Building a Strong Street Machine – Part 5: Pontiac Camshafts
- March 16th, 2010
- Posted in Building A Strong Street Machine . Technical Articles
- By D.A.P.A
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I’m always trying to improve the quarter-mile performance of my 1971 455 LeMans wagon, while retaining derivability, a 3.55:1 axle ratio, and a 5,500-rpm shift point. I recently tried several custom-ground cams, and I want to share the results with you.
“Performance”, as used in this article, includes idle quality in the 550 to 700 rpm range, throttle feel, and driving ease, in addition to actual quarter-mile elapsed times and speeds. I’d like to discuss basic camshaft operation, nomenclature, and application information for cams in Pontiac street and street/strip cars as well as the results of the cam changes on the LeMans wagon. Race-engine cam design and usage is significantly different and will be touched on only briefly.
Cams are vitally important to an engine’s performance, and a cam change must be carefully evaluated in order to obtain the expected benefits. Pontiac Engineering developed excellent cams for the various applications they were faced with in the glory days of the first musclecar era. Strong low-end performance will degrade the upper rpm range.
Conversely, good top-end power comes at the expense of low end torque. The Pontiac engineers knew that heavy automatic-trans cars would not perform properly with a longer-duration cam, such as the 9794041 Ram Air IV camshaft, so they were not installed in such cars. They were also aware that lobe separation would determine where in the rpm range optimum power would be developed, and how lobe location would affect idle quality. Accordingly, they selected the best overall combinations of lobe location and duration, and the factory cams perform very well when correctly applied.
A look back at six different cams that Pontiac used in the 1968 engines illustrates Pontiac’s design goals, and will provide a quick overview of the effects of cam timing/duration (see chart, below). Pontiac cams will be referred to by the last three digits of their part numbers.
During the horsepower wars of the first muscle car era, advertised horsepower was not always accurate, due to competition against other manufacturers and also to various racing associations’ rules. This is clear from the horsepower ratings for the last three cams listed in the chart.
A tighter LSA will also move the power range down in rpm and peak the power in a narrower range. Wider LSAs allow the engine to idle better, produce more manifold vacuum at both idle and cruise, give better fuel economy, and produce a wider power band. However, a wider LSA also slightly reduces cylinder pressure, and consequently the engine may produce less peak torque. Since Pontiacs were fairly heavy cars and most were delivered with automatic transmissions, adequate lower-rpm response and a wider power band were considered more important than peak torque. Therefore Pontiac used wide LSAs on all factory cams. Also, remember that peak horsepower occurs for only a very short time in the upper rpm range, and it has very little effect on the overall performance of a vehicle that uses the total rpm range, from idle up. However, peak horsepower is important for a race engine that is operated in a narrow rpm range that’s close to the peak horsepower point.
The chart below clearly illustrates that cam design is a compromise. If a cam design includes strong low-rpm power, the cam will not work as well at higher rpms. If it is designed for great high-rpm power, a very large hole will be left in the low-rpm power range. Remember, an automatic-equipped car with a factory-style cam always has to pass through the low rpm range, so a lack of power down low is crippling to real street performance. That is why Pontiac (and most other car makers) almost always installed a shorter-duration cam in automatic-trans performance cars as compared with their manual-trans counterparts.
|Cam No.||Duration Advert. .050||hp/rpm||Torque/rpm||Overlap||LSA||Intake Centerline|
|Notes: The 254 cam has .374″ lift on intake, .407″ on exhaust. The 041 cam has .407″ lift on both intake and exhaust (.313 lobe height). The other camshafts listed have .407″ lift on both intake and exhaust. These lifts are measured with a 1.5:1 rocker arm ratio. Intake centerlines indicate degrees of factory cam advance, usually 1 to 3 degrees. It should also be noted that during the horsepower wars of the first muscle car era, advertised horsepower was not always accurate, due both to competition from other manufacturers and from various racing association rules. This is apparent by looking at the horsepower ratings for the last three cams listed.|
The Camshaft Comparison
The test vehicle used in this series is a 1971 LeMans wagon powered by a relatively mild, moderate-rpm 455 running a Turbo 400 automatic with governor-controlled shifts (5,500 rpm); Edelbrock Performer RPM intake manifold; 800-cfm Rochester Quadrajet; 041 Pontiac-grind camshaft with Rhoads variable lifters and 1.65:1 rocker arms; owner-ported 1972 7K3 D-port heads with 9.7:1 compression ratio; HEI; custom-built Continental converter that flashes to approximately 2,700 rpm; stock rods and crankshaft; and a 3.55:1 axle ratio.
The vehicle has made an average of 200 runs (both quarter- and eighth mile) each year for the past seven years. It runs in the 12.35 to 12.60-second range at 107 to 110 mph (7.90/88 mph in the eighth-mile), with 60 foot times in the 1.80-second range. All parameters are dependent on weather and track conditions (local drag-strip altitudes are 800 to 1,000 feet above sea level). Because of the excellent weight transfer and traction of the wagon, the 60 foot times remain relatively consistent, which provides reliable test results.
The drivetrain makeup is the result of countless tests at the drag strip to determine the best performance combination of engine and chassis components, while maintaining good street manners. This combination idles in Drive smoothly at 570 rpm, with more than 14 inches of vacuum when warm, so the car is very easy to drive on the street.
Upon initial installation of a 455 in the wagon in 1986, an 067 cam was installed. At that time, stock heads and intake manifold were used, and several other engine components or accessories were slightly different. However, when a different camshaft was installed, only the cam was changed, so the following indicated differences in performance are a result only of the different cams. The wagon has undergone a number of evolutionary changes to improve its performance from the mid-13s to the present mid-12s, but only the differences attributable to camshafts will be discussed here. However, all relevant changes that significantly affect cam operation will be included in our discussion.
|Cam Terms and Definitions
The time in crankshaft degrees that the valve is open. Pontiac measured their advertised duration at different points on liver rise and fall. Most members of the cam industry now use a lifter rise of .050 inch as a standard reference to measure the total duration. The advertised duration of both Pontiac and aftermarket cams is somewhat meaningless because neither are/were based on a common standard.
The actual lift of the cam in inches at the maximum cam lobe height. Valve lift is derived by multiplying the cam lift by the rocker arm ratio.
The point(s) in crankshaft degrees that the valve(s) actually open and close.
Lobe Separation Angle (LSA)
The angular displacement between the centerline of the intake lobe and the centerline of the companion exhaust lobe. LSA is expressed in camshaft decrees.
The period of time in crankshaft decrees that both the intake and exhaust valves are open. Again, the overlap at .050 duration is significantly different than that at the advertised duration.
Factory Camshaft Comparison
The 041 cam, 1.65:1 rockers, and Rhoads variable lifters is the combination I’ve used over the past several years, and I used it during earlier testing of ported heads. In all cases, the Pontiac cams used for the controlled testing were the original Pontiac design, and not computer-enhanced versions.
Due to the rpm limitations (5,500) of the 455 engine’s stock internal parts, and my desire to retain comfortable street driving, any new cam I tried would have to provide a smooth idle at 560-580 rpm in Drive with at least 12 inches of vacuum, and develop strong power from idle to 5,500 with the 3.55 gears in this 4,000+ lb. vehicle. The Rhoads lifters were used in all cam tests in order to maintain the 500 to 600-rpm idle capability with adequate vacuum to operate the power brakes.
The factory-style torque converter was replaced several years ago with a custom-made unit. The new converter “flashes” to about 2,700 rpm before the car moves when the engine is accelerated to wide-open throttle from a stop. The factory-style units would flash to about 2,000 rpm on the heavy wagon with the high torque 455. Flash rpm will normally be higher than the “stall” of a given converter, because the engine will continue to accelerate as it “yanks” the converter to the point at which the car has to move. As the converter action has a tremendous effect on the load seen by the engine, cam operation is significantly affected by the converter used, and the effect on the wagon will be discussed later.
This article is not intended to be a test of cams. Rather, it is an attempt to discover the effects of different cam characteristics on the wagon. For this reason, the manufacturers of the custom cams will not be listed. In fact, cam operation is dependent on the technical specifications of duration, lobe location, lift, and timing. If identical specifications were obtained from each manufacturer, they would all perform similarly That is not to say that one manufacturer’s popular cam won’t perform better than a competitor’s, but if it does, the improvement would be attributable to different technical specifications more suitable to the application.
The first special cam (cam #1) was specified with a duration similar to the Pontiac 041, but with a tighter lobe separation of 110°, vs. the 113.5° of the 041. It measured 232° on the intake lobe with the lobe positioned at 104° (the 041 intake is 231° at 112°) and 239° with the exhaust lobe at 116° (the 041 is 240° at 115°). All durations are specified at a.050-inch lift.
The duration numbers were selected to be as close to the 041 as possible, so that any difference in performance would be the result of lobe separation rather than duration changes. Tighter lobe separation generally moves the power range down and tends to peak the horsepower into a narrower range. This becomes more pronounced on cams that measure 230° or more of intake duration. However, since tighter lobe separation increases overlap, idle quality will be degraded.
The first cam was installed with the intake centerline at 104°. This cam idled at the identical rpm, with the vacuum about the same, but it was slightly rougher at idle. Low-speed throttle feel was good, and at the drag strip it ran the same E.T. but lost 2 mph. The 60-foot times were slightly quicker. Several trips were made to the strip, but the 2 mph was gone.
In an attempt to move the power back up in the rpm range, the cam was retarded 3° (intake CL at 107°). This proved unsuccessful, as the E.T. deteriorated 0.1 sec. in addition to the lost 2 mph. The 041 was reinstalled, and the normal performance returned. l should note that performance with cam #1 was very good, and only a direct comparison test, such as conducted with the wagon, would prove that the 041 with Rhoads lifters is clearly a better performer than this typical non-Pontiac-grind cam with Rhoads lifters.
It was beginning to appear that Pontiac Engineering might actually have known what they were doing when the 041 was designed. Cam #2 was specified with the lobes at identical positions to the 041, with the intake at 112° and the exhaust at 115°. Increased intake duration of 236° was specified, with 240° exhaust. It was installed with the intake CL at 112°. These specifications were selected to determine if longer intake duration would help the engine from idle to 5,500 rpm. As the lobe positions were identical to the 041, any performance changes would be due to the increased duration. The idle of cam #2 was noticeably rougher, although it idled reasonably well at 600 rpm in Drive with 12 Inches of vacuum. Some low-speed throttle feel was lost.
At the strip, it ran the same E.T. and mph as the 041, but lost some 60-foot performance. Two trips were made to the strip, and tests were made with and without mufflers, but the quarter-mile performance was identical to the 041. As a last attempt to find some performance gain, 1.5 rockers were installed on the exhaust only (the 1.65s were left on the intake side) to ensure that the exhaust system was not over-scavenging the intake. This netted a 0.1-second loss of performance and little change in mph.
Cam #2 was subsequently removed and the 041 was reinstalled with the Rhoads lifters and 1.65:1 rockers. The quarter-mile performance with this longer-duration cam was comparable to that provided by the 041, but both the idle quality and derivability deteriorated.
What about lift? It appears that lift of around .500 inch is adequate for streetable engines running to the 5,500-rpm range. Cams #1 and 2 provided total lift of about .540 inch as compared to the 041′s rated .517 inch, but neither improved performance. We know that higher lift will theoretically flow more air earlier in the lift cycle, but we also know that a 5,500 rpm 455 engine is inducting as much air as it needs with the lower lift values. According to cam/engine design theory, the 455 would need vastly improved airflow and increased rpm capability to benefit from lifts of .550 inch and up.
At the end of the racing season, it was decided that it was time to retire the old cast-pistoned 455 short block. It had run more than 1,500 timed runs and about 8,000 street miles in the wagon since 1986, and an unrecorded number of drag strip runs and street miles in a 1977 Can Am. It had run 12.40s on nitrous oxide prior to its installation in the wagon.
Another junkyard 2-bolt-main block was bored .060 inch and deck-leveled. New forged pistons and factory-style moly rings were installed with the original cast factory rods and crank. The rods were checked and resized with new Pontiac rod bolts installed. The same ported 7K3 heads, Performer RPM intake, and reworked 13/4″ x 3″ headers were reinstalled. The compression ratio now calculates to 10.1:1. Both the 041 cam and the #2 cam with 236° of intake have been run in the new engine in order to develop a new baseline. Again, the two cams provided similar performance, but the car gained about .I second and I mph in the quarter-mile as a result of the new engine.
|About Rhoads Lifters
Variable or leakdown lifters are designed to lessen the cam duration and lift at low engine speeds by allowing more oil leakage within the lifter body while the lifter is under valve spring pressure (on the cam lobe). As the leakage is a function of time, the lifters leak the most at low rpm, because there is a relatively long time during the lift cycles. This allows the Rhoads lifter, with its machined leakdown groove in the body, to shorten the lobe duration by 10 to 15 degrees and reduce the lift at low rpm.
As the engine speed increases and the cycle time becomes shorter, less leakage occurs. At 3,000 to 4,000 rpm, the actual leakage and the effect on cam timing become inconsequential. The dramatic increase in vacuum and low-end torque provided by these lifters more than offsets the slight loss of upper-end horsepower in a heavy, automatic-equipped vehicle, such as my wagon. The overall result is the ability to use a fairly radical camshaft to maximize mid-range and top end power while developing strong lowed torque.
Additionally, a variable lifter will operate to a much higher rpm before it begins to hold the valve open due to lifter pump-up. The weight/viscosity of the engine oil used will have a significant effect on leakdown lifters. The thicker the oil, the less pronounced the leakdown eject becomes.
It is neither necessary nor advisable to use Rhoads lifters with a streetable cam such as the 068, because the cylinder pressure may rise too high at low rpm, causing detonation or pre-ignition. There are benefits and disadvantages in all types of variable fibers, but the Rhoads units provide the best overall performance in my wagon.
For the third cam test, a racing grind was tried. Cam #3 featured approximately 248° of intake and 255° exhaust with the lobes at 104° intake and 116° exhaust, for a 110° separation. The valve lift was approximately .540 inch. It was obvious that this was too much cam for the wagon as it is presently set up, but we wanted to feel first-hand what this type of cam does to real street performance.
With the Rhoads lifters, the engine would idle at 700 rpm, but at only about 10 inches of vacuum. In drive, the rpm dropped to 640, with 9 inches of vacuum. Although the idle was sustainable, the engine rolled and ran quite roughly in Drive and Park. This amount of vacuum is inadequate for normal power-brake operation.
During initial test drives, the transmission shift from second to third was much rougher, indicating a significant horsepower gain in the 3,500-4,000 rpm range. The shift was so severe that the valve-body spacer plate had to be modified to soften the shift enough to prevent drivetrain damage. At the track, the car was slower to 60 feet, but it ran the same E.T. with a slight gain of 0.2 mph. So what happened to the significant horsepower gain this cam obviously provided?
The horsepower increase was obtained at the expense of low-end torque, and when the power was averaged out through the three gears and the length of the strip, the engine developed about the same amount of usable power as the 041 and the other two custom units. However, this long-duration cam was totally unsuited for real-world street driving. In addition to the rough idle and low vacuum, it would not run smoothly at constant speeds until about 2,000 rpm. The #3 cam caused the engine to be more sensitive to poor air quality (higher temperature and humidity with lower atmospheric pressure), which resulted in proportionately greater performance loss than the 041 cam in poor weather.
The cam was advanced 4° (intake CL at 100°) in order to lower the power range. It idled slightly better with a gain of about 1.5 inches of vacuum in Park and Drive. At the track, however, performance suffered by.15 sec. and 1.5 mph.
We have received input from other Pontiac racers across the country that their cars run quicker with this type of cam, albeit with substantial losses in streetability. The cam manufacturers generally recommend a minimum compression ratio of 10.5:1, a 3,500-rpm stall-speed converter, and a lighter vehicle for this type of cam. Additionally, the carburetor must be optimized for such applications. Obviously, my wagon fails on all of these requirements. However, it must be emphasized that the wagon ran very well with this cam, but not any quicker than the much milder and infinitely more streetable 041 cam with Rhoads lifters.
The 041 was reinstalled, and the car immediately ran the same E.T.s and mph as the third test cam, but with a smooth idle and 15 inches of vacuum. In the cool and dry spring weather, with atmospheric pressure at 30-plus inches, the 041 and #2 and #3 cams ran in the 12.20s at more than 111 mph. This level of performance will not be retained in hot and humid summer weather, but will likely slow to 12.40s at 110+ mph.
A final test was performed to determine the effect of the Rhoads lifters when used with the higher-flash-point converter. Standard hydraulic lifters were installed with the 041 cam. The idle was extremely rough, with vacuum about 9 inches at 700 rpm in Park and falling to 7 inches at 600 rpm in Drive.
At the track, performance dropped to the tune of .3 seconds and .5 mph in comparison to the same cam with Rhoads lifters. The change to the custom converter, which allows the engine to accelerate to the 2,700 rpm flash point before it sees a significant load, replaced some of the gains originally found with the variable lifters from idle to 3,000 rpm. However, the Rhoads lifters were still beneficial for performance, and are almost mandatory for the 041 cam in a streetable, automatic transmission car.
What did we learn from this exercise? Cam #1, with its advanced timing and relatively short duration, moved the power range to a lower rpm and cut off some of the upper-rpm power, thus losing mph. Cam #2, with its longer intake duration, moved the power range up. This caused a loss of low-speed power, which degraded the idle and low-speed throttle feel, with no improvement in quarter-mile E.T. or speed. Cam #3 is designed for a lighter car with a looser converter, to allow the engine to reach the upper rpm range before encountering a load. The heavy wagon, with its relatively tight street-type converter, applies the load at much too low an rpm level. Even though the cam is developing more horsepower in the upper ranges, the overall quarter-mile performance is not improved.
Based on the feel of the car, the known effects of cam timing, and the impact on quarter-mile performance, it is likely that all three of these cams increased peak horsepower, which would have looked encouraging on a dyno test. However, all of them affected the power range, and as we have said numerous times, the range of power and the level of average power in that range is far more important than peak horsepower.
The 041 cam was designed for the highest level of power available from about 1,500 to about 5,800 rpm, which was the planned operating range of the 400 engines, for which this cam was originally designed. In our performance tests, the broad power range of this cam is obvious.
Clearly, choosing the right camshaft is not a simple matter. Vehicle weight, gear ratio, engine operational parameters—as well as the owner’s desired characteristics of idle quality, sound, and throttle response—are important factors when selecting a camshaft. The planned rpm ranges to be used must be considered, and the torque capability and torque range of the engine in question must also be factored in, as should the type of transmission torque converter used. Finally, the Rhoads variable lifters have an extremely beneficial effect on the low-rpm operation of the 455 engine and 041 cam.
Would the three custom cams have run better without the Rhoads lifters? On a dyno, more peak horsepower would probably be reflected, but at the strip, all three would have fared worse. Furthermore, all three would have been difficult, if not impossible, to run at normal idle speeds. We have found that automatic equipped, streetable cars will begin to lose adequate idle capability and some quarter-mile performance if the intake duration is increased beyond 230° (at .050inch lift) without the use of Rhoads lifters.
No, I didn’t find a “magic” cam for my Pontiac, but this limited testing indicated that the Pontiac 041 with Rhoads variable lifters and 1.65:1 rockers is very hard to beat in an automatic car with a 455. For great street driving with some drag-strip work the 068 with 1.65:1 rockers provides outstanding performance with automatic equipped 455s, including air-conditioned vehicles. The 041 cam is suitable for both the 400- and 455-equipped cars with 4speed transmissions, with or without the Rhoads lifters. The 400 automatics perform well with the 068 and 1.5:1 rockers. The Pontiac specifications for both duration and lobe placement should be used as the starting point for cam selections in both street and street/strip cars.
Both the 400 and 455 respond well to valve lifts of up to about .460″. Minor changes in lobe location and/or cam timing can compensate for unusual aspects of your engine/car combinations (such as excess compression or some need to narrow the power range of the engine), but radical departures in duration and/or cam timing from the Pontiac designs will be a compromise, and performance gained in one rpm area may be lost in another. As mentioned previously, all of the custom cams provided good performance at the strip, but in direct comparison, the 041 provided as good or better quarter-mile performance along with superior idle characteristics (with the Rhoads lifters).
It must be emphasized that this series of tests and the related comments are directed toward a 4,000-pound car that idles smoothly in Drive at 560 rpm. The relatively tight converter and 3.55:1 gear ratio allows extended driving at 65 mph with no overheating and yielding reasonable gas mileage. Almost any grind of cam can be made to work and even perform reasonably well if sacrifices are made in some or all of these parameters.
Well-informed suppliers who specialize in Pontiacs, such as the regular advertisers in Pontiac Enthusiast, should be able to assist you with special cam purchases for your street and street/strip machines.
It should be obvious that track testing cannot be controlled exactly, and it is impossible to repeat to 0.1 second or 0.1 mph on different days, or in different weather conditions. However, significant changes in performance are quickly detected in this very repeatable and consistent wagon, and none of the cams tested showed any substantial performance change other than the 2 mph lost by the first cam, and the significant loss when the #3 cam was advanced 4 degrees. It should also be stressed that dyno tests would have shown obvious horsepower and torque differences in the four cams. However, it is extremely difficult to correlate on-the-road and strip performance with dyno numbers without specific knowledge of the car’s weight, transmission characteristics, gears, tires, etc., and extensive experience in such dual testing.
It’s also impossible to determine how the cams will idle or operate in actual street conditions with a dyno.
We found this series of tests to be somewhat eye-opening with regard to how well the 041 cam and Rhoads lifters performed in the 600 to 5,500 rpm range in comparison to the non-Pontiac cams tested. We strongly recommend to the reader to give the old faithful 041 a chance to prove its overall performance in your street/strip machine before spending money on alternate cams.
|The effect of 1.65:1 Rocker Arms on Duration
We all know that 1.65:1 rocker arms provide 10 percent more lift than 1.5:1 arms, but how do they affect the cam durations. Technically, the 1.65 rockers do not change the duration of the cam lobes. However, the 1.65 rockers cause the valve to reach the .075-inch (.05-inch x 1.5 rocker arm ratio) lift point 1.5 to 2 cam degrees earlier than with the 1.5 rockers. Therefore, the use of 1.65:1 rockers will make a cam of about 230° (measured at .050 inch lift) act about 3° to 4° greater in duration, depending on the lobe design. This duration difference must be considered when attempting to duplicate the performance of a cam using 1.65 rockers with a different cam and 1.5 rockers.
Cams for Race Engines
The 041 cam is an excellent performer in a milder race engine, but there are many more options in cam design and selection for race-only applications. This is due simply to the fact that low-end power is not as critical. We run a solid-lifter cam in our 3,250-lb. 1967 LeMans with a stock-block 455 with 2-bolt mains, D-port heads, and alcohol. It runs high 10s to low 11s at 118 to 120 mph, while shifting at 5,500 rpm. This cam is a 246/246°-duration unit, with intake at 103° and exhaust at 113° with both sets of lobes lifting the valves .518 inch with 1.5:1 rocker arms. The actual strip performance is similar to that provided by the 041, but the totally different cam lobe locations on the non-Pontiac cam reduce cranking compression, making it much easier for the starter to crank this high-compression motor.
For more information on cams for race cars, contact the folks who race Pontiacs successfully and find out what works with their vehicles. A number of regular advertisers in Pontiac Enthusiast service and race Pontiacs exclusively, and they should be able to help you. However, do not make the mistake of confusing race engines with street engines. In most cases, an automatic-equipped street/strip Pontiac will lose performance if a race-only combination is used, due to the lack of low-end torque of such an engine.
The original Part 5 of this series was prepared a number of years ago. Since that time, we have learned more about Pontiac cams, and a comprehensive article on the subject was prepared for the Fall, ’94 issue of “Pontiac Enthusiast” magazine. The complete article (“Pontiac Cams: Is There A Magic One?”) has been duplicated with the magazine’s permission, and is furnished in place of the earlier Part 5. The article may be reprinted, but please give credit to the “Pontiac Enthusiast”.