Continuing fuel economy discussion --
Up until now, we've been looking at a 'package' of inexpensive ways to improve fuel mileage. Next up is more expensive options, starting with your motorhome's exhaust system.
Postulate: More power and torque can mean less fuel used in a given vehicle under identical driving scenarios. (Assuming you don't change your driving style and vehicle load.)
If you have a single exhaust system, a dual exhaust system can improve fuel mileage. A well designed dual exhaust system allows an engine to 'breathe' easier. This means less engine power is used to push the exhaust out the tailpipe and more power is sent to the drive wheels.
There's probably not enough cost benefit in replacing an existing functional exhaust. However, exhaust systems tend to 'wear out' due to corrosion. So, if your exhaust pipes and muffler need replacing, it'd be a good time to invest a bit extra for a dual exhaust system.
First, do NOT replace your stock exhaust manifolds with headers. I've read many detailed test reports comparing various exhaust system alternatives. These reports described controlled scientific tests using engine dynamometers and other expensive test equipment. For applications like our motorhomes, stock exhaust manifolds consistently came out on top.
(I know this is going to start an argument ... save your breathe. I prefer multiple tests, using controlled scientific methods, over anecdotal reports based on limited experience.)
Readily available, inexpensive headers are frequently just ****. The manufacturers are primarily focused on making a fast buck, so these tend to be sucker bait rather than serving a useful purpose. In addition to often reducing performance and economy, these headers tend to corrode easily, often lasting only a few years. (As one friend put it, "It really sucks when your headers fall apart before the mufflers.")
With one notable exception, even well designed headers aren't suited to our motorhomes. This is because virtually all headers are made for high RPM applications, like drag strips and race tracks. These headers don't start producing significant power improvements until the engine climbs above 6,000 RPMs. Below 6,000 RPMs, they often decrease performance compared to stock manifolds.
At cruising speed, the engines in our motorhomes run at 2,500-3,500 RPMs, well below the threshold that justifies headers. Additionally, our engines 'redline' around 6,000 RPMs. Most won't go much over 6,000 and if you manage to push them past that point, you risk damaging or destroying the engine. (I know ... I've destroyed a few engines over the years by over-revving them.)
The exception is something known as Tri-Y header design. These are designed for low-RPM, high torque engines, like those in our motorhomes. Unfortunately, these are complex and difficult to manufacture, so tend to be quite expensive. Likewise, there's not a lot of demand for Tri-Y headers, so they're equally difficult to find. (Your best bet is to search the off-road, rock-climber marketplaces.)
So, assuming your exhaust system needs replacement, and you've decided to stay with the OEM manifolds, you need to find a replacement dual exhaust system. You may be able to find a ready-made system but it's more likely you'll need to find reliable shop for a custom made system. (The muffler shop I rely on uses a mix of off-the-shelf and custom components and welds everything together ... and the slightly higher price is offset by a more durable -- and safer -- exhaust system.)
Be careful. As with carburetor jets, the "bigger is better" fallacy applies. If you find a ready-made system, make sure it's suitable for lower RPM applications ... many are made with larger pipes for high RPM race applications. Likewise, make sure the shop for a custom system has the knowledge, experience, and tools (e.g., pipe benders designed to prevent kinks and constrictions). A shop that does lots of muffler replacements may be better, with broader experience than shops specializing in high performance exhaust systems.
Regardless, you want a H- or X-pipe crossover in your dual exhaust. It'll cost more but is worth the improvement in performance and efficiency. (The crossover 'balances' the two sides of the exhaust system and helps 'scavenge' the exhaust.)
