High Performance Street Engines

Performance street engines are, in some ways, more challenging to build than racing engines. Because of the environment where street engines are used, they must run well in an extremely wide RPM range. From idle to maximum RPM, a street engine must be well mannered to be fun to drive. It must be consistent and repeatable in the way it runs; otherwise it becomes a frustrating thing to deal with. Because of the nature of street driving, component selection for a street engine is quite critical. All too often, street engines are patterned after drag racing engines, usually with disappointing results. An engine that has to get to high RPM's to get into its efficient power band is usually sluggish at RPM's below that power band. Unless a car with such an engine has a lot of gear in it, it isn't much fun to drive.

To build a engine that has a very wide power band, the choice of cylinder heads and camshaft is very important. Buy the best cylinder heads your engine budget will allow. Look for heads that have high flow AND small volume ports. The small ports create high velocities, which is very important in retaining low speed driveability and throttle response. The high flow capability helps make power in the RPM range where the engine is run at full throttle. Top end power in most performance engines is more dependant on exhaust flow than on intake flow, so look for heads that have that have exhaust flow that is at least 70 percent of the intake flow through most of the valve lift range. An engine with high flowing, high velocity ports (intake and exhaust) needs less camshaft duration to make excellent power. The shorter duration helps keep the valve overlap down, which really helps idle quality, drivability, throttle response, and torque. The availability of street appropriate cylinder heads and camshafts is getting better all the time, so it's getting easier to build engine combinations that are really fun to drive.

The name of the game in street performance is not top-end horsepower. The name of the game is torque, because it is torque that accelerates a car, not top end HP. Street performance is mostly about rapid acceleration, not top end speed. Torque is not what you get with long duration drag racing cams, at least not in the RPM range where a street engine needs that torque. The proper choice of heads and camshaft will allow a street engine to make excellent torque over a wide RPM range, while retaining good low speed -part throttle characteristics in normal street traffic.

A major player in a successful street engine combination is the carburetor. To allow the engine to achieve its full potential of driveability and power, the carburetor MUST meter fuel very efficiently, and must have high airflow. It is a common misconception that a street carb has to have small venturis to have good driveability and throttle response. A bigger carb that meters fuel efficiently at all engine speeds will have driveability as good as a smaller carb. The key is knowing how to calibrate the fuel metering circuits to achieve that efficiency. At CFS, we have been dealing with street engines for decades. Many of the fuel metering techniques that we have developed for street carbs, we also use in our racing engine carbs to improve the driveability of those engines, helping them make useable power over a wider RPM range.    © Norm Schenck