Over the weekend I sold my '15 TLX Advance-SH-AWD to a family member and picked up an '18 Sport Hybrid Adv. I drove it 3 hours home and decided to clean it the next day before putting it in the garage and discovered something interesting.

The mufflers actually have 2 outlets, 1 very small round outlet (maybe 1" to 1.5") that allows exhaust to be routed through the the bumper, and another outlet that is 3" or more that point down, like the '15 TLX (hidden exhaust), which I assume is like '14 to '17 RLX hybrids exhaust outlets.

I started the car and I found the hidden larger outlets were not functioning. A little bit of moisture was coming through the bumper outlets but not the larger outlet.

Using a flashlight I discovered there is a spring loaded flapper in the larger exhaust outlet and it was closed, which can be open manually.

I can not see any wiring that is hooked up to this flapper, but it is spring loaded. I'm not sure if these open during hard acceleration, or when put into Sport mode. I've tried googling and have found no further information.

Has anybody else discovered this and/or have any more information?

Thank-you,

 

The system could just be driven by the pressure of the exhaust system which opens the flapper to the larger pipe when higher revs are reached. This would likely help keep the exhaust note very quiet during sedate driving and then give a nice little "growl" when the engine revs up and needs more room to breath.

Thanks for the info ddswv!! I look forward to more of your remarks regarding the 2018 Sport Hybrid.

 

You are probably correct Hondamore, but I smell a PGEORG diagram shortly on the horizon with the answer to this little mystery

I too look forward to your observations of the 2018 SH. Congrats on the new ride.

 

I suspect as the system heats up, the spring's k factor gets smaller reducing the force required to open the flap. Like the old spring chokes on carburetors.

 

I believe you are correct. I got back up under there and there are no wiring harnesses connected anywhere. Here are a couple of photos.




The flapper on the left with the hidden exhaust and the small straight pipe on right that flows toward the bumper.

 

After I get my clear bra ordered and install it, I'll check that out. Right now she's going to stay in the garage till I get that done to avoid any chips beforehand.

 

All years have this mechanical valves, and it work with the exhaust pressure. When the exhaust pressure gets to a certain point, it is able to push open the flap. The sound changes to the throaty sound we all have heard before, because it it routed out of this pipe.

I took a couple of really crappy pics of my 2016 SH, and you can barely see the same valve. You can also see that the one that has the valve is much bigger in diameter.....

I will try to find the documentation I had read a while back.....

 

It's an old idea at Honda.

Keeping some back pressure on the valves increases the torque available for easy tuning, and once you've got going and HP becomes the motivator, you want to reduce back pressure.

K Motors are notable for needing some back pressure, more than some other high performance four cylinders. Some of the best exhausts for K Motors are also some of the quieter ones.

 

I've never totally understood how the back pressure is good.

 

I had a 1999 Prelude auto (wife's car) and a close friend had a 2000 SH model. He replaced his stock exhaust with a larger exhaust and that completely changed the dynamics of the car. He ended up putting the stock exhaust back on. Honda engrs. knew what they were doing with the design.