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2018 Ford Focus ST1 with a 2867 gen 2
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Discussion Starter · #1 ·
So I've noticed on the last couple map files that at the top of third I'm getting a pretty loud "pop" when I let off the gas (at around 6700 rpm or so). I used to get a small one with the stock exhaust on the WMI tune, but it really just started after I installed the NXT exhaust on the car on the big turbo setup. Maybe I wasn't hearing much with the stock muffler on it and the twin 4.5" tips are just acting like a megaphone, but dang... it's loud when doing a data log. The phone mic really didn't pick it up well, but you can hear the pop sound between 8 and 9 seconds in this video:
In the car, however, it was pretty loud.

Thus far the car doesn't have any issue while driving, mileage is great (31.6 so far on this tank), no DTCs, makes great power, etc... it's just having that weird pop at the top of 3rd when I let off the gas. Brian said that there's nothing in the tune files that should be contributing to that, so he asked about my purge valve, but it's a newer unit with only 12-15K on it, I think.

Any ideas?
 

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So I've noticed on the last couple map files that at the top of third I'm getting a pretty loud "pop" when I let off the gas (at around 6700 rpm or so). I used to get a small one with the stock exhaust on the WMI tune, but it really just started after I installed the NXT exhaust on the car on the big turbo setup. Maybe I wasn't hearing much with the stock muffler on it and the twin 4.5" tips are just acting like a megaphone, but dang... it's loud when doing a data log. The phone mic really didn't pick it up well, but you can hear the pop sound between 8 and 9 seconds in this video:
In the car, however, it was pretty loud.

Thus far the car doesn't have any issue while driving, mileage is great (31.6 so far on this tank), no DTCs, makes great power, etc... it's just having that weird pop at the top of 3rd when I let off the gas. Brian said that there's nothing in the tune files that should be contributing to that, so he asked about my purge valve, but it's a newer unit with only 12-15K on it, I think.

Any ideas?
I don't hear it, but only thought would be excess unburnt fuel.
 

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2018 Ford Focus ST1 with a 2867 gen 2
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Discussion Starter · #3 ·
I don't hear it, but only thought would be excess unburnt fuel.
It's running safely rich (11.4-11.6, I think), so I'm sure there's some unburnt fuel in there. Maybe I've got some exhaust joints that aren't sealed up all that well and it's pulling in enough fresh air to light it off?
 
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2018 Ford Focus ST1 with a 2867 gen 2
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Discussion Starter · #5 ·
I already texted him and am trying to pick his brain.
 
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· Super Moderator
2018 Ford Focus ST1 with a 2867 gen 2
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Discussion Starter · #6 ·
So I got another log tonight and when I shifted I didn't do it nearly as fast, and consequently, there was no pop/bang from the exhaust. Kyle (TurboGT) is theorizing that it may be the exhaust moving around and hitting the suspension or underside of the car on the shift, and given the circumstances of when it's happening, it seems like a plausible theory. I'll try to replicate it tomorrow and see what happens.
 

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I don't hear it, but only thought would be excess unburnt fuel.
It's most likely incompletely combusted fuel getting into the hot exhaust and igniting. Afterburn/afterfire is a common phenomenon among highly tuned engines, especially those with anti-lag systems that incorporate ignition delay.

It can happen in naturally aspirated engines too. I built an RX-7 that used to lay out a tail of flame out the exhaust when lifting after a straight, and when downshifting. It seemed to happen most frequently heading into turn 7 of the original Thunderhill layout.

I imagine an ST/RS with a larger turbocharger and high flow (especially catless) DP would be more prone to this.

Check out this video of the old BMW F1 engine in qualifying trim. If you watch at around 10:20 and 12:05 you can see Berger making quali runs, and hear the pops and occasional flame bursts out the exhaust as he upshifts. The old Group B turbo cars used to do this all the time.



I wouldn't worry too much about it.
 
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