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Towing Turbo Surging Severely
Question: I have a question about my '03 LB7 Duramax that I hope you can help me with. The truck has a Banks Six-Gun with the Speed-Loader stacked with a DiabloSport tuner, a wastegated Garrett 3788R turbo, Banks intercooler, PPE lift pump, K&N cold-air intake, 4-inch turbo-back exhaust, and manual transmission with a South Bend dual-disc clutch. The problem is the truck has a lot of what I call "turbo huff," at cruising speed and when towing at high elevations. It will go away if I accelerate, but I have problems with it when running empty or towing. It is definitely worse when towing. Is this just a simple wastegate adjustment issue, or is it something else?
Rory McManus
Cheyenne, Wyoming
| All turbochargers have what is called a surge line (arrow): a pressure-to-airflow ratio in which the turbocharger will start to stall out. As you can see on this compressor map, a high amount of pressure combined with a low airflow can result in some pretty severe surging. Limiting the amount of boost at lower rpms, or downshifting (and thus increasing the engine's airflow requirement) is the only way to cure a turbo that is starting to surge.
Answer: Your basic problem is that your turbocharger is creating too much boost under part-throttle, low-rpm situations. The intake pressure is overcoming the drive pressure, and the turbo is stalling out. We did a little bit of research and found that a stock LB7 turbocharger has a compressor inducer diameter of about 61mm. If we compare that to your GT3788R, we find it's only slightly bigger, at 63.5mm. In fact, stock '05 Chevys and GMCs with the LLY Duramax engine had 64mm turbochargers, so your upgraded turbo is actually smaller than the turbo on a stock '05 LLY Duramax engine. While the Garrett turbo is more efficient, the combination of part-throttle use and high elevation can cause the turbo to surge. While this turbocharger might have been fine for a daily driver at sea level, at higher altitudes you'll actually need a larger turbocharger. To get some advice, we talked to Fleece Performance Engineering (www.fleeceperformance.com or [765] 676-5063), which told us that although it was possible to upgrade your existing Garrett turbo for about $300, stepping up to an S300-frame turbo or a Garrett GT4094R would be a better choice.
While all this may sound counterintuitive, a larger turbocharger will cure your part-throttle, overboosting problems. How? A larger turbo has more mass, along with a larger turbine wheel that will take more drive pressure to spool, so it will be spinning slower and creating less boost at part-throttle, which will take it out of the surge zone. Under full-throttle acceleration, the turbo will also flow more air and keep EGT down. Although it will spool slower than your GT3788R, a larger turbo with a healthy exhaust wheel and a large housing will eliminate compressor surge completely. If you don't have the dough for a new turbo, downshifting and running higher engine speeds will increase the engine's part-throttle airflow requirement and help with the surging, but you then run the risk of overheating the truck.
Can You Lower a 4x4?
Question: Your October '09 issue got my attention to the point that I'm leaning toward trading in my gas truck for a diesel. However, the only thing that's keeping me from going through with this idea is that I enjoy lowered trucks. If I were to purchase a four-wheel-drive diesel truck, could I install an air-spring suspension to lower it for the sport truck look and performance on the dragstrip, while also having the ability to lift it up in case I would need to tow?
Joe Luna
Mission, Texas
Answer: Once you feel the power of a boosted four-wheel-drive launch, you'll never want to go back to a gas-powered rig. You can lower Ford and Dodge diesel trucks, but eventually their solid front axles will get in the way of the frame, which means those trucks can't be lowered that much. If you really want a slammed 4x4, we'd suggest a Duramax-powered GMC or Chevrolet, because of their independent front suspension. You can actually change the ride height up or down a tremendous amount just by adjusting the torsion bars. In fact, we've seen Duramax-powered rigs with the front tires stuffed in the fenders because they are so low-pretty rare for a four-wheel-drive truck, gas or diesel.
Also, a larger wheel (22- to 24-inch) will fill up the fender more, making the truck look lower than it actually is. Installing air springs in the rear for towing is common practice on diesels. You'd either have to get custom-made leaf springs for the rear, or maybe remove a few leaves, but we'd still stick with factory-based attachment points for the sake of reliability. You could probably leave the front low, or just adjust the torsion bars a bit. You won't be scraping the frame on the ground, but your truck will have a lowered look with Corvette-beating power, plus it will get 20 to 30 percent better fuel mileage than a comparable gas rig.
