The first part of this Series laid out the importance of carefully planning your engine build. In this month's segment, we will further discuss the planning process and begin addressing bottom-end component selection. The information should help you understand more about current techniques in circle track engines or, if you purchase your engines, is intended to make you a more educated buyer.
When planning a build, the first place to start is a careful review of the rules you'll face. You need to understand what is permitted in the class you'll be competing in. It is also important to evaluate what the engine will be required to do. Answers to the following questions will help guide you:1. On average, what is the minimum engine speed where wide open throttle (WOT) occurs?
2. What is the minimum WOT engine speed under worst possible cases (slippery track, ill-handling car, hot humid air with low barometric pressure)?
3. On average, what is the maximum engine speed?
4. What is the maximum speed (rpm) the engine should ever see (good track, good-handling car, cold dry air with a high barometer)?
5. How many miles will the engine run before a rebuild?
6. Will the engine be maintained or must it race without any regular maintenance?
7. What is the engine-building budget?
Questions one and two will help define the desired torque peak of the engine.
Questions three and four help define the maximum speed (rpm) for the engine and where power peak should occur.
Questions five and six will define the type of build (conservative or aggressive). If the engine will be required to survive many races with little or no maintenance, the bottom-end and valvetrain must be overly-strong for the power level produced. The engine should also be run at lower speeds.
Question seven is the all important one. How much do I have to spend? This simple phrase will make your engine build more successful: "Plan what you buy and buy what you plan." Also remember when planning your build that the greater the engine speed, the greater the cost. Don't forget that.
Power curve In many types of racing where gear changes (shifts) occur, the engine needs to operate at WOT an equal number of rpm both below and above the power peak rpm. This is the plan when rules are open and gearing choices are unlimited. However, most circle track racing requires a different approach. For example, if the track where you race generally has good traction, then the torque peak should occur just after corner exit with power peak occurring at a point about 75 percent of the longest straightaway. This will allow the car to have the greatest rate of acceleration when it is possible to pass.
When tracks are generally slippery, we would like to increase the rpm where peak torque and power occur so that the power can still be used. It is pointless to have the torque peak occur at an rpm where the engine never sees WOT. If the torque peak occurs at the point where the driver is just getting back to WOT, the car may be more difficult to drive.
Graph A will help illustrate this point.
The Short Block When building a house, you start with the foundation. Similarly, when planning an engine build, you start with the bottom of the engine. This is your foundation.
Selecting a block is of critical importance. Assuming that you are running an iron block, the block should be sonic checked to verify that the casting is solid at the thinnest sections. A good machine shop will have the equipment to check a block by this method.
Block mass Remember, in circle track racing, chassis performance is almost always more important than engine performance. If you spend time grinding or milling 5 or 10 pounds of material from the block, you will be able to place more ballast where you need it. Total mass (weight, in this instance) reduction often costs more than $100 per pound. The engine block is one of the easiest and cheapest places to remove weight.