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  • Fare Thee Well, Single-Sequential Turbo - Inside Perspective

Fare Thee Well, Single-Sequential Turbo - Inside Perspective

The next Generation Power Stroke Turbo

Mike McGlothlin
Feb 13, 2014
Photographers: Mike McGlothlin
It’s time for a little turbo truth regarding Ford’s revised 6.7L Power Stroke for ’15 Super Duty trucks. As you’ll read on page 72, Ford was once again outmatched by another manufacturer in a heavy-duty towing shootout. In both high-elevation acceleration and braking tests, the F-450 tested simply didn’t compare to Ram’s new 3500 HD. But why?
Single-Sequential Blues
From its inception, the biggest problem with Ford’s 6.7L Power Stroke has been its turbocharger. The Garrett GT32 SST (single sequential turbo)—a single turbine turbocharger with two compressor wheels—has led to Ford losing virtually every tow test in Diesel Power magazine. As you’ll read in this month’s “Tow King” article, the key area of Ford criticism stems from the 6.7L’s lack of aggressive exhaust braking with a considerable load in tow. This is because Ford feared overspeeding the GT32 SST charger. Overspeeding occurs when the turbo’s shaft speed becomes too excessive. Thanks to being privy to some aftermarket testing at Elite Diesel Engineering, we know shaft speeds easily exceed 100,000 rpm in stock form and can hit a whopping 150,000 rpm with a hot programmer in the mix (and the wastegate disabled). So in an effort to keep the turbo alive, Ford had to sacrifice exhaust braking ability on ’11 to ’14 trucks.
As for acceleration performance in elevation, the GT32 SST comes up short there as well. In layman’s terms, the smaller the turbo, the more compressor wheel speed you need in order to maintain efficiency at altitude. By having a 46mm compressor wheel (plus another compressor wheel behind it), the GT32 SST has to turn a lot of rpm to make adequate boost, and in thin mountain air it can only be pushed so hard. Essentially, the turbo is so far out of its map at high elevation that it becomes extremely inefficient. This is why the ’11 LML Duramax-powered Silverado 3500 beat the ’11 Power Stroke–powered F-350 to the top of the Eisenhower Tunnel by more than two full minutes in our King of the Hill shootout (Feb. ’11), and it’s also why the ’13 Ram 3500 HD beat the ’13 F-450 in the same exact test by more than a minute. I can’t lie—the GT32 SST is a neat piece. Its tiny turbine wheel (64mm inducer), variable-geometry turbine housing, and dual ball bearing center cartridge all combine for a very responsive, snappy truck at or near sea level. But it just doesn’t get the job done in elevation.
A Familiar Face
Want to know a secret? The single compressor Garrett GT37 Ford will be replacing the GT32 SST with for ’15 models is nothing new. The GT37 will be—or, at the very least, based off of—the Garrett Powermax (GT3788VA), a turbo many modified 6.0L Power Stroke owners have come to love in the aftermarket. Granted, it will be slightly different (requiring a different turbine housing for mounting purposes), but I suspect it will use the same 0.90 A/R. Other than that, I don’t see much else being different. We know, directly from Ford, that the GT37 will feature the same 88mm exducer compressor wheel diameter as the Powermax, which leads me to believe it will have the same 63.5mm inducer. Ford also released information that the turbine wheel will measure 72.5 mm, which happens to be the same inducer size as the Powermax’s turbine wheel. Finally, Ford stated its new turbo wouldn’t be wastegated, just like the current Powermax.
Photo 2/2   |   We have reason to believe a reworked version of Garrett’s GT3788VA Powermax will be the next turbo to grace Ford’s 6.7L Power Stroke. This same turbo made a name for itself in the 6.0L aftermarket for its drop-in nature and durability, and its ability to support up to 500 rwhp.
Better Performance
In theory, and because the GT37 turbo Ford plans to use utilizes larger wheels (more mass to get moving), shaft speed will be reduced under all operating conditions. This means engineers can get much more aggressive with the turbo’s exhaust brake function, something Ford says will be greatly improved on the ’15 trucks.
As for horsepower, we don’t know how much farther Ford will push the 6.7L V-8, but we can tell you the current Powermax available for 6.0L Power Strokes is capable of handling 500 hp at the wheels in the aftermarket. Furthermore, many modified versions of the GT3788VA retain the original 72.5mm turbine wheel because it flows well enough to support 600 rwhp. With this type of horsepower capability, there is really no reason Ford can’t add another 50 hp and 100 lb-ft of torque to the 6.7L’s output.
By changing turbos for 2015, Ford will get rid of the one thing that’s had a stranglehold on the performance of its state-of-the-art engine. Expect vastly improved exhaust braking, more horsepower and torque, and a big tow rating increase for the next-generation Super Duty. And, of course, expect another Tow King shootout in the future.
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