NPP muffler not flowing well - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums

Accarroll23 · 03-09-2021, 11:32 PM

I've searched the web for hours and could not get an answer. I added some longtubes with no cats, 2.4 upper pulley, injectors and CAI and had the car tuned. My tuner said the car was pulling timing and seemed to have high exhaust Temps and too much back pressure in the high RPM's. I want to say the knock sensor was making it pulltiming butim not 100 percent sure.
There was some steel wool looking wire starting to come out of one of the pipes. So I told him to just cut the mufflers off and see if that fixed the issue. Turns out it did and it put down 642hp...so I picked the car up and brought the mufflers home and cut the open. From what I could see the steel wool was coming loose. But...I don't think it was enough to cause an issue with creating to much back pressure being that the mufflers look pretty much like a straight through design. I pulled the stuff out and rewelded to top piece back on.
I called my tuner and sent him pics of the guts. I asked would they be safe to put back on and he suggested not to. He said the engine is moving to much exhaust and the mufflers are known to be restrictive. I'm having a hard time believing that because my friend has a 800whp SS and he's using the same exhaust as me.
I don't mind buying new mufflers but I just don't understand these mufflers being restrictive. Can anyone chime in on this? I love the NPP sound and I'd really like to keep the NPP on my car. Lmk what you guys think.

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Accarroll23 · 03-10-2021, 12:00 AM

Quote:

Originally Posted by Risingson05

Running on stock injectors ?

No, I have 65lb injectors at 75 duty cycle.

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larry_g · 03-10-2021, 06:23 AM

I'm putting down 680 through NPP.

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12oclockshadow · 03-10-2021, 06:37 AM

I've not heard the zl1 npp mufflers being a restriction...I'm running stock npp as well.

Accarroll23 · 03-10-2021, 06:41 AM

Quote:

Originally Posted by 12oclockshadow

I've not heard the zl1 npp mufflers being a restriction...I'm running stock npp as well.

I appreciate the input. I'm having a hard timing believing it as well.

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ZMEnow · 03-10-2021, 07:41 AM

Quote:

Originally Posted by Accarroll23

I've searched the web for hours and could not get an answer. I added some longtubes with no cats, 2.4 upper pulley, injectors and CAI and had the car tuned. My tuner said the car was pulling timing and seemed to have high exhaust Temps and too much back pressure in the high RPM's. I want to say the knock sensor was making it pulltiming butim not 100 percent sure.
There was some steel wool looking wire starting to come out of one of the pipes. So I told him to just cut the mufflers off and see if that fixed the issue. Turns out it did and it put down 642hp...so I picked the car up and brought the mufflers home and cut the open. From what I could see the steel wool was coming loose. But...I don't think it was enough to cause an issue with creating to much back pressure being that the mufflers look pretty much like a straight through design. I pulled the stuff out and rewelded to top piece back on.
I called my tuner and sent him pics of the guts. I asked would they be safe to put back on and he suggested not to. He said the engine is moving to much exhaust and the mufflers are known to be restrictive. I'm having a hard time believing that because my friend has a 800whp SS and he's using the same exhaust as me.
I don't mind buying new mufflers but I just don't understand these mufflers being restrictive. Can anyone chime in on this? I love the NPP sound and I'd really like to keep the NPP on my car. Lmk what you guys think.

Sent from my SM-N960U using Tapatalk

This problem existed with GM through several models but seemed to be worse with the 2013 z, mine came out at 3800 miles so GM replaced the complete exhaust, I tried to buy it back but GM made the dealer scrap it. You are correct in that if NPP is working during WOT there would be no restrictions as it’s a bypass.

Vegas Bound · 03-10-2021, 09:00 AM

I'm running 1 7/8 LTs, ultra high flow cats, 2.5" exhaust through NPP and close to 900 hp. No issues with heat, back pressure or blockage from the mufflers. I do run mine open 95% of the time, not original equipment. Open/close is through MyLink buttons.

Accarroll23 · 03-10-2021, 09:22 AM

Quote:

Originally Posted by ZMEnow

This problem existed with GM through several models but seemed to be worse with the 2013 z, mine came out at 3800 miles so GM replaced the complete exhaust, I tried to buy it back but GM made the dealer scrap it. You are correct in that if NPP is working during WOT there would be no restrictions as it’s a bypass.

Another thing I was thinking is that wouldn't cats cause more backlog than a muffler? Being that I'm catless i don't understand why it was pulling timing. I will say that one of my headers do rub the frame rail slightly at times. I have read that a rattle in the exhaust can cause the knock sensor to pull timing...does that seem more of the cause than the mufflers?

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Risingson05 · 03-10-2021, 11:16 AM

Quote:

Originally Posted by Accarroll23

Another thing I was thinking is that wouldn't cats cause more backlog than a muffler? Being that I'm catless i don't understand why it was pulling timing. I will say that one of my headers do rub the frame rail slightly at times. I have read that a rattle in the exhaust can cause the knock sensor to pull timing...does that seem more of the cause than the mufflers?

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There’s a lot of factors that can cause your ecu to pull timing . One sure fire way to tell what it actually is pulling timing is to look at your data logs . If you were dyno tuned then you’d have to talk to your tuner .

Risingson05 · 03-10-2021, 11:18 AM

What kind of fuel are you using ? Also if you could know what your iat’s are would help

Accarroll23 · 03-10-2021, 12:03 PM

Quote:

Originally Posted by Risingson05

What kind of fuel are you using ? Also if you could know what your iat’s are would help

He did mention my iat's were high at first. He checked my supercharger and it was low on coolant.

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chef-beavis · 03-10-2021, 03:38 PM

I recall seeing a batch of these on new cars years ago. I can't recall what the issue was, but it was warranty.

lbls1 · 03-11-2021, 09:13 AM

The ZL mufflers and exhaust are about the best oem exhaust system on the market. The wool material is how GM uses sound deadening for their exhaust (all or nearly all GM's oem mufflers feature this).

This is the first time I've heard anyone say that the exhaust is restrictive. The only known reason(s) I hear of people swapping out the oem catback is for louder sound.

03-09-2021, 11:32 PM	#1
Accarroll23 Drives: 2013 zl1 Join Date: Jan 2019 Location: Va Posts: 44 View My Garage	NPP muffler not flowing well I've searched the web for hours and could not get an answer. I added some longtubes with no cats, 2.4 upper pulley, injectors and CAI and had the car tuned. My tuner said the car was pulling timing and seemed to have high exhaust Temps and too much back pressure in the high RPM's. I want to say the knock sensor was making it pulltiming butim not 100 percent sure. There was some steel wool looking wire starting to come out of one of the pipes. So I told him to just cut the mufflers off and see if that fixed the issue. Turns out it did and it put down 642hp...so I picked the car up and brought the mufflers home and cut the open. From what I could see the steel wool was coming loose. But...I don't think it was enough to cause an issue with creating to much back pressure being that the mufflers look pretty much like a straight through design. I pulled the stuff out and rewelded to top piece back on. I called my tuner and sent him pics of the guts. I asked would they be safe to put back on and he suggested not to. He said the engine is moving to much exhaust and the mufflers are known to be restrictive. I'm having a hard time believing that because my friend has a 800whp SS and he's using the same exhaust as me. I don't mind buying new mufflers but I just don't understand these mufflers being restrictive. Can anyone chime in on this? I love the NPP sound and I'd really like to keep the NPP on my car. Lmk what you guys think. Sent from my SM-N960U using Tapatalk

03-10-2021, 06:23 AM	#3
larry_g Drives: 2014 1SS/1LE Join Date: Nov 2015 Location: Louisville, KY Posts: 826	I'm putting down 680 through NPP. Sent from my Pixel 5 using Tapatalk __________________ 830 WHP 700 WTQ, Whipple 2.9 @ 13lbs Boost, Flex Fuel Tune by Blevins Tuning, LSA SBE, 3.5 GripTec, ATI LS3 Damper, AFR 260cc Mongoose, ARP Studs, Yella Terra Ultralites, Johnson 2110R, BTR PDS Stage 3, Melling 10355, Improved Racing Baffle, ID1050X, DSX Triple Fuel Pump, RotoFab CAI, NW 103mm TB, Stainless Works 1-7/8" LTH to 3" Speed Eng w/ X-pipe to NPP, Powermaster 9509 XS, Detroit Speed Motor Mounts, MGW, Monster LT1-S, ZL1 Calipers, DBA Rotors & Pads, Bilstein B6 Struts & Shocks, BMR 1.25" Drop Springs & Adj Rear Bar, Pegasus Solid Bushings, Z/28 Wheels, other misc parts.

03-10-2021, 06:37 AM	#4
12oclockshadow Drives: 2012 ZL1 M6 Join Date: Feb 2019 Location: Conroe, Tx Posts: 221 View My Garage	I've not heard the zl1 npp mufflers being a restriction...I'm running stock npp as well. __________________ FBO. Cammed. Flex fuel.801whp/741wtq on e85

03-11-2021, 09:13 AM	#13
lbls1 Drives: 2002 Camaro SS SOM; 2015 Malibu LTZ Join Date: Dec 2010 Location: Queens, NY Posts: 4,030	The ZL mufflers and exhaust are about the best oem exhaust system on the market. The wool material is how GM uses sound deadening for their exhaust (all or nearly all GM's oem mufflers feature this). This is the first time I've heard anyone say that the exhaust is restrictive. The only known reason(s) I hear of people swapping out the oem catback is for louder sound. __________________ '02 CAMARO SS SOM; 5.7L LS1/FLS6B '08 TBSS AWD Black Granite Metallic '15 Malibu LTZ 2LZ Turbo *'14 CAMARO ZL1 Blue Ray Metallic*

03-10-2021, 09:00 AM	#7
Vegas Bound Account Suspended Drives: 2013 Sparkly Red One Join Date: Dec 2018 Location: Hospitality State Posts: 2,558	I'm running 1 7/8 LTs, ultra high flow cats, 2.5" exhaust through NPP and close to 900 hp. No issues with heat, back pressure or blockage from the mufflers. I do run mine open 95% of the time, not original equipment. Open/close is through MyLink buttons.

03-10-2021, 11:18 AM	#10
Risingson05 Drives: 2015 zl1 m6 Join Date: Jan 2020 Location: Alaska Posts: 702	What kind of fuel are you using ? Also if you could know what your iat’s are would help

03-10-2021, 03:38 PM	#12
chef-beavis Drives: IOM 2013 2SS/RS, Greenie, 06 FXSTBi Join Date: Apr 2009 Location: Turd Town, WI Posts: 1,588	I recall seeing a batch of these on new cars years ago. I can't recall what the issue was, but it was warranty.