Finesse 1 -2 -3: Power With Economy: Muffler vs Cat-back Exhaust

"New Blue" is a Plain-Jane XL model, so too much tailpipe would make it look like a cheap hooker.  The stainless 4-inch tip and throaty growl are the only giveaways.

You probably know that exhaust improvements offer a power increase and a bump in fuel economy. We thought you'd like to explore this topic with us involving one of the most popular and common light trucks around, the Ford F-150. While our test rig was of the '04-08 generation, what we discovered will largely apply to the previous '97-04 models and the spankin' new '09. It should also apply generally to any Ford truck that mounts the popular 5.4L two or three valve modular V8.
We explored but Dynomax was our guide. Dynomax has been in the performance exhaust business since the late '80s and has a huge application list. They also own the Thrush line of exhaust products and between these two established brands, they own a big part of the performance exhaust market. Scott Stutler

The stock muffler is huge. Cut it in half and it would make a fine bass boat!  The stock pipes are a decently sized 2.5-inches in diameter.

and Nick Macaluso from Dynomax were extraordinarily patient in putting up with the extra work our testing required.

Test Subject and Parameters

You're gonna see this truck in a few other upcoming stories. Allow us to introduce "New Blue," an '05 F-150 HD 4x4. The "HD" is our appellation to describe the rare 8,200 pound GVW version of the popular F-150. It comes with many heavy-duty extras, including a 10.25-inch ring gear rear axle and 4.10:1 ratios. The gears and axle add greatly to towing capacity and performance but tends to decrease the fuel economy compared to 3.55:1 or 3.73:1 F-150s, so any gains in mpg are welcome.
We subjected the truck to approximately 500 miles of test driving over two months, both in bone stock

The 1100 cfm Dynomax #17236 has 2.5-in inlets and outlets to match the stock pipes. A short extension was added to the tailpipe section, which was also cut from an original muffler.  The mufflers inlet and outlets are slightly different than the stock units, so some reconfiguration of hangers was needed.  It was also necessary to position the muffler perpendicular to the ground when installed. None of that is beyond a muffler shop or even a home wrencher with moderate skills.

and modified forms. We used the instant and average miles per gallon monitoring features of an Edge Evolution programmer to test fuel economy, after carefully measuring and calibrating the unit for the exact tire size of the truck. We used the same brand of 87 octane gas throughout and nothing else on the truck was altered. Since we were looking for the best mpg, we drove carefully and consistently throughout the tests.
To measure power and performance, we used two benchmarks: 0-60 times (times supplied by our Edge Evolution) and the Dynojet dynamometer at Paul's High Performance, in Jackson, Michigan. We also prevailed upon Dynomax to measure the cold flow of the stock versus the performance cat-back system on their flow bench and measure backpressure.

 

 

The Dynomax 3-in single cat-back system adapts up from 2.5 inches right after the front pipes "Y".  The straight pipe is either left out or cut according to the wheelbase of the truck.  The kit comes standard with U-clamps but band clamps are a worthwhile option. One clamp is included for the stainless tip.

Muffler Only vs Cat-back

 

We asked to test two setups; a Dynomax part number 17236, 2.5-inch Ultra Flo Welded straight thru muffler (rated for 1100 cfm) combined with the stock pipes and a Dynomax 3-inch mandrel bent cat-back system, part number 19387. The muffler-only test reflects a budget conscious upgrade made practical by the fact that the muffler is often the first part of the stock system to fail and also the source of most of the flow restriction after the catalytic converters. We wanted to test both the power and mileage aspects of this variation versus a full cat-back.

Installing the 3-iinch adapter right after the 2.5-inch headpipe "Y".  The addition of an aftermarket 2.5 into 3-inch collector installed here might result in a further increase in flow.

Dynomax offers two cat-back systems for the F-150, the aforementioned single 3-inch system, which exits to the side behind the rear wheel, or a dual 2.5-inch system which exits from under the rear bumper. Dynomax assured us that there is no notable difference in performance between them, though the 3-inch system has a higher overall flow rate. We opted for the single 3-inch because our F-150 is first and foremost a working truck and the dual pipes can interefere with towing operations. The 3-inch Dynomax Ultra Flo Welded muffler is rated for 1800 cfm, so it has more than enough capacity. 
Both kits use the Ultra Flow Welded muffler, a large-chamber, straight-thru design. The dual system

uses a dual-outlet version of the Ultra-Flow. The pipes are all mandrel bent for maximum flow. We opted for the stainless steel band clamps rather than using the

Installed and ready to rumble.

standard U-clamps. This not only helps the system last longer but makes it easier to remove and replace. That was important because, after Dynomax installed both systems for dyno tests, we had to switch systems again later to do mileage testing.

Go With the Flow

To further contrast stock vs modified, a flowbench shootout was performed between the Dynomax 3-inch cat-back and the stock system. At a steady 450 cfm, the equivalent of about 3,000 rpms worth of stock exhaust flow under a moderate 600 degree EGT (Exhaust Gas Temperature) load, the stock system produced 1.22 psi

Power to the ground. Paul's High Performance in Jackson, Michigan ran a total of 25 dyno tests on New Blue for us.

backpressure (2.48 inches of mercury). The Dynomax produced a mere 0.04 psi,    about as close to zero backpressure as you can measure. If we extrapolate the data using the 5.4L's maximum possible exhaust gas temp and 5,500 rpm, the Dynomax system might reach 0.50 psi. The stock system looks to produce somewhere around 6 psi in those conditions.  
 
 Installation

Installing the cat-back system was pretty much a no-brainer. Especially for Steve Grace, who's been prototyping and installing exhaust systems at Dynomax for 32 years. The only part of this install that required any brainpower at all was to measure the front section of pipe and cut it to length according to the

wheelbase. For the rest, the original rubber mounts were used and the aforementioned stainless steel band clamps made the job even easier.
Steve also built the muffler-only setup, much as a muffler shop or a home wrencher might do. Because the muffler is shorter than the monstrous stock one (it would make one heckuva boat!), pipe extensions were added at both ends.

Results

Our data is tabulated below but we'll sum it up here. Stock, our 9,000-mile-young 5.4L cranked out an average 8.43 second 0-60 time, 206.6 rear wheel horsepower and 251.1 lbs-ft of torque. It delivered an average of 15.2 mpg in

Common knowledge has long dictated mandrel bent pipes are superior and worth the price difference. Well, common knowledge needs to be challenged once in a while to keep it honest. We flowbenched two identically configured 2.25-inch diameter pipes, each with two 90 degree bends and a turndown tip. The only difference was that one was mandrill bent and the other wasn't. The difference in backpressure at 450 cfm was around 30 percent, namely 1.85 psi for the crimp bend and 1.30 psi  for the mandrel. That 30 percent difference should hold across any two sets of pipes. Buy mandrel bent!

our combined loop, consisting of 35 percent city type driving and 65 percent rural roads. On cruise at 55 mph on level ground we could eke out a solid 17 mpg max.
Our muffler-only improvement yielded us an 8.15 second average 0-60 time and a best gain of 9 hp and 12.5 lbs-ft over the best stock dyno run. As you will note by the dyno charts, the muffler only kept right along with the cat-back system until the higher rpms, when backpressure finally got the better of it. That's pretty good for about $100 in parts, plus labor. Fuel economy-wise, that change gained us an average of 0.5 mpg over the stock truck with a best of 17.2 mpg at 55 mph.
Our 0-60 runs with the cat-back system averaged 7.91 seconds and offered a best run of 18.1 hp and 27.8 lbs-ft over the best stock dyno run and 9.1 hp and 15.2 lbs-ft over the best muffler-only run. Those are better increases than you usually see from a cat-back, but we'll take 'em! From the mileage perspective, the cat-back didn't do significantly better than the muffler only. We saw virtually the same range of 0.5 mpg on average and a tiny increase at 55 mph. While the cat-back performed excellently, the best dollar/hp deal seems to be the muffler change. 
The sound quality is worthy of note as well. The single 3-inch Ultra Flo Welded cat-back is not raspy at all in normal driving but has a mellifluous bellow with the pedal-to-the-metal. The outside tone was just loud enough to attract admiring glances but not loud enough for annoyed looks. No unpleasant or obtrusive droning was noted from the inside. The muffler-only change was a bit quieter outside but did have a moderate interior drone at about 30 mph and again at 70. That just goes to show there's more to building and sound-tuning a system than just sticking on a muffler.

Final Word

This test shows what performance with economy is all about. If you can gain nearly 18 hp and 28 lbs-ft and still pick up an honest half a mpg, you can actually have the power when you need it and make it pay for itself in fuel savings when you don't.

Sources

Dynojet
www.dynojet.com

Dynomax
www.dynomax.com

Edge Products
www.edgeproducts.com

Paul's High Performance
www.paulshp.com