The wastegate is a simple device. Its main function is to regulate boost pressure by venting excess exhaust gases into the atmosphere. It provides excellent boost control on turbocharged vehicles, and is a critical piece of the puzzle to achieving maximum power output regardless of engine combination or turbocharger size.
Over the last several years, turbocharger technology has improved performance standards rapidly — some would say by leaps and bounds — through the use of computer-aided aerodynamic research, specialized machining techniques, and more robust, free-spinning bearings in the rotating assembly. But with these turbochargers that are capable of far more power than in years past comes a need to control them, especially when the turbocharger is limited in size by class rules.
This control is especially important in classes like the NMRA’s Limited Street, Modified Street, and Renegade, along with the NMCA’s Xtreme Street category, where turbocharger size is limited and it’s critical to make as much power as possible right away, without overpowering the turbocharger.
“For the cars we build, we set it up so that there is a wastegate for each bank of the engine. Cylinders one through four have one wastegate and five through eight has the second wastegate,” says JPC Racing’s Eric Holliday.
“At lower rpm—launch—you can get a lot better handle on it [the exhaust pressure]. And if it’s a street car, it really helps to get rid of the boost spike when the turbocharger initially spools. The compressor will pick up a ton of rpm really quickly and will overshoot the target pressure. If you have a less-than-stellar motor, and it overshoots it 4, 5, 6 pounds, right when it’s making a lot of torque, it can break. That’s why we use two on most everything.”
John Kolivas of KBX Performance agrees, but with a couple of caveats tailored toward his experience tuning for Ultra Street and X275 competitors.
“Before we got into these Pro Mod turbos, with a small engine and large-frame turbo that’s hard to spool, you can get away with running one wastegate. But with the progression of the turbo technology, and how much easier they are to spool, it takes less exhaust gas to spool them, so we’re running bigger and bigger engines. To control these smaller 80, 85, 88mm Pro Mod turbos, you just about have to have one wastegate per side to bleed off and control them,” says Kolivas.
A single 66mm wastegate would flow enough to control the turbo in most applications, but its physical size makes it impossible to position it properly in this type of application to properly vent the excess exhaust gases out.
Using two smaller ’gates makes sense—since each ’gate only has one side of the engine to contend with, control is easier for the tuner to achieve.
“In Modified Street, we’re running tons of rpm, and turbos don’t necessarily like a lot of rpm in a limited turbo deal; you have to be able to let the exhaust flow out so it doesn’t get corked up in the turbo,” says Holliday.
“You’re trying to make as much boost as humanly possible early in the run. What makes backpressure go up is when the turbo runs out of steam. You can overwork a turbo earlier in your rpm range than you can later on. You can shut the gates at 6,500 rpm and it will still work OK. As you rev it out past that, your boost will drop and the backpressure will go up, because the turbo basically becomes like a banana in the tailpipe. There’s more exhaust coming out than the turbo can accept, so now the backpressure is going up. The backpressure is the pressure in the pipe from the turbo to the cylinder head.”
In a Modified Street car, they might run 50 to 60 psi of backpressure. The issue Holliday and the JPC team discovered while tuning the multi-valve Coyote engines is that they can’t go super aggressive on the exhaust valve spring. With all of the excess pressure in the exhaust, it will push on the exhaust valve and cause a loss of spring control at higher rpm. So they are trying to spin higher rpm to help the limited-turbo engine make power, but the turbo is running out of steam at those higher rpm levels. When you consider that a multi-valve engine like the Coyote will typically use exhaust valve spring pressures in the neighborhood of 110 pounds of pressure on the seat, that’s considered a lot.
“You’ve got a 100-pound spring trying to hold back 50 psi of backpressure blowing on it, so you can see where the problem will arise. If you go too high on the valve spring pressure you’ll overwork the secondary chains for sure,” Holiday explains. “So we’re dumping boost as the rpm goes up — not so much in first or second gear with a three-speed, but definitely in high gear where it’s sustained at an RPM for a long period of time. We start dumping boost around 5.5-seconds into the run on a car that runs 7.50.”
Contrast that with Joel Greathouse’s Ultra Street Car, which runs out of the KBX stables.
“With that little cast 76mm turbocharger, we have to run two wastegates on it. If we tried to run one wastegate on it, it would probably be like not even having one at all. With a 400-plus-cubic-inch engine, small exhaust wheel, and small exhaust housing, you just have to have two wastegates to control the turbo,” Kolivas says. “The real problem is controlling the boost on the starting line and how fast it comes in. If you wanted to leave on 8 pounds of boost at 4,200 rpm, that single wastegate would not bleed off enough pressure to hold it at 8 psi. You’d end up leaving at 12, or 14, or 16 psi depending on how long you sat there.”
Ultimately, what it comes down to is a different way of thinking, and a different way of tuning the engine and its attendant electronic and mechanical controls. One thing is for sure: the level of accuracy provided by a pair of wastegates is clearly a major factor in the performance capabilities of limited-turbo cars.