Welcome to Top Tech Questions. Oftentimes, readers contact us with questions about articles, or to praise us on what a good job we are doing. But our favorite form of reader communication is tech questions. Our Top Tech section is a place where you ask what’s on your mind, and we answer it. Got a trouble code? Wondering how to get your engine to make more power? Send us an email at jason.sands@sorc.com and we’ll do our best to answer it.
Bigger Turbo = Worse Mileage?
Question: I have an ’04½ Dodge 3500 with the 5.9L Cummins engine, and I recently had the thrust bearing on the turbo fail. I replaced it with a Garrett PowerMax Stage 3, thinking it would help EGT under wide-open throttle conditions, and it definitely has. My concerns are about cruising EGT and fuel economy. Since installing the turbo, I have lost about 2 mpg, and my cruising EGT has gone up by nearly 200 degrees! I want to keep the power and wide-open EGT capabilities of this turbo, but I need my mileage back. Is there anything I can do?
Todd L. Cook
Hutto, Texas
| When Dodge first started offering the Ram with a Cummins engine back in ’89, they came with a mileage-sized 50mm turbocharger and 160 hp. With today’s 300-plus-hp diesels, turbos have had to be sized-up accordingly and mileage has suffered.
Answer: We initially thought you’d fallen into the “if it has more lag, it must be better” trap. With turbochargers, it’s all about engine efficiency, and a turbo that’s better suited for wide-open throttle use at high rpm might not be suited for low-rpm cruising, just as a smaller turbo that’s suited for low-rpm cruising might not work well for high-rpm power.
Like an engine, a turbo has an operating and efficiency range. With a larger turbocharger, that range is shifted up higher into the powerband. This means at the low-rpm cruising speeds that most are used to, a larger turbo just won’t be as efficient. People think turbochargers are geared toward peak power from the factory—but they’re not. Mileage and throttle response are just as much of a concern as power and torque ratings.
When we looked a little closer at your turbo specs, however, it seems as though you may still be in the ballpark. A Garrett GT3788R has a 63.5mm inducer, which isn’t all that large for the displacement you’re running. Certainly, it’s small enough that good fuel economy should still be a viable option. If we had to guess, we’d say you currently have an exhaust housing that’s too large for low-rpm efficiency. The Garretts come with either a 1.11, 0.99, or 0.89 A/R ratio, and it’s our guess that you have one of the larger housings on the turbocharger. With the 0.89 A/R ratio housing and ball-bearing design, the Garrett turbo should be able to perform very close to the stock turbo mileage-wise—but will have a lot more breathing room at the top of the rpm range.
Much Needed Injector Nomenclature Question: I am looking to upgrade the injectors on my ’99 Ford F-250 with the 7.3L Power Stroke engine. I see 155 or 238/80 or 400/400 injectors in the magazine all the time, and I have no idea what any of these things mean. I understand that the bigger numbers mean bigger injectors, but surely there has to be a rhyme or reason to these numbers. I’m looking for a good towing injector for 400 to 500 hp and only want to buy once. Any help would be greatly appreciated!
Thomas Moore Chicago, Illinois
| Feature Editor Mike McGlothlin chose a set of 238/80 hybrids for his own 500hp Power Stroke project. He broke the barrier with 502 rwhp, and the truck is virtually smoke-free on the street.
Answer: That’s a great question. Typical nomenclature for 7.3L and 6.0L Power Stroke engines lists the injector’s displacement (cc) first, followed by the nozzle size. This means a 238/80 injector can flow a maximum of 238 cc of fuel (per 1,000 shots) and is equipped with a nozzle that’s 80 percent larger than the stock nozzle. We chose to use the 238/80 example to explain the nomenclature because it would be the perfect injector for your needs (towing, and 400 to 500hp capability). Our ’97 F-350 has these injectors (known in the aftermarket as standard hybrids) and recently dyno’d 502 hp at the wheels. Our tow setting (roughly 350 to 370 hp at the wheels) is perfect for hauling loads, and exhaust gas temperature never exceeds 1,100 degrees.
Most aftermarket HEUI injectors are hybrids these days, named as such because they use internal parts that don’t require more high-pressure oil to fire them. This means you can have a 500, 600, and even 700hp-capable injector (with the 400/400 you mentioned), and you only need a single high-pressure oil pump (albeit an aftermarket version) for them to operate effectively. The larger the nozzle, the quicker the injector will empty all its fuel into the cylinder.
Head Gasket Woes
Question: I have a quick question about my ’05 Chevy truck with the 6.6L Duramax diesel. I started losing coolant in June, but I couldn’t find a leak. It gets worse when I tow with the truck. Coming home from the desert this weekend, it would only go about 60 miles before the low coolant light came on. Do you think it might be a head gasket? Anything else I should check?
Tim Foss
Los Angeles, California
| Head gasket jobs on a Duramax aren’t cheap, mainly because everything is in the way! Once the engine is torn down, aftermarket head studs should be installed to ensure another head gasket doesn’t pop in the future.
Answer: We hate to be the bearer of bad news, but you’re probably right—it probably is a head gasket. A leaky EGR cooler could also be the culprit, but since you’re leaking so much coolant and it gets worse while towing, it’s most likely a head gasket. LLY-code Duramaxes have head gasket issues more than most, probably because they also run a little hotter on the engine temperature than other versions. Factor towing into the mix, with heavy load and high mileage, and the head gasket just can’t take it. The bad news is that to replace both head gaskets on a Duramax is about $2,000 to $4,000 in parts and labor. The good news is that if you do the job with ARP head studs, you’ll probably never have to worry about it again.
