Changing my rear mufflers changes my STFT

[Xplaya]

LS3. Straight pipe guttered Cats. 3" Catback. Mafless Tune. LTFT disabled. STFT Only (The tuner did this, I only understood what was happening after learning up on this stuff)

I noticed with my 3" rear mufflers (Free flowing), my fuel trims show a bit leaner. Around 5% mark. When I swapped out to factory (Ultra Silent) mufflers, my fuel trims drop down to 0% mark.

Is this some type of back pressure effect? And how could I get my fuel trims to drop back to 0% with the 3" mufflers without going to my tuner first?

[blindsquirrel]

https://forum.hptuners.com/showthrea...change-my-STFT

[Xplaya]

https://forum.hptuners.com/showthrea...change-my-STFT

That was actually my original post, but I wrote a novel with multiple questions in the post and likely confused everyone. So I separated the threads as the questions were not really related. Unfortunately it wont let me change the thread title or delete the post to re-write the thread title.

[gtstorey]

were not really related.

Really, it's usually all related, you just don't realize how.

[eficalibrator]

Short answer: YES, exhaust configuration will impact volumetric efficiency.

When I was a calibration group manager at a major OEM, I had a dozen different programs that all used the same long block (displacement, CR, heads, cams, intake), but each one needed a unique VE model to account for the various exhaust systems in the different vehicles. Your example is no surprise.

[Alvin]

LS3. Straight pipe guttered Cats. 3" Catback. Mafless Tune. LTFT disabled. STFT Only (The tuner did this, I only understood what was happening after learning up on this stuff)

I noticed with my 3" rear mufflers (Free flowing), my fuel trims show a bit leaner. Around 5% mark. When I swapped out to factory (Ultra Silent) mufflers, my fuel trims drop down to 0% mark.

Is this some type of back pressure effect? And how could I get my fuel trims to drop back to 0% with the 3" mufflers without going to my tuner first?

Day to day will change your STFT. They will never be perfect in all conditions.

[edcmat-l1]

^^^^ I'd add a 5% swing (from 5% to zero%) is no big deal. You can't really get them much tighter than that. OE stuff typically isn't that close.

[Xplaya]

^^^^ I'd add a 5% swing (from 5% to zero%) is no big deal. You can't really get them much tighter than that. OE stuff typically isn't that close.

Is it OK to keep the car at +5%. My understanding anything outside -10 / =10 mark is the no go zone. But dont really know if the numbers are OK to stay between -10 to +10 besides 0

[gtstorey]

To me 10% is excessive, I would rather keep it below about 5%. Get more logs on long, typical drives and see what it does.

[edcmat-l1]

Is it OK to keep the car at +5%. My understanding anything outside -10 / =10 mark is the no go zone. But dont really know if the numbers are OK to stay between -10 to +10 besides 0

Is +5 the average or the peak? Does it bounce over +10? Just saying "+5" doesn't really explain how the vehicle is running. As far as I've seen you haven't posted any data logs, before or after, so it's all just speculation.

It can't be "+5" all the time, everywhere. So it's all going to depend what a long data log would look like plotted out against the airflow table of choice.

[JayRolla]

Living in high altitude I have seen stock cars swing 10-15% going from 5k altitude to 12k+. Dont chase your tail on the trims. Within 5% is perfectly fine.