What Makes An Engine Combination
Some careful research will help you find the best combination for your needs but its not always as straight forward as we would like it to be.
There are some important things to consider that make up the largest part of the combination. Its important to make sure all of these things are in since. For drag racing its easier to find your perfect combination. Your max power RPM can be reached and held thru your shifting rpm and will only drop the amount of gear ratio allows. Since you are building an engine for drag racing the rpm range the combination needs to work at is simple to figure and pick gearing for. Now when we get to road racing or street driving the combination is not nearly as simple.
What parts make up the engine combo
In no order of importance since any one part way off the combo’s needs can result in poor performance. A small intake on a largely built motor can stifle hp output. Same is true for cam and heads. Each has its own way it effects the combo and its performance.
- Heads (ports and chambers)
- Pistons (compression ratio and durability)
- Fuel Pump
The camshaft is the brain of the engine it makes all the decisions and tell the engine how it should preform. Best parts in the world can be let down by a bad camshaft choice. It determines the rpm the motor will try to reach and the rpm range the motor will produce power in. If the rest of the combo is not up to what the cam is asking it to do the engine will not perform well. It make still make some power but the real output of the engine is lost.
The Cylinder heads have several major functions in the sbc engine and of course most engines. They allow the motor to take in air/fuel mix and burn it rapidly and expelling gases out the exhaust pipes. The heads allow flow and are the way the heads can make more power for the engine. They pass the air fuel mix to the cylinders and allow power to be made. It is important to note the cfm flow of the heads and the port volume should match the power output of the engine as closely as possible. Too large of a port will slow down the air fuel mix and not let it be as snappy in the lower rpm range while smaller heads may improve the low rpm range they can really limit the maximum hp the engine can produce.
The intake manifold is used to distribute the fuel air mix to the cylinders. There are two major types used in about 90 percent of the sbc builds that is carburetor manifold where the carb sits onto of the manifold and the manifold reaches to all the cylinders allowing the engine to breath. Then there is EFI manifold used for fuel injection. In performance purposes the engine will use multi point fuel injection where the manifold has an injector point right into the ports of the engine one for each port and this allows for accurate fuel air mix to be passed right into the cylinders. Both can make lots of power and both come in many shapes and sizes. From single carbs to multi carb units with varying heights and volumes all of which effects performance.
Headers serve the same function as the manifold but in reverse. these are often used for tuning and can help a lot in making good power across a wider or narrow rpm band.
Several types of pistons are available for the sbc. They also come in several materials that help to make the engine run and last as intended to. Cast pitons are often used for stock applications with hypereutectic being used for mild performance builds and forged piston being used for boosted and higher performance engines. Most racing engines use forged pistons and would not be able to produce the power they do without the strength of the forged pistons. Forged is the go to performance piston.
Pretty much all rods are forged with the exception of PMD rods from factory small blocks in later years chevy went to PMD rods made from powdered metal that is heated into a solid shape of a rod. these are very close to each other and do not require as much balance work in mild performance engines. For just about any real performance engine aftermarket performance rods are going to be a must.
Crankshaft is used to put it all together and is another important choice in performance engines. they can also control the size of the engines. Some stroker cranks can be used to make more cubes under the same bore size. Stronger cranks in many forms can be had from cast to forged materials that respond well to performance. Most mild engines will be fine with the stock crank since its a well made piece in the sbc engine. But can be made stronger and lighter to support more hp and spin faster.
Used to mix the Air and fuel together and pass it into the engine. Without a proper carb or efi the engine will not make good power.
Often over looked part of the build but its important the fuel pump can provide fuel to the engine at proper pressure levels all the way thru the rpm range.
Some builds will require better electronics and is often a soar spot in many engine combos. Some will need hotter spark or timing adjustment. That simply cant come form mechanical means. This will be very important for EFI engines and Boosted engines. If using large amounts of boost are needed to make the hp you need to have electronics that can turn back th timing and control turbos and other forced induction systems. Some of this can be done with conventional equipment but as the hp is raised the engines needs will change quickly and need a system that can keep up with these changes.
RPM limits HP limits
RPM and hp can both have very hard limits. Its important to base your combo around the rpm limits of the engines internal parts such as the crank and rods. You cant ask the engine to make hp beyond the limits of the rotating assembly or your engine will be short lived. This gives you some goals and limits to your build. You can build a much stronger engine than you need and this is not a bad thing.
Hp limits are also based on the rotating assembly and strength of the block. Once the hp limit is reached its going to start to break parts. Some race engines will build more hp than the engine can safely handle all the time but can handle it for short periods.
The big moving parts
Let look at the major factors of the engine combination. First thing i like to consider is rpm range, size of engine available and the horsepower that will be needed to get the job done. Now its always possible to go up in engine size. But in most cases the engine rpm range needs to be increased to make the power requirements. Now we get to the HP issue its never as much as we thought. If it is the next guy still has more. This is why an engine that sings the write song is so important. A screaming drag racing engine will most likely be outran by a Honda on the street for many reason none of them are hp. The greatest bad ass road racing engine will look a fool at the drag strip next to purpose built drag race engine.
Camshaft Picked last but thought of first
So the camshaft must support the rpm range needed to make power in the rpm range of the heads and the heads need to support the hp requirements needed. The camshaft also needs to have the lift needed to reach the cfm requirements needed to make the hp mark. Then the compression must also support the needs of the camshaft to make the proper power levels at all rpms ranges and still hit the max rpm range.
The exhaust also needs to scavenge the cylinder heads of spent exhaust gases and the carb must be able to provide air and fuel at a level that lets the engine breath easy. Then the weight of the car and its gearing must also be correct to support the engines rpm range. If its got tall gears and 8K rpm engine you will be going 90mph in first gear and not move the car well.
With a low rpm engine long stroke and off idle power you can run a taller gearing and still move the car well. An engine that falls on its face at 4500 rpm the and has 4.96:1 gearing it will only be going 15 mph at the top of first gear. Then it will again not be fast. That same gearing with a 9K rpm engine will jump to 40 mph in first gear.
Lets go over the basic needs of a good combo.
Engine Compression Ratio:
Needed to make sure the cam works as it should. The Dynamic Compression Ratio needs to be correct for the fuel. It is also good for a few extra ponies but fall off as you reach 14:1 compression. It also offers better combustion when running fuel that can support the ratio.
Material can effect power output and fuel octane requirements Aluminum heads can take more CR and cast iron can make more power with better heat transfer. Iron heads are prone to heat soaked in hot weather and hard running. Combustion chamber design and shape can also effect efficiency and performance.
Port Cross Section:
Is the width and height of the port and allows more air to flow thru the head to the engine. Larger cross sections provide more power at higher rpm ranges and for larger engines. Smaller cross sections work best with smaller engines and provide faster movement thru the port. Air speed thru the port is very important to making hp above the 100 percent volumetric efficiency level. Engine and port size must allow for the highest air speed possible without limiting flow.
The length of the port effects the power with longer ports making more torque and the shorter ports allow faster travel of the air and make more high rpm power.
Head Flow Potential:
This is the CFM rating of the heads as measured by a flow bench. When the flow bench reads about 500 cfm the rule of thumb is the engine can make 1000 hp. This is not written in stone boosted engine can make more than that. A well built engine will make about 20% above the cfm number. 2.2HP per cfm.
Moving the intake or exhaust center line can effect performance at a given rpm range. With the camshaft event timing with Intake closing and exhaust opening being the prime event that effect the engine output.
This is the lobe separation angle. LSA is measured as the difference in degrees between the intake and exhaust center line.
Lift is the amount the valve is push into the cylinder by the camshaft. More lift usually means more flow but only helps to the heads potential. Some heads will flow more at higher lifts. Some heads flow max cfm at .550 and some flow best at .700 lift. For many sbc engines higher lift means more hp. With the valve springs taking the abuse.
Is the length of the time in degrees the valves are open in degrees.
The length of the headers collector effects power output and rpm range that power will be is increased. Assume the pipes are well designed for the engine the collector can be used as a tuning aid to make the engine work better in a wider rpm range.
Exhaust Header Design:
Manifolds, Logs and other type of factory manifolds do not produce as much power nor do they produce good scavenging from the cylinder. Performance engines use long tube headers with lengths matched to the engine to make the most power and torque for the given rpm range. Lots of math goes into making a good set of headers.
Scavenging Rpm Range:
This is the rpm the exhaust pulls the spent air fuel charge out of the cylinders. in most engines this is at the higher rpm band but is not limited to any one rpm. It also produces the effect often none as being on the cam. A point where the cam starts to work in its rpm range. Outside of this rpm range the air flow is reduced producing less power.
Carburetor Air/Fuel Ratio:
This is the mix of air to fuel 14.7:1 many performance engine like more fuel than this and make a bit more power when they are just a bit rich. Other engines run good with leaner mixture.
Not all fuel mixes properly. The carb puts out Atomized fuel the engine only uses fuel vapor. Fuel vapor that enters the engine is will burn completely. Atomized fuel entering the cylinder some of it is converted to vapor by the heat and pressure but not all of it some is just passed out the exhaust system as hydrocarbons.
This is the amount of time in degrees the spark plug fires before the piston reaches the top of the cylinder. The type of heads, octane rating of the fuel and rpm the engine is turning.
This is the amount of timing degrees the ignition timing is at. When raised from idle highest engines rpm and the curve as it goes up.
These are the rear end gear ratio or final drive ratio.
This effects the amount of gear ratio needed to reach the rpm range required.
The transmission ratio is used to plan out the power band. The first few gears are almost always lower than the final drive and the highest gears now a days are normally over drive where the gear ratio is less than one to one.