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Supercharge Your Diesel Truck!

No lag, less smoke, and instant throttle response

Text By , Photography by The Diesel Power Staff, The Readers

Supercharger. Blower. Huffer. There’s just a certain ring to these words. With huge airflow capabilities and a nasty whine, superchargers are a common road to horsepower in the gasoline world, so why not with diesels? Well, a few brave souls have tried blowers, usually in conjunction with nitrous oxide or turbochargers. A lot have failed, but a few setups have worked pretty well. And, in addition to being cool and making power, there are a few other benefits of supercharging, which we’ll gladly explore now.

Why Supercharge? A Broad Power Curve, with Instant Response!
With turbochargers, especially large ones, spool-up time can be two or even three seconds before the turbo is making full boost, and during that spool-up time most diesels are only making about 100 hp. Even with smaller or VGT turbos, the fueling always has to be ahead of the turbo to get it to spool, resulting in black smoke and limited power. Superchargers, on the other hand, will already be up on boost—independent of fueling—and can make as much as 10 psi at idle when mated to a diesel engine. This means there is no turbo lag, only instant response, because there is already enough airflow through the engine to make substantial horsepower.

Draw-Through, Blow-Through, or Standalone?
Another commonly asked question about superchargers involves their setup and use with turbochargers. We’ve seen three different popular arrangements, including the blower compounding air into the turbocharger, the turbo compounding air into the supercharger, or just running the supercharger by itself. We’ve also seen setups in which the supercharger is only used for low-rpm operation and is then bypassed at higher engine speeds. Most of these arrangements have their merits, although some are definitely better than others.

Blowing into the Supercharger with a Turbo
Detroit Diesel’s two-stroke engines made this setup popular, but it’s also one of the least efficient. Blowers take a lot of power to drive, so putting one downstream of a turbocharger only makes things worse. In addition, most roots or screw superchargers have oiling issues if you try to push more than about 15 psi into them. Centrifugal blowers don’t fare any better, as they will take more power to drive, and the front compressor cover can pop off when pressurized. The only way we would suggest this type of setup is with some type of bypass that circumvents the blower entirely when the turbo is creating boost.

Running a Blower as a Single Compressor
Stepping up the ladder a bit in our idea ladder is that of running a supercharger as the only source of compressed air. This isn’t that great of an idea, either, mainly due to the fact that most diesel engines benefit from very high-pressure ratios (lots of boost). Superchargers are very effective in the 5- to 15-psi range, but beyond that they start to sap a lot of power. A large centrifugal supercharger operating at 50 psi might take upward of 500 hp to drive! A turbo of the same size would take about 100 hp at the most, so it’s clear who the winner is. The only way we’d run a supercharger as a single would be in a low boost (less than 30-psi) application with a healthy dose of nitrous oxide to make up the airflow deficit.

Drawing through a Supercharger with a Turbo
This is an interesting theory and could work well in a few different applications. In this setup, both turbocharger and supercharger are sized very similarly, and the turbocharger sees positive boost from the supercharger at low rpm, and no (or even negative) pressure at higher rpm. Basically, the supercharger is used as a low-end enhancer, but without bypass valves or any other sophisticated hardware. During high-rpm operation, the supercharger is moving enough air so it doesn’t become a restriction or take a lot of power to drive. We believe the aftermarket may soon offer kits with this type of arrangement.

Blowing into a Turbo with a Supercharger
If you’re going to try a supercharger setup, this is probably the best arrangement one could come up with. This setup works just like a basic compound arrangement, only instead of another turbocharger, the large compressor is a supercharger. This way, the supercharger still makes boost at idle, and as the turbocharger starts making pressure, the strain is taken off the supercharger, which means less horsepower is taken away by driving the blower. For the ultimate in street driveability, a very large centrifugal supercharger could be added to a set of compound turbos, making a three-stage system capable of 2,000 hp in which the blower could be limited to 15 psi or less. This means the big supercharger might only take about 50 to 75 hp to spin—a small price to pay for the response and horsepower potential.

So, Does it Actually Work?
We’ve come to the point in diesel performance where we can definitely say that yes, supercharged diesels can make power. However, driveability is where they really shine, or in very high-horsepower applications in which turbos can take enormous amounts of fuel and rpm to drive. As of right now, the lack of commercially available kits is probably the only reason superchargers haven’t caught on more with diesels, and this is something we hope will change.

How Much Power Does It Take To Drive A Supercharger?

Blower Speed (rpm) Boost Pressure (psi) Drive Power Requirement (hp)
36,855 11.7 74.1
38,745 12.2 88
40,635 13.7 118.1
42,525 15.1 127.1
44,415 16.4 140.6
46,305 18.2 156.6
48,195 20 173.4
50,085 21.5 190.5
51,975 23.7 213.4
53,865 25.4 239.2
55,755 27 279.2
57,645 29.1 304.8
59,535 31.3 340.1
61,264 32.9 353.6

If you want to keep drive losses to a minimum in a compound arrangement, keep thepressure ratio of the turbocharger high and the blower boost low. This graph providedby Steve Morris Racing Engines shows how much power it takes to spin a superchargerat high boost levels. The good news is that the 115mm F2 Procharger used to generate this test data is good for between 1,500 and 2,000 hp!

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