12/14/2023 0 Comments Dodge variable valve timing![]() ![]() ![]() An aftermarket phaser limiter kit is shown installed in this cam phaser. Get any of these wrong, and the cam will be a mismatch for the application and deliver less than optimal performance. On a street application, a cam has to match the gearing of the drivetrain, the weight of the car, the stall speed of the transmission (if it is an automatic), the rpm capabilities of the engine (valve spring and pushrod stiffness), the flow characteristics of the cylinder heads, valves, intake manifold and carburetor or throttle body, and the engine’s compression ratio. The cam may not make the most peak power, but it will deliver the power where it does the most good – and that’s what it takes to win races. Based on this information, they can then choose a cam grind that delivers the most usable power within the rpm range where they can really use it. Some racers use a data recorder to plot their engine rpms as they run laps around a track. Racers know this and try to choose a cam grind that will deliver the most usable power within the rpm range where they need it most – which for a dirt track engine might be from 4,500 to 6,500 rpm, or 5,500 to 8,500 rpm for a drag engine. The breathing characteristics of any camshaft works best within a fairly narrow rpm range, and everything on either side of that range is less than optimal. Many racing cams actually produce less torque at low rpms than a stock cam and don’t start to make serious power until 4,500 rpm or higher! That’s why cam selection is such an important factor when building an engine.īecause of these factors, most cams (including performance cams) are still a compromise at best. By comparison, a performance cam in a circle track car, drag car or even a street/strip machine is usually ground to deliver peak power at a higher rpm range. A cam with minimal duration and overlap also creates lots of intake vacuum, which allows for good throttle response, but it also creates pumping losses as the pistons struggle to pull air past the nearly closed throttle plates.Ī good mid-range cam (2,500 to 4,500 rpm) gives up some low-speed torque and peak high-speed horsepower to obtain a fatter power curve in the middle rpm ranges. Street engines spend most of their time from idle to about 3,500 rpm with occasional bursts to 5,500 rpm, so street cams are typically ground with less duration and overlap to increase low end torque. What’s more, the profiles and locations of the lobes were ground to optimize power within a certain rpm range. Prior to VVT, camshaft timing, lift, duration and overlap were all fixed values and were determined by the location and shape of the lobes on the camshaft when the cam was ground. The valve timing overlap between the intake and exhaust lobes is clearly visible on this rebuilt cylinder head. At higher engine speeds, valve lift and duration are increased to add more power. This boosts fuel economy about 10% while also lowering emissions. The BMW Valvetronic system uses a stepper motor and secondary eccentric shaft to actuate a series of intermediate rocker arms.īy varying valve lift and duration, the engine can breathe freely like a diesel with minimal pumping losses at idle and low rpm. The main advantage of VVT is that all of the factors traditionally associated with a given cam grind are no longer fixed, but can change in response to changing engine speed and operating conditions.īMW has even figured out a way to eliminate the need for a throttle by using VVT and direct injection to control idle speed and acceleration. Think of VVT as the valvetrain equivalent of the powertrain control module (PCM) advancing/retarding spark timing and readjusting the air/fuel mixture as engine load and speed change to optimize performance, fuel economy and emissions. If that weren’t enough, VVT can also reduce pumping losses at idle for increased engine efficiency and fuel economy. It can even change an engine’s effective compression ratio by altering the opening and closing points of the intake valves during the intake and compression strokes. ![]() On overhead cam engines where there are separate cams for the intake and exhaust valves, changing the timing of one cam also changes the effective duration and overlap, and on engines where VVT also involves activating extra cam lobes or changing the rocker arm fulcrum point, VVT can also change total valve lift. VVT has been used on numerous Japanese (Honda, Nissan and Toyota) and European (Audi, BMW, Mercedes and VW) engines since the late 1980s and early 1990s, but only in the last decade or so on domestic engines (such as Ford 4.6L V8s, Chrysler 2.4L and 3.6L VVT engines, Chevy 2.4L Ecotec, etc.). VVT is computer-controlled and typically uses oil pressure to change the position of a phaser mechanism on the end of the camshaft to advance or retard cam timing. ![]()
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