Baby Got Back: Turbo Exhaust Size Comparison Test

It should come as no surprise that turbo LS motors are a hot commodity in the performance world. Looking back, several things transpired that allowed us to enjoy the performance we have at our fingertips today. First and foremost, the advent of the LS engine family ushered in a new era of performance. Replacing the original small-block was no easy task, but GM hit a serious performance homerun with the new design. Much like the original 5.0L did for the blue oval boys back in the 1980s, the LS started a new revolution. 

I ain’t gonna’ fall for no banana in the tailpipe!

Think about it: how often did guys go to the wrecking yard, snatch up a Vortec small-block, and add a turbo? That was all but unheard of back in the day, but it seems like every LS in the yard is a candidate for boost, which brings us to the second event that changed performance. The introduction of low-buck, off-shore turbos cemented the LS turbo deal, as boost was now attainable for the regular guy. Most newbies won’t remember this, but there was a time was when turbos, intercoolers, and all the associated plumbing were much more expensive. The current trade tariffs recently increased the cost of off-shore boost, but even now, 1,000 horsepower has never been easier or more affordable. 

The SBE 6.0L LY6 (with increased ring gap) was upgraded with a BTR Stage-3 twin-turbo cam.

A walk down memory lane is always fun, but only if the walk ends up at the dyno for some turbo testing. Turbo power is alive and well, and with any luck will remain so. But how do we improve upon what is already there? The general consensus is that the route to more power with any turbo motor is actually easy: just add boost. True enough, in most instances, increasing the boost will increase power. We need only look at what we call the multiplier effect to demonstrate this. Believe it or not, any naturally aspirated motor already runs under pressure, something we call atmospheric pressure, 1 Bar or 14.7 psi (at sea level, at a given temperature, etc.). The important point is that when the piston goes down, and the intake valve opens, it is this atmospheric pressure that pushes air into the low-pressure area created by the downward moving piston. Since the naturally-aspirated motor produces a given amount of power at 1 atmosphere, all we need to do to double that power is add another atmosphere, meaning 14.7 psi of boost. There are reasons why the formula works better on paper than in the real world, but it is definitely possible to increase the power output of your 400 horsepower (naturally-aspirated) 5.3L to 800 horsepower with 14.7 psi of boost from the proper turbo system.

Additional mods included a set of cathedral-port, Gen X 225 heads from TFS. The heads featured a dual valve spring upgrade and were run with hardened pushrods from Comp Cams and stock rockers.

To provide an effective power curve, we selected a Dorman LS6 intake manifold from Summit Racing. The Dorman intake was combined with Wilson fuel rails and Holley 120-pound injectors and Fast ignition coils.

Using this formula, we can add power at whatever rate we decide. If we want to increase the power output by 50-percent, we simply apply 7.35-pounds of boost. The application of 10 psi will bring an increase in power of 68-percent, while 29.4 psi (3 bar) will triple the power output. As you can no doubt tell from the multiplier effect, having a powerful naturally-aspirated combination pays big dividends once you add boost. Running 14.7 psi on a 300 horsepower combination will yield 600 horsepower, but the same 14.7 psi run on a 400 horsepower engine produces an impressive 800 ponies. For this, it should be evident that modifications to the naturally-aspirated combination should be given high priority, as their gains are often multiplied under boost. Though the math portion of turbocharging an LS motor is certainly fun, there are many reasons why certain combinations fail to follow this formula. Things like turbo efficiency, air inlet temperature, and restrictions come to mind. Our focus is on the last of the three, and while inlet restrictions to the turbo can certainly hurt power, we decided to take a look at exhaust restrictions. Many enthusiasts seem to forget that all the wonderful air that gets forced into a turbo engine must also find its way out.

Boost for the exhaust test was supplied by BorgWarner S480 turbo from Little John Motor Sports (LJMS). As the name implied, the powerful turbo featured a standard compressor housing around an 80mm, billet compressor wheel. The hot side consisted of a 92mm turbine wheel tucked inside a 1.25 AR, T4 turbine housing.

We know from the movie Beverly Hills Cop that completely closing the exhaust system with bananas will stop a motor from running (it must be true, it was in the movie), but what happens with more minor restrictions? To illustrate the impact of exhaust restrictions on a turbo LS application, we set up the following test. Using a 6.0L turbo motor, we configured three different exhaust systems to be installed after the turbo. System number one was designed to simulate a street exhaust and consisted of 3.0-inch tubing with several bends (the type you might see running under a street car) and a single muffler. The muffler was a straight-through design from MagnaFlow and featured a 3.0-inch inlet and outlet. To ensure the muffler was not a major restriction itself, we compared it to a length of straight pipe and found no difference in power, just a change in exhaust note. Exhaust system number two was stepped up to 3.5-inches and featured 45-degree bends in place of the 90-degree bends used on the 3.0-inch system. The 3.5-inch exhaust was designed to simulate a more race-oriented street/strip application, with exhaust noise still somewhat of a concern. The final system tested was a race-only, 4.0-inch open pipe. This system represented the typical fender-exit exhaust common on high-horsepower, turbo LS applications. 

To keep things cool, we supplied a steady diet of 85-degree dyno water to this air-to-water intercooler from ProCharger. Capable of supporting over 1,300 horsepower (we know from experience), the core was more than adequate for our low-boost exhaust test.

Since exhaust restrictions are a function of flow, we needed a suitable turbo motor to run our test properly. The 6.0L LY6 was an ideal candidate, having recently produced 1,543 horsepower at 29.2 psi with a stock bottom end (only increased ring gap). Though we would be nowhere near that power level in this test, it was nice to know the 6.0L could handle the intended power level. To coax the power out of the LY6, we applied a trio of upgrades to the top end, including a stage 3, twin-turbo cam from Brian Tooley Racing, a set of CNC-ported, Gen X 225 heads from Trick Flow Specialties, and a Dorman LS6 intake from Summit Racing. For our exhaust test, this combination was fed boost by a single BorgWarner S480 turbo through a ProCharger air-to-water intercooler. The single BorgWarner turbo was capable of supporting over 1,200 horsepower, and featured a billet compressor wheel, 92mm turbine wheel and 1.25 AR. Boost was controlled by a pair of TurboSmart Gen-V wastegates using a custom Y-pipe fed by stock truck manifolds. The ignition and 120-pound Holley injectors were controlled by a Holley HP management system, while boost was kept in check using a manual boost controller.

The DIY turbo system featured a set of 6.0L truck manifolds. We reversed the truck manifolds to allow them to feed our forward-mounted Y-pipe. Note the generous plug and plug-wire access. We don’t have to worry about burned plug wires with this setup.

The custom 2.5-inch Y-pipe (from JFab) featured provisions for a pair of Gen-V, Hyper-Gate45 wastegates from Turbosmart. There were zero issues with boost control on our turbo motor.

First up on the dyno was the 3.0-inch street exhaust system. 

The Y-pipe also featured a fitting that allowed us to data-log the backpressure present before the turbo.

It is evident from the first pull that the turbo motor did not like the restrictive exhaust. It sounded like it was working extra hard to process all the airflow. We made sure to keep the air/fuel and timing constant during the pulls and monitored the backpressure present both before and after the turbo. Run with the 3.0-inch exhaust; the turbo 6.0L produced 847 horsepower at 6,500 RPM and 755 lb-ft of torque at 4,900 RPM. The backpressure readings before the turbo started at 6.9 psi then rose to 19.4 psi, with a peak boost pressure of 11.7 psi. The backpressure reading after the turbo (in the 3.0-inch exhaust) registered 1.9 psi. After installation of the 3.5-inch exhaust, the power output jumped to 884 horsepower at 6,400 rpm and 786 lb-ft of torque at 4,800 RPM. The backpressure before the turbo rose from 6.9 psi to 18.7 psi, while the peak boost reached 12.9 psi. The post-turbo back pressure dropped from 1.9 psi with the 3.0-inch exhaust to just 0.9 psi with the 3.5-inch system. The final step was to install the 4.0-inch race exhaust, which pushed the power output to an even 919.9 horsepower at the same 6,400 RPM, while peak torque rose to 807 lb-ft of torque at 5,500 RPM. The pre-turbo backpressure rose from 6.8 psi to 18.6 psi, nearly matching the 3.5-inch system, while the post-turbo pressure dropped slightly to 0.7 psi.

The air/fuel and timing curves were dialed in using a Holley HP Management system. The air/fuel and timing curves remained constant for each exhaust system.

There are a lot of facts and figures here, so what is the take away from all this? Well, the most obvious thing is turbo motors like free-flowing exhaust. Turbo enthusiasts often associate backpressure with turbo response, but nothing could be further from the truth –at least when it comes to post-turbo backpressure. Once we removed the banana in the tailpipe (eliminated the exhaust restriction), the boost response and absolute power both improved substantially. In truth, we expected more significant changes in the pre and post turbo backpressure readings, but we were certainly happy with the sizable power gains. The difference between the 3.0 and 4.0 exhaust was over 70 horsepower! True, a portion of that came from the change in boost pressure (nearly 1 psi), but any LS owner would experience the same thing with no changes to a manual boost controller. On turbo motors, we tend to focus on getting lots of boost and airflow going into the engine, but this test shows that if your baby’s got backpressure, it’s best to get it all out!                   

Naturally, each exhaust system also featured a bung for the oxygen sensor.

To allow easy installation, we utilized these v-band adapters to connect each exhaust system to the turbo.

Designed to simulate a street exhaust, we started the test with a 3-inch system that included a straight-through MagnaFlow muffler. We tested the muffler itself against a straight section of open 3-inch pipe, but the muffler itself represented no restriction. Run with the 3-inch street exhaust; the turbo 6.0L produced 847 horsepower at 6,400 rpm and 751 lb-ft of torque at 4,900 RPM. The peak boost registered (controlled by a manual TS controller) was 11.8 psi.

Next up was the 3.5-inch super street system. This exhaust represented what an enthusiast might do on a more serious street/strip application while retaining a muffler. Equipped with the 3.5-inch exhaust, the turbo motor produced 884 horsepower at 6,400 RPM and 786 lb-ft of torque at 4,800 RPM. The peak boost registered was 12.7 psi with the larger exhaust — we made no changes to the wastegate controller. As you can see from the power curves, the turbo motor responded very well to the improvements in exhaust flow and was much more responsive.

The final test involved a race-only, 4.0-inch open exhaust. This is the type of system you might find exiting the fender and is designed to maximize exhaust flow. Equipped with the 4.0-inch exhaust, the turbo motor produced 920 horsepower at 6,400 RPM and 807 lb-ft of torque at 5,500 RPM. The peak boost registered was also 12.7 psi, nearly the same as the 3.5-inch exhaust. It is obvious from this test that turbo motors run best with an unrestricted exhaust system after the turbo.

Graph 1: Turbo 6.0-Liter Exhaust Test-3.0 vs 3.5 vs 4.0 (Horsepower and Torque)

The three different power and torque curves clearly illustrate that turbo motors like maximum exhaust flow. Each step up in exhaust size and flow resulted in a significant change in power. The power output jumped from 846 horsepower with the 3.0-inch, street exhaust to 884 horsepower with the 3.5-inch system. The final step up to the 4.0-inch race system resulted in 920 horsepower and 807 lb-ft of torque. The extra exhaust flow improved power and boost response through the entire curve, clearly busting the myth that backpressure improves turbo response.          

 

Graph 2: Turbo 6.0-Liter Exhaust Test-3.0 vs. 3.5 vs. 4.0 (Pre-Turbo Back Pressure)

This graph shows the back pressure present in the Y-pipe before the turbo. Changing the exhaust flow after the turbo changed the backpressure before the turbo. Despite having the lowest boost pressure, the 3.0-inch exhaust had the highest backpressure. Though the difference was only 1.5 psi or so, there was a definite trend in the power and pre-turbo backpressure. If you compare the results of graphs 2 and 3, you will also see the trend between the pre-turbo and post-turbo backpressure.    

 

Graph 3: Turbo 6.0-Liter Exhaust Test-3.0 vs. 3.5 vs. 4.0 (Post-Turbo Back Pressure)

This graph shows the change in exhaust backpressure in the exhaust system after the turbo. Not surprisingly, the smaller 3.0-inch street exhaust posted the highest post-turbo backpressure, with a peak of 1.9 psi. There was a significant drop in backpressure (from 1.9 psi-0.9 psi) when we installed the larger 3.5-inch system. The final increase in exhaust size to 4.0-inches dropped the peak back pressure to 0.7 psi. These changes in post-turbo back pressure coincide perfectly with the changes in power, including the biggest gain going from 3.0-inches to 3.5-inches.  

   

 

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About the author

Richard Holdener

Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. Despite receiving a Bachelor of Arts in Advertising, Richard decided to follow his passion for all things automotive.
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