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. This is also why 500 CI funny cars are so fast. They are making peak torque at 6500 RPM and maintaining high torque to over 8000 RPM! So, as you can see, there are different ways to build a high horsepower engine.
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). Now for one of the favorites, BOOST! Boost is measured in PSI ans is simply a way to increase the DENSITY of air flowing through the engine. Engines are air pumps. The more air they pump the more power they generate.
The MASS of air flowing through an engine can be increased in 2 ways. You can increase the air VOLUME by increasing the cfm the engine is capable of flowing or you can increase the air DENSITY by increasing how much air gets packed into that CFM number the engine already has. Things that increase airflow would be larger throttle body, ported cylinder head, more aggressive camshafts, and low restriction exhaust. Things that would increase air density would be forced induction, nitrous, charge air cooling, and to an extent induction.
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". Every time I see this I shake my head. The engine does not know you are running 30 PSI of boost. Its just flowing its xxx CFM of air. The volumetric efficiency will actually be negatively affected with boost as higher density air has minimally higher frictional losses and the turbo or superchargers ability to flow can change with the more boost it generates. If you layout dyno graphs on an engine with a high boost pull and a low boost pull you will notice the shape of the power curve is identical provided there is no excessive exhaust restriction or large compressor efficiency changes, its just higher overall. This is because the engine volume flow dynamics don't change appreciably with boost.
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. When designing high powered motors for use here this has to be taken into account. So if our air density is limited the only way to increase that power number is air volume. This brings me to another thing I hear all the time "The stock (cams)(intake manifold)(cylinder head)(throttle body) is good for XXXXX high HP".
While this may be the truth it can be misleading. The first question I ask is "Yes, but at what boost?". Many of the people that claim high hp with stock airflow components are getting that power by increasing boost. As we determined, due to fuel quality that is not a possibility for us. So that intake manifold that supported 600 hp in the States on race fuel will likely not do so here on Vancouver Island. At least not without reducing your pistons to chunks. To get more mass we need to increase the engines ability to flow air at higher engine speeds. There are disadvantages to building a high air volume,high RPM engine. This typically requires aggressive cams, lightened and stiffened valve train, cylinder head porting, short runner intake manifolds, and large exhausts. Idle quality and low rpm torque will suffer due to valve overlap and port velocity however that big power number will be achievable. Building for high rpm can also be very expensive. Getting lightweight but stiffer valve train requires the use of more exotic materials such as stainless steel and titanium. Higher spring rates are required as well which adds more load to valve train components. Piston speeds have to be determined and limited. Inertial loading on the reciprocating assembly also goes through the roof requiring stronger rods and pistons however these also have to be lighter to reduce the stress on the rod bolts, crankshaft, main bearings, main caps, and main cap bolts. Clearances have to be larger to account for heat and flex. The list goes on but EVERYTHING has to be taken into account. Everything you do with one component affects another.
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. After the camshaft is selected compression ratio can be decided upon which allows pistons to be selected. After pistons and rods are selected and the machine work is complete the head gasket thickness is decided upon. Building engines this way takes more time but ensures everything works well together.
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. Decide on a logical intended use for the engine you are building. Building your street engine from specs designed for a drag car is a poor decision and will likely be less enjoyable on the street than a milder build. Every application has different requirements.
If you have any questions or would like help with your engine build give me a shout.