STFT, LTFT with VPC

[Mike 99GSX]

I was hoping that KPT4321 or one of the other knowledgable tuners could explain something to me. My mods are listed in my profile and I have a good handle on how to tune my VPC/GCC at WOT. My question is about the STFT and LTFT that I see on my OBDII pocketlogger. I understand that these numbers are supposed to register near 100 or 0% depending on the year of your car.(It seems we've seen both). And that these numbers are relevant only during a closed throttle loop. If any of thats wrong, correct me, but heres my question. I guess there is no way to compensate for these with a VPC/GCC because the fuel setting are global and not dependent on throttle position. I have no experience with an AFC but I would guess this would be how you would set the low throttle settings and the high would be set more by using the other parameters like timing and A/F ratio. I'm asking because I pulled my spark plugs out the other day and after 500 miles they look very bad. The tips are very white which would indicate a lean condition but the car is definately tuned correct for WOT at 20 PSI boost on 94 pump gas. Does this mean that during normal driving (say a 5th gear cruise control highway trip) that the car is running very lean? Is there nothing I can do about it because of the tuning limitations of the VPC/GCC? Am I worrying about nothing?

 

[Mike 99GSX]

One other thing. My old school V8 dad told me that I should consider going 1 range colder on my plugs and it would help guard against detonation. I'm currently running BPR7ES and I haven't heard of anyone having to run anything colder who is making my power levels. Let me know if this is a good idea or not.

 

[Ronin17]

I've heard some people who've had good luck with the BPR8ES's. The downside to the plugs are that they might get alot of carbon buildup on them if you don't get them hot enough when cruising around. Just be sure to do some WOT pulls on the highway every now and then to make sure no carbon gets built up .

For 10 bucks it wouldn't be a bad thing to just try out. Good luck!

 

[Blk_99gst]

Originally posted by Ronin17
I've heard some people who've had good luck with the BPR8ES's.

I've only heard on people running the 7ES's, not the 8ES's. I think the 7's will take you a very long way.

 

[Blk_99gst]

Originally posted by Mike 99GSX
My question is about the STFT and LTFT that I see on my OBDII pocketlogger. I understand that these numbers are supposed to register near 100 or 0% depending on the year of your car.

-1g's shoot for 100%, 2g cars shoot for 0% LTFT.

Originally posted by Mike 99GSX
And that these numbers are relevant only during a closed throttle loop

-That is correct.

Originally posted by Mike 99GSX
I guess there is no way to compensate for these with a VPC/GCC because the fuel setting are global and not dependent on throttle position

-First off, what are your LTFT over the entire rpm range; they will change over th rpm range by the way.

-Second, I have very little experience with vpc but I can see the problem you're in. You have your correction settings based on what's best for WOT. To digress a little, the safc has two settings, low and high. Low are used for normal driving, closed loop. High settings are used for WOT. Just for an example in my case with 660cc, my low value is -25% (25% less fuel) and my highs are -37% (37% less fuel). Anyway, back to your situation.

Bottom line is I think you are probably doing it right, tuning for WOT, but that depends on your answer to what are the LTFT off the logger. The problem you'll have is if you use the global vpc settings that you set up for WOT, these settings will most likely be too lean for normal closed loop mode. But the reason fuel trims are there is so the ecu can compensate (add more fuel or take away fuel) as it sees fit. So the ecu will see your vpc settings as too lean (see this by o2 sensor voltages and stft) and will add more fuel on it's on to compensate, i.e LTFT will be in the + range, maybe +8% to +15%. So the ecu is probably correctly things to run properly at normal levels/closed loop mode, but I would not run my LTFT up that high. The goal in tuning is to get LTFT as close to 0% as possible.

So post up what your current LTFT are. And other advice from vpc users would help. You could also post up a picture of your spark plug tips. I thought my were white until I showed poeple a picture, then they said that looked normal.

 

[Mike 99GSX]

Thanks for the reply. Next question. What is the best way for me to log the LTFT data that you need to see? Do you just need say 5th gear at 70 MPH closed loop? Do you need different gears and RPMS? Idle? Let me know what I should log and I'll get you guys some info. Thanks again.

 

[Blk_99gst]

Log RPM, STFT, LTFT, throttle position, and o2 bank1 voltage. Start the log, and let the car idle for 10-20 sec. Then just drive normal. 2nd gear is good, start at 2000rpm stay there for 5sec, then make your way up to 3000rpm, stay there for 5sec, go up to 4000rpm, stay there for 5sec, and then let off the gas and decelerate back to 2000rpm. You can also do the above in 3rd gear too, but I think 2nd is fine. Stay under 30% throttle the whole time. Post that up and we'll take a look.

 

[Mike 99GSX]

great. i have to go out of town for work for a few days but i'll do this over the weekend and get you the numbers. thanks again.

 

[Mike 99GSX]

OK I did the test just now and here's what I got:

RPM STFT LTFT Throttle
725 -3% -10% 0%
835 -1% -10% 0%
750 -2% -10% 0%
789 -3% -10% 0%
825 -4% -10% 0%
750 -2% -10% 0%
This was during about 10 seconds of idling then I started in 2nd gear..

RPM STFT LTFT Throttle
2000 -5to1 -10% 7% held for 5 seconds

3000 -20to-5 -5% 12% held for 5 seconds

4200 0% 0% 18% held for 5 seconds

I then decelerated down to a stop. The LTFT went back to -10% during the deceleration and held the same at idle after. Any feedback on this would be appreciated.

 

[Blk_99gst]

Originally posted by Mike 99GSX
This was during about 10 seconds of idling then I started in 2nd gear..
RPM STFT LTFT Throttle
725 -3% -10% 0%

O.K, my whole theory is now shot. My theory was that if you tune for high settings on a safc or vpc, then those settings will be leaner than the ecu wants to see for normal driving; aka it wants to see richer values for normal driving than at wot.

So at idle, with a LTFT of -10%, the ecu is having to remove 10% of fuel that it thinks it needs to deliever; i.e. the normal driving settings would be too rich if the ecu is having to remove fuel. Hmm, I guess I don't know vpc that well at all; so maybe it doesn't use your WOT settings for normal driving mode? I don't know. Either way, your settings would be on the rich side, leaning them out by 10% would probably make the LTFT value go back to a normal 0% value.

Originally posted by Mike 99GSX
RPM STFT LTFT Throttle
2000 -5to1 -10% 7% held for 5 seconds

3000 -20to-5 -5% 12% held for 5 seconds

Still the same, the ecu is removing 10% of fuel at 2000rpm, and removing 5% of fuel at 3000 rpm. The normal driving settings for 2k and 3k are still too rich.

Originally posted by Mike 99GSX
RPM STFT LTFT Throttle
4200 0% 0% 18% held for 5 seconds

This is very very odd. The ecu goes into open loop mode at 4000rpm, and your LTFT value of +18% is saying the ecu is having to add 18% more fuel to reach ideal air/fuel condictions. I don't understand why this one is +18% and all the others are -10ish. But 18% away from the desired 0% is pretty bad; I know the ecu will throw check engine lights if fuel trims get too out of wack; you might be getting one soon.

Sorry, I guess I don't know vpc enough to know how its affecting the air signal to the ecu. You need some vpc users/tuners to give you some sugguestions on how to correct your normal driving fuel trims. Good Luck...

 

[Mike 99GSX]

Originally posted by Blk_99gst



This is very very odd. The ecu goes into open loop mode at 4000rpm, and your LTFT value of +18% is saying the ecu is having to add 18% more fuel to reach ideal air/fuel condictions. I don't understand why this one is +18% and all the others are -10ish. But 18% away from the desired 0% is pretty bad; I know the ecu will throw check engine lights if fuel trims get too out of wack; you might be getting one soon.

Sorry, I guess I don't know vpc enough to know how its affecting the air signal to the ecu. You need some vpc users/tuners to give you some sugguestions on how to correct your normal driving fuel trims. Good Luck...

No. The throttle is at 18%. The LTFT and STFT are at 0% at 4200rpm. Sorry about the confusion.

 

[Blk_99gst]

Originally posted by Mike 99GSX


No. The throttle is at 18%. The LTFT and STFT are at 0% at 4200rpm. Sorry about the confusion.

Opps, your right. That's a lot better than; so your STFT and LTFT at 4000rpm look good. Whatever your low settings are or how they come from the vpc look to be on the rich side and the ecu is trying to compensate to make them leaner. It's decent, but hit some vpc users up for some help and try to get those LTFT values back to 0%.

 

[Mike 99GSX]

OK with that said here's another question. Would this possibly explain why I think my 50 trim is spooling a little slow (3800 RPM for 20 PSI, .48 A/R). Maybe I'm still too rich in the low band. I'm repairing a sripped out exhaust stud this weekend that could be part of the problem also but I don't think its leeking if at any(inside one). Leaning out the mixture at the 2400 RPM and 4000 RPM points would make the turbo spool a little faster right?

 

[kpt4321]

Sorry I didn't get here sooner.

Although the GCC and the VPC do not have fuel maps for different throttle conditions, you should still be able to get the cruise fuel maps to the point where they are well within the adjustment range of the fuel trims.

Also, keep in mind that the VPC does have an idle adjustment knob. It would appear to me as though you have to turn that down, to get the fuel trims up a litle bit at idle.

Now, for tuning under cruise, you have to be a little more careful. In a perfect world, your WOT and cruise corrections would be the same, and a device like the GCC with only one map would work great. However, this is not the case.

On the GCC, you can use the 1k and 2k (if those are rpm points on the GCC...I cannot remember exactly what the rpm points are) to set the fuel trims while idling or cruising, because you will not be under boost at these engine speeds. Then, as you get close to 3k and 4k, you have to gradually taper the corrections from where they need to be to keep the fuel trims happy at cruise, to where they have to be to give you good timing at WOT.

White tips are not horribly bad, what you really have to watch for are the aluminum speckles you will get when you start detonating.

Just keep an eye on the timing advance.

So, in short, use the VPC idle knob, and then use the lower rpms on the GCC to tune for cruise, and taper into the WOT settings.

....Kyle T.
93 TSi AWD "TURBO4"
14b - 12.8 @ 108

 

[Mike 99GSX]

Thanks alot. The 1st 2 tuning points on the GCC are 750 and then 2400. I've got the idle knob almost all the way lean but it sounds like I should go more. I figured I probably needed to go leaner at 2400 to correct the LTFT but I was concerned about going from such a lean setting at 2400 to almost a 0 setting on the next knob which is 4000rpm. I understand what your saying though about the 2400 adjustment not really affecting WOT or on boost because I'm not really in that range. I just did the helicoil on the exhaust stud and will get everything put back together. I'm going to the track this weekend hopefully with some good results.thanks.

 

[kpt4321]

I doubt you will really build much boost with your setup at 2400 rpm, so do not worry about it too much.

However, it is pretty easy to see what you need to do. Just get on it at a low rpm, and see how much boost you build by that point, and a little past there (because the GCC extrapolates between rpm settings). Also, watch the logger, and see if, as you transition to boost, the A/F ratio slowly curves down to rich (assuming your WOT A/F is richer than that at cruise ) or if it "jumps" down. You can also do this by applying medium throttle up a hill, so that the LTFT is still active, but you are under a decent amount of load. Watch the LTFT and see if it says near the middle, or if it jumps lean and then richens out.

Hope this helps. )

What is the GCC set at right now, all the way across?

.....Kyle T.
93 TSi AWD "TURBO4"

 

[Mike 99GSX]

Here's some info on the settings:
VPC
response- 4 oclock
gain- 8 oclock
idle 7 0clock
the vpcs knobs aren't numbered

GCC
750= -8
2400= -4
4100= -3
5800= -4
7500= -4

I just got the car put back together tonight after repairing some head studs. I didn't do anymore logs expect check the LTFT at idle. It was at -8 and if I leaned out the 750 knob to -12(as far as it goes), it did not make the LTFT change. Does there have to be a load on the car for this number to accurately register. I also tried to turn the idle knob down one more click(6 oclock) but the car stalls out everytime so I guess I'm maxed out there. Hope this helps some.

 

[1LE]

Don't worry too much about STFT and LTFT with a VPC. As long as you don't get more than 15% off while cruising the computer won't CEL you.

My car works best like this:

idle - full lean, gain +8%, response -6% (I have a 2g, 1g people generally have to go +8%, 2g ECU just dumps fuel on tip in while the 1g ecu doesn't add anywhere near enough fuel). My LTFT at idle is -25%, but the computer doesn't seem to care when it's in idle. I see LTFT from -10 to -8% while cruising... GCC is 0 until the 6400(?)rpm range that's +1, and the 7100 range is +2... If you mostly tune on the GCC with close to zero gain, you may experience problems with the car stalling while dropping to idle. tuning the gain, then just tweaking a little bit with the GCC was the ticket for me!!! FWIW, YMMV, if you blow up your engine, you did it, etc...

 

[Mike 99GSX]

Thanks for the advice but I think if I put my gain at +8 that my o2s would be so high off the charts that it would not even register! I've got a lot of mixed opinions on the response setting also like you said. It sounds safe to say that all the 1g guys have this at about 5 oclock but 2g guys go either way. I've got bigger problems to figure out now. I've got a boost leek somewhere that I can't find. I've done a pressure test and checked every vaccuum hose that I can find but nothing. I can't get more than 10 psi to hold through the system using a 5.5hp air compressor. I can't hear any hissing coming from anywhere either so I'm really getting stumped. The car ran really bad at the track last weekend(low 13's), it hesitated going into every gear and my EGTs were skyrocketing no matter what I did tuning wise. Hopefully I can get it figured out.

 

[1LE]

Yeah, don't use my settings exactly My engine is so far from stock that you're almost guaranteed to need less fuel Just trying to give you some idea of how it *could* end up. Setting the response is an exercise in futility on a 2g... Set it towards max and you shoot flames on tip in. Set it towards min and you shoot flames of lift I found my compromise As long as I don't mash the gas real suddenly after shifts I don't flame on tip-in. Doesn't flame on lift unless I lift at 4000rpms. It's controllable on my car with that setting.
Define skyrocketing EGT's? I see a max of 750C while normal cruising, reaches a max cruising of 800C on downhills. Reaches 810C on WOT runs. LOOKS like I could lean it out a smidge more from the EGT's, BUT, then the car doesn't give me as much timing...

 

[1LE]

Yeah, don't use my settings exactly My engine is so far from stock that you're almost guaranteed to need less fuel Just trying to give you some idea of how it *could* end up. Setting the response is an exercise in futility on a 2g... Set it towards max and you shoot flames on tip in. Set it towards min and you shoot flames of lift I found my compromise As long as I don't mash the gas real suddenly after shifts I don't flame on tip-in. Doesn't flame on lift unless I lift at 4000rpms. It's controllable on my car with that setting.
Define skyrocketing EGT's? I see a max of 750C while normal cruising, reaches a max cruising of 800C on downhills. Reaches 810C on WOT runs. LOOKS like I could lean it out a smidge more from the EGT's, BUT, then the car doesn't give me as much timing...