The scene is the local cruise hangout. The hood is open on a young car crafter’s black-primer Pro Street Mustang, and a small group eavesdrops on a deluge of overlapping technical discussions regarding turbos, boost, camshafts, and a dizzying array of other power-related topics. The Mustang driver innocently asks for feedback on his cam selection and is quickly barraged with several contradictory recommendations, each of which are vehemently defended as gospel. The young driver is soon overwhelmed and quickly closes the hood of his car and leaves while a pair of turbo true believers square off in a technical fencing match that looks as if it will devolve into a jihad-like religious riot complete with willing martyrs.
While this scenario might be fictional, the debate is real enough and rages across the Internet in forums and tech chat rooms dedicated to anything related to boost. The problem with opinions is that everybody has one, with few rooted in real-world experience or established combustion theory. So we decided to seek out those who talk less and race more. Our participants include Kenny Duttweiler, who began experimenting with and racing turbo Buick V6 engines virtually from the moment those black sedans hit the showroom. He now builds the maxi-turbo’d, mini displacement, 299ci small-block Chevy powering the George Poteet-driven Speed Liner Bonneville streamliner that blitzed the Salt with a 436-mph exit speed last year. We also talked to street and dragstrip turbo expert Kurt Urban, who lets his 100,000-mile turbocharged street truck speak for his knowledge of how to use boost to become an Urban guerrilla. Then we quizzed our favorite rocket scientist, Comp Cams’ lobe designer Billy Godbold, who lent his insight into eloquently combining cam timing with boost pressure and not getting squeezed in the process.
Turbo Cam Basics
The one thing that all three of our noted sources emphasized is that the knowledge base established from turbochargers designed 10 or 15 years ago is antiquated when applied to the current crop of high-efficiency turbochargers—unless you’re trying to get by on old, cheap turbos. “In the old days it was typical to see 1.5 to 2:1 backpressure ratios,” Duttweiler says. “Today the backpressure is actually less than the boost pressure.” The ratio Duttweiler is talking about is the relationship of exhaust backpressure to inlet boost pressure. Exhaust gas backpressure is naturally created when the hot gas exiting the exhaust ports arrives at the turbocharger’s turbine wheel. The exhaust gas “stacks up” between the exhaust port and the turbine wheel, creating pressure as it would with any restriction. All internal combustion engines perform best when tuned with a certain amount of camshaft overlap in which both the intake and exhaust valves are open at the same time. If the exhaust backpressure is greater than the inlet pressure, the exhaust will push back into the cylinder and (given enough time) up into the inlet manifold. Exhaust gas doesn’t burn a second time, so it works just like an emissions-era exhaust gas recirculation (EGR) system, reducing power, except it’s happening at wide-open throttle (WOT). Because of the high backpressure ratio, older turbos required an earlier-closing exhaust valve, which was most easily achieved with a wider lobe-separation angle (LSA). This could be where the now-common wide-lobe-separation-angle theory propagated. According to Duttweiler, today’s more efficient, larger turbos reduce that backpressure, which minimizes the power-robbing effect of exhaust dilution. That means the LSA can be tightened, which is contrary to the contention that all turbo cams must have wider 112- to 114-degree LSAs. With newer turbos, the reduced backpressure also means the exhaust valve can be opened sooner and held open longer, which is generally accepted as beneficial to high-rpm power production, just like on a normally aspirated engine. According to Duttweiler, to make good power, turbo engine efficiency depends more on low exhaust backpressure than tricks with the cam.
Duttweiler also mentioned that attempting to build a turbocharged engine with a set LSA (such as 112 or 114 degrees) can lead you astray. He mentioned some work he did way back in the early Buick Turbo V6 days while racing these engines in NHRA Stock Eliminator. Stock class rules required the intake and exhaust lift and duration specs to remain stock, so to improve power, he tightened the LSA on these engines to 109 degrees to help the Buick’s really small cam improve power. The engine responded by building boost much quicker. “When you spread the lobe-separation angle way out, the engine gets lazy,” Duttweiler says. As an example of a good V6 engine, Duttweiler says he built a V6 turbo Buick with a 215-degree-at-0.050 intake lobe camshaft that made 900 lb-ft of torque and 580 hp and idled at 16 inches of manifold vacuum. The concept of lobe-separation angle and duration is addressed more fully in the accompanying sidebar “Overlap Chronicles.”
Duttweiler says this idea reinforces the concept that it’s not necessary to use radical lobe designs with turbo engines. “A turbo allows you to run a milder lobe and valvespring package that’s much easier on the engine.” An example of this is a slew of very reliable turbocharged Bonneville engines built by several turbo engine builders, notably Duttweiler and turbo engine guru Mike Lefevers. Often these engines will make multiple 5-mile-long sustained WOT passes and require little more than pulling a spark plug for maintenance. A properly designed valvetrain and a less aggressive lobe design can virtually eliminate valvetrain problems such as broken springs, bent pushrods, and mangled rockers. There are caveats however. Duttweiler and Urban both emphasize that attempting to open the exhaust valve too early can cause bent pushrods. This is mainly due to the surface area of the exhaust valve attempting to open against high cylinder pressures. “You will need bigger pushrods,” Duttweiler warns. “I see more bad things happen on the exhaust side trying to open against cylinder pressure.”