level6

I'm one of those guys with the base exhaust that has been wanting to get a Corsa or the just arriving non-npp Borla. Even with the popularity of npp for the c7, with the total sales numbers of new Stingrays, there are plenty of others like me out there.

So how does that exhaust valve located before the non-npp mufflers work? Does it close somewhat when AFM sends the engine to 4 cylinder mode? When my car goes to 4 cylinder mode, will there be a big change in sound with an aftermarket back exhaust? The npp people complain about that sudden drop in volume from these valves that all c7s have.

Did I explain myself well? I have yet to see my question addressed on this forum. It might be too soon to ask this question, but I'd imagine others who have non-npp with added louder aftermarket exhaust are out there reading this? Thanks.

Theta

Mark,

Those valves before the mufflers are the AFM valves, and are actually the same (and installed) on every C7 Stingray.

When the engine management kicks over to 4-cylinder mode (V4), those AFM valves actuate to control the exhaust pulses for a few reasons - one of the primary reasons being exhaust tone.

Regardless of the exhaust you choose to put on the C7, you will still have the AFM valves - and honestly, you want them there. You will see decreased sound in V4 mode by design - the sound without the valves is quite awful, especially when amplified.

The NPP volume drop has more to do with the actual NPP valves, tip-in, mode selection, etc. Essentially, we have ways to open them by selector mode (normal), almost all the time (by screen selection), and always (by pulling fuse 41).

I believe that's what you were looking for judging by your PM and post here.

level6

Thank you Sean/Theta for all the detailed help you provide in the C7 Technical section of the forum. (for anyone that wants to read of a monster fast modified C7, check out some of Theta's posts about his in-process Stingray in the Tech section)

One of these days, I'd like to actually see one of those valves and see what it's doing to control the sound. i.e., is it just a gate?

Whatever, it's there to stay. My biggest concern is: I don't hear the AFM now that I have whisper quiet stock exhaust, but with a louder aftermarket exhaust I'm wondering if this actuation will prove to be annoying.

People who have npp now speak of this AFM exhaust valve being so obvious, that passenger's look over and ask "what was that all about?"

If I never go to a loud exhaust I'll never be bothered by this sudden change in tone.

But I want to hear this LT1.

Anyone out there change from a stock non-npp to BB's, Magnaflow, Corsa or Flowmaster yet? What are your experiences with this subject? Thanks

Theta

Well it's certainly a noticeable difference when it kicks down into V4 (in more ways than one, of course). The valve actuates similarly to a cutout, in that it's an electrically-powered, gear-driven valve (think of a throttle body if you're not familiar with a cutout).

When the AFM kicks in and you enter V4 mode, the throttle blade opens significantly to allow air to enter the system more efficiently. The vibration, harmonics, etc. also change during the AFM V4 operation. Without the AFM valves, you would perceive/hear drone inside the cabin, and an unpleasant sound outside the vehicle.

While I can't source an LT1/C7 video of AFM aftermarket exhaust sounds, I can give you an example of a G8 with a loud exhaust system going in and out of AFM V4 mode.

If you have a manual, you will only enter AFM in Eco mode. If you have an automatic, the change occurs far more often. You can bypass this by keeping the car in 'manual shift' mode, from what I've seen here.

On this clip, start around 3:15 and you'll hear it going in and out of AFM.

level6

Thank you Theta That didn't sound bad.

I've been waiting for Borla's non-npp and from a recent post in the Tech section it sounds like it's release is finally forthcoming. I liked their's better than the other sales leader: Corsa, on the C6 as the Borla has a slightly deeper sound. From what I gather, drone won't be an issue on either of these systems.

But the Borla is packed with a wire mesh like substance that will clog over the years? while Corsa just uses refractory walls for drone cancellation. I'm sure I'm overthinking this, but hey, this our hobby and it's all there to be discussed.

Theta

Perhaps a bit, yes.

The Corsa install does require cutting/welding, while I believe the Borla will be a true full axle-back replacement. Something else to keep in mind between the two.

I'm a fan of Corsa products, but I'm not big on the way they implemented their C7 exhausts.

SASprof

I had a Borla S-Type II full cat-back with X-pipe on my C6 and luved it. Great deep low sound, no drone, and awesome back-burble when off throttle.

Last time I checked Borla's website, they had axle-back systems only for C7. Has that changed?

Another thing I'm considering for my C7 is straight pipes. My boy put straight pipes on his Camaro and it sounded great, very much like my C6's Borla S-Type II. Not harsh or obnoxious at all. I believe the reason for that is that the cats have muffling properties.

I would be interested in anybody's advice/experience. Thank you.

AORoads

Didn't know that. Could be difficult to do installations on C7s at Carlisle as is normally done with that added installation factor.

level6

Motohead279 Has installed the Corsa and stated that cutting is involved but no welding. They supply the flanges and brackets to connect. I think the welding subject has come up in the scenario where you might want to remove the Corsa before you sold the car and it's been suggested that you would have to weld your stock muffler back on. It wasn't like that on the c6. But Corsa didn't want to create a pipe with the AFM valves because of added expense, so they have you cut the pipe after the valve but before the muffler.

Theta

Ah, that's fair. He and I have talked several times, but I didn't notice that the install didn't require the welding aspect.

From the looks of the kit as they send it out, it doesn't appear to be able to be clamped (using the traditional exhaust band clamps with the staggered ID / OD), but I guess they found a way.

As for the going back to stock part, mating the pipes is usually just done by tacking the two together to line them up, then inserting a larger pipe to span and welding the circumference of both ends of that stainless span (4" or so will do it). Usually doesn't look bad at all if a good welder does it with SS/409 stick or TIG.

Theta

Their initial release was only for NPP-equipped vehicles, as they had the four valve outlets. They were close to bringing the non-NPP axle-backs to market as of last month.

I believe they're close to shipping the non-NPP, but it's best to talk to David (Borla) directly about that. He's posted his info a few times around here, which is a nice gesture.

Quentin Green

I put the flowmaster axle back system on my 2015 Stingray without NPP valves and the check engine light been on since. Bought it to two Chevy dealers, who didn't have a clue of what's going on with (from phone conversation)

boa721 boa721

After all the reading and still not coming to a conclusion I would like to know if I cut my non NPP exhaust nufflers out will I have backflow pressure issues?
my exhaust is very quiet my last car I removed the Mufflers but I had resonators to make up the back pressure and it sounded really good. Can I buy NPP Mufflers as a replacement? Where's my stock 2017 Stingray not equipped wiring wise to handle npp exhaust?