Gonzoid, the theory for experimenting with my truck modding was formed from these thoughts.
1. I think the 3.5" pipe is a little too big for good lower rpm flow. a 3.5" pipe can support 460-470 hp. When driving, the closer I'm pushing to 400 hp, the more efficiently the pipe is flowing exhaust gas. But in normal daily driving, I'm probably varying between 200-375 hp. The further away I am from 400 hp the less efficient the pipe is flowing as the 3.5" diameter allows the slower rpm exhaust gases more room to expand, cool, slow down and impeded flow. The only time I get near 400 hp output on the road is if I manual shift through first and second and peg the tach needle. But that's not how I normally drive. In auto and during spirited take offs the tranny changes gears by 5k rpm. I think a stock 3.25" exhaust would be a better compromise but it's not a standard size. It would support the 403 hp just fine but would be a little better flowing at low and mid rpms.
2. My magnaflow, or any straight through muffler, although freer flowing than stock, also allows the exhaust to cool a little sooner. The stock muffler traps heat better, keeping the exhaust gas hotter further down the pipe. Hotter exhaust flows better. The longer then pipe from muffler to exit, the more important it is to keep it hot.
3. Narrower pipes will enhance low end torque whereas larger pipes will enhance top end hp. Capacity favors hp, velocity favors torque.
4. On motorcycles, people often swap stock pipe/mufflers for straight through pipes, aka drag pipes, because they look and sound better and shed significant weight. Stock motorcycle exhaust are typically 1.75". Drag pipes are usually 2" to 2.25" diameter. Drag pipes are ideal on the drag strip but hurt low end because of #1 and #2 above. These drag pipes, being larger diameter than needed and without a muffler to keep the exhaust hot allow the exhaust to expand more, cool more, slow down more and ultimately impede low rpm power. Drag pipes become most efficient at drag strip rpms, hence their name.
So, we've established that bigger pipes with lower exhaust temperatures flow most efficiently at max rpm but increasingly less efficiently the lower the rpm until you hit idle, where they are the least efficient at flowing. But what about daily driving exhaust flow efficiency? What if the pipe on my truck was designed for better exhaust flow efficiency from idle to say 5200 rpm, where I spend all of my time, and not closer to 6000 rpm where I spend very little time? Would it improve my daily driving experience? The answer, for me, is yes.
Increasing velocity is how I accomplished improving the efficiency. A 50 year old technique used to improve low end on motorcycle drag pipes is to install an eye bolt about two inches inside the exhaust exit. This little obstruction forces the gas, upon exit, to speed up or increase velocity. When you increase velocity you reduce pressure. So by adding the eye bolt it increases exit velocity to help clear the slower rpm gases sooner, allowing less time to expand, cool and slow down but it also reduces exhaust pressure. Both of these elements, faster and less pressure, decrease low end pumping losses which translates to better lower rpm power and street drivability. From that, motorcycles exhaust manufacturers began playing with narrowing the end of drag pipes to add velocity/reduce pressure. Further, they began tuning them by varying the amount of narrowing and the length of the narrowed sections for individual motorcycle engines and performance goals.
When I added my magnaflow muffler I lost some low end. Then I looked at the magnaflow + 3.5" pipe as being like putting drag pipes on a motorcycle in that the pipe was a little too big and, with no reversion muffler, a heat source was eliminated. I tried the eyebolt in the exhaust trick on my 3.5" pipe and noticed some low end improvement, so I knew I was dealing with a velocity issue and not a capacity issue. I chose to incorporate a narrowed section in the back half of the exhaust, like manufacturers do on motorcycle exhaust. I felt the total exhaust might work better for my daily driving by adding velocity, clearing exhaust gases faster and, since velocity and pressure are inversely proportional, that increased velocity would mean lower exhaust pressure.
Subjective gains are more torque vs stock/magnaflow on take off, more torque vs stock/magnaflow when I want to increase speed, and smoother, more even power throughout the powerband. Before it seemed like there was a point around 2400-2800 rpm, both stock and just the magnaflow, where I started to feel a stepped increase in power as the 3.5" pipe began flowing exhaust more efficiently. "Getting into the meat of the power band" is how most would characterize it. The narrower section shifted the power band down some so it's much sooner and I spend more time in it rather than working up to it. Objective gains are decrease in downshifting on hills, tires breaking loose occasionally on normal take offs (I'm a laid back driver), faster acceleration and mpg increase from not pressing the pedal down as much.
For clarification of my previous post- you can't just go narrowing any exhaust. Most stock exhausts produce flow that matches the engine hp well. But you can go narrower on an exhaust that is larger than needed, meaning it has the capacity to handle more hp than the engine can produce.
