There are no free lunches when it comes to getting performance. Making power is like making money. Were it easy, everyone would be doing it. It takes a lot of effort to gain modest amounts of power because it’s all about heat energy and how we turn it into power at the crankshaft. The challenge with heat energy is harnessing as much of it as possible to turn it into power at the rear wheels.
Did you know just 25 percent of the heat energy generated in the combustion chamber is used to actually make power? That means 75 percent of the heat energy created by the light-off above the piston is lost to the atmosphere. A full 50 percent of it is lost out the tailpipe. Another 25 percent is lost to the cooling system.
Making power is straight physics. We’re taking intense fury of fire—thermal expansion—and acting on the pistons, connecting rods, and crankshaft to turn linear action into a rotary motion at the flywheel or flexplate. When you observe how engine power has evolved over the past century, it is remarkable how far we’ve come even in the past 50 years. We have a better understanding about how power is made than we did a half-century ago. Computer analysis along with pressure from Washington and the buying public has gotten us more powerful, fuel efficient, cleaner-burning engines.
There was a quantum leap in the ’80s when Detroit began offering roller cams with more aggressive profiles, better cylinder heads and induction, and finally electronic fuel injection. Performance only got better in the years to follow thanks to the extensive effort of Motown’s best engineers and product planners. Variable valve timing and intake runners got us more power and a broader torque curve. These days, Detroit has moved toward direct injection in its quest for even better fuel economy, reduced emissions, and vast amounts of power.
And by the way—there’s more to channeling power than just the engine. Electronic engine control has evolved into powertrain management where automatic transmissions have become an integral part of engine management where the two work together for better overall performance. We see this most from having a greater number of speeds in automatic transmissions, which keeps engine rpm more constant as we navigate the cogs.
Internal combustion has always been about taking heat energy and turning it into mechanical motion. Although everything is computer-controlled today, internal combustion engines are still suck, squeeze, bang, blow, and make motion. We haul air and atomized fuel into the combustion chamber, close the intake valve, compress the mixture, ignite it, use the heat energy released in the light-off, and exhaust the spent mixture. The trick is getting as much power from the light-off as possible.
Fuel doesn’t “explode” in the combustion chambers. Fuel and air mix in a reaction known as atomization, ignition, and something of a quick-fire above the piston. The quick-fire or light-off generates tremendous amounts of heat and thermal expansion powerful enough to thrust the piston downward in the bore to exert force on the crank journal, putting that energy to work for us. This is done in a timed sequence across multiple cylinders to get us rolling.
It has always been the physics of air and fuel and how much of it we can place above the piston that determines how much power we’re going to make. We do that via bore size and stroke. A bigger bore harbors more air and fuel. And if we can drag the piston deeper into the bore, we get more air and fuel there too. However, there’s more to stroke than greater amounts of air and fuel. With stroke comes mechanical advantage—leverage—that gives us more twist when it’s time to get it on.
Although we spend a lot of time focused on horsepower, usable power is mostly about torque. Madison Avenue likes to use the word “horsepower” in automotive advertising. However, torque is the hero—that down-low grunt that gets us started to begin with. Horsepower gets the spotlight once we are rolling when the hardest work has already been accomplished by torque. In fact, in our opinion horsepower gets way too much credit for torque’s work.
Before you begin planning an engine build you must first know what you want the engine to do. Naturally aspirated or forced induction? How much horsepower and torque do you want and when? Are you building an engine for drag racing or road racing? Perhaps you’re building a weekend warrior or daily commuter. Each category calls for a different type of engine build. Road racing engines need a broad torque curve meaning low-end torque for turns and high-end horsepower for the straights. Drag racing engines are all about high-rpm horsepower. When you’re going cruising you want a broad torque curve that gives you power under most driving conditions.
And finally, plan your build and stick with it. It gets expensive when you change direction in the course of an engine build. Work it all out ahead of time and resist the temptation to change your plan. If you get stuck on direction consult with a reputable engine builder or experienced hot rod shop. Know what you want beforehand.
Engine theory is all about math. If you’re the machinist, it is straight math and making allowances for expansion as an engine heats up. Perhaps you’re the engine builder, which calls for many of the same thought processes as the machinist with an understanding of what parts do hot and in motion. If you’re planning the engine build you need to think about elements such as forged versus hypereutectic pistons, compression ratio, cam profile, induction, cylinder heads, displacement, and exhaust.
Let’s use a typical Chevrolet 350ci as an example. Bore x Bore x Stroke x 0.7854. Then, 4 x 4 x 3.48 x 0.7854 = 43.73 ci per cylinder. Multiply 43.73 ci by 8 and you get 349.85 ci, which is true displacement.
To calculate compression ratio you need the following elements: Displacement (D), Piston Volume (PV), Deck Clearance Volume (DC), Head Gasket Volume (G), and Combustion Chamber Volume (CC). Compression ratio equals (D + PV + DC + G + CC) / (PV + DC + G + CC). Translated based on our 350ci Chevy formula is (43.73 + 0.305 + 0.1885 + 4.272) / (0.305 + 0.1885 + 4.272) making compression ratio 10.18:1.
As with any engine formula there are variables. Piston variables are domes and dishes, including valve reliefs. This also includes the area above the top ring groove. Piston manufacturers can tell you this volume, which also takes away from compression.
Deck clearance volume is calculated by the distance between the top of the piston at top dead center and block deck. It is always possible the piston has a zero deck height or even extends above the block deck, in which case it should be given a negative value. You can measure deck clearance using a bridge and dial indicator with the piston at top dead center. Because a room temperature piston will tend to wobble on the pin, you need to allow for this movement in your calculations. If the piston is above the block deck, you will need the appropriate head gasket thickness to where there’s no cylinder head contact. Compressed head gasket thickness is typically between 0.005 and 0.015 inch.
Combustion chamber volume is calculated by simple volume in cubic centimeters. Chamber size has a very direct effect on compression ratio. Cylinder head manufacturers can tell you chamber size. However, it’s always a good idea to measure chamber size yourself due to manufacturing irregularities and any machine work you may have performed.
Dyno rooms have always been the best place to prove out engine theory. You can make changes in an engine to prove out each of these changes, be it ignition timing, fuel mixture, valve timing and cam profile, compression, induction, and cylinder heads. The next phase of testing is the real world out there on the open road. A dyno room is very different than the open road because a dyno room is a controlled environment.
Compression ratio is the quickest path to power. So is chamber size and shape. You want good quench from a combustion chamber. Quench is that area between the piston and the flat portion of the head around the combustion chamber pocket. Good quench generates turbulence in the combustion chamber and, in theory, pushes the air/fuel mixture toward the spark plug for more complete combustion reducing emissions and making the most of the air/fuel mixture.
Combustion chamber physics has changed a lot over the years. This is a vintage small-block Ford 64cc shovel-style chamber from the ’70s. This is not the cylinder head you want for the Ford because it does not offer the compression or quench desired.
Here’s the Chevrolet Performance LT4 cylinder head released in the ’90s. When you study these high-swirl chambers it is clear how far technology has come. The LT4 head offers superior quench around its tight chambers. This head has received extensive port and bowl work, plus additional work around the valves to improve airflow.
Early small-block Ford heads (289/302 ci) sported smaller 53-57cc chambers, which makes them a good choice in terms of compression and quench. If you’re doing a stroker you could wind up with too much compression with these smaller chambers. Where they fall short is port size in and out. Vintage Ford heads have always struggled with poor flow because ports are so small.
Open chambers like this one lack the quench needed for real power. They tend to detonate (ping or spark knock) under even light acceleration. This is not what you want.
Today’s aftermarket cylinder heads yield high-swirl and better quench. Quench should be as close as possible without the piston touching the head surface. You can run as little as 0.038 to 0.043 inch with steel connecting rods on the street. With street engines you can get it as tight as 0.032 inch without consequence.
Valve and valvestem shape affect airflow through intake and exhaust ports. A multi-angle valvejob smooths airflow across the seat and valve face during the brief moment valves are off their seats.
Gregg Jacobson of PHD Speedcenter in Bakersfield, CA, stresses the use of larger valves works quite well if there is no valve shrouding to where you lose airflow. Valve shrouding can rob you of airflow.
When calculating compression ratio, keep in mind head gasket thickness takes away from compression because you’ve added volume to the chamber.
Swept volume of a cylinder bore is the distance between the piston dome at bottom dead center (BDC) and top dead center (TDC). This is the distance the piston “sweeps” from bottom to top, hence the term “swept” volume.
Compression height is the distance from the centerline of the piston pin to the crown. You will need to know this number when it’s time to buy pistons so the piston will settle in the right place in relation to the block deck. To properly calculate compression height, you must know the block’s deck height, connecting rod length, and stroke.
Compression isn’t calculated by piston alone. Displacement, combustion chamber size, swept volume, deck, and compression height all calculate into compression ratio. Were this a flat-top piston you’d have greater compression than you do with this sizable dish known as negative dome.
Here’s another example of negative dish in a big-block Ford piston. The dish and valve reliefs take away compression yet the 0.040-inch oversize adds compression when we keep the same chamber size. Increasing stroke also increases compression. Pistons alone are only a part of calculating compression.
It is important to check true TDC as a part of your engine-building regiment. Most builders check true TDC on one bore only. It is suggested you check true TDC at all four corner cylinder bores. You can even check all eight bores and come up with an average. True TDC is when the crank journal is at 12 o’clock and the piston at peak dwell.
Rod length as it relates to stroke affects the geometry of piston-to-stroke, which is known as rod ratio. Rod ratio also affects piston and cylinder wall wear. A short rod, or lower rod ratio, also increases cylinder wall wear, driving engine temperature skyward. A longer rod or higher rod ratio sports greater advantages because it decreases piston side loading, thereby reducing friction. There’s also greater mechanical advantage thanks to longer piston dwell time at each end of the bore.
How much air and fuel we introduce to the chambers has a very direct effect on power. Cylinder head porting, depending on how it is done, offers a great benefit to power. Here, exhaust ports are being hogged out to improve scavenging.
Proof of the effectiveness of cylinder head porting is examined on a flow bench by checking airflow at various levels of valve lift. Making ports larger doesn’t always ensure success. The main purpose of porting is to reduce turbulence and restriction.
Intake ports have been opened up via an extensive port job. You want a good port match between the intake manifold and cylinder head, plus a smooth transition through the port to the valve. You want a certain roughness in the port to keep fuel droplets in suspension. Not every head porter is going to agree.
Exhaust port design takes on a different dynamic. You want reduced restriction for better scavenging. Yet, at the same time, you also want velocity, which helps scavenging. Exhaust ports are only the beginning. Header tube size and length determine the rest. You don’t want pipe diameter that’s too large because you lose velocity and back pressure, which enhance power. Too small and you lose power via restriction.
Easily one of the greatest leaps in engine technology has been camshaft design. This is a flat-tappet camshaft, which can offer adequate performance via duration, lift, and lobe centers. However, it’s never going to deliver like a roller-tappet cam.
Roller cams and rocker arms not only reduce internal friction by a huge margin; you can also do more with a roller cam lobe profile than you could a flat-tappet. Roller tappets and lobes allow you to go with a more aggressive profile, which yields a greater level of performance.
Intake manifold design and function baffles a lot of enthusiasts, but it’s actually quite simple. A dual-plane intake manifold yields longer intake runners, which deliver better low to midrange torque for street use. Low to midrange torque is what you want on the street.
A closer look at a dual-plane intake manifold shows the plenum, which is where velocity begins and heads into the long intake runners. Velocity at lower engine speeds is where we get torque. The waffle surface in the plenum keeps fuel droplets in suspension.
A single-plane intake manifold like this one for a big-block Ford is strictly a high-rpm horsepower piece and not engineered for low-rpm operation due to its huge plenum and shorter intake runners. This manifold sports a Dominator flange.
Carburetor size is another bench racing debate that doesn’t call for a complex answer. Larger carburetors deliver horsepower. Smaller ones generally deliver torque. We have learned via dyno testing that every combination of engine and parts is different.
Carburetor jetting directly affects air/fuel ratio when you’re on the carburetor’s main metering circuit. Optimum air/fuel ratio is 14.7:1, meaning 14.7 parts air to one part fuel. Of course you’re not always going to get 14.7:1. When it comes to lean versus rich, it is always best to err on the side of rich. A lean mixture not only robs you of power, it can also do extensive engine damage.
Carburetor spacers generally improve velocity by adding to intake runner/plenum length. You can ascertain success by trying a variety of different spacer sizes and testing the result on the dyno or on the track. Be sure your spacer doesn’t create hood clearance issues before slamming the hood.
Header tube sizing and length affects performance as much as the induction side does by having a direct effect on exhaust scavenging. Long-tube headers deliver better performance than shorties, especially at high rpm. Shorties are popular because they consume less space and are easier to install. Smaller header primary tubes offer greater velocity and scavenging, depending upon displacement and expected horsepower.
Aside from a good healthy ignition system, we stress proper installation, which calls for all ignition wires to be routed neatly and far enough apart to prevent crossfire. If you bundle ignition wires together you will get crossfire.
Although electronic engine control makes engines seem more complex they remain “suck-squeeze-bang-blow” with the same basic principles of internal combustion. Fuel and spark curves work the same basic way only with more precision function and virtually no misfire. Two fuel rails harbor eight injectors on this 5.0L Ford lower intake manifold. The injectors are fired in timed sequence with intake valve timing events.
The Ford Modular V-8 eliminates the humble distributor instead going to coil-on-plug or coil-pack with ignition wires. Fuel rails and injectors are being installed here as a complete unit, including fuel pressure regulator. Fuel enters the rails under pressure to the injectors. The fuel pressure regulator controls fuel pressure by regulating return flow.
Aftermarket bolt-on EFI systems make it easy to plug-and-play and get turnkey performance. Summit Racing Equipment’s MAX-efi500 (PN SUM-240500) throttle body fuel injection system is good for up to 500 hp. It is debatable whether EFI or carburetors yield more power. However, EFI holds its tune.