Considerable information has been recorded about numerous aspects of the four stroke internal combustion engine. Nevertheless, only a small percentage of people really understand how it works and even fewer still know how to modify an engine to suit their needs. I will try to simplify this complex subject by discussing some basic principles that may be overlooked or misunderstood by the average person. First, it is very important to understand the relationship between piston travel directions and valve timing events. The reason this relationship is important is because it is one of the few things that is relatively easy to adjust/change. The camshaft which opens and closes the valves makes ONE complete revolution (360 degrees) while the crankshaft moving the piston up and down the cylinder rotates TWICE (720 degrees). Camshaft timing is usually expressed in terms of crankshaft degrees relative to the piston location in the cylinder. That is, relative to Top Dead Center (TDC) and Bottom Dead Center (BDC), respectively. Note that during the four strokes of a piston in an internal combustion engine the crankshaft will rotate 720 degrees and the piston will be at each TDC and BDC twice.


Starting at TDC, the piston starts from zero velocity and moves down the cylinder during the intake stroke; first picking up speed and then slowing down again when it reaches the bottom of the stroke. As the piston moves down the cylinder, the intake valve is opening. Some air/gas mixture starts to flow into the cylinder as the valve opens, but the greatest gulp comes when the pressure differential is the greatest. This occurs when the piston reaches its maximum velocity somewhere between 70 to 80 degrees ATDC. What governs piston velocity is the stroke, rod length, RPM, and piston pin off-set. The maximum piston speed of the engine is then limited by the resistance to gas flow of the engine and/or the stresses due to the inertia of the moving parts. You must be wondering why I'm talking about piston velocity during the first stroke.

FACT ONE: Volumetric efficiency is directly related to piston velocity!

Volumetric efficiency is a measure of the effectiveness of an engine's intake system and there are about 200 miles of air above the engine just waiting to fill the cylinder with 14.7 psi at sea level. The intake valve is almost closed as the piston reaches BDC, but it does not close completely until after BDC, when the piston is on its way back up the cylinder. The reason for this is because the incoming air/fuel mixture still has momentum even though the piston has slowed way down. We are now starting,


The piston compresses the air/fuel mixture to a high enough pressure and temperature to permit spark plug ignition. We hope that this results in a CONTROLLED BURN, rather than an explosion (detonation), that produces POWER and moves the piston down for,


Power is produced while the gases in the cylinder expand and cool. In most instances, the gases are at a relatively low pressure by the time the crankshaft reaches 90 degrees After Top Dead Center (ATDC), so we can safely open the exhaust valve Before Bottom Dead Center (BBDC) to take advantage of blow- down. Otherwise, the piston would have to push ALL the exhaust out. When the piston reaches BDC we begin,


The exhaust valve is opening at a fairly rapid rate, the piston is going up, and if the exhaust valve is not open a lot by the time the piston reaches maximum velocity, there will be resistance in the cylinder caused by excessive exhaust gas pressure. This produces conditions which are referred to as pumping losses. As the piston reaches the top of the cylinder, the end of the fourth stroke, you will see the exhaust valve is almost closed, but, lo and behold, the intake valve is just beginning to rise off the seat! At TDC at the end of the fourth stroke, both the intake and exhaust valves are open just a little. For this reason, this part of the stroke is called the OVERLAP PERIOD.

During the overlap period you will often find that both valves will be open an equal amount. This condition is referred to as SPLIT OVERLAP. On standard engines, the valves are only open together for 15 - 30 degrees of crankshaft rotation. In a race engine operating at 5 - 7000 RPM, you will find the overlap period to be in the neighborhood of 60 - 100 degrees (which also translates to more total duration)! As you might expect, with this much overlap the low speed running is very poor and a lot of the intake charge goes right out the exhaust pipe.


Let us review the four strokes again and add some timing events to calculate the total valve duration. For illustrative purposes, we can discuss a good street cam with a 268 degree duration and 108 degree lobe centers. (The lobe center angle is the angle in camshaft degrees between full intake cam lift and full exhaust cam lift). As we discussed above, at the end of the fourth stroke both valves are open and the next stroke is the intake stroke. Referring to fig. 1, we see that the intake valve began to open at 26 degrees BTDC. The piston moves down the cylinder after the crankshaft passes TDC, and the valve reaches full lift at 108 degrees ATDC (lobe center). Note also that the intake valve is still open when the piston reaches BDC. We can start to add things up now. The crankshaft has rotated 180 degrees from TDC to BDC on the first stroke and the intake valve opened 26 degrees BTDC, so the total crankshaft rotation so far is 26 + 180 = 206 degrees. We started with a 268 degree camshaft so that tells us when the intake valve will close: 268 - 206 = 62 degrees ABDC. Note that even though the second stroke is the compression stroke, we see that it starts while the intake valve is still open!

FACT TWO: In the lower RPM range, the engine does not have any compression until the intake valve closes. As the engine speed increases, there is a ram or inertia effect which begins compression progressively sooner with engine speed.

Now, we compress the air/fuel mixture and ignite it at the proper time in order to maximize the push down on the power stroke, or stroke three. Remember, I said most of the cylinder pressure is gone by 90 degrees ATDC, and you can see that with our 268 degree cam, that the exhaust valve begins to open 62 degrees BBDC, that is, before the exhaust stroke actually begins. So adding again, we have 62 + 180 (stroke four) = 242 degrees. Thus at TDC at the end of the exhaust stroke, the intake valve has opened but the exhaust has not closed. The exhaust valve remains open for 268 - 242 = 26 degrees ATDC. With the intake valve opening at 26 degrees BTDC and the exhaust closing at 26 degrees ATDC we have a total of 52 degrees of overlap.

Now, with the basics down, we can start discussing duration, lift, lobe centers, compression, and cylinder flow.


Let us now take the four valve timing events and put them in order of importance. The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power, stroke and it will be easier to pass a smog test, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down. You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.
The next least important timing point is the exhaust valve closing. If it closes early, say around 15 degrees ATDC, you will have a short valve overlap period. Less overlap makes it easier to pass the smog test, but it does not help power at the higher engine speeds. Closing the exhaust valve later, in the vicinity of 40 degrees ATDC, will mean a longer valve overlap period and a lot more intake charge dilution that will translate into poor low-speed operation. Some compromise must clearly be made to determine just how much overlap one needs to use. Many factors such as idle quality, low speed throttle response, fuel economy, port size, and combustion chamber design must be considered in making this choice.

A somewhat more important timing event is the intake valve opening. Early opening allows for a greater valve overlap period and adds to poor response at low engine speeds. Now, for the high performance enthusiast, low engine speed could mean 3000 RPM, but I would not consider such an engine as appropriate for normal street use! If you are not concerned about passing the smog test, then early intake valve opening will help the power output of the engine. That is, earlier valve opening will have the valve open further when the piston reaches maximum velocity and that, in turn, will increase volumetric efficiency.
I must stop now and ask you a question about your engine. If a 1500 Cortina head does not flow much air above 0.350 in. of valve lift, and it is possible to have the intake valve open that much by the time the piston reaches maximum velocity, WHY DO MOST PEOPLE THINK THEY WANT AT LEAST 0.500 in. VALVE LIFT???
Now, the last timing event is the most important, and the most critical to engine performance - THE CLOSING OF THE INTAKE VALVE. This event governs both the engine's RPM range and its effective compression ratio. If the intake valve closes early, say about 50 degrees ABDC, then it limits how much air/fuel mixture can enter the cylinder. Such an early closing will provide very nice low speed engine operation, but at the same time it limits the ultimate power output as well as RPM. Another problem with early intake valve closing that most people do not consider is that if you have a high compression engine, say 10:1 or higher, you will have more pumping loss trying to compress the mixture. This might even lead to head gasket and/or piston failure! These observations suggest that if you close the intake valve later the cylinder will have more time to take in more air/fuel and the RPM will move up. That seems simple enough, doesn't it? The later the intake valve closes the higher the RPM and therefore the more power, MAYBE? It turns out that if the intake valve closes past 75 degrees ABDC, you could lose most of your low-speed torque and if your static compression ratio is only 8:1, the engine will not be able to reach its horsepower potential. This should give you a better understanding of why the intake valve closing is the most important timing event.


So, now you ask, "What do I need to know to make a proper camshaft selection for my particular application?" The list is long. First of all, in what RPM range will you want power: 1-4000 RPM, 3-6000 RPM, 5-8000 RPM, etc.? What is the size of the engine? What are the bore and stroke dimensions? How long is the center-to-center distance on the connecting rod? How much piston pin offset is there? What is the static compression ratio? In the cylinder head, what is the maximum air flow (in cubic feet per minute or CFM) in the intake track with the intake manifold and carburetor installed? At what valve lift does the air flow level out on both the intake and exhaust valves? What is the percentage of air flow of the exhaust versus the intake? What are the valve sizes? What are the lengths and sizes of the intake and exhaust systems? Once you have this data, you should be able to make a logical cam choice; but sometimes you might have to face the reality that your basic engine parameters are wrong for the RPM range you are after. How can a layperson look in a cam catalog and make an intelligent choice? First the parts supplier must supply the proper information in order to help the customer choose the right camshaft for his/her application. But, in addition, you need to be prepared with the right information about your engine and what you ultimately want to be driving.


Let us now review some basic cylinder head data that one must consider before selecting a camshaft. Most people will agree with the statement that larger valves are required for more power. But now we need to ask several questions. What happens to the volumetric flow rate (in CFM) when valve sizes are increased? What about the port velocities, both intake and exhaust? How are the exhaust and intake flows effected? IS BIGGER REALLY BETTER? It has been my experience that when you are dealing with a stock cam, say 250 degrees duration, it does indeed help to increase the valve size to get more flow through the engine. Low to mid-lift flow is very important on the exhaust valve and mid-lift to full lift flow is very important on the intake valve. Some engines respond to increasing the exhaust flow so that it almost matches the intake flow. Based on valve diameters, you will find that the exhaust flow is about 80% of the intake flow in your typical engine. Design guidelines developed by the Society of Automotive Engineers (SAE) suggest that the exhaust flow should be 75-80% of the intake. I prefer to be in the 80-85% range and port the head to achieve about 75-80% exhaust CFM flow compared to intake CFM flow. When using a stock cam, you can get good results even at exhaust/intake ratios of 90-95%. Such high ratios will also work in drag racing applications where the engine is intended to operate at wide open throttle (WOT) conditions. However, when a camshaft with more duration is installed in a "hot" street, auto cross, or road racing engine, a 90-95% exhaust/intake flow will over scavenge the cylinder resulting in wasted fuel and an undesirable reduction in torque. Now let's see how these comments have been translated into some popular 1600 Lotus Twin Cam street and racing motors. Valve sizes for various twin cam heads are summarized in the following table:

Type : Intake Exhaust : Exhaust : % of Intake :
Standard Engine 1.530 1.320 86 %
Sprint 1.560 1.320 84 %
Racing 1.625 1.375 84 %
Brian Hart 1.690 1.440 85 %

In the above table, the standard engine is an early Weber head and the Sprint is a late Stromberg head. Flow measurements of the Stromberg, Racing, and Brian Hart heads are shown in figures 1 a-c. The Stromberg head (fig. 1a) was "cleaned up" but not fully ported, and the flow curves show a high exhaust to intake flow up to a lift of 0.100". This flow ratio then levels off to about 80% for higher valve openings. Note that the intake flow doesn't increase much past 0.400" lift, and the exhaust levels off at 0.400". The racing head (fig. 1b) is a Weber that has been prepared (supposedly), but you can see that it has a very poor flow ratio at low lifts where the exhaust flow actually EXCEEDS the intake flow! Things look better above 0.150" lift and the intake flow is good past 0.450" lift while the exhaust flow levels off at about 0.400". Finally, the Brain Hart head (fig. 1c) shows some really deep breathing capabilities! A lot more CFM overall, and great intake flow up to a 0.450" lift with the exhaust good to 0.400".

Most push rod and twin cam cylinder heads flow very well up to 0.350" lift, but flow increases really start to level out beyond that lift. The larger the valve the higher the CFM is over what you normally expect, and you can see that the twin cam head will flow well even above 0.400" lift when it has been reworked by increasing the valve sizes, grinding, polishing, and blending the valves and ports. The bottom line is clear: a well developed cylinder head on an engine will really pay off in increased horsepower. However, as I have said before, the individual making an engine modification has to be realistic about where he/she wants the power range.


Just about any engine would benefit from a prepared cylinder head, a good exhaust system (with a relatively small diameter for street use), and maybe a little larger carburetor. As you increase the RPM band, you'll need to increase the compression ratio and add some more duration to the cam. The more duration you add, the more compression you'll need and that combination will increase the upper mid-range and top-end power. It is very important to keep your combinations balanced; for example, you can not use a 270 degree camshaft with 8:1 compression. 9.5:1 would be a lot better. Conversely, you can not have 10:1 compression and use a cam with only 250 or 260 degrees of duration! As soon as the duration is above 270 degrees, the standard exhaust system will likely restrict the breathing ability of the engine. As a result, it may become difficult to make the idle mechanism work properly due to reduced vacuum and extra exhaust back pressure.


You probably have figured out by now that I am not an advocate of extra high lift, unnecessarily long duration, or very high compression for any street driven car. I prefer instead to use maximum velocity in the camshaft design which allows my cams to have more duration at 0.050", 0.100", and 0.200" lift compared to the "Brand X" cams you might get from other sources. As a side benefit of this design choice, it turns out that when you have more duration at 0.200" - 0.300" lift and not as high a cam lift, you end up with a cam lobe with a rounder nose radius which will support higher valve spring loads and therefore will last longer than a "pointed" high lift cam. I learned a long time ago that dwell on the nose, or top, portion of the cam lobe is equivalent to lift provided that you have the valve open far enough when the piston reaches its maximum velocity. On a normally aspirated engine, I have never seen power increased by adding valve lift above and beyond the flow capacity of the head.

You now have all the info you need to make the important performance enhancement choices appropriate for your own application - so there is not much more to say except HAPPY TUNING.

By: Dimitri N. Elgin

Camshaft No. Seat to Seat Duration In/Ex Duration @ .50 Cam Lift Cam Lift In/Ex Valve Lift In/Ex 1.25 Rockers Valve Lift In/Ex 1.5 Rockers Application
CAM001 244/264 198/215 .264/.264 327/327 393/393 Mild road, smooth idle.
CAM002 250/264 208/215 .250/.264 312/327 375/393 One up from Cooper 'S' spec.
CAM6608 264/264 217/217 .262/.262 327/327 393/393 Max. for smog emissions.
CAM003 264/268 215/222 .264/.289 327/358 393/430 Fast road cam.
CAM004 262/268 225/230 .293/.318 366/397 439/477 Fast road cam. 9.1:1 compression recommended.
CAM6708 268/268 222/222 .289/.289 361/361 433/433 Mini & 1275. Will work with stock engine.
CAM6707 268/268 230/230 .318/.318 397/397 477/477 Heavy duty springs reqd. Cylinder head porting recommended
CAM005 268/278 230/240 .318/.321 397/401 477/401 Hot street. Header recommended. 9.5:1 compression.
CAM7008 280/280 228/228 .263/.263 329/329 395/395 Hot street. Must have header to idle. 9.5:1 compression, heavy duty valve springs & good cyl. head are req. Adv. cam +2 deg.
CAM7007 280/280 228/228 .278/.278 348/348 417/417 Hot street grind. Same reqs as above.
CAM7107 284/284 235/235 .290/.290 363/393 435/435 Autocross. 9.5:1 compression required.
CAM71506 286/286 242/242 .302/.302 378/378 453/453 Autocross. 10:1 compression required.
CAM7207 288/288 235/235 .290/.290 363/363 435/435 Autocross. 10:1 compression required.
CAM006 288/292 235/254 .293/.321 366/401 439/481 Autocross. 11:1 compression required. Pump gas okay.
CAM7505 300/300 250/250 .297/.297 371/371 446/446 Race grind. 11:1 compression reqd.
CAM007 300/300 250/250 .323/.323 404/404 485/485 Race grind. 11:1 compression reqd.
CAM76506 306/306 261/261 .341/.341 426/426 512/512 Full race. 13:1 compression reqd.
CAM7706-32 308/308 255/255 .322/.322 403/403 483/483 Full race. 13:1 compression reqd. Power range from 4500-7500
CAM7706-34 308/308 263/263 .348/.348 435/435 522/522 Full race. 13:1 compression reqd.
CAM3503 310/310 268/268 .380/.380 475/457 570/570 Race. Works best for 1275-1380cc engines.