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Discussion Starter · #1 ·
Is it the right procedure to tune the LTFTs to +16.4 at idle when using a cam with overlap?

Is this what should be done with the Stage2 cam?
 

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If you at 16.4 at idle your PCM is dumping fuel at your engine. In my opinion. I never heard of anyone wanting 16.4 at idle. 16.4 is MAXIMUM LT fuel trim. I believe you may have a problem. Maybe a vac leak or something.
 

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Discussion Starter · #4 ·
I dont get +16.4 LTFTs at idle. But Zoomer recommends a cam with overlap be tuned to +16.4 at idle.

I am wondering if this is a good idea and why.
I dont understand why you would want to be dumping fuel either.
 

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seriesIIIChump said:
But Zoomer recommends a cam with overlap be tuned to +16.4 at idle.
Hate to say it, but he is the only vendor I have ever heard of that says this.
I've tuned 3 different cams in my car and quite a few other friends as well now and have always had my LTFT's as close to zero as possible through the entire RPM range.
Go look at LS1tech.com and see if anyone else thinks the same thing.
V6 or V8 doesn't matter, a tuned engine is a tuned engine.
 

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John F said:
Hate to say it, but he is the only vendor I have ever heard of that says this.
I've tuned 3 different cams in my car and quite a few other friends as well now and have always had my LTFT's as close to zero as possible through the entire RPM range.
Go look at LS1tech.com and see if anyone else thinks the same thing.
V6 or V8 doesn't matter, a tuned engine is a tuned engine.
When I installed my cam, I get -16.4 to +16.4 (at idle). It jumps around everywhere. I've played with my afc so much and it doesn't do jack. From what I gather, the only time LTFT's become in effect is at cruising and WOT. Now when I cruise and go WOT, I see around +.7-.0. I know I don't have a leak anywhere.

There are other cars on CGP who are having the same issue. I do believe that Matt M (I think) said that what I said above. LTFT's don't mean anything unless cruising or WOT.

Am I correct or incorrect?
 

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Discussion Starter · #7 ·
That was my understanding as well thanks to the help from Cook/Yates/Morad.

But my LTFTs are just as steady at idle as they are at cruise. I was just worried about gas mileage and carbon buildup. If you are dumping fuel in at idle isn't that going to hurt the mpg and add black soot to the tips?
 

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I've always heard your LTFT values should be as close to zero as possible at Cruise and WOT, for POM you could tune your idle LTFT values to ~0.

Zooomer is the only vendor i've ever heard to recommend that. You're dumping too much fuel in the motor. You will see a decrease in gas milage and such.
 

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Every where other than idle my LTFT's are nearly Zero. I've attempted to play around with the afc at the 1-3 values. No matter what I change them to, it's always +16. Most of the time it will go from +16 to -16 in a few minutes. It fluctuates that much. How do I tune for that when it's moving around all the time?
 

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+16 LTFTs is super lean and your pcm is sending more fuel to compensate. The most common cause of this is vacuum leak. I'm running the GPstore #2 aka HL cam and have no problems tuning it to near 0. Id be willing to bet you've got a significant leak somewhere introducing air after the MAF.
 

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I believe Zooomer states that +16.4 LTFT's are normal for the aftermarket cams , because these cams create more valve overlap, and the extra air causes the O2 to report to the PCM that the car is running lean, hence the max'd out LTFT's in the positive range.

To test and see for a vac leak, check your IAC counts. If they are below 25 or so at idle then you probably have one...

Here is link explaining about LTFT's and aftermarket cams:

http://www.clubgp.com/newforum/tm.a...ltft&language=single&tmode=&smode=&s=#1744574

Hope this helps a bit.
 

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John F said:
That's one explanation, but it sure isn't the correct one :rolleyes:
Like I mentioned earlier in this post, go check LS1tech.com and see if anyone else holds to that practice.........
:rolleyes:
 

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Ok guys what did we come up with? This topic sort of just stopped. I am curious if anyone came to a conclusion as to what the LTFT should be at during idle with a cam? It was a pretty divided discussion. I have mine set to +16.4 at idle with the VS cam and I just did some idle,cruise,wot scans and sent them to Matt(STATTAMA) for his imput. I have a csc. He said that everything looks really good. My ITA is good, MAFF is good,02's are right on, and KR is 0. This thing is also pulling like a beast. So I'm not sure what everyone else decided on but the +16.4 is working great for me.
 

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I think people have misinterpreted what ZZP (Zooomer) meant. An aftermarket cam has higher emissions because it sends some amount of raw fuel out the tailpipe due to the increased overlap. To make matters worse, the raw fuel is also accompanied by unburned oxygen which is picked up by the O2 sensor and interpreted as slightly lean because of this. In return it increases the amount of fuel it sends into the motor to "balance" things out. The result is a overly rich car and exhaust. What ZZP recommends is to lean out the car out at idle via the PCM. Since the car is in closed loop mode, the PCM will just try to compensate for it by trying to add more fuel. But if you've already tuned it so lean so that the LTFTs go up to 16.4, the PCM will not be able to add any more. That is at the core of their recommendation and in principle it makes sense. To tell you the truth though, I'm not sure just how well it works in practice though. I would think that the PCM could/would just bump up the STFT when it can't raise the LTFT, but I really haven't played with this so I can't say for sure.

Just the same though, this is only intended to reduce the rich exhaust smell on cammed cars, and has nothing to do with power.

HTH,
John
 

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Just the same though, this is only intended to reduce the rich exhaust smell on cammed cars, and has nothing to do with power.
But if the car is super rich or super lean won't it effect the idle? I mean if the car is choking on fuel it would idle poorly and vise versa for lean. Right? :confused:
 

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Yes, but I don't think it ever gets to the point of choking on fuel. It's definitely richer that 14.7:1 but not to the point where the engine runs rough.

John
 

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Buck- you must have misunderstood something that I said. Perhaps I told you not to worry about the short-term trim.

Anyhow- John T is correct as to why Zooomer recommended that procedure. If you have no way to correct for the additional oxygen released into the exhaust system due to a cam with a lot of overlap, then the o2 sensor will be wrong. It tells the PCM that there is a lean condition and the PCM adds fuel. The only easy way to overcome this is to set the AFC low enough to make the Ltrim max out at 16.4. Then the PCM will not keep adding more fuel than it needs to. It works surprisingly well. I will be working on a device that will allow the use of long-duration cams without having to max the fuel trim in order to run stoich.

Matt
 
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