Pontiac Cams - Shake or Roll?

By: Jim Hand

Written for the 

1997 Pontiac Southern Nationals

This section is a recreation of the technical seminar that Jim Hand presented to Pontiac enthusiasts at the 1997 Pontiac Southern Nationals in Dallas Texas. There is a lot of technical data presented below, so please take your time in reading. Jim Hand has become a regular at the Pontiac Southern Nationals, and I, the members of the Dallas Area Pontiac Association, as well as Pontiac fans across the country, greatly respect and admire him for the knowledge he has and is willing to share with us.

Pontiac Cams - Shake or Roll?

What kind of title is that? It is very easy to make a Pontiac shake - Misadjust the carb - Screw up the timing - Install very cold spark plugs - or install a radical cam. However, to make a street Pontiac really roll requires some knowledge and experience in cam selection. That is what we are going to cover today. Some things to consider when selecting a cam for best street and strip performance. The definitions and discussions about cams will also apply to race cams. Some technical aspects of cam design will be discussed, but primarily, we will talk about the effects of various cam parameters on engine performance.

We will begin with a short video of two vehicles. The first vehicle is a '75 Grand Ville convertible that the owner has had customized. He picked the paint colors! The second vehicle is my '71 LeMans wagon.

My son Joe did all under hood work (except pick the paint colors), and installed the Ford 3.50 rear axle and one piece driveshaft. Joe and I built the 469 engine and installed it and the aluminum radiator. The second vehicle is my '71 LeMans wagon. The engines are very similar with identical cams, RPM manifolds, and 750 Q-Jets. Both run on 92 octane pump gas, and both idle well at 650-700 in Drive with about 14" vacuum. The Grand Ville weighs 4800+ with driver, fully operational AC, PB, and PS, and was run with all belts tight and air filter element installed. The wagon weighs 4100# with driver. The Grand Ville is an excellent example of what we mean when referring to a streetable car with excellent throttle response and smooth idle with great performance. The wagon is more oriented towards strip work, but also is very easy to drive with smooth idle. During our discussion today, a reference to idle means engine RPM in the 500 to 800 range. When we talk about low end power, we mean power in the range of 1500 to 3000. Mid range is 2500 to 4000, and top end means 4000 and up.

1.    We will start with a quick look ( SLIDE #1 below ) at a computer prediction of relative performance of two cams. The computer program is "Engine Analyzer" from Performance Trends. The base engine is my 462 9.8cr unit with mildly ported '71 #96 heads. The two cams are:

1. A Pontiac 744 with 1.65 rockers. It has 224 degrees intake and 236 degrees exhaust duration at .050 tappet lift. With the 1.65 rockers, it has about .450 lift.

2. The second cam is a Crane 3/4 Race unit with 226 degrees intake and 236 degrees exhaust at .050 tappet lift. It has about .469 lift.

Although these two cams are very similar in duration and lift, the predicted performance is vastly different. What is causing such a variance in performance? Please remember that the computer generated torque and HP numbers are a prediction, and not actual numbers. I will be using several computer generated predictions to review the effects of cam parameters. Do not get hung up with any computer predictions, regardless of the program type. These programs are developed using the most common engines, and Pontiacs are not one of the most common units. Computer programs can indicate trends in performance, and that is how they should be used. This particular program recognizes air flow only at .525 valve lift for the 2.11 valve, so it is more oriented towards peak HP. It does predict the peak power of my 462 reasonably close, based on drag strip performance of my wagon. We will discuss this plot in more detail later, but first, let's talk about basic engine operation and cam parameters.

SLIDE #1

Basic Operation:

This slide illustrates the four cycles of a normal engine.

FOUR STROKE OPERATION

           

                       

1 Power Stroke

2 Exhaust Stroke

3 Intake Stroke

4 Compression Stroke

 

 

SLIDE #3

This slide above shows a typical exhaust and intake valve cycle for one cylinder cycle.

1. Duration; The length of time in crank degrees the valve is off the seat.

2. Advertised duration: Who knows? Measured at .004 tappet lift, .006, .009? Unable to compare one cam to another.

3. Duration at .050 tappet lift. An industry standard that allows direct duration comparison from one cam to another. The lobe design may be slightly different and provide a steeper opening and/or closing curve, but using .050 tappet lift to compare duration is the best method.

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1. Lobe Lift; The actual lift caused by the cam lobe from the base circle to the lobe center.

2. Valve lift; The lobe lift multiplied by the rocker arm ratio.

3. The valve lift generally controls peak torque. It will also affect the RPM range if extremely high or low. Our street engines will respond well to a valve lift up to about .480. More does not hurt engine output, but doesn't seem to help.

4. Rocker arm ratio has an impact on lift as well as duration. Within a given duration, peak lift is limited due to the required dynamics of the lifter, lobe, springs, and valves. The lifter must be accelerated as rapidly as possible, yet slowed enough to follow the lobe nose and closing side ramp. A high lift lobe with aggressive lift and closing ramps can be very hard on valve train reliability. That is why Pontiac used 1.65 ratio rockers with the 041 rather than redesigning the cam. More lift and quicker lift rate can be obtained with the same lobe profile, lifters, and similar pressure valve springs. ( see SLIDE #4 below )

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Lobe Centerlines.                            ( reference SLIDE #3 above )

The lobe centerline is that point in reference to the crankshaft that the lobe reaches maximum lift. Note that the intake centerline will always be after TDC, and the exhaust centerline will always be before TDC.

If we advance the cam 4 crankshaft degrees, the intake centerline moves from 106 to 102 and the exhaust centerline moves from 114 to 118. However, the distance between the two lobes does not change. That distance between the two lobes is known by either of two names: The cam centerline, or as we prefer, the lobe separation angle. When we reposition the cam by advance or retard, the lobes advance or retard, but the separation angle remains the same. To determine the separation angle, add the crankshaft degrees positions of the two lobes and divide by two. The result is separation angle in camshaft degrees. This is the only cam parameter that is stated in cam degrees. The separation angle is ground into the cam and cannot be changed. The lobe positions determine the cam advance or retard as ground. For example, to determine the advance/retard of this cam, we subtract the intake CL from the exhaust CL and divide by two. Advance/Retard is a reference to the positions of the intake and exhaust valves in reference to TDC. This cam is ground 4 degrees advance. If we advance the cam 4 degrees, we now have a total of 8 degrees advance in reference to TDC. What would be the advance/retard of a Pontiac 041 cam with the intake at 112 and the exhaust at 115? (1.5 A) How about a Pontiac 066 with 107 intake and 116 exhaust? (4.5 A). What would be the lobe separation of an 066? (112.5) The lobe centerlines and the lobe separation are extremely important as we will show later, and you should become familiar with cam specs and how to determine both of these parameters.

Timing Events:

( reference SLIDE #3 above )

Exhaust Opening.

The exhaust valve is opened a considerable time before the end of the power stroke. Much of the exhaust leaves the chamber early in the cycle during the initial "Blow Down". Very little exhaust is moved during the last portion of the stroke due to the crank/rod angle. Extending the exhaust further into the power stroke can help mid and upper RPM power, but will degrade lower RPM output. An early opening allows some of the burning exhaust to escape out the pipes and causes that distinctive exhaust sound associated with HI PO engines. Pontiacs are not as sensitive to this point as some other engines. However, Mac McKellar reports that they determined that a longer duration exhaust event improved both torque and HP within the desired RPM range.

Must be optimized to obtain the best cylinder filling (also known as volumetric efficiency) at the desired engine RPM range. Too early an opening will lower manifold vacuum, idle quality and low speed torque because we are further into the exhaust stroke of the engine. If the event is too late, poor cylinder filling will occur at all engine RPM's because the intake charge is not started earlier enough in the cycle.

This point seriously affects idle quality, because it occurs at the beginning of the intake event. If it is late, the initial intake flow will be blown out the exhaust. The exhaust closing event affects the torque curve peaking speed due to the scavenging effect of the exhaust gases on the incoming intake charge. If too early, more of the exhaust gas will remain, and will pollute the intake charge.

Most authorities feel this is the most important event of the four, because it tends to determine the engine RPM at which the maximum torque and HP occurs. Remember that this event occurs within the beginning of the compression stroke. The intake charge has developed considerable inertia, and hopefully is roaring into the chamber, so we can leave the intake valve open longer. At some point however, the piston can begin to shove some of the intake charge back out of the chamber, or cause turbulence back up the intake tract and carb. If the intake is closed to early, the cylinder filling will be poor, especially at higher RPM. We can see why higher compression is required with later closing intakes. The dynamic CR drops as the intake closes later in the compression stroke. (7.5 actual on most good engines).

There is no specifically good or bad timing for each of the four events. They all are important, and will effect the RPM range of the cam, the idle and low RPM power, and the points within the RPM range at which torque and HP will peak. We do not have enough time to go into detail on these timing points. Instead, we will discuss the broader effects of duration, lobe centers, and lobe separation angles.

The duration of the intake event is all important in establishing the RPM operating range of the engine. As RPM increases, there is less time for the carb, manifold port, and head intake port to fill the cylinder. By extending the intake duration, more fill time is available at higher RPM. However, high RPM HP is not automatically increased unless the intake and exhaust systems can effectively provide/handle the increased volume of air/fuel and exhaust. As previously noted, opening the intake valve earlier will always adversely affect idle and low speed operation, thus changing the low RPM operation. So longer duration will always hurt low end power, but may help high end power. The exhaust duration is not as critical, except as it infringes into the overlap phase and affects idle/low RPM power. Max power and torque can be developed with a wider range of exhaust duration values. It is impossible to totally predict engine operation based on cam duration, unless the lobe positions are also known.

These are the most overlooked parameters in cam selection and operation. The position of the lobes in reference to the crankshaft have a dramatic effect on cam operation.

Lobe center is that point at which the lifter is at maximum lift. It occurs very close to midway between open and close points, but most modern cam lobes are asymmetrical - The opening ramps are steeper than the closing side, and the center may occur slightly before midway. However, for general reference, half way between opening and closing will be very close. If a given lobe is extended in duration, it's opening, closing, and/or center may change. This will probably change the lobe separation. Add in changes to the exhaust lobe and it is apparent why there are so many variations of cam timing. It is possible to extend duration without changing the lobe separation by adding timing on the opening or the closing sides of both lobes.

So what happens when we change lobe separation?. Let's go back to the first slide.

( see SLIDE #1 above )

These two cams were selected because the familiar parameters of duration and lift are very similar. However, the lobe timing is vastly different! The Pontiac cam has a lobe separation of 115.5, and the Crane cam has 106. The Pontiac 744 has the intake lobe at 113, but the Crane is at 101. This computer prediction shows that peak torque and HP will be developed much earlier on the Crane tight lobe unit. It also predicts higher torque levels for the Crane, but notice that both torque and HP drop very early in comparison to the 744. Look at Slide 5 for relative positions of the lobes.

There are somewhat different interpretations among cam makers about the effect of lobe separation.

Crane states the followingCrane states the following: " Lobe separation determines where peak torque will occur within the engine's power range. Tight lobe separations (such as 106) cause the peak torque to build early in basic RPM range of the cam. The torque will be concentrated, build quickly, and peak out. Broader lobe separations (such as 112) allows the torque to be spread over a broader portion of the basic RPM range and shows better power through the upper RPM." That is exactly what the computer predicted with the two cams.

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Last Updated: 09/26/04