Turbochargers are perhaps the single most important component of today’s diesel engine, and perhaps the least understood as well. Why do we even upgrade our turbochargers? Is it for power, reliability, exhaust gas temperature (EGT) control, or all three? We set out to find the answer to these questions and more by performing one of the most comprehensive turbo tests ever.
Since they’ve been around forever (and many of our readers are running them), we chose the line of Aurora turbochargers built by ATS Diesel Performance for our comparison. They range in inducer diameter from 57 mm to 71 mm, so we’d have a pretty broad range of compressors to choose from. During our testing, we’ve monitored peak horsepower, torque, boost, drive pressure, discharge temperature, peak EGT readings, cruising EGT readings, and spool rpm. We’ve also racked up thousands of miles of driving impressions on each to let everyone know just how they performed.
The pickup we chose for this test is our ’95 Dodge Ram 2500, known as Project Triple Threat. With its 5.9L Cummins 12-valve engine making 433 rwhp at a toasty 1,600 degrees EGT on a stock Holset HX35 turbocharger, we felt this would be the perfect testbed for various-sized turbos. Going into this test, we didn’t know whether the turbos would increase power or not, as the engine already seemed to have enough air (the exhaust produced very little soot) with the HX35. As it turned out, we’d learn more than we could have imagined.
Aurora 3000 Turbo
The first turbo we tested was the 57mm-inducer Aurora 3000 offered by ATS. As it was advertised to support 350 to 450 hp, we surmised this turbocharger would be comparable to our stocker—only without the possibility of explosion. As it turned out, the Aurora 3000 actually spooled quicker than the stock version, especially during part-throttle conditions. It also made more boost, peaking at 49 psi (compared to 42 psi for the HX35). And although EGT still hit 1,550 degrees, that peak temperature came a lot later in the dyno run. The Aurora 3000 also made more power, peaking at 449 rwhp and 1,041 lb-ft of torque. There were no driveability issues (such as lag or surging), but our EGT level showed we were pushing the limits of the 3000’s horsepower range.
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Aurora 4000 Turbo
Our second turbo in the test was the billet-wheel Aurora 4000. With a 63mm inducer, it represented a mid-step in performance between large and small turbos. With the 4000 installed, one could tell the truck had a larger-than-stock turbo on it. It spooled later and would surge slightly on locked shifts into Overdrive at maximum boost. We also got some part-throttle surge with this turbo, probably due to the fact that we were at sea level with a tight exhaust housing. On the other hand, running the 4000 (which is rated for up to 625 rwhp) did have its benefits. Boost was up to 56 psi, while EGT dropped a whopping 200 degrees at wide-open throttle, to a mere 1,400 degrees. Remember, this was with no intake or intercooling modifications—just the turbo itself. Power was also up slightly, to 452 hp. What was surprising was that at 3,000 rpm, the Aurora 4000 was worth an additional 20 hp as compared to the Aurora 3000. This power increase came with no additional fuel and indicated a broader powerband that would translate into a faster truck. We ran the truck at the dragstrip with the 4000, and it made a 13.50-second pass (vs. 13.76 seconds with the stock turbo), which indicated it was indeed making more power.
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Aurora 5000 Turbo
ATS offers its Aurora 5000 turbo as its entry level large turbocharger. The 5000 is rated at 750 rwhp, and with its 71mm inducer, it’s no joke. The turbo is also based on a different frame (BorgWarner K31 versus BorgWarner S300), so we were interested to see how it would perform. When we rolled the truck onto the rollers with the 5000, we were surprised to see 464 rwhp, a good 12hp gain at the peak over the 4000 turbo. It also carried out power even further than the 4000 and made nearly 20 additional horsepower at the top of the powerband. ATS recommends a cam, headwork, and intercooler to make full use of this turbo, so we were surprised at how driveable it was with our stock engine.
Surging was minimal, especially considering it made a whopping 59 psi of boost at full tilt. Exhaust gas temperature dropped even further—to 1,350 degrees at wide-open conditions—but was a little toasty at part throttle. ATS doesn’t recommend this turbo for towing (we could hit 1,200 degrees uphill with our empty truck), but as long as you’re willing to drop down a gear and keep engine speed at 2,500 to 3,000 rpm, we could still see using it. It’s definitely a large turbo, but it’s one we could see living with every day—if we had a 600 to 700hp truck and wanted a single turbo.
Aurora Compound Turbos
The idea behind using two ATS Aurora turbochargers in compound is simple: You get the spooling ability of the 57mm Aurora 3000 turbo, with the airflow capability of the 71mm Aurora 5000 charger. Each turbocharger does half the work, which makes everything a lot more efficient and the vehicle much more driveable.
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When we first drove the truck with the Aurora compounds on it, it felt even better than in theory. No matter how hard we tried, we could only hit 1,200 degrees on the pyrometer, and that was with a puzzlingly low 56 psi of boost. Since our Aurora 3000 was wastegated, we figured it was what was limiting our overall boost pressures. Plugging off the wastegate resulted in 65 psi, but the truck really didn’t feel any faster. When we hit the dyno, our seat-of-the-pants theories evaporated, as we only put down 432 rwhp and 900 lb-ft of torque. While boost hit 65 psi, drive pressure hit a scary 99 psi, so we connected the wastegate back up, which resulted in 57 psi of boost with 72 psi of drive pressure. Then we tried removing the air filter. No gain. We checked for leaks, nothing. When we checked boost pressure between stages, we found the 71mm Aurora 5000 charger was producing 40 psi, while the smaller 3000 was barely spinning, at a pressure ratio of only 1.3:1.
Our working theory for the 30hp power drop was that the power required to spin a second turbocharger and maintain the same boost level took a bit more horsepower in the compound setup. As for why the compounds were able to keep EGT so low (on the dyno they were only 1,100 degrees) yet not make any more power, we have no solution; that’s a puzzler for us.