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Choosing A Replacement Camshaft
A number of cam suppliers we interviewed for this article say they can make a custom cam for virtually any engine – provided the engine builder provides them with technically accurate specifications as well as detailed information as to what exactly they want the engine to do.
By Larry Carley
Unless you are doing a totally stock rebuild and reusing the original camshaft, selecting a camshaft depends on what kind of engine you are building and how that engine will be used. A stock engine for a daily driver is obviously an entirely different application than an big stroker motor for a Pro Stock racer. So how do you navigate the daunting process of selecting the “best” camshaft for a particular engine?
One approach is to stick with what works. If you’ve used a particular cam grind before that delivers good torque and horsepower for a certain kind of application, you might want to play it safe and stick with a tried-and-true grind that has worked well in the past. But in today’s highly competitive world of professional racing, the hot cam, cylinder head and valvetrain combination that worked well last season may not be the best choice for this season.
Technology is constantly changing, and to stay competitive you have to be on the cutting edge (or not far behind it). New aftermarket cylinder heads are proliferating like crazy, as are cylinder head port configurations. Small blocks are now becoming big blocks with longer and longer stroke crankshafts. Intake manifold manufacturers have had to redesign many of their manifold plenums and runners to flow more air for these stroker motors. Aftermarket engine blocks with larger cylinder bores and bore spacing are adding more and more cubic inches of displacement.
All of these different cylinder head, manifold and engine combinations mean new camshaft profiles have to be developed to fill the gaps not covered by currently available cams. This makes it much more difficult for an engine builder to pick an off-the-shelf cam that will deliver the best possible performance for a given combination of engine parts, gearing and usage. But it is good news for cam suppliers who can create custom camshafts for engine builders.
A number of cam suppliers we interviewed for this article say they can make a custom cam for virtually any engine provided the engine builder provides them with technically accurate specifications as well as detailed information as to what exactly they want the engine to do.
Chase Knight of Crane Cams said the more information he can get from a cam customer, the better. “Detailed information helps us help them pick the best cam for their application,” he said. He stressed the fact that the information the customer gives him has to be as accurate as possible, not guesstimates or approximations or vague statements. “I need to know the exact compression ratio of the engine, the exact ratio of the rocker arms, the rpm range where the customer wants the engine to make the most power, and so on. If the engine is a stroker, have the rods been clearanced to clear the cam?”
Gordon Johnson of Elgin Industries said cam selection depends on what you want to achieve with an engine. “Some customers just want a little more performance from a relatively stock street engine while others are building an all-out performance engine for a particular kind of racing.” Johnson said Elgin Industries has cams for every application from stock on up, including classic OEM grinds for older GM, Ford and Chrysler engines as well as current generation LS1 and other engines. “Our catalog lists all the cam specifications, so a customer can usually find a cam that matches his needs.
Steve Tanzi of Erson Cams (which is owned by PBM Performance Products) said he asks engine builders lots and lots of questions before he recommends a particular cam grind. “After I’ve evaluated the information the customer has given me, I’ll narrow down the potential list of possible profiles to my ‘A’ choice. If we have something on the shelf that matches, we can go with that. Otherwise, I will make them a custom grind.”
Tanzi says he keeps his customer notes for years, and has compiled quite a database of engine specifications and combinations for choosing cams. “Everything matters in selecting a cam. No detail is too small.”
Scott Scovrowski of Howards Cams said the airflow numbers on ported or aftermarket cylinder heads are important to know when picking a cam because the profile of the cam affects port velocity. The cam’s airflow capabilities should closely match those of the cylinder head to produce maximum torque. “If a cam is capable of flowing up to 700 cfm of air, but the heads peak out at 600 cfm, the cam is too big.”
MUCH TO CONSIDER
The variables that must be considered when choosing a cam or having a custom cam ground include all of the following:
Engine Displacement A smaller displacement engine usually needs a shorter duration camshaft for good low end torque and throttle response. A large displacement stroker motor, on the other hand, can usually handle more cam duration without sacrificing low end torque and throttle response.
Bore & Stroke Are you building an over-square engine with a stroke that is longer than the bore diameter, or an under-square engine with bigger bores and a shorter stroke? Long stroke engines develop more torque than short stroke motors, but short stroke motors can typically rev much higher. So for a high revving engine, you want a cam that develops power from maybe 3,500 rpm up to 9,500 rpm. A big stroker motor, on the other hand, may never see the high side of 6,000 rpm, so it would need a cam that works better at lower rpms.
Rod Length Are you building the engine with long rods or short rods? Rod length affects the angularity and torque on the crankshaft as well as piston speed. These factors have to be considered to optimize airflow in the desired rpm range.
Engine RPM Range Are you building a low rpm torquer motor or a high revving engine? The cam grinder has to know where you want maximum torque to occur so he can choose the optimum duration, lift and lobe separation numbers for your cam.
Compression Ratio The higher the static compression ratio, the more duration the engine can handle without going into detonation.
Cylinder Heads Are the heads stock, modified stock or aftermarket? If aftermarket, which brand and model of aftermarket head? Are they off-the-shelf heads or have the ports been reworked or CNC machined? If so, what are the port runner volumes? How big are the intake and exhaust valves? How about the volume of the combustion chambers (open chamber or small chamber)? Are the heads aluminum or cast iron? Aluminum runs cooler and can handle more heat with less risk of detonation. All of these factors influence the lift, duration and overlap that will be ground into the new cam profile.
Valvetrain What is the ratio of the rocker arms? Are they stock or high-lift, and if so what is their exact ratio? This will affect both cam lift and duration at the valves. Are the rockers steel or aluminum? How stiff are the valve springs? Can the springs and pushrods handle the anticipated rpms? The cam has to work with the lifters, pushrods, rocker arms and valve springs to achieve the desired lift, duration and rpm you want to achieve.
What Type of Cam and Lifters? Do you want a flat tappet cam with solid or hydraulic lifters, or do you want a roller cam with flat or hydraulic lifters? If you want a flat tappet, will you be using oversize lifters? Roller cams cost more but provide a significant reduction in internal engine friction, and typically make more torque and horsepower because a roller lifter can handle a much steeper ramp on a cam lobe than a flat bottom lifter. This allows roller cams to open the valves more quickly so they can reach maximum lift earlier in the timing cycle. If you plot valve lift and duration on a graph, the area under the curve of a roller cam with an aggressive lobe profile will be greater than that of a flat lifter cam for the same lift and duration specifications. This makes more power.
Induction System Will the engine be naturally aspirated, boosted with a supercharger or turbocharger (if so, how much boost pressure?), and/or fitted with a power adder (nitrous oxide)? Blown engines typically run better with slightly milder cams that have a wider lobe separation to reduce valve overlap. Does it have a carburetor or more than one carburetor? If so, what’s the cfm rating of the carburetor(s). If the engine is fuel injected, what type is it (multiport or throttle body, original equipment or aftermarket)? How big is the throttle body? What kind of manifold is on the engine (brand, split-plane or single-plane, low rise, high rise, tunnel ram or multi carburetor)? Split-plane manifolds typically produce more low end torque and are better for street engines.
Firing Order Is it stock, or are you swapping the firing order on cylinders #4 and #7 on a big block or small block Chevy for a broader torque curve? Obviously, you’ll need a different cam if the firing order has been changed.
Street, Street/Strip or Race Only If the engine is being built for the street, will it be emissions exempt or will it have to meet emission regulations? If a customer wants you to build him or her a street/strip engine, how much time will it actually be run on the strip? The customer may believe it will be a 50/50 split, but realistically it will likely be more like 95 percent street and 5 percent strip. That affects where you want the engine to develop peak torque for everyday drivability.
Transmission Manual or Automatic? Automatics typically need more low end torque, especially in a heavier vehicle. But this can be affected by the stall speed of the converter. A higher stall speed allows the engine to rev higher before it grabs, but hurts fuel economy in a street driven vehicle. If the engine will go in front of a manual transmission, how many gears does the transmission have, and is the gear spacing wide ratio or close ratio?
Final Gearing The final drive ratio of the differential and the size of the tires will also affect the rpm of the engine as it accelerates and cruises. The engine’s power curve should match the gearing for optimum performance.
Exhaust System If the engine has headers, what is the style, diameter and length of the headers? Does the exhaust system have mufflers or a cross-over H-pipe or X-pipe?
What Does Your Customer Want? Does the customer you are building the engine for want lots of low end torque or high rpm power? Does he want a lumpy idle or a smooth idle? Is he willing to spend more for a custom cam or will an off-the-shelf cam be good enough?
PICKING THE RIGHT GRIND
All of this may seem like a lot of information just to choose a camshaft. But if you want the best possible grind for a particular engine, all of these variables have to be considered when picking a particular grind. It all boils down to picking a cam that will develop peak power within the rpm range where the engine needs to make it.
If you want the engine to make gobs of low end torque, then you need a cam grind with lift, duration and lobe spacing that matches those requirements. On the other hand, if you want the engine to make tons of high end horsepower, you need a cam that provides maximum flow in the higher rpm range.
As you read through the various camshaft manufacturer’s catalogs, you’ll notice that most offer a broad range of different cam grinds for given engine and vehicle applications. The cams may be classified by usage including stock replacement, mileage, low speed torque, towing, marine, mild performance and economy, intermediate performance, high performance, street/strip and racing cams. These off-the-shelf cam grinds generally provide good results provided you follow the cam supplier’s recommendations. If you don’t, you can end up with a bad mismatch that just doesn’t work.
Comp Cams has a neat software program that helps you pick a cam for a particular combination of engine factors. The program is called CamQuest 6, and it can be downloaded from the www.compcams.com Web site for free. Chris Douglas of Comp Cams says this program has been downloaded over 15,000 times since the first of this year. “You just plug in the specifications for your engine, and the program shows you which of our cams provides the best match.”
The program lists the best matches from the Comp catalog, and also includes a dyno simulator function that estimates how much power a particular engine and camshaft combination will produce. Douglas says the dyno simulator software is about 95 percent accurate compared to real world results, and allows a user to play around with many variables.
“The Cam Quest 6 software is great for choosing a cam for a stock or performance engine, but for serious racing applications you should really give us a call.” Douglas says custom cams is a big part of their business, and they can usually grind and ship a custom cam within 48 hours. “Every application is unique, so we try to match the caller with a product expert who knows their particular type of racing best.”
One of the most common mistakes novice engine builders and DIYers alike make is over-camming an engine.
They want the cam with the biggest numbers, never mind the fact that such a cam may be a poor match for the engine or vehicle they are sticking it into. A wild cam in a slightly modified engine may sound great with a loping erratic idle. But if the engine has no low end torque or throttle response and never reaches the rpm range where the cam starts to really work, the engine will be a dog.
UNDERSTANDING DURATION NUMBERS
Duration is one of the factors in cam design that affects where the cam develops peak power. Duration is how long the cam lobe holds a valve open and is specified in degrees of crankshaft rotation. The duration specification will vary depending upon how and where it is measured.
As the cam spins around and a lobe begins to push its lifter up, the valve starts to open. But if the engine has solid lifters, the lash in the valvetrain must first be compressed before the valve starts to open. If duration is measured from the point where the lifter has risen .004˝ above the base circle of the cam lobe to when it comes back down to within .004˝ of base circle, it creates a somewhat inflated value. Some cam manufacturers refer to this as the “advertised duration” because it gives the biggest numbers, and thus appeal to engine builders who subscribe to the “bigger is always better” philosophy of cam selection.
By comparison, the Society of Automotive Engineers (SAE) method for measuring duration says it is to be measured at .006˝ above the base circle for hydraulic cams, and .006˝ plus the specified valve lash for mechanical solid lifter cams.
Another way duration may be specified is to measure it at .050˝. above the base circle of the cam lobe. The .050˝ specs are the ones most commonly cited in aftermarket catalogs.
What does a duration spec tell you about a cam? It reveals something about the rpm range where the cam will make the most power. Generally speaking, the longer the duration the higher the rpm where the cam makes power. Short duration cams are good for low speed torque and throttle response while long duration cams hold the valves open longer for better high speed breathing and top end power.
Cams with durations in the 195 to 210 degree range (measured at .050˝ cam lift) are usually considered best for stock unmodified engines and those with computerized engine controls. Once you go beyond 210 to 220 degrees of duration, intake vacuum starts to drop. This upsets idle quality and affects the operation of computerized engine control systems.
Performance cams typically have durations ranging from 220 up to 280 degrees or more. The longer the duration, the choppier the idle and the higher the cam’s power range on the rpm scale. A cam with a duration of 240 degrees or higher will typically produce the most power from 3,500 rpm to 7,000 rpm.
If you try to compare duration specs between different cams, though, you don’t always get an accurate comparison because duration specs don’t tell you anything about the lobes themselves. Though cams from two different manufacturers may have identical lift and duration specs, the lobes on one cam may be ground differently from those on the other. One cam may have more of a peak shaped lobe while the other has a “fatter” lobe. A “V” shaped lobe will breath differently from a “U” shaped lobe because it doesn’t hold the valve at its maximum opening as long.
One lobe profile may also close the valve more softly than the other, to reduce valve bounce at high speed. Valve float can also be a problem with lobes that change shape abruptly unless valve spring pressure is increased. The profile of the lobes on one cam may also be the same on both the up and down sides of the lobe (which is the norm for most stock and street performance cams) compared to an “asymmetrical” grind (different profiles on each side of the lobe) on the other cam.
The only way to really compare cam grinds, therefore, is to measure and plot lift versus rotation on a graph. This can be done manually with a degree wheel and dial indicator (which is a tedious job), or with an electronic stylus (such as the Cam Doctor) that plots the results on a computer.
OVERLAP AND LOBE SEPARATION
Another spec you need to look at when selecting a cam is the relative timing of the intake and exhaust valves. This can be expressed either as “valve overlap” (the time during which both the intake and exhaust valves are both open) or “lobe separation” (the number of degrees or angle between the centerlines of the intake and exhaust lobes). Decreasing the lobe separation increases overlap, while increasing the separation decreases overlap.
Most stock replacement cams with durations of less than 200 degrees will have lobe separations of 112 to 114 degrees. Higher duration cams for mid-range performance typically have 110 to 112 degrees of lobe separation. With racing cams, you’ll find lobe separations that range from 106 to 108 degrees for more valve overlap.
Overlap occurs when the intake valve starts to open before the exhaust valve has finished closing. Increasing overlap can be a desirable thing in a naturally aspirated high rpm engine because the outgoing exhaust actually helps scavenge the cylinder to draw more air and fuel into the combustion chamber. But too much overlap at low rpm kills low end torque and throttle response by reducing intake vacuum excessively. It can also create idle emission problems by allowing unburned fuel to be drawn through into the exhaust.
For stock, street performance and some racing applications, a cam kit that includes lifters is a good way to go. Some kits also include new, stiffer valve springs for higher revving applications. But for professional racing, most cams are sold outright. This gives the engine builder more flexibility in choosing lifter configurations.
Regardless of what kind of engine you are building, or the application it is going into, always use new lifters with a new camshaft. Reusing old, worn lifters can ruin a new cam.
Be sure to check valve-to-piston clearance, especially with high lift cams and/or high lift rocker arms. Also, make sure the springs do not bind with high lift cams or rockers.
Lubrication is also critical. All cam lobes should be coated with a high pressure assembly lube, not just motor oil (which will run off if the engine sits for any period of time). Also, if the engine has a flat tappet cam and will be used for racing, special racing oil that contains higher levels of zinc than ordinary motor oil may be necessary to prevent premature lobe and lifter wear.
Camshaft break-in after the initial engine start up typically requires running the engine at 2,000 to 2,500 rpm for up to 30 minutes. Don’t let it idle or you may wipe out a lobe.