Engine Building Basics
This article is going to cover the “Which, Whys and How Much” of engine building.
By reading this article, you be able to comprehend the importance of how choosing components for your engine build will be reliant on deciding other components first and how their decisions are related.
There seems to be one question that is asked a multitude of times on all forums. That question is: “What parts should I use to build my engine?”
I’m going to give my opinion and also try to explain the theories so that people might understand and be helped in making their own decisions of what parts that they might use on their build.
Before I get started, you may want to look at my website articles addressing “exhaust” and “induction” to get a better understanding of this article:
http://hemrickperformance.com/default.aspx
The number one basic to creating power is to get as much oxygen into the combustion chamber as possible. Notice that I didn’t say “air”. Oxygen and fuel is what creates power. A “forced induction” system (turbo, supercharger) takes air and forces it into the combustion chamber. Air has oxygen in it so, by cramming more air into the cylinder you are basically forcing more oxygen into the cylinder. The same thing happens when using nitrous. Nitrous is two parts nitrogen and one part oxygen. That doesn’t seem like much but, nitrous is introduced in the induction system under high pressure. That is a large volume amount or “volume” of N2O with the emphases on the “O” (oxygen).
How do we get more oxygen into the engine without forced induction or N20? One way is to increase the cubic inches of the engine. Larger cylinders and longer stroke will allow the engine hold more oxygen. Porting the heads will reduce restriction of the flow of oxygen into the engine. The correct exhaust, intake, camshaft can give the engine the ability to flow more oxygen into the cylinders. Now the problem is deciding which parts to use out of the multitude of parts out on the market. One of the problems with performance parts manufacturers is that they want you to by their product. It seems that every one of these companies have the “best product”. Decisions, decisions…who do you believe?
I am a firm believer that the exhaust, cam and induction of an engine will determine where the torque will begin, peak, end, and how it will flow. Remember that horsepower will follow torque. Increase TQ and HP will increase also. My opinion is that all other performance enhancements will only increase the TQ and HP numbers except for forced induction and N2O.
Where to start?
You will need to start by deciding on the camshaft.
Most of the parts and modifications are dependant on each other and will be affected by each other, such as choosing the compression ratio. You can change the “static” C/R by the size of the combustion chamber, increasing cubic inches, milling the head or cylinder deck, also by the type of piston dome that you are using.
Before you decide on your “static” C/R, you must calculate your “dynamic” C/R.
Your camshaft timing will play a major part in this decision.
What cam to use?
Since cam timing, exhaust and induction controls the way that TQ flows and HP follows TQ…You will need to choose these three components so that they will match each other. This is easier said than done. To chose the combination for your bike and riding needs, I would suggest that you research Dyno Graphs that list a detailed list of parts used on the bike that is dynoed. I hope to be able to add a dyno library to this forum later.
Granted, all dyno graphs will not be the same and the results can be manipulated. The numbers on the graph may not be correct but, the flow of the TQ & HP lines should still be accurate. If you look at the TQ flow of the graph and not the numbers, you can determine what RPM that the TQ starts, peaks, ends and how smooth it flows across all RPM ranges. Remember, HP follows TQ. If you have a lighter bike, you may want your TQ in the upper RPM range and vice versa with a heavy bike.
So, start by selecting your cam, exhaust and induction by researching dyno graphs. After you have made your selection you can go on to choosing components that will increase the HP and TQ numbers.
Choosing a cylinder head:
Basically it would be great to have a head ported so that it could flow a lot of volume of air (oxygen) without restrictions. Porting is a highly debated issue and I would suggest that you research thoroughly before deciding on the person to port your heads. The thing to keep in mind is that no matter how well that the port job is, it will not help much if the exhaust, cam and induction is not properly matched. Most of you have read my beliefs on this subject but the fact is that the timing of the exhaust and induction waves will either help to flow air through the engine or it will hinder the flow of air through the engine.
Another thing to remember is that the intake port diameter and length will affect where the TQ peak will fall. Here is a online calculator found on RB Racing website that will help you understand this theory.
Inlet Runner and Peak Torque Calculator
This should help you understand the importance of the size of the port when intake porting is performed. If you by a set of heads that has been ported, they may not work well on your engine even if they were ported by the “worlds best” porter, just because the port size may not match you engine needs. One porting job does not fit all engine applications.
Choosing the correct size intake valve for the head:
First, you will need to decide the maximum RPM that you will target. You will need to calculate the “MACH” number to determine the size intake valve they you will need. The area between the valve head and the valve seat will be the area of most restriction in the induction system. The area that is restricted will have an increase of velocity (speed). This theory measures the speed of the air in mach numbers. Mach 1 being the speed of sound, Mach .5 is half the speed of sound. Studies have shown that when air in an induction system reaches six tenths the speed of sound or Mach .6 the air is disrupted enough that the engine will no longer make power. There are several Mach calculators online that will assist you in determining the Mach number, valve size and cam lobe lift. Here is a link to a more common one:
Inlet Valve Mach Index Calculator
If looking at a dyno chart you can determine approximately where the Mach numbers are. Mach .5 will be where the HP will peak. It will then flat line through Mach .55 and the HP will drop at Mach .6
When playing with the calculator, you can input different “max. valve lift” vs. “valve diameter“. If you are shooting for a particular Mach number at a certain RPM, notice that you will have to add a lot of “max. valve lift” compared to only having to increase the valve size a little to get the same results. So basically you can increase the valve size while keeping a lower lift cam. Remember that a high lift cam will put more strain on the valve train compared to a lower lift cam. Like I said earlier, all of the parts of an engine will affect how the other parts perform. So when choosing the valve size, you will also determine the cam lobe lift that you will need.
One advantage of a high lift cam is that the ramp will be more aggressive. Aggressive meaning: If you looked at the sides of a cam lobe and imagine that the sides cannot be increased.
Now still looking at the lobe, imagine that the lobe is taller. When the lifter rides on the taller cam lobe it will open the valve at the same degree but, since the lobe is steeper, it will open the valve further than the original cam even at the same degree. Basically an aggressive lobe or steeper lobe will open the valve more, even if it is just starting to open, compared to a non aggressive lobe. The reason that this is important is that when the piston is starting to travel downward on the intake stroke, you will want to have the intake valve opened as much as possible during this crucial time because the piston down stroke will have a lot of suction during the down stroke. This will allow more air (oxygen) to enter the cylinder. Again, it’s a give and take situation. An aggressive cam lobe will produce more power but, it will also produce more strain on the valve train.
The combustion chamber:
There are several combustion chamber designs out there. One of the most important thing to consider about your combustion chamber is determining the “static compression ratio” that you want to use. You shouldn’t choose your static compression ratio until you have determined the dynamic compression ratio. First you will need to understand the difference between “Static” and “Dynamic” compression ratio. Static compression is what most people are more familiar with. It calculates the amount of area that that is compressed into the combustion chamber. The area of the bore, stroke, deck height, head gasket thickness, piston dome or dish, and combustion chamber is added then compared to the size of the combustion chamber that it will be squeezed into. There are several good online calculators that you can use. Here is a link to the RB Racing calculator:
RSR Static Compression Ratio Calculator
Dynamic compression takes the same figures calculated by “static compression” and recalculates it by including the camshaft timing. This will also be related to the “cranking pressure” of the engine. The reason that this is so important is that the cam timing can increase or decrease the actual cranking pressure of the engine. Think of it this way: If you have a static compression ratio of 14:1 but, if the valves do not seat, what will your engines compression be? It will be “0”. So, if you have a long duration cam, the valve will be opened longer which will result in a lower dynamic compression (lower cylinder pressure). You will need to raise the static compression to compensate for the loss of cylinder pressure. You can do this by selecting a head with a smaller combustion chamber, increase the piston dome, install a thinner head gasket… The opposite will be true if you are using a short duration cam. This again shows how that all of the parts have to be chosen to match properly.
Here is a link to the RB Racing calculator:
Boost Compression Ratio Calculator Choosing the air filter:
Of course the filter needs to be as free flowing as possible. Here one thing that is over looked by most people. Some air filters such as the “Force Winder” has an extended throat (tube). How does this affect the engine build? Remember the wave theory in the induction system? You just lengthened the induction system which changed the timing of the induction waves.
Throttle Body Size:
T/B size is another highly debated subject. Basically you can get a rough estimate of how many “CFM” that your engine will require, then match a T/B to that CFM. From my experience, a T/B that is too small will hurt your power but, a T/B that is too lager will not hurt the power. An example of this is my 103 ci motor with a max RPM of 6,000 and using a 57mm T/B. It runs great at any throttle position. Remember that it doesn’t matter how much air that you T/B lets enter the engine as long as there is the proper amount of fuel add also. As far as that goes. You could take the T/B off as long as you can supply the proper amount of fuel. Here is an online CFM Calculator:
Big Boyz Head Porting - Volumetric Efficiency and CFM Calculator
Another thing that is normally over looked is where the air comes from before it enters the induction system. Most people realize that colder air is more dense than hot air. This also means that the oxygen is more dense when the air is cold. More dense means more molecules compressed in the same space. The normally used HD air cleaner cover will pull “hot” from the cylinders. Air cleaners such as the “Force Winder” will get “colder” air in front of the hot cylinders. Here is a picture of a simple way to pull colder air through the SE air filter housing. There is no cover restricting the cold air.
Choosing the correct injector size:
There is no need to purchase injectors until you have calculated you target horsepower.
You can only make as much power as the injector size will let you. Injectors are rated in lbs per hour, grams per second… The injector can only deliver a certain amount of fuel per hour or per second. Here is an example: Calculating a 3.91 gm/sec Harley injector.
3.91 x 60 seconds = 234.60 gm/min. 324.60 x 60 minutes = 10476 gm/hr. 10476 grams = approx. 31 lbs/hr.
Injector Selection Guide
Using this online calculator “Fuel Injector Max Capacity Worksheet” the 3.91gm/sec Harley injector can supply enough fuel to produce 99.2 horse power. So when someone is bragging about their engine making 120 HP, ask them what injectors they are using. If it has 3.91 gm/sec injectors, you will know that it will be impossible to produce 120 HP using two 3.91 gm/sec injectors. You will also be able to increase the injector fuel rate by increasing the fuel pressure.
All of the engine components influence each other and they have to match, as I stated at the beginning of this article. After understanding this theory, it’s now wonder that there are so many engine builds that won’t produce the power that is expected. You can’t take advice on what parts to use from several different people and hope that it performs correctly. You have to decide what you want or target, then plan before you purchase and build the engine.
I hope that this will help people reading this article. I’ll be glad to
try to answer any questions that you may have. I’ll add any other subjects to this article if needed. This covers the basics but there is a ton of other information and theories out there to consider if you care to go more advanced.
Thanks, Clark Hemrick
How To Decide What Parts To Use On Your Engine Build 
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