Displacement vs Back Pressure

By Richard Holdener/Photos By Author

Turbos build boost, but did you know they also build back pressure?

It is hard to believe that it was even possible to replace the original, and wildly successful, Small Block Chevy. After all, the original mouse motor was successful in just about every form of motorsports. What we are saying is the LS had some mighty big shoes to fill, but not only did the new motor step right it, they hit the ground running.

Blessed with an abundance of head flow, LS enthusiasts immediately discovered how well the new Chevy responded to performance upgrades, especially cam swaps. Once the newfound power from the cam upgrade had worn off, LS enthusiasts immediately turned to forced induction. Combine that with an overabundance of affordable turbochargers, and you have the makings of a revolution. Not long ago, it would be hard to imagine reaching (or exceeding) the 1,000-HP mark with nothing more than a cam, springs, and boost, but LS owners do exactly that.

Given the popularity of turbocharging in the LS industry, we decided to take a closer look at what happens when you apply boost pressure to a pair of LS combinations. To keep things interesting, we decided to run the same turbo system on both motors and even maintain the same boost level. This would allow us to demonstrate the power gains offered by boost, but more specifically, we wanted to see what happened to back pressure in the system when we change the normally aspirated power output of the test motor.

You see, fellow LS enthusiasts, boost pressure is only part of the equation. We all know that adding boost pressure to any motor will increase the power output. Unfortunately, all the exhaust from the extra power produced has to find a way out past the exhaust side of the turbo. More air in always equals more air out, which is why turbo sizing is so critical. Miss on the sizing of the turbine wheel, housing, or A/R and the result can be excessive back pressure. Of course, back pressure can also come from the exhaust system itself, but for this test, we ran the same system on both motors to isolate the change from the displacement and attending power output. To show what happens to the back pressure when you increase the power output of the test motor, we assembled a pair of LS combinations for testing.

Test mule number 1 was an SBE 5.3L equipped with a few modifications. The high-mileage truck motor was augmented with a COMP Cams 54-454-11 cam that offered a .614./.624 lift split, a 227/243-degree duration split, and 113-degree lsa. The 5.3L was sporting the stock 706 heads equipped with a 26918 valve spring upgrade. Topping the 706 heads was a factory LS6 intake and manual throttle body.

Test mule number 2 was a GM LS3 crate motor equipped with a cam upgrade. The 6.2L short block was given a COMP Cams 54-469-11 cam that offered a .617/.624 lift split, a 231/247-degree duration split, and 113 lsa. Instead of the LS3 heads, the aluminum short block was topped with a set of 317 truck heads (from a previous test) and FAST LSXRT intake.

Both combinations were run with long-tube headers, a FAST XFI management system and FAST 89-lb injectors. The modified 5.3L produced 448 HP at 6,800 RPM and 398 lb-ft of torque at 5,000 RPM. By contrast, the larger 6.2L produced 551 HP at 6,600 RPM and 507 lb-ft of torque at 4,900 RPM.

After running the two motors in normally aspirated rim, we installed the single turbo system on both for testing. The turbo system featured a T4, 7675 Precision turbo providing boost through an air-to-water intercooler supplied by Procharger. The discharge tube also featured a Race Port blow-off valve from Turbo Smart. The hot side of the equation included a pair of stainless turbo headers feeding a custom Y-pipe from JFab. The Y-pipe featured the required T4 turbo flange and a pair of wastegate flanges designed to accept the dual Hypergate45s, also from Turbo Smart.

The Precision turbo was configured with a 4.0-inch exhaust, which housed the required oxygen sensor. We took the liberty of drilling and tapping the Y-pipe to accept a pressure sensor to monitor the back pressure of each combination. Naturally, the dyno was configured to also register boost pressure in the intake manifold. To keep the boost pressure consistent on the two test motors, the dual wastegates were configured with 7-psi wastegate springs. Things like air/fuel and timing were kept constant for both motors.

Run first on the 5.3L, the single turbo supplied boost that varied slightly from 7.1 psi at the start of the pull, to 8.0 psi in the middle, and finally ending at 7.6 psi. The slight variation in boost pressure was the waste gates doing their job at regulating the pressure and power output of the motor, as the turbo was capable of supporting well over 1,100 HP.

Run with the single turbo, the 5.3L produced 691 HP and 612 lb-ft of torque. During the run, the back pressure varied from a low of 5.4 psi to a high of 12.8 psi. Note that the back pressure started out lower than boost pressure, but the two crossed over at 4,300 RPM, where the back pressure rose rapidly. The back pressure continued to climb and reached a back pressure-to-boost pressure ratio of 1.68:1 at our shut off point of 7,000 RPM.

After installation of the turbo on the 6.2L, the combination produced 807 HP and 747 lb-ft of torque. The boost pressure started out at 7.5 psi, rose slightly to 7.6 psi, then dropped to 6.9 psi. By contrast, the back pressure started at 6.6 psi then rose to 13.8 psi (both higher than the 5.3L). Note that the back pressure reached a 2:1 ratio on the larger motor, a point where you might start to consider changes to the hot side, as things will only get worse at higher boost levels.

To get things started, we ran both the modified 5.3L and 6.2L in normally aspirated trim.   The 5.3L retained the stock 706 heads but the LS3 was equipped with a set of 317, 6.0L truck heads. The lone upgrade to the heads was a set of beehive valve springs. Both motors were equipped with cam upgrades. The larger 6.2L also featured a FAST LSXRT intake, where the 5.3L ran a factory LS6 intake. The 5.3L produced 448 HP at 6,800 RPM and 398 lb-ft of torque at 5,000 RPM. The larger 6.2L stepped things up to 551 HP at 6,600 RPM and 507 lb-ft of torque at 4,900 RPM. Not surprisingly, the bigger motor made more power everywhere, by a consistent amount. The question now is how would this affect the boost and back pressure once we added the turbo?

After running both the modified 5.3L and 6.2L in normally aspirated trim, both were then subjected to boost from the same single turbo! Using dual 45mm HyperGate wastegates from Turbo Smart, the 76mm Precision turbo was limited to 7 psi. The same turbo system was run on both combinations, including the ATW intercooler. Run on the 7-psi wastegate springs, the turbocharged 5.3L produced 691 HP at 6,700 RPM and 612 lb-ft of torque at 5,000 RPM. After installation of the turbo system on the larger 6.2L, the combination produced 807 HP at 6,800 RPM and 747 lb-ft of torque at 4,900 RPM. As with the normally aspirated combinations, the turbo 6.2L produced consistently more power through the entire rev range.

These graphs represent the boost and back pressure curves generated during the turbo runs. For the smaller 5.3L, the boost pressure started out slightly lower at 7.1 psi, rose to a peak of 8.0 psi, then fell off slightly to 7.6 psi. This was with no change to any boost controller, just running on the wastegate springs. By contrast, the boost curve of the larger 6.2L started out at 7.5 psi, rose to 7.6 psi, then fell off to 6.9 psi. In reality, this is well within the normal range of boost from a 7-psi wastegate spring. If we check out the back pressure readings, they give us even more information. On the 5.3L, the back pressure started out below boost pressure at 5.4 psi, then rose to a peak of 12.8 psi. The larger (more powerful) 6.2L produced even more back pressure, starting at 6.6 psi, then rose to a maximum of 13.8 psi. The back pressure of the larger motor was consistently higher than the smaller motor, while the boost pressure was lower. In truth, the lower boost pressure was caused by the increase in back pressure, as the opening of the waste gate was a function of the delta pressure on both sides of the valves. The back pressure pushed against the valve (working with boost pressure) to prematurely open the wastegate.