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flap in exhaust, what's it for?

ES44AC
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ES44AC Enthusiast

ES44AC

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What is the flap in the exhaust for? Looks like it opens as it gets hot. My step dad put duels on his 2018 Silverado and he kept the old muffler and pipe.

rav3 Senior Enthusiast

rav3

Posted
Posted
7 minutes ago, ES44AC said:

What is the flap in the exhaust for? Looks like it opens as it gets hot. My step dad put duels on his 2018 Silverado and he kept the old muffler and pipe.

Good question as related to the Chevy Silverado's. Our BMW X3 & X5 both have duel exhaust with a "flap" that is closed one of the tips during cold starts at idle, then would open after a few minuets or at higher RPM's due to the noise issue. I have modified it to stay open all the time, so am familiar with the "flapper" on them. However, my '17 5.3L only has a single exhaust, so there isn't any 'flap' to delete, that's why I don't understand where the "flap" is on our trucks. Please help explain it to me. 

Daly Senior Enthusiast

Daly

Posted
Posted
6 minutes ago, rav3 said:

Good question as related to the Chevy Silverado's. Our BMW X3 & X5 both have duel exhaust with a "flap" that is closed one of the tips during cold starts at idle, then would open after a few minuets or at higher RPM's due to the noise issue. I have modified it to stay open all the time, so am familiar with the "flapper" on them. However, my '17 5.3L only has a single exhaust, so there isn't any 'flap' to delete, that's why I don't understand where the "flap" is on our trucks. Please help explain it to me. 

There is a "flapper". It's ahead of the muffler under the truck. 

full-39399-28950-valve.thumb.jpg.65feefab9f13fa4e66b49eb9431186e0.jpg

It's sole purpose is to deal with AFM.

  • Like 1
T-Town-Z Enthusiast

T-Town-Z

Posted
Posted

Here is what a GM exhaust flapper valve looks like on the inside.  Photo of it closed and open.  Even when fully open, it blocks at least a third of the exhaust pipe diameter !

Vehicles - GMC Flapper Valve - closed.jpg

Vehicles - GMC Flapper Valve - open.jpg

  • Like 1
2016 Sierra SLT Enthusiast

2016 Sierra SLT

Posted
Posted

As stated it’s to prevent the helicopter noise when going in to V4 mode , when I did the muffler delete on my 16 Sierra I took the flap out also.

ES44AC Enthusiast

ES44AC

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Posted
23 hours ago, 2016 Sierra SLT said:

As stated it’s to prevent the helicopter noise when going in to V4 mode , when I did the muffler delete on my 16 Sierra I took the flap out also.

Thanks.

Ed S Enthusiast

Ed S

Posted
Posted
On 6/11/2020 at 7:05 PM, ES44AC said:

What is the flap in the exhaust for? Looks like it opens as it gets hot. My step dad put duels on his 2018 Silverado and he kept the old muffler and pipe.

If you remove the flapper and install an aftermarket dual exhaust system like a Borla system you will get whats called a "drone" sound or what some call "helicopter" sound. That's just another reason I installed a Range AFM delete module in my 14 GMC. No more of the V4 drone for me. Plus saving a little oil consumption and future problems caused by AFM on the engine that they are finding out is happening to some.

ES44AC Enthusiast

ES44AC

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I'm not changing my exhaust, but my step dad did on his 2018 5.3l truck. I'm leaving my 2018 6.2l truck exhaust alone ,I definitely don't want that drone sound.

flyingfool Senior Enthusiast

flyingfool

Posted
Posted (edited)

it adds torque to the engine when in v4 mode,  i just bolted mine open with 2 large fender washers and a 3" long 1/4x20 screw and nut, easy it works well, you rotate the flapper, and slide the bolt thru the brackets, and lock it down

Edited by flyingfool
00pooterSS Enthusiast

00pooterSS

Posted
Posted

It's for cutting out and throwing in the trash along with the stock muffler. Both of them are really restrictive. I put a magnaflow straight through muffler on my truck and took that valve out and gained fuel mileage and power. I have a completely different exhaust setup now but back when I did the mod my truck picked up 3 mph at my test spot and a couple of mpg. I had already disabled the DOD/AFM on my truck though. 

 

 

flyingfool Senior Enthusiast

flyingfool

Posted
Posted (edited)

for a daily street truck you kinda want the restriction on the 5.3 motor since it builds torque, the 3mph increase means you made power but power is fuel and not economical for a daily driver. i see all these guys bolting on the 6.2 exhaust system and now the little 5.3 has to rev out, since it lost its factory  torque curve  . And on a 6000lbs truck TOrque is  more valuable to me than HP.. I would keep the factory exhaust just take out the flapper. 

 

otherwise sell your 5.3 truck and buy a 6.2 truck or get the new ugly T1 model with the stroker 6.6L

Edited by flyingfool
poconojoe Enthusiast

poconojoe

Posted
Posted

My boat has exhaust flappers! But for a completely different reason. They're called exhaust shutters. They prevent the back-flow of water into the engine which would result in hydro-lock. Basically water forced into the cylinders if you suddenly stop the boat too quickly. This is a very bad scenario! All kinds of internal damage can happen, as you can imagine. The Mercury I/O engine is a 4.3L GM truck block they adapt for marine use. It's carbureted, not injected and puts out 190 HP.

  • 1 year later...
Saul Cruz New Member

Saul Cruz

Posted
Posted

Just had mine welded open today at the shop. Sounds louder when accelerating, but levels out once your coasting. 
I also had the squashed y pipe replaced with a round pipe. Smoother shifting and an increase of a mile or two mpg

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