When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Jerry, at first glance, the NPP exhaust system would also appear to be straight through from the outer pair tailpipes to the catalytic converters when the tailpipe flow control valves are open. There is a crossover. There are small holes just before the outer tailpipes which become the only path of escape into the shroud chamber if the tailpipe flow control valves are closed. But with those valves open, I wonder what is causing the sound attenuation, relative to other aftermarket systems. Is it the cats, the cross over, of perhaps even when open, those small holes result opening to the shroud chamber (which is a muffler I suppose) is still resulting in some attenuation?
Managing sound, tones, harshness etc is very complex. That Muffler housing has sound bouncing off the walls in very carefully managed ways.
The top two in the bottom Pic are how my Borla short Hot Rod mufflers in my street Rod are constructed. You can look straight thru and all you see is some perforations in the 3 inch diameter pipe. But as low flows, if just cruising, it's much quitter than just a pipe. At WOT it's more like a pipe.
This is an interesting thread. Seems everyone has neighbors who built houses out of paper or something if a 10 second cold start is bothering them. The garbage trucks that come through cause way more disturbance than a quick cold start.
On cold start, the engine runs rich for a brief period to heat up the catalytic converter faster. Trading a brief period of a little more pollution for a much quicker reduction in pollution. It's the rich mixture that makes it louder. And it's only cold start: that's why when you fire it up after parking for just a few minutes it doesn't have that same loud bark.
All documentation from GM says it was for the "experience", not to warm cats faster. "Warm cats" faster is a cover story, 100% user/forum sourced.
I hated it at first, but now I like that my neighbors think I'm 19 and blipping it intentionally during 4am starts.
Correct. The service manual (not the forum) says "To provide a more aggressive exhaust note when the vehicle is started, the exhaust tailpipe valves are opened during an engine crank event".
The manual (not forum sourced) also discusses the cold start strategy as follows: "The cold start strategy is to reduce the amount of time it takes to warm the catalyst. During a cold start, the engine spark timing is altered to allow the catalyst to warm quickly. .......Fuel trim bias is used to keep the post catalyst air/fuel ratio within a predetermined range."
I interpret this to say that spark timing and fuel trim bias (mixture) are modified during a cold start to warm up the cats. We also know from experience that the idle speed is increased during a cold start.
The Service Manual never says the the cold start process is the reason for the start up bark. But we also know that forum members who do not have the NPP exhaust also report a start-up bark and an increase in exhaust sound after a cold start. So it seem reasonable that the whatever is going on during the cold start process is the cause of the louder exhaust - at least that is my conclusion if not a "forum concensus".
I have not seen any other explanations offered other than the postion of the exhaust flow control valves and the cold start process that is used to warm up the cat.
This is an interesting thread. Seems everyone has neighbors who built houses out of paper or something if a 10 second cold start is bothering them. The garbage trucks that come through cause way more disturbance than a quick cold start.
Or the neighbor's dog barking for a 1/2-hour causes way more disturbance than a quick cold start!!! Actually, the quick cold start shuts the dog up for a few seconds!
Or the neighbor's dog barking for a 1/2-hour causes way more disturbance than a quick cold start!!! Actually, the quick cold start shuts the dog up for a few seconds!
Yes, that would do it. Or even when people do yard work like mowing, leaf blower, trimmer, etc. Or construction work/home projects (hammering, cutting, etc). Those are all loud but last much longer than 10 seconds.
I leave at 5am most days, I remote start my car and keep the garage door shut for about 15 seconds. It muffles the sound enough.
Im always backed in too, but I’m not sure if that helps or not. Part of me thinks it makes it echo more inside
I think this 'feature' is pretty common on later model GM's. Neighbor has a late model Camaro Z28 and it barks loud on start-up, then quiets right down. Not that I care, but he's apologetic about it
My C7 also gives a loud cold start but not as loud as the C8. No neighbors have said anything to me but I’ve begun starting it with garage door closed for 10 seconds. Only had the car for 3 weeks at this point. Of course this is only in the mornings, any other time I don’t care if it s loud.
Just at start up I wish I had straight pipes to **** off my neighbor more.
Back-in-the-Day it was common to use cut-outs. Recall my buddies '56 Olds where we had one activated by a choke cable. Remember going thru the Lincoln Tunnel and hearing the exhaust eco off the walls.
Back-in-the-Day it was common to use cut-outs. Recall my buddies '56 Olds where we had one activated by a choke cable. Remember going thru the Lincoln Tunnel and hearing the exhaust eco off the walls.
Today can do cheap remotely:
Try to find the Stainless Steel Remote Controlled Weld Flange to C8 Pipe After CATs units to understand more how they work. Couldn’t find anything about them and did not find them on eBay.
All documentation from GM says it was for the "experience", not to warm cats faster. "Warm cats" faster is a cover story, 100% user/forum sourced.
I hated it at first, but now I like that my neighbors think I'm 19 and blipping it intentionally during 4am starts.
Warming the cats faster is what every car sold in most of the world does, whether you believe it or not. Leaving the valves wide open is "for the experience,"
11-15-2023, 02:49 PM
