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Turbo Camshaft Guide - The Truth About Camshafts And Turbochargers

Tech Editor Smith debunks some long-held turbo cam myths

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Cylinder head selection also comes into play when selecting a camshaft. Just like with a normally aspirated engine, Duttweiler says a well-flowing cylinder head will allow shorter duration figures to carry the power to higher engine speeds. Less efficient heads do the exact opposite, requiring more duration to compensate for the weak flow. This is reinforced by what we’ve noticed here at Car Craft on our dyno testing of normally aspirated engines. With a given cam timing, adding better heads extends the peak power rpm point, while weaker heads will do just the opposite. But typically there are limitations when dealing with a turbo. Duttweiler warns that a properly designed turbo “will run the engine well past its ability to control the valvetrain and will go right into valve float!” That means valvesprings are just as critical as with normally aspirated engines. This tends to emphasize mechanical roller cams rather than hydraulic rollers, although hydraulic rollers seem to have an edge on durability for street engines.

If all this sounds just like what normally aspirated engines want (and it is), then go to the head of the class. Duttweiler says you can and probably should treat a properly sized, high-efficiency turbo motor like a normally aspirated engine. An early-opening exhaust valve can be beneficial for top-end power because even high-efficiency turbos still have to work against some exhaust backpressure. The earlier-opening exhaust helps to reduce residual pressure in the cylinder before the intake valve opens.

Advanced Classes
There’s much more to cam timing selection, however, than simply tightening the LSA when using a more efficient turbocharger. The difficulty in choosing camshaft specs is that much of the advice that comes from professional engine builders is usually aimed at ultimate power applications such as Bonne­ville or 6-second, 2,400hp drag cars. Expanding on our comparison to normally aspirated engines, you wouldn’t choose the same camshaft for a 500ci NHRA Pro Stocker as you would for a 350ci, 400hp street engine. Given this, there are many factors that go into choosing a camshaft for a street-driven turbocharged engine. Kurt Urban has tons of experience in such things, and he’s used a variety of different cam designs based on how the engine will be used and the car it will power. When it comes to cams Urban says, “For me, it’s what works in the car.”

Urban went on to say, “I try to design a cam around what I think the driver needs. Everybody wants big power, but what you really need is a cam that races well.” As an example, Urban says, “With a Powerglide, a big camshaft and a big turbo don’t work well together. The car would be lazy off the line, and the power would only come on hard at the top end.” So in that case, he says a shorter-duration camshaft would probably work better to launch the car because so much of elapsed time is based on starting line acceleration. “In drag racing you want the car to leave, so I take into account what the car weighs, the displacement, how good the driver is, and probably a dozen other details to fit the cam. Sometimes I’ll cam the motor smaller to make the car easier to drive and add duration later as the driver improves.”

As an example, Urban says, “I built an engine for a drag radial car with a 427 LS. The camshaft had the normal split at 260/272 degrees (at 0.050) with a 115 separation angle. The motor used 1,400 pounds of fuel at max power, and the car runs 7.20s at more than 200 mph. At 1,400 pounds per hour—that’s well over 2,000 hp. I tried a 272/280 cam to make more peak power, but the car ran slower because it wouldn’t leave even though it made 5 more pounds of boost at the top.” As both Duttweiler and Urban have emphasized, this is much like a normally aspirated engine in which too much duration killed the bottom-end power and the car ran slower.

To emphasize that taking into consideration how the car will be used is critical to camshaft selection, Urban built an engine for a heavy street car that appears to fly in the face of conventional turbo cam wisdom. “I built an LS engine with a 227/223 cam with 0.614/0.610 lift with a 72mm turbo—it makes 900 lb-ft of torque at 3,500 rpm in my all-wheel-drive Chevy truck. It runs 11.40s at 120 mph and I’ve got over 100,000 miles on it!” Here the application demands lots of torque because the engine builder is faced with a very heavy vehicle with a tight torque converter and not much gear ratio to help it get moving. The combination of a short-duration intake lobe with an even shorter-duration exhaust lobe than the intake (sometimes called a reverse split cam) means the cam timing emphasizes low-speed torque as evidenced by the incredible torque at a relatively low engine speed.

Urban went on to say “Turbo engines run faster (at the track) when you open the exhaust valve sooner. With short intake duration, the engine responds with speed at the track. By opening the exhaust valve sooner with either a tighter lobe-separation angle or long exhaust duration, the engine generally responds better. You will lose scavenge if you open the exhaust valve too late.”

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