I found out recently that Volvo was generous enough to include adjustable camshaft gears on both the intake and exhaust camshafts in my car. Realizing that I really don’t know a whole lot about how the valve timing, I did some research on the matter. I’m not going to go into things like lift, duration, and cam profile too much today, and wait until I understand how all the factors interact well enough to write about it.
To set things straight, you don’t really gain or lose power when you change the valve timing, unless you change it so far away from the “optimal” spot that the engine can’t breathe properly.
Most modern engines have two camshafts, one for the intake and one for the exhaust, and they can be adjusted independently. This setup makes things pretty straightforward:
Adjust the intake to put the power band where you want it. Advancing the timing will “move” the torque lower in the RPM band, to make less peak horsepower, but greater power when in the low end to midrange. Retarding the timing will do the opposite, and will generate more peak horsepower, but at the expense of power in the low end.
Changing the exhaust camshaft will do the same thing to a lessor extent, but changing the timing between the exhaust and intake changes the overlap. For this part, I’m going to assume that you have a turbocharged car, as it significantly affects how much overlap is desirable.
Less overlap (advance the exhaust more than the intake) improve how well the cylinder fills with air in the upper RPM range and/or at high boost. It can also smooth idle some, depending on how it was configured before. One serious downside is that it raises the engine RPM needed to generate boost.
More overlap increases the amount of exhaust gas recirculation which reduces power a bit, but also lowers the temperatures, allowing for more aggressive ignition timing. Too much overlap creates an over-scavenging condition where a portion of the intake gasses flow right through the cylinder and out the exhaust to create a lean condition which would increase cylinder temperature. Alternately, under high load, the exhaust back pressure is much higher than the boost pressure due to the inefficiencies of the turbocharger, and the exhaust flows back into the cylinder.
All in all, overlap is a bad thing for an engine with forced induction, but if you’re messing around with it, experiment to see what gives you the best compromise between low end and high end power, fuel economy and smoothness. If there was a dyno I could use to test out a bunch of things, I would show how the shape of the power curve changes with different camshaft timing setups, but unfortunately, like many other things, that will have to wait a while.
I’ll be back with a part two of this article sometime in the future, so stay tuned!
November 14th, 2008 |
