In this third installment of the camshaft shootout I'll start to look at the details of cam timing and how it can affect power. The writing of these articles has really re-ignited my interest in the theory of the internal combustion engine. I've even dusted off the old college engineering books. Unfortunately my math and thermodynamics skills aren't what they used to be and it was quite a challenge to refresh even some of the basics. I spent nights pouring over books and Society of Automotive Engineers' papers about internal combustions engines and realized just how complex the subject really is. It seems simple-just get the air in the engine and then get it out, how hard can it be? If you really take a look at how many things are going on in an engine and how it's all dynamic, it really poses a puzzle that I can only barely scratch the surface of. What you'll read in this article is just a basic assumption of what I've learned from my research and testing and by no means does it apply across the board to every engine. If there's one thing I've learned about camshafts it's that it's really a game of sacrifices and complete dependence on the engine combination. Encouraging news right? So, you may ask, what are you to do to with your naturally aspirated Nissan VQ35 engine, and what cams do you choose? Although you can follow some of the same basic principles that I discuss here, it really comes down to hard science and experimentation to learn what your combination needs.
Timing Events And How They Affect Performance
Timing events are particular to the engine setup as a whole, and one generalization does not work the same for every engine. Contrary to popular belief, retarding an intake cam won't always make power at higher rpm.
Intake Valve Opening
Early intake valve opening is beneficial to both good and poor flowing intake ports. It allows more time for the intake charge to start moving into the cylinder. The exhaust port flow and exhaust-closing event will really determine how early you can open the intake valve. An earlier intake valve opening will increase the overlap period when both the intake and exhaust valves are open. With a poor flowing exhaust port an earlier intake valve opening can cause the exhaust charge to pollute the intake charge. This needs to be taken into consideration in our turbocharged application since a turbo is an exhaust restriction that reduces exhaust flow. Typically, as we near the limits of a particular turbocharger, the exhaust backpressure rises. On my test car, I fitted a pressure probe into the collector of the header and recorded the pressure through the AEM EMS. I found that at 30 psi of boost, I was seeing about 45 psi of exhaust pressure at redline. This is a good time to really explain the overlap period. Overlap period is how much time both the intake valve and exhaust valve are open. While it seems to not make sense to have both the intake and exhaust valve open at the same time, there is logic behind it. As the piston nears the top, the exhaust gas is still moving past the rapidly closing exhaust valve. This exhaust gas has energy and can actually help to pull in fresh intake charge as the intake valve starts to open. If the timing is right the exhaust gas will leave a depression, or low-pressure area, behind it, literally sucking in the fresh intake charge. You can actually have intake charge flowing into the cylinder before the piston starts moving down the bore and drawing in the intake charge. This overlap tuning effect occurs near a certain rpm window and, in our example, can be impeded by the exhaust backpressure. I was running my test car at 20 psi of boost and I was only seeing 20 psi of exhaust backpressure, as opposed to the 45psi exhaust pressure at 30 psi of boost. Since this exhaust-to-intake pressure ratio changes at different boost levels, the overlap period requirements can change, making valve timing a little tricky on turbocharged cars.