photos by: the author
We Take a Bottom Feeder 4.8L LR4 and Totally Transform it with Boost!
Who among us isn’t looking for the proverbial bang for the buck? When it comes to LS motors, the hot setup is to combine a used 4.8L or 5.3L truck motor with boost from an inexpensive turbo. Both of the major players are dirt cheap and the combination can yield significant power. Given the abundance of affordable turbochargers on the web, we’ve decided to compare a couple different units to demonstrate (if nothing else) their true value. On one side, the old motto that you get what you pay for holds true, but on the flipside, the allure of low-buck boost is hard to ignore.
When it comes to boost, the ideal situation is to select a turbo designed for the specific needs of your engine and intended usage. Simply rating a turbo by power production tells only part of the story, as the configuration for a 7.0L V8 making 700 hp is considerably different than a 2.0L four-cylinder making the same power. Things like pressure ratio, spool up and surge line should all be considered when matching a turbo for your motor, but often as not, enthusiasts simple grab a shelf turbo and call it a day. It is with this simplistic selection process in mind that we designed this test.
Like many budget LS enthusiasts, we let our keyboard do the walking and sourced a couple of turbos for our test. First on the list was the ever-popular, 76-mm unit from CX Racing. These turbos were made famous by their use in a number of Big Bang Theory tests on LS, Ford and Dodge motors alike. On paper, the 76-mm turbos from CX Racing fit the power and price needs of the project, so we selected one for this test. The T4, 76-mm turbos were capable of supporting over 700 hp, or more than enough for our little 4.8L LR4.
Next on the turbo list was a GT45-style turbo from DNA Motoring. Capable of supporting over 800 hp, we hoped the larger DNA turbo would demonstrate some difference in response rate, as we planned on running them at the same boost level. Obviously the larger turbo had more power potential, but did this extra potential come with a penalty in response rate. Since compressor maps were available for neither turbo, we could only speculate at how well each might be matched to our test motor, but that’s where the dyno came in.
To test the turbos we needed both a test motor and the remaining components in the turbo system. A lone turbo does not a turbo kit make. The test motor was a rebuilt 4.8L LR4 augmented with forged JE SRP pistons, Fel Pro head gaskets and ARP head studs. Topside we installed a set of CNC-ported, GenX 205 heads from Trick Flow Specialties. To further tax the turbos, we installed a performance camshaft from Crane Cams that offered .590 lift, a 224/232-degree duration split and 115-degre lsa. Small for most LS motors, the 224 cam was a healthy customer for the little 4.8L. The remainder of the test mule components included a truck intake equipped with an Accufab throttle body, 50-pound Holley injectors and Holley Dominator ECU.
The modified 4.8L fed a custom Y-pipe through a pair of stainless turbo headers from DNA Motoring. Also present were a pair of 45-mm Hypergate wastegates (and BOV) from Turbo Smart, an air-to-water intercooler from CX Racing and a plenty of aluminum (and steel) tubing, couplers and clamps to complete the system. Finish-up components included oil supply and return lines for the turbo which required drilling a hole in the stock pan.
The low-buck turbo test procedure was simple enough. All we had to do was run each turbo at the same boost level and the dyno would provide the results. Of course we also had to run each test using the same air/fuel, timing and temperatures. To ensure the same the boost (or so we thought), the Turbo Smart wastegates were set up with seven-psi springs, meaning the maximum boost allowed by the system would be something very close to seven-psi. First up was the 76-mm turbo from CX Racing.
After dialing in the air/fuel mixture at 11.8:1 and the total timing at 24-degrees (7-psi., intercooled and race fuel), we were rewarded with peak numbers of 573 hp at 6,300 rpm and 523 lb-ft of torque at 5,100 rpm. The power numbers were fine (if a little low), but the boost curve was what surprised us most. Despite the seven-psi. wastegate springs, the boost pressure never reached seven-psi. In fact, the peak boost of 6.5-psi. occurred at 4,300 rpm and the boost fell off thereafter to a low of 5.4-psi. at the power peak. So concerned were we with the shape of the curve that we actually swapped out the CX turbo for a brand new one-same results.
Concerned as we were about the results of the first test, we continued on by swapping out the CX turbo for the GT45 from DNA Motoring. The turbo swap included the installation of a larger 3.5-inch exhaust pipe (up from 3.0-inch) exiting the turbo. The difference in the two turbos was immediately apparent as the GT45 offered not only significantly more peak power (648 hp & 569 lb-ft), but more boost and power through the entire curve. The GT45 boost curve was much more what we expected with a wastegate setting of 7psi. The curve started at 6.3-psi., rose to a max of 7.4-psi. then leveled of to a steady 7.0-psi. at the power peak.
The stationary load imposed by the engine dyno artificially improved the response rate of both turbos. Despite the extra load, the larger turbo made more power than the smaller one, even at lower engine speeds. We know that both turbos had more power to give, but that was not the point of the test. If you purchased one of these turbos, you would be dealing with a falling boost curve, even at lower power levels. You can, of course, turn up the boost, but the overall shape of the curve will remain the same (just elevated). This will continue until the flow rate of the turbo can no longer keep pace with the boost/power needs of the engine. The result will be an even more pronounced drop in the boost curve.
There is one critical bit of information missing here that might shed light on the falling boost curve offered by the CX Racing turbo. Had we elected to hook up a sensor to monitor back pressure, we might have seen that the smaller turbine side of 76-mm turbo creating elevated back pressures. The wastegate functions by applying boost pressure to the bottom of a diaphragm. This pressure pushes against a spring located on top of the diaphragm. When the boost pressure is sufficient to overcome the spring pressure, the wastegate(s) open to bleed exhaust energy to limit the boost.
If we elevate the back pressure also trying to force the waste gate open, it combines with the boost pressure. The combined pressure overcomes the wastegate at a lower boost level than had there been no back pressure, thus the extra back pressure caused a drop in supplied boost pressure. Had we monitored back pressure data, we could correlate the back vs boost pressure, but like most enthusiasts, we did not hook up the proper sensors. Also like most enthusiast, we’d cure this by just cranking up the boost and get what we get. If nothing else, this turbo test illustrates that all budget boost is not created equal.
The interesting thing about this graph is that it displays the boost curve provided by the two turbos with the waste gate springs set to allow just seven psi of boost. The boost curve supplied by the DNA turbo started at 6.3-psi., rose to a maximum of 7.4-psi. then leveled off to 7.0 psi, exactly what you would expect from a turbo with the waste gate with a seven-psi spring. By comparison, the CX turbo started out at 6.1-psi., rose to a max of just 6.5-psi. then fell off to 5.4-psi. We suspect the back pressure was higher on the .96 A/R CX turbo. The wastegate operation is a function of boost vs back pressure, as the back pressure works against spring pressure to prematurely open the wastegate.
The sizable difference in the boost curves meant the two turbos produced dramatically different power curves. Run with the seven-psi wastegate setting, the turbo 4.8L produced peak numbers of 648 hp at 6,300 rpm and 569 lb-ft of torque at 5,500 rpm with the DNA turbo. By contrast, the lower boost produced by the CX turbo brought the power down to 573 hp at 6,300 rpm and 523 lb-ft of torque at 5,100 rpm. The obvious thing here is that it is possible to increase the boost pressure on either turbo with a wastegate controller, but this test illustrates the power and boost differences that can occur with different low-buck turbos. Given the size and back pressure difference, the GT45-style turbo will ultimately produce more power (by as much as 150 hp) than the smaller 76 mm, but even at lower boost and power levels might be a better choice.
For the past 25 years, Richard Holdener has specialized in direct back-to-back dyno testing. Both on staff and as a freelance editor, over 2000 technical articles have appeared in all of the major automotive enthusiast magazines including Hot Rod, Car Craft and Super Chevy. Holdener’s work, on the dyno has generated no less than 9 different books for Cartech/SA design and HP Books, as well as a website dedicated to LS-specific dyno results.