Engines – Where Does the Power Come From?

Okay, so I mentioned in a previous post that we can think of your engine as an air pump.  Really?  Okay, not so much, but why is this concept important?  Because it will help to understand how some speed components ‘increase horsepower’, and it will also help us to understand why some of these horsepower increases have no real benefit to your everyday hot rod driving experience.

 Let’s begin.  If an internal combustion engine is being viewed as a large air pump, we naturally assume that air is flowing through it.  The first and most important theory is that air, not unlike any other fluid, will flow better and faster if it has fewer turns to make and larger openings to travel through.  So imagine the path that air must take to get through your motor.  It travels through the air cleaner and through the carburetor.  Not many turns there.  Then the air enters the intake manifold.  Now the air flow has been divided into several different passages to deliver the fuel air mixture to each of the cylinders.  Out of necessity, there are quite a few twists and turns to deliver the fuel/air mixture to the cylinders.  But why is that?  Why isn’t the intake manifold just one big empty space that attaches the carburetor to the heads?  Why is there any need for all those separate passages? Good and important question.  Remember a few pages back when I mentioned how a motor’s optimal functionality occurs when all of the cylinders are acting as one?  Well oppositely, a motor performs less efficiently when one cylinder is functioning better or worse than the rest.  Ideally, we want each cylinder to receive the same amount of air and fuel.  Consequently, the design of an intake manifold is such that the fuel/air mixture has to travel the same distance to reach every cylinder, no matter where it is located.  Think about it, if the intake manifold was just one big empty cavity that attached to all the cylinders, the cylinders closest to the centre, or the carburetor, would get much more fuel than the ones at the ends of the block.  In effect, the cylinders in the centre would steal fuel from the cylinders on the end.  The fact to keep in mind is that it is necessary and beneficial for all the cylinders to have access to the same volume and quality of air/fuel.  So, to accomplish this, intake manifolds are designed to force the air/fuel to travel the same distance in order to reach each cylinder.  With all that in mind, it seems like a good time to discuss how a high performance intake can benefit the performance of your motor.  In general, a high performance intake manifold will have larger passages to carry the air/fuel to the heads.  As well, in very high horsepower applications, the walls of the passages inside the manifold may be significantly smoother than any stock variety. This smoothness will reduce the friction as air flows through the passages.  Friction …. really?  Yep, that’s the theory.   In racing applications, a tunnel ram style intake will be used.  Tunnel rams illustrate very well the concept of giving each cylinder access to the same amount of fuel.  In a tunnel ram, each cylinder has its own nearly straight passage to carry fuel/air from the carburetor to the heads; the obvious sacrifice is space.


In realistic applications, intake manifolds are one of the components that will not have a noticeable affect on a stock motor.  Most of the benefits provided by a high performance intake manifold are only realized at high rpms (2500 and over).  In fact, if you were to add a high performance intake to a stock motor, the only benefit that you might notice is that the motor may rev higher.  That is, if you were to open the throttle wide open, your engine might gain an extra few hundred rpm.  But, in all honesty, if you are in the habit of running your hot rod quite often with the throttle wide open, you will probably need a new motor to bolt onto your intake manifold in the not too distant future.  One other benefit to high performance intakes is a slightly increased throttle response.  It is debatable whether or not any increase in throttle response will be noticeable to the driver.


High performance intake manifolds are a good example of how a product might claim to increase horsepower, but might not actually provide any real beneficial power gains.  An intake manifold will not increase the torque of a motor.  The only horsepower gain will come from the engine being able to rev higher.  So, in effect, unless you are consistently red-lining your hot rod, the horsepower gains provided by a high performance intake are not particularly useful.  If there are other ‘bottom end’ upgrades in an engine, a racing intake will be much more beneficial and most often, utterly necessary.  But for most stock applications, the only benefit in buying a high performance intake is that they are more esthetically pleasing than the stock unit.


As long as we are in the general neighborhood of the intake manifold, I thought that I would take the opportunity to briefly cover a couple of optional speed components that can be installed instead of, or sometimes in conjunction with an intake manifold.  These are turbochargers and superchargers.  I know I know, how can I so casually mention what many hot rodders would call the most effective speed components you can buy?  Well because they are expensive…and most times overkill in my opinion.  But I digress; both turbochargers and superchargers greatly increase horsepower and torque.  How do they do it?  By increasing the compression in your engine; in more precise terms by pre-compressing the air/fuel mixture before it even enters the cylinders in an engine.  A turbocharger uses the exhaust air flow of the engine to compress the intake air.  As the exhaust flows out of the engine it spins a turbine which is connected to a similar turbine located in the intake stream. For this reason, turbochargers have to be connected to both the exhaust system and the intake air system.  Retrofitting a turbocharger is often difficult as re-routing exhaust pipes can be very problematic.  Most examples of turbocharged engines are seen installed at the factory, in fact turbocharging is most often used and most beneficial on diesel engines.  As well, due to some thermodynamic properties of internal combustion engines that are beyond the scope of this blog, turbochargers are largely ineffective at low rpms.  It is only when a motor is at higher rpms that the exhaust flow is significant and steady enough to provide compressive assistance.  So most hot rodders will opt for a supercharger.  A supercharger uses energy obtained from the rotation of the motor to drive a compressor.  It is usually connected to the crankshaft with a belt at the front of the engine.  The advantage of a supercharger is that it is driven as long as the engine is running.  At low rpms or at high rpms, a supercharger will always be able to provide compressive assistance.  In fact, often engines that are being built with the intent of receiving a supercharger will have their compression lowered so that the supercharger does not overload the combustion cycle and work against the motor.  Overall, superchargers and turbochargers greatly increase the efficiency and power of an engine.  They come in many different shapes and configurations.  It is also worthy to note that a supercharger or turbocharger does not replace a fuel delivery system.  That is, these components get added in between the carburetor and the intake manifold, or in some cases instead of an intake manifold.  The most important thing to consider when investigating one of these units for your hot rod is the price.  I have rarely seen these units available for less than $3500.  They definitely represent the greatest power increase for the most money.


Let’s move on past the intake now.  Back to our concept of analyzing an engine as an air pump.  Once the air/fuel has passed through the intake manifold, it enters the heads.  All of a sudden, there is a lot more going on.  The heads serve several functions.  (Oh by the way, I am going to confine my discussions to overhead valve engines.  Sorry to all you Ford guys working with flat heads.) The heads serve to seal the top of the cylinder, and allow air into and out of the cylinder by way of the valves.  The heads basically direct traffic into and out of the cylinders.  As well, they usually house the spark plugs and contain some coolant passages.  So, while the function of the heads is of the utmost importance, there are surprisingly few factors that will affect the performance of an engine.  The most important factor if we think about our air pump analogy is the valve openings.  In the whole internal combustion system, the valve opening is the smallest orifice that the air must pass through on its journey through a motor.  For that reason, to increase the air flow, the valves and valve openings must be larger, or else the valves must open farther and for the maximum amount of time between each stroke.  The shape and size of the valves are somewhat fixed without replacing the heads, however the timing and opening distances are controlled by the camshaft.  A high performance camshaft will essentially open the valves of an engine wider and for longer to allow fuel/ air and exhaust to flow more easily.  A camshaft will also, because it changes the timing of the strokes slightly, cause an engine to idle quite roughly.  This is cool.  I have not met many hot rodders who don’t appreciate the random sound of a nicely cammed motor.


Overall, a camshaft is the brain of a motor.  It basically controls when and where all the compression, intake, combustion, and exhaust activities take place.  For this reason, there are very significant power gains to be had from adding a high performance camshaft to your motor.  Here again, a camshaft cannot really add much torque, but can increase the horsepower of a motor significantly.  And, unlike an intake manifold, a camshaft is much more likely to affect horsepower in a useable range of rpms.  Overall, high performance camshafts probably provide the most significant power gains for any one single component in an engine.  Not only will they make your hot rod sound mean, but they will add a noticeable performance increase in a lower range of rpms.  The frustrating part about replacing camshafts is that they are difficult to remove and replace.  It is most often recommended to replace the cam bearings and lifters at the same time that the cam is replaced.  Replacing cam bearings requires special tools and preferably some experience. Then on top of all of that, most engine rebuilders will try to convince you that the lift rods and rockers need to be replaced at the same time.  Pretty soon your whole engine has been rebuilt and you have spent your kid’s college fund putting a new motor in your hot rod.  A camshaft replacement is an intensive and finicky job.  It can usually be done with the motor still in the vehicle, but is much easier with the engine out.  Basically, adding a high performance camshaft is worth the time and money, but it is probably some thing to do when you are ready to rebuild your engine.


Incidentally, while we are on the topic of camshafts, it may be interesting to acknowledge where camshaft technology has moved in the last twenty years.  Nowadays, most engines large and small use over-head camshafts (OHC).  An even more familiar term is duel over-head camshaft (DOHC).  The basic theory behind this new design was to move the camshaft from the centre of the engine and mount it above the heads, right on top of the valves that the camshaft has to open and close.  Makes sense right?  Why transfer the force from the rotating camshaft through a whole mess of lifters and push rods and then reverse the force in a rocker arm just to get a valve to open?  Why not just mount the camshaft right on top of the valves that need to be controlled?  Brilliant.  There are so many advantages to this system, I need to restrain myself and only mention a few.  First of all, there is much more precision available when the camshaft is directly opening the valves.  Think of how many different parts are eliminated: Lifters, push rods, rockers.  Without all these components, the camshaft in an OHC engine is moving a lot fewer parts to get the valves open and closed.  So what is the benefit?  Think about it this way.  Remember when we talked about the maximum rpm of an engine?  When the valves start to float; basically the valves are opening and closing so fast that they don’t have time to close before they are required to open again.  Part of the reason that the valves cannot close fast enough is that the valve spring has to push the valve back into place, but also it has to push the rocker back, and the push rod, and the lifter.  But how much difference would there be if the valve spring only had to push the valve closed?  The answer is a lot.  This is one of the basic reason why OHC engines rev much higher than older V-8 engines.  An OHC system allows the valves to close faster, so in essence, the motor can rev higher before the valves start to float.  Among other benefits, the throttle response is also significantly increased.  Overall, the move to over-head cam engine designs is the reason we are seeing so much more power coming out of significantly smaller engines.  The unfortunate news for hot rodders is that most newer OHC or DOHC engines are fuel injected and thus very difficult to retrofit into hot rods.  This is not to say that newer production motors cannot be incorporated into you street rod project, in fact I would argue almost the opposite, but the average street rodder may have to alter the definition of a hot rod project in order to properly assimilate late model drive trains.  Don’t worry, I will cover all of these topics in the drive train section.


Let us get back to the discussion of engines in general.  We had just talked about intake manifolds and camshafts.  Next in the path through an engine is the heads.  As for heads, there are several factors that will translate into a power increase.  Again to reduce the friction, the ports inside some high performance heads will be polished.  Adding heads with larger valves will increase air flow significantly, but will usually only make a difference in power at higher rpm ranges or when other speed components have been added to the bottom end of an engine.  Ironically, a very significant factor in some aftermarket high performance heads is that they are aluminum.  Aluminum heads are very expensive, but the substantial reduction in weight alone can be a significant factor, especially in racing environments.  In summary, high performance heads can provide power gains for a variety of reasons.  The science of valve design and combustion chamber shape are beyond the scope of our discussion, but are all factors that will affect the power of a motor.  However, in general, high performance heads are outrageously expensive and will not result in a noticeable power increase unless combined with some other bottom end upgrades.  As a side note, some people may have noticed that in the past few decades, auto manufacturers have started listing the number of valves right in the main label of the motor.  One such example is the 32 valve Northstar engine made by Cadillac.  This engine is a perfect example of how increasing the number of valves or generally increasing the volume of passage that allows air into the cylinders can increase power.  This design was only made possible by the advent of over-head camshafts, but the basic science is simple: they found a way to squeeze two valves where there was only one before.  Two valve openings will allow more air through than one valve, so the result is an increase in horsepower.  Incidentally, these motors are phenomenally smooth and very powerful.  If they weren’t so complicated to retro-fit, they would be a great choice for the power plant in any hot rod.


Now in our air pump analogy, we are nearing the end of the journey; the exhaust.  Luckily enough, the exhaust is straightforward so to speak.  Here again, the less turns and bends and the wider the passage, the easier it is for the exhaust to escape from the motor.  A stock exhaust manifold usually routes all the exhaust ports into one cavity almost immediately as the exhaust exits the block.  It is then routed into the exhaust pipe, through a muffler and out the tailpipe.  The only real upgrades to an exhaust system are the addition of headers, dual exhaust, and performance mufflers.  You will notice that headers accomplish the same task as an intake manifold.  That is, they provide an equidistant path for exhaust to travel from the exhaust ports to the collection point.  And, similar to intake manifolds, the power gains provided by the addition of headers becomes more noticeable as engine rpms increase.   Headers will provide more benefit for increased air flow because there is a higher volume of exhaust leaving the engine than there is air entering.  As for dual exhaust and high performance mufflers, the increase in power will probably not be particularly noticeable.  I would imagine that performance exhaust is one component that gets installed on nearly every hot rod.  It is of coarse, not so much for the speed, but for the sound.  So, that being said, I must provide you with fair warning.  Be careful when selecting how loud your exhaust is going to be.  If you plan on doing long road trips or cross country cruises, quieter exhaust is the way to go.  Six or eight hours on the highway listening to a gentle roar will not only leave your with a headache, but can do some permanent hearing damage.   Seriously.

I hope that this brief description of engines might help to brighten the bigger picture when it comes to engines, but more so I hope that it will provide a foundation to understand how different motor accessories will affect the performance of the motor.

As always, we are all ears.  Please send us your comments, questions, and pictures.

And tell your friends!