By Dema Elgin
Introduction--(by Joel Lipkin) This year's winter "project" is a new motor for my Super 7. My goal is reliable power--what else is new?--so I am starting out with a tried-and-true Cortina 1500 and will make appropriate changes to improve it in the best way I know how. Early on in the process, I decided that I will need a new "hot" cam, but I quickly discovered that my knowledge in this specialized area is sadly lacking. Figuring the trade-offs between lift, duration, and low-end torque, for example, seemed like some kind of black magic. Almost as quickly, I discovered that the Bay Area is blessed with a talented and experienced cam grinder who is also eager to share his knowledge with local amateurs. This is Demitri "Dema" Elgin who owns and runs Elgin's Cams in Redwood City. In fact, Dema gets asked so many questions that he decided to write down some of the answers in terms we can all understand. The result is a series of three articles which he has generously offered for publication in the Chapman Report. The following is the first of these, the others will appear in subsequent issues.
PART I -- THE BASICS
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 that opens and closes the valves makes ONE complete revolution (360 deg) while the crankshaft moving the pistons up and down the cylinder rotates TWICE (720 deg). Camshaft timing is usually expressed in terms of crankshaft degrees relative to piston placement in the cylinder. That is, relative to Top Dead Center (TDC) and Bottom Dead Center (BDC), where the piston is at the top and bottom of its stroke, respectively. Note that during the four strokes of a piston in an internal combustion engine the crankshaft will rotate 720 deg and the piston will be at each of TDC and BDC twice.
THE FIRST STROKE: Starting at TDC, the piston moves down the cylinder during the intake stroke; first picking up speed 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 biggest. This occurs when the piston reaches its maximum velocity. The things that govern piston velocity are the stroke, rod length and piston pin off-set. 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! 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 pack up the cylinder. We are now starting
THE SECOND STROKE: The piston COMPRESSES the air/fuel mixture to a high enough pressure and temperature to permit spark plug ignition. We hope this results in a CONTROLLED BURN, rather than an explosion (detonation), that produces POWER and moves the piston down for
THE THIRD STROKE: Power is produced while the gases in the cylinder expand and cool. In most instances the expanding gases are at a low pressure by the time the crankshaft reaches 90 deg after TDC (ATDC), so we can safely open the exhaust valve before BDC (BBDC). When the piston reaches BDC we begin
THE FOURTH STROKE: 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 its 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 a SPLIT OVERLAP. On standard engines the valves are only open together for 15-30 deg 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 deg (which also translates to more valve opening 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 total valve duration. For illustrative purposes, we can discuss a good street cam with a 268 deg duration and 108 deg lobe centers. (Lobe center is the relationship between the centerlines of the intake and exhaust cam lobes, expressed in camshaft degrees.) As we discussed above, at the end of the fourth stroke both valves are open and the next stroke is the new intake stroke. Referring to Fig. 1, we see that the intake valve began to open 26 deg BTDC. The piston moves down the cylinder after the crankshaft passes TDC, and the valve reaches full lift at 108 deg (lobe center) ATDC. 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 deg 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 deg + 180 deg = 206 deg. We started with a 268 deg camshaft so that tells us when the intake valve will close: 268 deg - 206 deg = 62 deg ATDC. 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: The engine does not have any compression until the intake valve is fully closed!
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 that most of the cylinder pressure is gone by 90 deg ATDC, and you can see that with our 268 deg cam, that the exhaust valve begins to open 62 deg BBDC, that is, before the exhaust stroke actually begins. So adding again, we have 62 deg + 180 deg (stroke 4) = 242 deg. Thus at TDC at the end of the exhaust stroke the intake valve has opened but the exhaust is not closed. It remains open for 268 deg - 242 deg = 26 deg ATDC. With the intake opening a 26 deg BTDC and the exhaust closing at 26 deg ATDC we have a total of 52 deg of overlap.
Now, with the basics down, we can start discussing duration, lift, lobe centers, compression and cylinder flow.
Camshafts - Part 2: Choices
In the 1st part I discussed the four strokes that consume 720 degrees of crankshaft rotation in the reciprocating engine. I gave a very brief explanation of what was happening to the valves and pistons during each stroke. I finished the article by examining the timing events of a good "street" cam.
Let us now take the four timing events and put them in order of importance. The LEAST important is exhaust valve opening. It could open anywhere from 50 to 90 degrees before bottom dead center (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 loss by not having enough time to let the cylinder blow-down. You must let the residual gas start out the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the 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 earlier.
The next least important timing point is the exhaust valve closing. If it closes early, say around 15 degrees after top dead center (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 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 in 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 sizes, 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 engines. If a 1500 Cortina head does not flow much air above 0.350" valve lift, and it is possible to have then intake valve open that much by the time the piston reaches maximum velocity, WHY DO MOST PEOPLE THINK WANT AT LEAST 0.500" 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 after bottom dead center (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 ultimate power output as well as rpm. Another potential 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 is only 8:1, the engine willnot be able to reach its horsepower potential. This should give you a better understanding 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, 3 - 6000, 5 - 8000, 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 pistons offset is ther? What is the static compression ratio? In the cylinder head, what is the maximum air flow (cubic feet per minute, or CFM) in the intake track with the 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 the air flow of the exhaust versus the intake? What are the valve sizes? What re 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.
Next time we will look at how to choose a cam for the venerable, and still potent, 1600 Lotus Twin Cam motor.
Part 3 -- Lotus Twin Cam Variations
In previous articles, I covered a variety of topics related to the workings of the four-stroke, internal combustion engine. For Part 1, I discussed a little about volumetric efficiency and how it is related to piston velocity, cylinder pressures and how they determine normal or abnormal combustion, pumping losses that occur on the compression stroke as well as the exhaust stroke, overlap period, lobe centers and how to establish camshaft duration in terms of crankshaft degrees. In Part 2, we went deeper into the four strokes by listing the order of importance of each stroke and how it affected engine performance. We talked about compression ratio versus intake valve closing, what RPM range one might choose for their individual application, some information about cylinder head flow and finally some ideas about camshaft decisions when the engine data is known.
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 cubic feet per minute, or CFM) when valve sizes are increased? What about the port velocities (both intake and exhaust)? How are the exhaust flow and intake flows affected? IS BIGGER REALLY BETTER? It has been my experience that when you are dealing with a stock cam, say 250 degree duration, it does indeed help to increase the valve sizes to get more flow through the engine. 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 is about 80% of the intake in your typical engine. Design guidelines developed by the Society of Automotive Engineers (SAE) suggest that the exhaust diameter should be 75-80% of the intake. I prefer to be in the 80-85% range and port the head to achieve about 80% exhaust CFM flow compared to the 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 conditions. However, when a camshaft with more duration is installed in a "hot" street, autocross or road racing engine, a 90-95% Ex/In flow ratio will over scavange 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 Lotus Twin Cam street and racing motors. Valve sizes for various twin cam heads are summarized in the following table:
TYPICAL LOTUS/FORD TWIN CAM VALVE SIZES (inches)
|Brian Hart Engine||1.690||1.44||85|
Where 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 Figure 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 still good past 0.450" lift while the exhaust flow levels off at about 0.400". Finally, the Brian Hart head (Fig. 1c) shows some really deep breathing capabilities! A lot more CFM overall, and great intake flow up to 0.450" lift with the exhaust good to 0.400".
Now, 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 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 and add some duration to the cam. The more duration that you add the more compression you'll need, and that combination will increase the upper midrange and top-end power. It is very important to keep your combinations balanced; for example, you cannot use a 270 degree cam with 8 to 1 compression--9.5 would be a lot better--and conversely you cannot have 10 to 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 restrict the breathing ability of the engine. As a result, it may be difficult to make the idle mechanism work properly due to the 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" 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 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's not much more to say, except, HAPPY TUNING.