If you’re a turbo guy (or gal), there has never been a better time to be an enthusiast. Just look around, turbos are everywhere and on everything.
Not long ago, many people didn’t even understand how turbochargers work, let alone attempt to install them on their hot rod. Unless your name was Gale Banks or Corky Bell, you left turbos alone.
Fast forward a few decades and the advent of automotive internet forums, YouTube, and eBay have ushered in the modern turbo era.
Never in history have turbos been so available and affordable, but it is this over-abundance that creates confusion. The massive availability of cost-effective, offshore turbos has everyone jumping on the boost wagon, but many are doing so without a basic understanding of the differences between various turbo offerings.
The offshore turbos have many enthusiasts selecting them based solely on price—because a turbo is just a turbo, right?
While everyone wants a good deal, there is more to turbo selection than simply price.
To illustrate the difference a turbo cam make, we selected a pair of popular turbos, one from eBay and the other from Summit Racing. Though eBay doesn’t manufacture their turbos, this popular GT45-style turbo offered the best combination of price and power potential.
This turbo can be sourced through a variety of resellers, as most repackage or simply resell the same product under their own name. Our eBay turbo actually came from the folks over at DNA Motorsports.
There might be some confusion about this turbo, as there seems to be different versions available. If our spec sheet is to be believed, our GT45 featured a 50-trim compressor wheel, with a 68.9-mm inducer, and 98-mm exducer.
The T4 exhaust featured a 1.05 A/R ratio, an 88.7-mm inducer, and 77.5-mm exducer. Though offered at a variety of different price points, ranging from our cost of $163 to over $400, the eBay turbo was certainly affordable.
By contrast, the $649 S475 turbo offered by Summit Racing was considerably more expensive.
The spec sheet on the Summit turbo made up for at least some of the cost difference, as the T6 turbo offered a larger 75-mm inducer, and 100-mm exducer on the compressor side and a hot side equipped with an 88-mm exducer, and 96-mm inducer. The T6 housing also featured a 1.32 A/R ratio to further improve exhaust flow.
Spec sheets are all well and good, but how do they actually perform?
For most people, a turbo is just about boost, more boost equals more power, right?
I mean, all you have to do is keep cranking up the boost, and the turbo will keep making more power.
Unfortunately, like any air pump, there is a limit to how much a turbo will flow.
Forget about the boost and think more about airflow.
A turbo will only flow so much, and boost is simply a measurement of the restriction inherent in the engine it happens to be flowing air into. At some point, the turbo will be unable to flow additional air, either because of a limitation in the compressor or turbine side, and then boost will drop.
This is exactly what we saw in this comparison, but first we need to quickly cover the test engine.
The 5.3L LM7 Test Engine
On loan from Roadkill’s David Freiburger, the 5.3L LM7 featured a stock block and crank combined with forged rods and pistons. The powdered-metal rods and cast pistons were ditched in favor of a set of 6.125-inch, K1 rods and Wiseco forged pistons. The motor also received a set of TFS 205 heads and FAST LSXR induction system. In addition to the intake, FAST also supplied a set of coils, injectors and a 102-mm, Big-Mouth throttle body.
On loan from Roadkill’s David Freiburger, the 5.3L LM7 featured a stock block and crank combined with forged rods and pistons. The powdered-metal rods and cast pistons were ditched in favor of a set of 6.125-inch, K1 rods and Wiseco forged pistons.
Because we were testing a pair of turbos, naturally we needed some kind of turbo kit.
Less a kit then a DIY project, the turbo system consisted of a pair of stock truck exhaust manifolds feeding a custom Y-pipe.
The Y-pipe merged the exhaust flow from each bank of cylinders in the V8 to a single three-inch, V-band outlet. The three-inch V-band was used to allow us to run both T4 and T6 turbos, using the respective adapters.
One of the mistakes we made—and why there will be a part two follow-up to this test—is that we relied on a manual waste-gate controller to raise the boost.
The manual controller resulted in slightly different boost curves for the two turbos, despite the use of a pair of Turbo Smart Gen-5 waste gates.
Boost from each of the turbos was run through an air-to-water intercooler from ProCharger. Both turbos were run with the same 11.8:1 A/F ratio and with 20 degrees of total timing at the power peak (less at the torque peak). With our DIY turbo system ready to accept both turbos, it was time to test.
eBay GT45 Turbo Test
First up was the eBay GT45 turbo. To ensure we had the tune nailed down, we started out running on the waste-gate springs of 7 psi. Using the manual controller, we raised the boost pressure until we started to get diminishing returns from the turbo.
After reaching a peak boost of 14.5 psi early in the run, the boost pressure dropped down to just 10.1 psi at 6,500 rpm. The falling boost curve produced a peak power reading of 776 horsepower at just 5,600 rpm. The upside to having lots of boost at lower engine speeds was a healthy peak-torque reading of 786 foot-pounds at 4,500 rpm.
To give you an idea, run naturally aspirated, this combination would make peak power closer to 6,500 rpm, so the turbo and dropping boost curve were definitely altering the power curve. Though available maximum flow, from what is likely an 800-hp turbo, was certainly the major contributor, the method of boost control was also a factor.
Had we run an electronic (three- or four-port) boost controller, the boost curve would be more consistent. We know the eBay turbo should be able to run a consistent boost level of 10 psi, but we doubt it will go more than one or two extra psi before it runs out of flow.
Summit Racing S475 Turbo Test
After running the eBay GT45 turbo, off it came and on went the T6 Summit S475 turbo. The turbo was configured to flow through the same ProCharger intercooler and was run at a variety of different boost levels up to a peak of 17.3 psi.
Because the larger S475 turbo was capable of supporting more power, the boost curves were slightly flatter on the Summit turbo. It wasn’t until we ran the boost up that we started seeing a dropping curve, but nowhere near as dramatic as the smaller turbo.
Run at a peak boost of 17.3 psi (at 3,700 rpm), the boost dropped to 14.7 psi at at 6,500 rpm. The dropping boost curve meant the turbo motor produced peak power at 6,100 rpm, but this was still at a higher engine speed than the smaller GT45 turbo. Even with the dropping boost curves, it is obvious the Summit turbo had more to offer than the smaller GT45, so if all you need is 800 (flywheel) hp, then go for the eBay unit. If, like most turbo owners, you plan on cranking it up, then the Summit turbo is definitely worth a look.
On our next test, we plan to monitor the backpressure to show how the power output affects the exhaust flow, and therefore the backpressure. This backpressure works to open the waste-gate valve, reducing the boost. With boost pressure applied to the top of the valve, we hope to counter this effect, but the change in boost curves and absolute max outputs will have to wait until next time.