Negative LTFT, new BNR S3 and Bigger TIP

I'm in process tuning my engine with a new S3. I changed my TIP only to an HTP 3" but kept the same MAF housing, which is a modified Corksport Stage 2 (2.8" ID). I'm on 2 port OEM BCS.

Previous to the S3 install, MAF CAL was pretty spot on. After the S3 install, I am experiencing some weird trims. On OL and CL LTFT, getting as much as -10 at low RPM / boost, but gets closer to 0 at higher RPM / boost.

Question is, can a bigger TIP change the maf table? Nothing I have read indicates this is possible, but I have never read about someone doing what I did. Trying to decide if the trim change is the TIP or a leak.

BTW: I plan to do a boost leak test in the near future.

 

-10 isn't THAT far off. Usually +/- 5% is what you try to stick to. Remember when you increase flow you're going to be pulling more air in, so you're going to be at a higher MAF voltage, so maybe that voltage range wasn't as well calibrated. Just do a quick mafcal and see if you can get it closer to +/- 5%.

You could also just have a weird batch of gas and that could be effecting the trims a bit. It could also be a difference in temperature outside if it took you a long time to do the install.

When you're in Open loop you shouldn't have any ltft or stft logged at all. It'll just show 0.

 

Good point on -10 not too far off, I guess I forgot that.

Can you explain more "It could also be a difference in temperature outside if it took you a long time to do the install." Are you just making a note of ambient air temp due to seasonal changes? Its just funny you say that since it took almost a month to finish (long story I won't bore you with).

 

Cold air is more dense, so if the temperature has changed a bunch (or altitude) you can see differences in your trims as well.

Honestly I'd just recalibrate and then go from there. This isn't that far off, so I wouldn't stress it much.

 

Cool, Thx!

 

You need to give the trims about 50 miles to adjust themselves after any tune. If you're still negative fuel trims after then, considering you just swapped turbos that would point almost immediately to a leak in your exhaust. Did you reuse the gaskets or get new ones?

 

LTFTs are just an average of STFTs over a period or time, so if he's seeing -10 in STFT for a long time at a certain MAF V it'll eventually just move over to LTFT and STFT will reduce. No matter how much time he lets it trim, the total between LTFT and STFT will somehow add up to what his fuel trims are.

@mattyhawk I thought I read that -10 was what your trim total was at, but what does the STFT+LTFT look like? That'll be what your trims really are. Eventually if you're seeing -20 in STFT for long enough it'll push it over to LTFT and make minor adjustments with STFT usually +/- 1-3% or so. So you'll have -17 in LTFT at that MAF V range and -1-3 in the STFT after driving it for a while.. Might check that out just to see how far off you actually are.