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I can tell you that NPP's closed fall 20hp short of NPP's open. That's on a 470rwhp cam and bolt on LS3. Saw that on a dyno. NPP's open get you an extra 6 hp vs stock narrow body mufflers on a stock LS3...per GM. I suspect that difference may grow if/when you had HP.
Base muffler pic. Inlet on the left and exit on the right. There was fiberglass in the center compartment.
Originally Posted by old motorhead
I can tell you that NPP's closed fall 20hp short of NPP's open. That's on a 470rwhp cam and bolt on LS3. Saw that on a dyno. NPP's open get you an extra 6 hp vs stock narrow body mufflers on a stock LS3...per GM. I suspect that difference may grow if/when you had HP.
I wonder if the fiberglass helps with drone or the muffler design itself? My non-npp mufflers had zero drone and worked great, changed to npp few years back when I went head/cam/nitrous. I know NPP open vs closed is huge difference.. Now I'm procharged and the npp open drone is awful. Car will not run worth a crap with npp closed, like falls on it's face with just a few lbs of boost. I'm debating on going back to base mufflers but wondering how much power I might lose.
I'm assuming you have NPP in a box to control the mufflers. If so, the controller is in your right hand cubby just inside the trunk. Take the controller out of the system by hooking the vacuum feed from the engine direct to the vacuum line going to the mufflers. You're just by-passing the controller. You'll have to cut the line going from the box to the mufflers as it's connected inside the controller. Just cut it as long as possible in case you want to put the controller back in the loop. The hose from the engine is connected by a hose barb on the outside of the box. It just pulls off. You may have a zip tie there that you'll need to cut off. You'll need a vacuum line butt connector that is made for different size vacuum lines as the line going to the mufflers is much smaller than the line coming from the engine. They are commonly available at just about any auto parts store. This is about a 1 minute job that's cheap, easily done, and easily reversible.
Running it this way means your NPP's are closed anytime there is 5" of vacuum or more at the engine. Just driving abound easy and it's quiet. The moment you step on it hard, they will open immediately.
I'm assuming you have NPP in a box to control the mufflers. If so, the controller is in your right hand cubby just inside the trunk. Take the controller out of the system by hooking the vacuum feed from the engine direct to the vacuum line going to the mufflers. You're just by-passing the controller. You'll have to cut the line going from the box to the mufflers as it's connected inside the controller. Just cut it as long as possible in case you want to put the controller back in the loop. The hose from the engine is connected by a hose barb on the outside of the box. It just pulls off. You may have a zip tie there that you'll need to cut off. You'll need a vacuum line butt connector that is made for different size vacuum lines as the line going to the mufflers is much smaller than the line coming from the engine. They are commonly available at just about any auto parts store. This is about a 1 minute job that's cheap, easily done, and easily reversible.
Running it this way means your NPP's are closed anytime there is 5" of vacuum or more at the engine. Just driving abound easy and it's quiet. The moment you step on it hard, they will open immediately.
I know how the NPP works, I have it on my car, installed it myself as it didn't come with it..
I was thinking of ditching the NPP for a base muffler but was wondering how much more restrictive the base muffler is compared to an NPP open.
06-24-2019, 10:50 AM
NPP vs Non-Npp/Base Muffler Cans
