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I've stated this before and will state this again. FLOWMASTERS ARE GARBAGE. Their engineers are stupid. Their customer support is worthless.
Here are the facts. My previous motorhome had a stock 454. The previous owner had installed Flowmaster dual 50 series. Due to a cracked manifold, I installed Thorely headers. There was a very small performance increase, which was disappointing. Over time, the rest of the exhaust needed replaced. Along with that replacement, I was talked into installing 40 series Flowmasters. While there was a very noticeable performance increase, the noise was unbearable, both the drone inside, as well as outside. I question if they were legal, it was so loud. As a result, I installed 70 series Flowmasters. While these were quieter, the performance took a nose dive. Zero to 60 dropped by almost 3 seconds. ~20%. I lost 2.5 mpg. Several calls to Flowmaster's tech support, they suggest the mixture may be off, or there may be too much scavenging. Between an electronic fuel pump, and a fuel pressure regulator, I could skew my mixture from too lean, to to rich. This made no improvement on performance. They also suggested I needed some sort of restriction to prevent over scavenging. I put on an adapter for a slightly smaller pipe. This made the performance worse. I finally gave up on Flowmaster and installed Magnaflows. These are a straight through muffler, with an inner perforated tube, similar to a glass pack. They have stainless steel mesh for absorption. The performance came back completely, and so did my mileage. These performed every bit as good as the 40 series, but were very quiet. Under normal throttle, you could not hear them inside the coach. At full throttle, there was a little bit of rumble, to let you know they were there. I did eventually burn out the first set of Magnaflows, however Magnaflow did replace them under warranty. The second set lasted for more than 8 years, and as far as I know are still fine. In that time, I did change to the 502. They still handled the doubling of the HP, without any performance loss. The big 502 was slightly louder, but still very exceptable.
Besides the performance issue, Flowmasters have a serious drone problem. The exhaust gas flows through chambers that are directly part of the outside shell. As such, all the exhaust noise radiates through the shell to the outside of the muffler. They are actually louder at the muffler than they are at the end of the tail pipe! On the other hand, Magnaflow has absorption between the inner pipe and the outside shell. This absorbs the drone, so it does not radiate through the shell.
Incidentally, when the Magnaflows did burn out, I had a little fun driving down a country road at wide open throttle, 5200 RPM. It was an amazing sound to hear those big pistons scream. I was towing a boat, so I was staying in the high RPM for extended periods. Hanging my head out the window, smiling like a fool. Was great fun, but for long term, I was glad Magnaflow replaced them.
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2004 AllegroBay 34XB Nov 2017 Banks, Front & Rear Trac bars, Konis
Sold:'83 Revcon Prince 31' FWD GM Performance 502 w/Edelbrock MPFI, Thorley Tri-Ys & Magnaflows, 4L85E 4 spd. Tested to exceed 100 mph.
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