Aftermarket Exhaust: Quantifying Power and Fuel Economy Improvements

Exhaust upgrades are usually expected to result in increased power and improved fuel economy. We decided to test these claims on the Ford F-150, one of the best-selling light trucks on the market today. While the truck we tested falls into the 2004-2008 generation of F-150s, the results should generally apply to almost any Ford truck with a 5.4L two- or three-valve, modular V8. This includes those in the 1997-2004 generation, as well as the 2009 model year.

Our tests focus on a Dynomax exhaust. This brand entered the performance exhaust segment in the 1980s, and has filled out its application list to impressive proportions. Because Dynomax also owns Thrush exhaust products, the company in total commands a sizeable piece of the performance exhaust market.

The test truck

The truck used for testing was a 2005 Ford F-150 4x4, the heavy-duty model that weighs in at 8,200 lb. This truck is equipped with a 10.25-inch ring gear axle and 4.10:1 ratios, which improve towing capacity and performance. On the downside, the gears and axle depress fuel efficiency relative to other F-150 models. Needless to say, we were interested in quantifying the fuel economy gains associated with the installation of aftermarket exhaust.

Recording the fuel economy

To record the fuel economy, we used an Edge Evolution programmer that had been carefully calibrated to the truck’s specific tire size. We recorded the instant mpg and the average mpg readings generated by the unit. The truck was driven roughly 500 miles over 60 days, both with and without the upgrade. Nothing but the exhaust was changed on the truck between the two sets of tests. Given that fuel economy was the focus, we deliberately drove calmly and evenly throughout the testing. Our fuel of choice was one brand of 87 octane gas.

Recording power gains

We used two measures to evaluate the change in performance, the 0-60 times as recorded by the Edge Evolution and a dyno. Paul’s Higher Performance in Jackson, Michigan, let us use its Dynojet dynamometer. Also, the folks at Dynomax graciously helped us out by measuring the cold flow of the stock and cat-back systems and by quantifying backpressure.

Cat-back against the muffler only

We wanted to test a cat-back system against a muffler-only setup, to determine the added benefits of the more expensive upgrade. The muffler is usually the first part of the exhaust system that would need replacing. Behind the catalytic converters, the muffler is also the most restrictive to the airflow. If you are budget-constrained, you might leave your stock pipes, but replace your failed stock muffler with an aftermarket product. We used Dynomax 2.5” Ultra Flo Welded muffler (Part No. 17236). This unit was a straight-through design, rated for 1,100 cfm. This was tested against a Dynomax 3” mandrel bent cat-back system (Part No. 19387).

For the F-150, Dynomax produces two cat-backs, a 3” and a 2.5”. The 3” exits behind the rear wheel on the side of the truck. The dual 2.5” system exits underneath the rear bumper. The 3” unit delivers a higher overall flow rate, but the engineers at Dynomax tell us the performance profiles of the two systems are the same. Our selection between the two was based on how we use this truck. The dual pipes of the 2.5” system could be inconvenient obstructions when towing. The 3” would be sufficient for our purposes anyway, since it’s rated for 1,800 cfm. Both the 3” and 2.5” cat-back kits include the Ultra Flow Welded muffler, although the dual system uses a dual-outlet version. The Ultra Flow is a large chamber, straight-through design. All pipes are mandrel bent, a design feature that optimizes flow.

For this installation, we used stainless steel band clamps instead of standard U-clamps. The stainless steel clamps add to the durability of the system. Plus, they’re easier to remove, which was crucial for our project. Dynomax did the dyno testing on both systems, and then we had to switch them out again to test the fuel economy.

Flow bench test

We also used a flow bench to compare airflow between the stock set-up and the Dynomax 3” cat-back system.

• The stock exhaust flow produced 1.22 psi backpressure (or 2.8” of mercury) at a stable 450 cfm, which equates to about 3,000 rpm, under a temperature 600-degree exhaust gas temperature. Assuming the truck’s maximum possible exhaust gas temperature and 5,500 rpm, the stock system could produce around 6 psi maximum.
• The backpressure of the Dynomax was 0.04, or close to zero. Using the figures noted above, this system might max out at 0.50 psi.

Installation of cat-back system and muffler-only setup

The Dynomax exhaust was relatively simple to install. The most difficult part was measuring the front pipe section and cutting it to fit the wheelbase. We did use the original rubber mounts, plus the stainless steel band clamps.

The muffler-only setup required pipe extensions on both ends of the new muffler, because the stock muffler is so huge. This installation could easily be done by any muffler shop or a savvy DIY mechanic.

Test results

We tested the fuel economy driving 35% on city roads and 65% in rural areas.

The results for the stock exhaust were:

• Average 0-60 time: 8.43 seconds
• Rear-wheel horsepower: 206.6
• Torque: 251.1 lb-ft.
• Average mpg: 15.2
• Maximum mpg: 17

The results for the muffler-only setup were:

• Average 0-60 time: 8.15 seconds
• Rear-wheel horsepower: +9 vs. stock
• Torque: +12.5 lb-ft. vs. stock
• Average mpg: +0.5 mpg vs. stock
• Maximum mpg: +0.2 mpg vs. stock

Notably, the muffler-only system was competitive with the cat-back system in performance at lower rpm. At higher rpm, the backpressure became an issue. Considering that this upgrade cost about $100 in parts and labor, the performance improvement with this setup was impressive.

The results for the cat-back system were:

• Average 0-60 time: 7.91 seconds
• Rear-wheel horsepower: +18 vs. stock
• Torque: +27.8 lb-ft. vs. stock
• Average mpg: +0.5 mpg vs. stock
• Maximum mpg: negligible increase over muffler-only setup

Sound comparison

The cat-back system wasn’t harsh at all, but did develop a deeper roar with quick acceleration. Outside the truck, the rumble was noisy enough to attractive positive attention, but not so loud as to be obnoxious. Inside the cabin, we couldn’t detect any unpleasant noise. With the muffler-only setup, the truck produced more noise in the cabin, and less noise outside.

Conclusion

If you’re looking for performance and fuel economy gains, the muffler-only upgrade appears to produce the best bang for your buck. Gaining 18 hp, 12.5 lb-ft. of torque, and 0.5 mpg for a low input cost means you get power when you need it, plus the mileage gains to cover your costs.