It’s wrong to say that a diesel engine absolutely needs a turbocharger, but boy do they help. Naturally aspirated, a 7.0L diesel (basically the size of a new pickup engine) might only make a little over 100 horsepower, so it’s clear that boost is the only way to go. That’s why virtually all modern diesels are turbocharged, so much so that the term turbodiesel is actually recognized as one word.
In the early years (think 1989 Dodge Ram with a Cummins) turbo systems were fairly simple. A turbo was hung off the exhaust manifold and was spun by exhaust pressure in order to feed air into the engine. That was it. No wastegate, no intercooler, no variable geometry–nothin’. As technology progressed, however, all of these things were added, as all of them are beneficial to the turbodiesel engine. One of the very first items that made its way into turbodiesels was the wastegate. But what exactly does a wastegate do?
The most popular wastegate design for turbocharged diesels is the internal wastegate. It’s called this because the wastegate is incorporated into the turbine housing, and is used to bypass the turbine through a small hole right before the downpipe.
The Advantages of Running a Wastegate
Turbochargers spin at a very high rate of speed–up to 100,000 rpm in some cases–in order to windmill air into an engine at pressures that are much higher than atmospheric. A modern OEM turbo may run at 30-psi or more, while competition versions can run upwards of 70, 80–even 100psi! In the end, though, every turbo has its limit. Over-pressurizing or over-speeding a turbo can result in catastrophic failure and that will ruin a turbo. Possibly even damage the engine.
Simply put, a wastegate bleeds off the exhaust pressure that drives the turbocharger. This in term limits the amount of boost the turbocharger creates, and also the maximum compressor speed. Score one for the turbo that was just saved! There’s more, however, as turbo sizing can also be adjusted, which means a smaller, quicker spooling exhaust side can be fitted for low-rpm response, and then pressure can be bled off up top in the rpm range. This is the main reason that Ram, Ford, and GM all went with wastegated setups; it protected the engine from damage, and it gave a more usable and extended rpm range.
When Don’t You Need a Wastegate?
If there are times that you need a wastegate, surely there are times where the opposite is true, and you don’t need one. Many competition vehicles that aren’t using nitrous oxide injection (we’ll get into that later) will run non-wastegated turbochargers, as they’re looking for literally the most boost pressure the turbo can produce, and are also not too concerned with elevation changes or drivability.
Other applications that run at a fairly specific rpm like tractors or generators can also get away without running a wastegate, mainly for cost reasons. But for the diesel performance industry, boy are wastegates beneficial.
A wastegate is a fairly simple system. Most mechanical wastegates use a simple diaphragm with a spring inside the opens at a preset pressure. This pressure can be changed anywhere from 5psi to 50psi (or more) depending on the spring setups and boost referencing available. This assembly in turn actuates a valve, that opens towards the atmosphere, venting excess pressure.
This type of wastegate is simple and effective and can be used outside the turbocharger (an external wastegate) or integrated into the turbo (internal wastegate). Variable geometry turbos can also create a wastegating effect through a nozzle or vanes, but since they are complex and computer-controlled, we’ll stick mainly to valved gates.
At first, this may sound counter-intuitive, after all the wastegate is the control right? Technically, yes that’s correct, but there’s still a lot of “dialing in” that has to occur. A good starter spring for a wastegate is somewhere around a 15-pound spring. This means that once the turbocharger reaches 15psi, a boost line to the bottom of the diaphragm will open the wastegate and relieve some backpressure.
On a diesel, however, you may want a lot more boost, something along the order of 40 to 70-psi for most trucks, so, therefore, regulated air must be run to the top of the diaphragm to keep it shut. Adding a regulated 30-psi to the top of the gate now means that the turbo won’t overcome the spring pressure (and boost on top of it) until about 45-psi. Adding full boost to the top of the gate will effectively keep the wastegate shut; eliminating it completely.
Sometimes it’s best to have a few examples in order to get the hang of how something works. Here we have a couple of street trucks, a drag truck, and a sled-pulling rig all to show how wastegating is used (or not used) on each setup.
Truck #1- 1996 Dodge Ram, minor fueling upgrades, 57mm/71mm ATS compound turbos with a single internal wastegate, street truck.
We had the pleasure of tuning this truck on the dyno with pressure monitors everywhere, so we can report on how much power it made under various configurations. Before we hit the dyno, we ran it on the street where it only made 57psi with its limited fueling. That seemed low so we pinched off the wastegate line, effectively closing it. Boost hit 65psi, but the truck didn’t feel any faster.
The dyno would tell the story. After our first run, the truck made 400rwhp at 65psi, but with a whopping 99psi of backpressure (or drive pressure) which was far and away from the magical 1:1 boost to drive pressure that most folks aim for. Opening the wastegate saw a drop in boost of 8psi to 57psi, but drive pressure was a mild 64psi, and the truck actually picked up in power to 432rwhp! In this case, wastegating netted an increase in power even at a lower boost level, due to an increase in engine efficiency.
Truck #2- 2003 GMC 2500, Heavy fueling upgrades and tuning, stock turbo with internal wastegate, lots of nitrous, drag truck.
After this truck ran 6.60s in the eighth mile we had to adjust our glasses, because there was no way it should have been that fast on the factory turbocharger. The racer had initially been running the turbo in the danger zone at nearly 40psi, but he found that by putting a larger diaphragm actuator (Banks Big Head) he could lower the boost to 30psi and just add more nitrous and run even faster! This was a case where the wastegate was being used to just keep the turbocharger alive with nitrous, since adding N2O to the mix dramatically increases drive pressure.
Truck #3- 1999 Dodge Ram, Heavy fueling upgrades, 3.0-inch turbo with no wastegate, sled puller
Sled pulling trucks are great examples of diesels that may be run without a wastegate, even though they make extreme amounts of boost and horsepower. Since these engines operate at an extremely high (4000-6000rpm) rev range and go down track at a fairly constant rpm (say 4,500-5,500rpm) they actually don’t necessarily need a wastegate.
Instead, boost is controlled by very large exhaust sides and effective intercooling on the intake side. If the turbo explodes because of excess speed or pressure, a “better turbo” is simply found instead of a wastegate. It’s for this reason that top pulling turbochargers can cost $5,000 or more, and while you could drive this type of setup on the street, it would be downright miserable. There have also been claims (mostly on tractors) of 100psi or more of a single turbo, which is downright insane!
Truck #4- 2008 Ford F-250, Heavy fueling upgrades, Compound Turbo system with a VGT high-pressure turbo and twin external wastegates, street truck
The turbo setup on 6.4-Liter Fords is a factory compound setup that includes a variable geometry high-pressure turbo, so right from the factory, they’re fairly complicated. This particular truck has probably the most extensive wastegate-tuning time on the dyno we’ve ever heard of, and took nearly 100 hours of fuel, timing, and tweaking to dial in.
After some initial runs the truck made 730rwhp, but the tuner decided there had to be more in the combination because the 82mm turbo made enough air to flow more than 1,000rwhp. So the tweaking began. As boost began to rise, the backpressure climbed fast, and at a little over 40-psi, the truck had backpressure of more than 80psi. The trend continued until the sensor peaked out at 85psi–because that was all that it could read!
True backpressure was over 100psi. So, the decision was made to incorporate two external wastegates (one per bank since it was a V8) and dial it into work with the factory VGT curve. After a bunch of tweaking the truck arrived at 67psi of boost with 84psi of backpressure, and 908 hp! It was also still dead reliable and driveable, although the quick-spooling turbos ate transmissions after a few dynos run with more than 1,800 lb-ft of torque. Again some changes were made and the wastegates and VGT were used to limit power down low to 1,600 lb-ft, and the truck lived a long and happy life.