It's time to wrap up our Castrol Syntec Top Shop Challenge VQ35DE engine build. The opportunity to work with Cosworth Engineering has given us a lot of insight as to what it takes to design and build a race engine, but with the competition just around the corner, it's time to wrap it up. Now that the bottom end is built and the Cosworth CNC heads are bolted down, all that's left is to look at the gas exchange process, or sucking and blowing as we call it. This should be the part that's most interesting to tuners since manifolds, cams, and how they provide the most efficient and useful gas flow through the combustion chamber is what we have some margin of control over. Since few of us are engine builders, making the most of the bottom end is all that we're left with. Unfortunately it's a black art of sorts. But instead of just telling you to buy this part and bolt it on because it makes power, we want to talk about what the manufacturer has to account for in order to make a part that makes power. Knowing more about the process will help you figure out the difference between duds and truly engineered pieces.
In our case, much like any aftermarket product, there are certain constraints when it comes to cam and manifold design. The displacement, compression, fuel octane, valve sizing and, most likely, peak RPM is already set. This actually makes life easier when designing parts instead of starting from a blank sheet. We will now explore the "black arts" in the detail-design process that are necessary in order to maximize peak horsepower and the area under the power curve while running on 100-octane fuel. Since we're keeping it naturally aspirated, enhancing the gas exchange process is crucial to our success.
The rules of the valvetrain game
One of the most important systems in the gas exchange process is the valvetrain system. As it is the primary passageway in and out of the combustion chamber, properly designing valve operating characteristics, or cam profile, is essential to a high-performance engine. Three important characteristics describe valve dynamics: valve lift, valve event duration, and valve event phasing. Theoretically, we would like to open the valve as far as possible instantaneously and at different points in the combustion process, depending upon engine speed and load. But that's a pipe dream since most modern engines are of an interference design where the valves can hit the piston near the top of its stroke. Physics also makes life hard because there are mechanical limitations to how fast we can open the valves. Instantaneously opening a valve requires infinite acceleration and thus infinite force!
So the first rule is that we must face reality and have smooth, gentle valve motion. Since the engine is a complex, dynamic system, the physics driving engine characteristics change based on engine speed and loading. Since we are more interested in power than fuel economy, the discussion from here on out will focus only on wide-open throttle or full-load operation.