What exactly determined if the exhaust crackles when slowing down and shifting to 1st?

[Dory]

2015 GT auto...
So in my experience the engine has two phases after a down shift, a brief period of sort of continuing on, but less power and then moving to actual engine braking. Maybe something for emissions?

But for the question at hand, when slowing down for a stop it seems the sweet spot is about 24 mph for the shift to 1st. There seems to be some combination of speed and time since the gas pedal was pressed that determines if you get annoying rev down or nice crackles.

it’s embarrassing when you don’t get the pops and get the loud rev down.

Does this happen on the manual to? What is actually happening here?

Sponsored

 

[engineermike]

I believe when dfco is on you don’t get the crackles. No fuel = no crackles.

 

[Dory]

I am not familiar with "dfco", but I do have a ford performance branded power pack 2 installed so my tune may not be stock.

 

[engineermike]

Deceleration fuel cut off

 

[dx2]

Got a MY2020 GT manual and I can hear crackles/pops sometimes as well, I believ eit was Normal mode, same conditions: slowing down while still in gear. It's not too consistent though.

Might be a emission thing / getting rid of unburnt fuel for better emissions. I think what happens is either the fuel gets burnt in the exhaust or the exhaust values are open while remaining fuel is ignited in the chamber causing this sound. Covoyte has TiVCT (and no EGR).

 

[NoVaGT]

Interesting question, as with DFCO these cars should never crackle & pop.

I can, however, confirm that my car does that all the time when in Track mode. So I'm guessing track mode turns of DFCO.

That's a thought when it comes to MPGs on a long trip.

 

[Torched10]

Interesting question, as with DFCO these cars should never crackle & pop.

I can, however, confirm that my car does that all the time when in Track mode. So I'm guessing track mode turns of DFCO.

That's a thought when it comes to MPGs on a long trip.

Isn't it just rev matching

 

[NoVaGT]

Isn't it just rev matching

I'm talking on deceleration. With the car in gear.

But to he honest, I'll have to pay better attention driving home this evening.

 

[engineermike]

DFCO has to meet certain conditions, like speed, engine speed, and coolant temp, to turn on. I don’t see anything in the cal that ties it to drive mode.

 

[Bronco2fan]

That's why I like driving the 03 GT Conv in the summer. SLP Loudmouth catback on it and it sounds awesome downshifting lol.

 

[cmxPPL219]

2015 GT auto...
So in my experience the engine has two phases after a down shift, a brief period of sort of continuing on, but less power and then moving to actual engine braking. Maybe something for emissions?

But for the question at hand, when slowing down for a stop it seems the sweet spot is about 24 mph for the shift to 1st. There seems to be some combination of speed and time since the gas pedal was pressed that determines if you get annoying rev down or nice crackles.

it’s embarrassing when you don’t get the pops and get the loud rev down.

Does this happen on the manual to? What is actually happening here?

OP, do you have an aftermarket exhaust?

Often, free-flowing exhausts have more crackling and popping, due to the reduction in restriction. You can see this in comparisons of the GT350 and GT350R exhaust decels, where the GT350R always has slightly more crackles and pops.

In more freer-flowing exhausts, decel pops are much more noticeable, because they allow some small amounts of air to be “sucked up the pipe” (vacuum) during closed throttle and any amount of unburned fuel ignites, resulting in the crackles and pops.

@engineermike is entirely correct that there are certain conditions for DFCO/DFSO to occur, so when the conditions aren't met for the injectors to stop on decel, there will still be a possibility for small amounts of unburned fuel on decel to burn.

This is different than If there's a problem with the car - popping can be caused by intake manifold leaks or exhaust manifold leaks, lean or rich conditions.

 

[NoVaGT]

Ya know, understanding the pops & crackles, it's surprising that damage isn't done to the exhaust system.

But cars have done this for many decades now, so I suppose it's all good.

 

[Dory]

Yes, Roush axel back and x pipe. If you hit the “zone” it’s amazing nice crackles. If you miss it and downshift even 1 mph too fast it’s a terrible noise.

 

[beetle6986]

I think the crackles are because of a rich fuel mixture that occurs right after you back off the gas. My 335 has this crackle tune option on it. I tried it once and it basically adds fuel on deceleration for whatever time frame you pick for the crackle in the tune. Instead of being above 20 AFR, I can see it keeps it rich around 14 even during decel.

 

[Dory]

I think the crackles are because of a rich fuel mixture that occurs right after you back off the gas. My 335 has this crackle tune option on it. I tried it once and it basically adds fuel on deceleration for whatever time frame you pick for the crackle in the tune. Instead of being above 20 AFR, I can see it keeps it rich around 14 even during decel.

If that’s the case maybe I need to watch the air fuel gauge and experiment to find the sweet spot!

Sponsored