The only benefit of zeroing the LTFTs is that the car doesn't end up "hunting" and changing the mixture as much and will not take so long to adjust itself after a battery disconnect/reset. I still don't agree with the recommendation to lean the WOT mixture by leaning the PE table when you have positive LTFT. It just doesn't make sense and from my own testing of WOT O2 sensor readings, doesn't work there either. In my own testing, I had LTFTs about +10. Once I zeroed the LTFT's, I found that leaving PE's alone (zero) produced the same WOT fuel mixture that I was getting before (with the +10 LTFTs). In my case, I even had to richen the WOT mixture a bit to get the ideal WOT O2 readings I was looking for. Trying it by the quoted (RWTD) method gave me a WOT fuel mixture that was much too lean.
Comments? RWTD?
Mike
Positive LTFT's - For a positive value you subtract that value in percent in the Predator Injector Slope end-user option. It is also recommended to lean both fuel tables by the same percentage.
i.e. if you have i.e. a +6 on the LTFT's as average, then subtract in % the same amount from the Injector Slope. This will also globally add fueling everywhere, even in PE, so you generally will want to lean out the PE fueling by the same % you just added in the overall fueling.
i.e. if you have i.e. a +6 on the LTFT's as average, then subtract in % the same amount from the Injector Slope. This will also globally add fueling everywhere, even in PE, so you generally will want to lean out the PE fueling by the same % you just added in the overall fueling.
Now, when you change injector slope, you are changing the base value. Decreasing slope by 6% will cause 6% more fuel to be used as the base calculation. Now LTFTs are zero because the base table already gives you the proper amount of fuel. At WOT, the base table also is giving you the right amount of fuel because the WOT fuel mixture is calculated from a point on the base table. So the fact that LTFTs are zero at WOT doesn't mean you are running leaner or richer. Technically, you should not have to adjust anything in the PE.
Example 1 (LTFTs are +6):
Part throttle (closed loop) mixture: 25ms+6%=26.5ms
WOT (open loop) mixture: 105ms+6%=111.3ms
Example 2 (after zeroing the +6 LTFT):
Part throttle (closed loop) mixture: 26.5ms+0%=26.5ms
WOT (open loop) mixture: 111.3ms+0%=111.3ms
So zeroing LTFTs simply shifts the 6% from the LTFT to the base table. Base+LTFT+STFT will still give you the same mixture as before. All you've done is move the 6% from the LTFT to the base. The only benefit of zeroing the LTFTs is that the car doesn't end up "hunting" and changing the mixture as much and will not take so long to adjust itself after a battery disconnect/reset. I still don't agree with the recommendation to lean the WOT mixture by leaning the PE table when you have positive LTFT. It just doesn't make sense and from my own testing of WOT O2 sensor readings, doesn't work there either. In my own testing, I had LTFTs about +10. Once I zeroed the LTFT's, I found that leaving PE's alone (zero) produced the same WOT fuel mixture that I was getting before (with the +10 LTFTs). In my case, I even had to richen the WOT mixture a bit to get the ideal WOT O2 readings I was looking for. Trying it by the quoted (RWTD) method gave me a WOT fuel mixture that was much too lean.
Comments? RWTD?
Mike
02-10-2006, 08:09 AM
#2
Race Director
If the ltft's are positive, then the car is adding fuel, and YOU need to add fuel. I can't speak for your program, but in LS2edit, you would raise the VE table by that same percentage.
I think you've gotten bad advice, but you seem to know the correct thing to do.
About the PE table. I've noticed (on my race car) that it doesn't alway effect my wot as much as it should. I'm still working on getting it dialed in.
I think you've gotten bad advice, but you seem to know the correct thing to do.
About the PE table. I've noticed (on my race car) that it doesn't alway effect my wot as much as it should. I'm still working on getting it dialed in.
