tech tips

Engine Understanding?

Contribution: Scott Collier

Previous posts have talked about some of the aftermarket pieces and accessories that are needed to go fast.  But, the real heart of going fast is the engine.  Pushing the limits of your engine requires skills and knowledge and a lot of maintenance.    More then most, it requires much knowledge.  Just adding parts and not making proper changes to accommodate these aftermarket parts can hinder engine performance.  This is not our goal!  I thought a general overview of the 4 cycle engine would help the inexperienced mechanic and provide a refresher to some with a little education.

Before getting into the heart of this there are a few things that need to be covered.

TDC: Top Dead Center

This is a point where the piston has reached its maximum height and briefly tops out before it swings downward.  On a degree wheel 0/360 degrees.  General basic terms - On a clock think of it as 12:00

BDC: Bottom Dead Center

This is the point where the piston has reached its maximum downward swing and briefly bottoms out before it starts to swing upward.  On a degree wheel 180 degrees.  General basic terms ? on a clock 6:00

There is also...

BTDC - Before top dead center - an area prior to 360 degrees

ATDC - After Top dead center and area after 0 degree 

Ok, let's get into it.

The basics of a four stroke 

A four cycle or four stroke engine requires 4 distinguished steps that allow it to operate.  They are as follows:

  1. Intake
  2. Ignition
  3. Combustion
  4. Exhaust

First thing to discuss is the carburetors position in this mix (pardon the pun).  A carburetors job is to provide a mixture of fuel and air in the proper amounts to promote a strong burn or ignition.  Too much air and not enough fuel creates a condition called a lean mixture.  This condition leads to excessive heat and will break down the engine quickly.  Too much fuel creates a condition know as a rich mixture.  This condition will run cooler and burn or ignition may not be complete causing a lack of power.  The goal of the carburetor is to provide a balanced mixture (to be discussed in a later post). 

Now we have a fuel air mixture...lets move on to how this gives us power.  We know the piston travels up and down in the cylinder turning the crank giving us a rotational output for power.  The up and down of the piston is governed by many things.


On the intake stroke, the piston is on its way down.  This movement creates a vacuum that draws the fuel/air mixture into the cylinder.  Of course the valve timing plays a part in this as it determines how long the valve (door) is open allowing the flow of this mixture from the carburetor.  If valve timing is off, you are restricting or lengthening the amount of time that valve is open allowing a condition of too much or not enough fuel into the cylinder.  Therefore check you valve clearances regularly! 


Fuel and its properties are the basics of this stroke.  The fuel and air will burn very well under atmospheric pressure.  But, place that same concentration of fuel and air under pressure and it will burn much quicker.  Hence, the compression of the fuel air mixture allows an explosive burn producing the punch to push that piston back down allowing power to be created.  Yes...power is only created on one of those four stroke... make it count!  So the compression stroke is the upward movement of the piston, with both valves closed, that allows the fuel air mixture to be compressed.


This is where the power is created.  After the spark plug ignites the compressed fuel air mixture, the piston is driven downwards with help from the explosion.  Again, the valves will maintain a closed position when this occurs.


After the power stroke travel downwards, it must return to the top to start again.  However, the cylinder is now charged with vapors and exhaust gases that do not promote good combustion.  So during the upward movement of the piston, the exhaust valve opens, and bad air is forced out of the cylinder allowing for a good clean return to the intake stroke.  Hence, it is called the Exhaust cycle.

At this point the exhaust valve has allowed the bad air out and the cylinder is ready to accept a new clean mixture into the cylinder as the four cycle process starts again.

So keeping that piston going up and down and the valves opening and closing at the correct time will provide a good ride.

Other areas of future focus...and future articles will include:

Valve timing

This all seems easy to understand but the real performance comes with the valve timing and knowing how to push the limits of a specific engine.  Valve timing is all related to position of the piston.  Timing is based on the 360 degree circle. 

Valves open and close before and after top dead center and at a time, there is an overlap of the opening of the intake and exhaust valves!


Getting the proper mixture is almost the one key factor that will allow a smooth trouble free run.  Too lean or too rich leads to a lack of power and poor performance, not optimum for a race machine.  Finding the balance is a matter of learning your machine and watching your plug. 

Follow the flow

An important piece of information besides the timing and correct mixture is the flow.  This is the flow of the fuel/air mixture into the cylinder and the exhaust out of the cylinder.  Better flow means better performance.  There are various ways to create better performance.  Topic to be discussed later

Aftermarket parts

In some cases, aftermarket parts are just a simple bolt on procedure.  However, when it comes to disturbing the flow to create more horsepower, usually there is more to it then the required wrench and the single part.

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Thanks to our Contibutors ....
Phillip Small
Aaron Hesmer
Scott Collier
Ian March
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