How to choose a camshaft
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==Overview== | ==Overview== | ||
| − | The camshaft can be thought of as the brain of the engine, and it has a very large effect on the amount of power an engine makes as well as where in the rpm range that power occurs. | + | The camshaft can be thought of as the brain of the engine, and it has a very large effect on the amount of power an engine makes as well as where in the rpm range that power occurs. The main focus of this article will be on a conventional OHV (overhead valve), cam-in-block engine configuration using two valves per cylinder, as shown above. |
| − | The | + | ==The valve train== |
| + | Mechanically speaking, the '''camshaft''' is linked to the crankshaft and turns 1/2 the speed of the crank. As the cam turns, the eccentric-shaped '''cam lobe''' lifts and lowers a cam follower, or '''lifter'''. The lifter is linked to a fulcrum, or '''rocker arm''' by a '''pushrod'''. The rocker arm directs the motion of the cam lobe to the '''valve''', lifting the valve open and closing it shut with the aid of the '''valve spring(s)'''. | ||
| − | This article assumes that | + | The shape of the cam lobe dictate when the valve opens and closes in relation to crankshaft position (aka the '''cam timing'''), and how far the valve is opened (aka the '''valve lift'''), as well as how long the valve is open and closed (aka the '''duration'''). |
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| + | This article assumes that the basics of how and what a camshaft does is understood. If further explanation or a refresher is needed, see [http://www.dragzine.com/news/camshaft-101/ '''Camshaft 101: How do cams work?''']. The article and video will make many of the ideas and terms used in this article much clearer. | ||
For some more advanced information on camshaft operation and definition the specs of a camshaft, see [http://www.carcraft.com/techarticles/ccrp_9812_secrets_of_camshaft_power/index.html '''Secrets of Camshaft Power'''] by Marlan Davis (Car Craft, December, 1998). | For some more advanced information on camshaft operation and definition the specs of a camshaft, see [http://www.carcraft.com/techarticles/ccrp_9812_secrets_of_camshaft_power/index.html '''Secrets of Camshaft Power'''] by Marlan Davis (Car Craft, December, 1998). | ||
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===Lift=== | ===Lift=== | ||
| − | + | The valve lift is found by multiplying the lobe lift by the rocker arm ratio. Lobe lift is how far the lobe of the cam will move the lifter in a linear fashion. Lobe lift is measured by subtracting the base circle diameter from the maximum height of the eccentric (including the base circle). The lobe lift is ground into the cam, however the actual lift seen at the valve can be changed by using a different rocker arm ratio. Read more: [http://www.chevyhiperformance.com/techarticles/148_0307_basic_camshaft_info/index.html '''The Basics Of Lift, Duration, And A Whole Lot More'''] by Jeff Smith (February, 2009 Chevy High Performance). | |
| − | + | ====Port flow==== | |
| + | The amount of lift that can be used by a particular head depends on how much flow the cylinder head ports can deliver and at what valve lift the port flow stops increasing. More lift is generally better, provided the valves, retainers, pushrods and springs are properly matched to the cam profile and rpm the engine will turn, and if the port flow will support the valve lift without the port "stalling" or going into turbulence that keeps the flow from increasing. If head port flow stalls or starts decreasing above a certain lift, there is no reason to try to use more than that amount lift. But more lift is better, up to the point where the heads start losing flow. | ||
| − | Head flow for common domestic head castings can be found [http://users.erols.com/srweiss/tablehdc.htm '''here''']. Heads are flow tested at different valve lifts, and at different amounts of "depression" (usually measured in inches of water or "in/H2O"). When comparing heads, be sure the depression is similar (28 in/H2O is a commonly used depression), or be prepared to convert the results from one depression to another depression, using a calculator. | + | Head flow for common domestic head castings can be found [http://users.erols.com/srweiss/tablehdc.htm '''here''']. Heads are flow tested at different valve lifts, and many times the ports are tested at different amounts of "depression" (usually measured in inches of water or "in/H2O"). The results will be expressed as cubic foot per minute (CFM) of air flow. When comparing heads and their ports, be sure the depression is similar (28 in/H2O is a commonly used depression), or be prepared to convert the results from one depression to another depression, using a calculator. |
| − | Another difference that is often found when comparing | + | Another difference that is often found when comparing flow is the size of the cylinder the head is sitting over (a larger cylinder usually means better flow numbers). Some heads are wet flowed (the air is mixed with a fluid to simulate a working engine) while other heads are dry flowed (just air is used). And yet another difference that may be found is whether an intake manifold is in place during testing (rarely done), or if an exhaust tube is in place (more common), or if clay is used to radius the openings (fairly common). Unfortunately, these differences can make comparing different heads much more difficult. |
===Duration=== | ===Duration=== | ||
| − | This is the amount of time (stated in crankshaft degrees) that the cam will hold the valve open. Advertised duration is greater than duration at | + | This is the amount of time (stated in crankshaft degrees) that the cam will hold the valve open. Advertised duration is greater than duration at 0.050" lift, but the duration at 0.050" lift is more useful when comparing cams or estimating how the cam will perform. |
Some cams have the same duration and lift for the intake and exhaust valves. They are typically called '''single pattern''' cams. Those with different lift/duration numbers for the intake and exhaust are typically called '''split pattern''' or ''dual pattern''. The seat-to-seat duration is ground into the cam and can't be altered without physically changing the camshaft lobe profile, although changing the rocker arm ratio changes the open duration a small amount. | Some cams have the same duration and lift for the intake and exhaust valves. They are typically called '''single pattern''' cams. Those with different lift/duration numbers for the intake and exhaust are typically called '''split pattern''' or ''dual pattern''. The seat-to-seat duration is ground into the cam and can't be altered without physically changing the camshaft lobe profile, although changing the rocker arm ratio changes the open duration a small amount. | ||
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*Hot street performance cam: 0.320" or so lobe lift (0.460" to 0.480” lift w/1.5 rockers); duration @ 0.050" lift around 230° | *Hot street performance cam: 0.320" or so lobe lift (0.460" to 0.480” lift w/1.5 rockers); duration @ 0.050" lift around 230° | ||
*Racy street/strip cam: 0.333" and higher lobe lift (0.500” and higher lift w/1.5 rockers); duration @ 0.050” lift around 232° and higher | *Racy street/strip cam: 0.333" and higher lobe lift (0.500” and higher lift w/1.5 rockers); duration @ 0.050” lift around 232° and higher | ||
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| + | ==Solid vs. hydraulic camshaft== | ||
| + | [[File:Roller L ft R.jpg|thumb|400px|left|Roller cam, left; flat tappet cam, right]] | ||
| + | Telling one from the other visually is relatively easy. The roller cam will have much more rounded lobes, like the big end of an egg. A flat tappet cam will be much more pointed, similar to the small end of an egg. | ||
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| + | Above is a hydraulic roller cam, the rounded lobes are readily appearant compared to the flat tappet cam to the right of it. | ||
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| + | Often roller cams will be made of steel and will be shiny instead of a flat black color of a flat tappet cam caused by the wear treatment it is given. | ||
| + | <br style="clear:both"/> | ||
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| + | {{Warning}} Roller cams cannot use flat tappet lifters, and vice versa. Besides the possible mechanical interference between a flat tappet and a roller cam lobe, the timing events will be skewed much to badly for this to work. A roller lifter on a flat tappet lobe would have very little duration, a flat tappet on a roller lobe would have way too much duration, even if it could work without mechanical interference. | ||
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| + | [[File:Ft vs roller.jpg]] <br style="clear:both"/> | ||
==Things that can "frag" a camshaft and lifters== | ==Things that can "frag" a camshaft and lifters== | ||
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*[[Adjusting hydraulic lifters]] | *[[Adjusting hydraulic lifters]] | ||
*[[How to prep and start a rebuilt engine]] | *[[How to prep and start a rebuilt engine]] | ||
| + | *[[Identifying camshafts]] | ||
[[Category:Engine]] | [[Category:Engine]] | ||
[[Category:Camshaft]] | [[Category:Camshaft]] | ||
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