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 MagnaFlow Performance Exhaust Systems Do you really need to install a performance exhaust system before adding power? Does an aftermarket exhaust system really make a difference in dyno or towing performance? Real-life testing is the best way to answer these questions. By Jim Bigley Factory exhaust systems have improved considerably though the years, with mandrel-bent 3-1/2" or 4" tubing now being the norm. You've probably read quite a few exhaust system reviews through the years, and saw the rear-wheel horsepower improvement following the installation of a performance exhaust system. The installation photos were nice and the dyno numbers usually reinforce the notion that an aftermarket exhaust system allows the engine to make more power and help to provide a safety margin when installing performance products. And, if we truck owners are honest with ourselves, we also install a larger exhaust system for the improvement in looks, which is also important. We're told that we need to install a performance exhaust system as well as a set of boost pressure/exhaust temperature gauges before adding power, and that not following that advice could result in excessive exhaust temperatures. This has been and remains excellent advice, but more people than you know have added power while maintaining the factory exhaust system. Let's find out what really happens. I bought the GMC pictured here in the fall of 2000. Nearly 8 years had elapsed and more than 70,000 miles had been accumulated by the time these photos were taken - all with the original factory exhaust system. In addition, somewhere near 20,000 of those miles were accumulated with a 7,000-10,000-lb trailer in tow. I had an opportunity earlier this year to perform a series of towing tests, comparing exhaust gas temperatures (EGT) while running at stock power and while using a VanAaken performance module for the LB7 Duramax. These tests also compared EGT with both with the factory exhaust system and following the installation of a MagnaFlow performance 4" system. The results were illuminating. One of the very first modifications I made to my 2001 GMC was to add a 2-gauge pillar-pod along with a set of Isspro boost pressure and exhaust temperature gauges. Aside from allowing you to help protect the engine after adding power, gauges also allow you see the relationship between boost pressure and exhaust temperature, which can also help you to manage the powertrain for both better performance and fuel economy. I installed a VanAaken power module more than 5 years ago, which incorporated a 3-position rotary switch. This power module was advertised to produce stock power on level 1, an additional 50 horsepower on level 2, and up to 150 additional horsepower on the lights & siren setting. I rarely use level 3 because I know the stock Allison would remind me at some point what it costs to play. However, I do use level 2 quite a lot, especially while towing - though I am careful with the power, so as to not unnecessarily stress the transmission. With 70,000+ miles on the Duramax/Allison drivetrain, there have been no problems. Even though the VanAaken module doesn't have the bells & whistles some of the other products currently available, the relatively inexpensive and simple to use VanAaken has performed very well. It's too bad the VA is no longer available. There is a 4-lane grade nearby that provides an undulating 6% climb for more than 2 miles. The speed limit on this stretch of highway is 65-MPH, and you can safely tow at that speed, provided your truck has the jewels to do it. The travel-trailer shown here weighs approximately 7,000-lbs, and its boxy front-end adds significant wind drag. The combination of a 6% grade and the travel-trailer helped answer the question of whether adding power increases exhaust temperatures while towing and whether a performance exhaust system helped to lower that EGT. Outside temperature averaged approximately 67°F throughout the testing period. For each test, I approached the bottom of the hill with the cruise-control set at 65-MPH. How much we slowed and what gear the onboard Engine Control Module chose during the climb was determined by the VanAaken power setting and the vehicle ECM/TCM programming. The first run up the hill at stock power and with the stock exhaust system caused the truck/trailer to slow to 62-MPH and the Allison dropped to 4th gear. EGT stabilized at 1275°F and boost pressure reached a maximum of 21-PSI during the 2-mile pull. 1350°F is considered by most to be a safe exhaust temperature for the time it takes to pull just about any grade. The 1275 I saw was not a problem. During the second run with the stock exhaust system, I approached the bottom of the hill at the same 65-MPH, but this time VanAaken lent us a hand. Adding another 50 rear-wheel horsepower does wonders for hill-climbing ability. The speedo never budged from 65-MPH with the added power, but I kept one eye glued to the EGT gauge, expecting it to climb well above the previous level. Such was not the case. Amazingly, the needle on the gauge stabilized 65°F lower than when we were using stock power - 1210°F., even though we were running 3-MPH faster. Like the stock power run, the Allison also dropped to 4th gear. Boost pressure remained exactly the same as before, topping out at 21-PSI. Following the installation of the MagnaFlow 4" exhaust system, we returned to the hill for an identical set of runs - one using stock power and another with VanAaken set to level 1. About 2 hours had elapsed between these two sets of runs. When using stock power, the MagnaFlow exhaust system helped to provide just enough extra power to add 2-mph to the top speed of 64-mph, though the Allison still dropped to 4th gear. EGT stabilized at 1275°F and boost pressure reached a maximum of 21.5-PSI during the 2-mile pull. Surprisingly, the maximum exhaust temperature remained exactly the same as when running the hill with the factory exhaust system and with stock power. Repeating the 65-MPH towing with the VanAaken module set to its tow setting and with the MagnaFlow exhaust system produced an uneventful run up the hill that kept the speedometer glued to 65-MPH. However, this time the needle on the EGT gauge stabilized at 1200°F, which was 75°F lower than when using stock power, even though we were running a little faster. Like the stock power run, the Allison also dropped to 4th gear. Boost pressure topped out 1-PSI lower at 20.5-PSI. Lower exhaust temperatures, slightly lower boost pressure, but with a higher overall speed - I'm good with that. These tests show that, at least with the combination of products used here, upgrading to a performance exhaust system isn't absolutely necessary as long as you have gauges and know how to drive your truck in ways that take advantage of its strengths. Keep the engine RPMs in the 1800-2500 range, and you'll not likely see excessive exhaust temperatures when towing reasonable loads even with more than stock power. The highest exhaust temperatures I've seen with this truck occurred while towing heavy at 3000+RPM. 1400° is not uncommon during these conditions, even when running with stock power and stock exhaust system. I believe the exhaust backpressure created by the factory turbocharger and a reduction in turbo compressor efficiency at higher engine RPMs are what can lead to high exhaust temperatures. Because turbocharger efficiency is best at moderate engine RPMs, managing the powertrain for 1800-2500 RPM will produce lower exhaust temperatures, improved fuel economy and an improvement in overall towing performance. The most noticeable performance benefit I discovered following the installation of the MagnaFlow exhaust system occurred during the short 10 mile drive to the hill. Once exiting the Interstate, the highway takes an undulating run up a small valley that caused the truck with the factory exhaust system to drop out of overdrive on one of the small inclines. With the MagnaFlow system installed and while using stock power, the truck remained in overdrive the whole way. The 3.3 hp and 10.9 lb-ft stock power advantage provided by the performance exhaust system made just enough difference to allow the truck to remain in overdrive. Interstates all across the country include overpasses and small hills that can cause your diesel pickup to drop a gear while towing. Remaining in overdrive not only helps to improve fuel economy, but it also makes towing a lot more pleasant. Staying in overdrive more of the time is a big deal to those who use their truck to tow. As mentioned above, we saw a 3.3 hp and 10.9 lb-ft stock power gain following the MagnaFlow exhaust system installation. We also saw a gain of 8 hp and 23.8 lb-ft torque gain when the VanAaken was set to its tow setting. This illustrates that using the higher VanAaken power settings produce more of a gain in rear-wheel performance and more of a reduction in exhaust temperatures following the installation of a performance exhaust system. In other words, the more power your engine is producing, the more your engine benefits from running with a free-flowing performance exhaust system. There are many options in exhaust systems these days, and I've installed quite a few different systems through the years. Some of these fit well and some didn't. The MagnaFlow system shown here installed easily, fit very well, looks great and best of all provided a real-world performance improvement. Hill Test: Stock power w/stock exhaust: Boost 21 lbs, EGT 1275 degrees, speed 62-mph 4th gear Stock power w/Magnaflow exhaust: Boost 21.5-lbs, EGT 1275 degrees, speed 64-mph, 4th gear Dyno: W/MagnaFlow Exhaust3.3 hp gain using stock power with MF exhaust 8 hp gain when using VA +50-hp setting #1 with MF exhaust  The photo at the beginning of this article shows the truck and trailer parked at the top of the 6% grade that we used to measure any changes in hill climbing ability after installing a MagnaFlow performance exhaust system. Shown above is USDP's Mustang chassis dyno we used to measure any changes in rear wheel power/torque.  The above photo shows how close the factory exhaust pipe is to an Allison ATF cooling line. Only about 1/4"-3/8" clearance exists. Shown below is the clearance with the MagnaFlow system installed. Virtually identical clearance. During installation, you'll need to push the pipe away from the ATF cooling line as the various clamps are tightened.   Getting the first section of the exhaust system to clear the ATF cooling line begins with pipe design. On the right of this photo is the factory original pipe. You can see here that the MagnaFlow pipe incorporates approximately the same bends and angles as the factory pipe. The bends are important for best fit.  The inset photo shows a special tool and a spray can of silicone spray. These make dislodging the exhaust system hangers easy.  This factory exhaust system hanger bracket usually needs to be unbolted to simplify removing the forward section of the exhaust system. The bracket is held onto the Allison using 3 bolts. It's easy to remove and re-install, and I recommend taking it off to help speed the exhaust system replacement.  This is MagnaFlow's system #16900, which was produced for the Duramax equipped 2001 GMC crew cab shortbox. This system is made using 4" 100% stainless-steel pipe, and comes with a welded-on 5" double-wall slant cut tip. The rather small muffler includes a 14" main body length and 4" inlet/outlet, though there is a step-down inside the muffler to approximately 3". As with all MagnaFlow systems, having the right sound is an important aspect of having a performance exhaust system. As their TV ads say, "You not only want to feel the performance, you want to hear it too." This system does produce a low throaty rumble when into the power, but it is not offensive.  Clearance over the torsion bar crossmember is about an inch. All of the various clearances should be checked and verified before the final tightening sequence. In some instances, you'll need to hold the pipe in the position you need as the clamps are being tightened.  The tip exit position is more or less predetermined by the pipe bends made at the factory. Again, you'll need to hold the individual sections of the exhaust system while the pipe clamps are being tightened. Begin tightening the clamps at the front, and work you way to the rear. Verify all final clearances before calling it a day. It should be noted that stainless-steel exhaust systems turn various shades of amber as the metal is heated - including the tip. This system was bright and shiny when first installed, but the various sections of pipe and tip turned amber after about a dozen pulls on the dyno and several loaded runs up our local 6% grade. The stainless-steel tip can be easily polished in just a couple of minutes using metal polish and a drill-powered Mother's Powerball, but if you absolutely must have a bright and shiny exhaust tip, another system with a chrome tip might be a better choice.  This is the VanAaken performance module used during these tests. I suppose the move toward more features made modules like this less attractive to diesel truck consumers, ending with it being pulled from production. I've been using this module for more than 5 years, and I have not experienced a single problem. I normally use the +50 setting while towing, and that allows the truck to tow any size trailer with ease, yet keeps EGT at or below the temps seen when running at stock power.  The above dyno graph illustrates the rear-wheel power and torque produced by the 2001 LB7 with the VanAaken set to its +50 setting and with the truck running the stock exhaust system. The graph below illustrates the measured performance improvement after installing the new MagnaFlow exhaust system.  MagnaFlow offers a wide range of diesel performance exhaust systems that should cover just about anyone's needs or vehicle choice. Each ISO 9001 certified system carries a lifetime warranty. MagnaFlow does NOT sell directly to customers, but you can contact U.S. Diesel Parts for any retail questions. TDP Source: U.S. Diesel PartsCall: 1-800-823-4444 www.USDieselParts.com
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