How to get the most power
Why do some modifications increase bottom end power and some increase top end
power? What can I do to get the best of both worlds?
Unfortunately there are trade-offs for everything in life. You usually pay for peak power out of your bottom end wallet. When tuning for top end power, you lose torque and your power band becomes narrower at the bottom end. A wider, higher torque power band at lower rpm will nail the quick 0-60' times but a top end, high-horsepower power band will give you the ¼ mile advantage. Horsepower is the rate that torque can be delivered at. Your best setup delivers the highest horsepower over the power band for the transmission's gear range for the longest amount of time.
So... how do we do that? What goes into determining where we get the most power? Get your brain thinking for this quick intro on power. Your peak power band lies wherever the engine is most efficient. There are 3 efficiencies we are interested in maximizing for a torque/hp monster: volumetric efficiency, mechanical efficiency, and thermal efficiency. Volumetric efficiency refers to how well your engine breathes. An engine has 100% volumetric efficiency if the cylinders are 100% filled with air/fuel at atmospheric pressure. You can optimize volumetric efficiency through headers, cams, exhaust systems, and the intake manifold. Extrude hone the intake manifold and port the cylinder head to increase airflow. A longer runner improves airflow at lower rpm moving the torque peak to the left or improving bottom end power at the expense of top end power; a shorter runner does the opposite improving top end power. The exhaust system affects the torque curve in the same way. Smaller diameter, longer primary tubes help increase bottom end power at the expense of top end. Bigger diameter, shorter primaries improve the top end at the expense of bottom end. Forced induction motors increase the pressure entering the cylinder and the engine can have more than 100% volumetric efficiency.
However if you're not running a turbo, supercharger, or NOS, a high velocity, low backpressure exhaust is the key. If the exhaust pipe diameter is too large, the vacuum created behind the closed exhaust valve will not be as great. When the exhaust valve opens at the beginning of the next exhaust cycle, the air won't flow at such a high velocity. Headers are also crucial to increasing volumetric efficiency. They are designed to exploit the energy created by the charging exhaust gasses during the overlap period at the end of the exhaust stoke and beginning of the intake stroke, when both the intake and exhaust valves are open at the same time. These high-flowing exhaust gasses create a suction that is manipulated by the pipe's length and diameter.
The best setup for a streetable race car is a header with long runners and smaller diameters. A Tri Y (or 4-2-1) header pairs opposite cylinders in the firing order into two "Y" shapes, and then joins both "Y"s into one collector. The pulses created by the pipes work off each other to widen the power band but they don't offer as optimal peak power as a 4-1 header does. On a 4-1 design, the main pulse sent down the primary is stronger, which maximizes peak power but for a shorter band of rpm. These headers are used mostly on all out race cars where peak power is more important. Cams specs also manipulate where your power band lies. You can shift the valve opening overlap period to earlier in the rpm by advancing both the intake and exhaust cams. This will produce bottom end power at the expense of top end power. By advancing the intake cam only, you are increasing the overlap period and starting it earlier giving you more bottom end power while decreasing top end power; retarding the intake cam only will result in the opposite power gain. Advancing the exhaust cam shortens the power stroke by reducing overlap and opening the exhaust valve sooner helping top end power at the expense of bottom end power; the opposite for retarding the exhaust cam. Retarding both cams moves the overlap period later increasing top end power at the cost of bottom end power. Cams duration also greatly affects the engine's power band. Short duration cams tend to idle smoother and have more bottom end power and a broader power band while long duration cams have a rough idle due to increased overlap and more top end power over a more narrow range.
Thermal efficiency is how well your engine can convert the fuel's energy into rotating energy. It is mostly affected by the compression ratio, timing, and combustion chamber design. The more air/fuel mixture filling the cylinders, the more energy is available to produce power. There isn't much you can do to improve the combustion chamber but you can increase the compression ratio by increasing the quench area with welding, milling the cylinder head to reduce compression chamber volume, or using higher domed pistons. A higher compression ratio will increase thermal efficiency. Timing can be adjusted manually or programmed into the engine's ECU.
Mechanical efficiency is how well your engine limits the amount of energy exhausted to power the engine itself from friction between parts, and to run accessories such as the water pump and alternator. These losses happen in all moving engine parts, such as rocker friction, bearing friction, and piston skirt area. Mechanical efficiency decreases as rpm increase because it takes more power the turn the engine at higher rpm. Some ways to improve mechanical efficiency is to carefully control piston to wall clearances, bearing clearance and using special coatings. Using longer connecting rods can improve both mechanical and volumetric efficiency. Underdrive pulleys also increase mechanical efficiency because they drain less power operating the driving accessories, passing more power to the wheels.
By increasing these efficiencies, you are increasing the amount of fuel/air in the cylinder, how much is converted to useable power, and how much of that power is used to push your car forward.
Write up supplied by Kev Shek
Many thanks to Kev Shek for supplying this write up for Club Nissan