SBC Camshafts – A primer
Sonix Oct 12 2006 - 9:58pm
SBC Camshafts – A primer
Updated September 16, 2006
By Jay Knopp – “Sonix”
The following is a basic crash course in camshafts. This was designed to clarify some of the terminology behind camshafts, and to help give a basic understanding into the types of camshafts, lifter types, and a brief view on selection.
Definitions of terms
Advertised Duration – This is the time, in degrees, where the lifter is off the base circle of the camshaft. You can also think of it as how long your valve is open. Higher duration means the engine will have its powerband higher up in the RPM range. Different camshaft manufacturers use a different measurement, ie, Comp Cams says that Advertised duration is when the lifter is .006” off the base circle, where Crane cams says .004” etc. This means that if comp and crane both made an identical camshaft, the crane would have a larger advertised duration. This is a poor method of comparing camshafts between brands. Expect a number like 275*. Something like 282*/292* means 282 intake duration, 292 exhaust duration.
Split pattern, aka dual pattern – This is when a camshaft has unequal intake and exhaust lobes. Some cams have a single pattern, ie, 264/264* duration, where a dual pattern would have 264*/274* for example. The dual pattern usually has a longer exhaust duration, and more exhaust lift. This is the make up for the fact that factory cylinder heads, and most aftermarket ones have far less exhaust port flow, so more cam timing is needed to make up for it. Most times a split pattern makes more power on stock heads, or mild aftermarket heads. High end CNC machined race heads would probably benefit from a single pattern.
Duration @ .050” – This is the duration when your lifter is .050” off your base circle of the camshaft. This number is best used to compare camshafts between brands. Expect a number like 220*. A “small”, or stock cam might be in the 202 range, where as a full blown race cam would be in the 260* range.
Ramp rate – An easy way to tell how fast a camshafts ramp rate is to take the advertised duration, and subtract the duration @ .050”. The larger the number, the slower the ramp rate, the more gently the cam lifts the valve. Slower ramp rates usually tend to have higher advertised duration, with less duration @.050”, and less lift. This tends to give the “big cam feel” of large advertised duration, without the big cam power of high lift. These cams are easier on valvetrain, at the cost of performance and mileage. Solid roller cams will have the shortest ramp rate, and old hydraulic flat tappet grinds will have the largest ramp rate.
Here is an illustration showing how roller tappets give more duration at a given lift, or more lift at a given during, depending on how you like to look at it. You can see the steeper ramp rate on the roller lobe.
Power band – This term is somewhat loosely used. Most cam manufacturers will list a “guesstimated power range” with a cam. This is usually assuming many things, such as it having 350 cubic inches, a basic dual plane intake manifold, etc. If you have a larger motor (400cid for example), it “eats up the cam”, hence the powerband is lower. A smaller motor (305 for example) will shift the powerband higher. Other factors effect the powerband as well, so paper specs will be a guess at best. The true way of finding YOUR power band is to dyno test the engine.
LSA – Lobe Seperation angle – This is somewhat complicated, but it’s basically how many degrees of separation there is between the lobes (duh). The shorter the number, the more overlap there is. This overlap has the same effect as having a larger duration cam. A shorter LSA, (say 106), will have a peakier power band, and a rougher idle. A larger LSA (say 114), will have a flatter torque curve, and a smoother idle. The larger LSA is recommended for computer controlled vehicles, like TPI motors, as it makes it easier (or possible) to tune for.
Lift – This is how far the valve gets lifted. The more lift, the more airflow (assuming your heads can support it), so the more power. Rarely, a cam will give “lobe lift”, this is the base lift your cam gives. This might be in the low .3xx” range. Most of the time however, lift is measured with an assumed 1.5 ratio rocker arm. Lift might be .460”. If you go to a 1.6 ratio rocker, you will get 1.6/1.5 or 6.6666% more lift. This artificially increases your duration, but that’s another story. The angle shown in the following picture is the duration of the lobe, since the camshaft spins at half the rate of the crankshaft, this lobe would be a 282* lobe.
Retrofit roller – This is what you call it, when you put a roller cam in an older non roller motor. In order to do this, you need to use a retrofit roller camshaft, and retrofit roller lifters. A retrofit roller camshaft differs from a stock camshaft, in that has no provision for a thrust button. Retrofit roller lifters, do not require a spider in the lifter valley, they are simple linked in pairs, via link bars, which keep them from rotating. They are also longer than stock roller lifters, meaning you can use flat tappet length pushrods. These are extremely expensive, hence the reason why retrofit roller setups are not very common.
Camshaft materials – Camshafts are made of different materials, depending on the style of the cam. A flat tappet cam, will generally be made of cast iron. Roller camshafts, are typically made of cast steel, or can be made from a billet. This is one reason why roller camshafts are more expensive. Pay attention to this when you are selecting your distributor gear, some aftermarket billet roller camshafts require the use of a softer brass distributor gear.
Brands – Without showing any brand loyalty, here is a list of camshaft manufacturers to consider:
Picking the RIGHT cam
CR requirements – The bigger the cam (duration), the more compression ratio it needs to work right. A low CR motor, will be extremely sluggish at low RPMs, when used with a “big” cam. Likewise, a small cam in a high CR motor, will ping like crazy, all the time, due to excessively high dynamic compression ratio. Here is a ROUGH guide:
Gearing requirements – A larger cam, will give you more top end power at the expensive of less low end power. This means you want your car to have “more gear”, to get it rolling, and into the higher RPM sooner. So with a larger cam, you want “steeper” gears. 3.73 gears, match up well with a Compxe274, as an example. The right gear ratio to pick for ¼ mile racing is more complex, as you want your car to run through the traps at peak HP, however for street use, that isn’t a necessary consideration.
Convertor Stall speed – For those of you forsaken with an automatic transmission, the stall speed of your torque convertor is another factor to consider. This goes hand in hand with the abovementioned point of gearing. You want your car to get into it’s powerband sooner, so if your powerband is 2500-6500RPM, a 1700RPM stall speed is inadequate, 2500RPM would be better. True racers can dyno their motor, and help them pick the right torque convertor, the rest of us just guess, or ask a reputable torque convertor manufacturer.
Power brakes – A large cam has another side effect – power brakes. Due to the fact that a larger than stock cam will produce less vacuum at idle, it will give less power to your power brake booster. There is no set in stone size of cam where this happens, but as a ballpark, over 230* @.050” duration in a 350 CID, vacuum will start to get dicey. Tuning skill can go a long way here, some people have trouble with a 225* cam, and others can make a 240* cam work.
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