Here at Carmena Performance we build engines. I say build rather than assemble as there is considerably more to making a high output engine last than just bolting bunch of expensive parts together. Over the course of the last 20 years I have had the pleasure of building everything from single cylinder air cooled motorcycle engines to 12 cylinder overhead cam automobile engines. I do not fancy myself as the best engine builder in the world however I am very meticulous with my builds and have learned many important things over the years. I have also had the displeasure of visiting MANY online forums and seeing horrible or uninformed advice perpetuated by people who , a good portion of which, have no idea of what they are talking about. In the information age that we live in post count is mistaken for technical expertise. In this series of posts I am going to share some of the basics of what I have learned over the years. I am going to try to keep everything as simple as possible.
Simply put, torque is the amount of twisting force the crankshaft provides to the transmission. There are only three things that directly affect this, cylinder pressure, bore size, and stroke. An indirect affect is also caused by ignition timing as this determines how much leverage there is when all that cylinder pressure is applied to the rod journal.
Horsepower is a measure of torque over rpm. Horsepower wins races. Some people say torque is what makes a car fast. This is not entirely true. With my 150 pounds of weight and a 4 foot cheater bar I can supply 600 fl/lbs of torque to the input shaft of a transmission. You can bet your ass the car wont be moving very fast though. To make a fast car this torque has to be supplied over a range of RPM.The higher the range of rpm the faster the car is going to be. This is why some 3 liter motors can destroy some 7.4 liter motors on the track. If you make half the torque but maintain it for twice the RPM you are still making the same horsepower.
Air Mass vs Air Volume vs Air Density
This is in my opinion one of the most important aspects to explain for someone looking for big horsepower numbers. Total airflow through the engine is known a MASS airflow. Mass airflow takes into account both volume and density of the air and is the ultimate judge of how much power the engine will produce. Most people are familiar with the term CFM. This is a measurement most often associated with throttle bodies and Carburetors. It stands for “Cubic Feet per Minute”. As you can see this is a measurement of VOLUME (cubic feet).
As I said before an engine is an air pump. Volumetric efficiency is a measurement of how well it pumps air. As a technical term it is a percentage of how much air the engine pumps in relation to its displacement. If the engines displacement was 1 liter and it pumped 1 liter in a full engine cycle it would have 100% Volumetric efficiency. As the name implies it applies only to the engines air VOLUME. It has NOTHING to do with air density. The engine flows X amount of volume. This brings me to the first thing I hear all the time mainly to do with smooth airflow on forced induction engines “It doesn’t matter, your’e forcing the air through”.
Making Big POWER!
It seems like the majority of my customers are after big horsepower numbers with smaller displacement engines. The biggest hindrance we have to making power on Vancouver Island is fuel quality. Ask any high power street car owner and they will tell you that our fuel is crap. On many stateside produced handheld tuners we have had to flash 91 octane tunes just to be able to get by on our 94 octane. We also do not have that E85 stuff that all the cool kids are using. The reason this is important is that it limits the amount of cylinder pressure (air density) and heat that we can get away with.
A powerplant is a system of components. Every component is designed to provide a benefit in a specific area. When building engines the system components have to be selected to work together for an end goal. Everything interacts with the others. My first consideration is what rpm is needed for an application and in forced induction applications what kind of boost threshold is required. This determines camshaft choice.
Whether you are building your engine yourself or having it done by someone else take your time. Ask questions. Do your homework. The forums can be a wealth of information however dont trust any one person. Just because “Ubercarguy64” has earned his “Expert” badge from making 2000 posts that does not make those 2000 posts technically sound. Take that information and research. Read technical documentation. Come to intelligent decisions about engine modifications.