SBC Camshafts – A primer

Sonix  Oct 12 2006 - 9:58pm   

SBC Camshafts – A primer

Updated September 16, 2006

By Jay Knopp – “Sonix”

for http://www.thirdgen.org

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.

Types:

  • Hydraulic Flat tappet (aka hydraulic lifter cam)

    • This is the factory style for SBC’s from the dawn of time, to ~1987. Good performance can be achieved, and there is little to no maintenance required. Limitations are that valve float can occur, due to “lifter pump up”, limiting your max RPM to around 6500RPM. The tappets, aka lifters, simply have flat bottoms, this limits the camshafts ramp rates. Flat tappet

    • Cost is roughly $120 for the camshaft, $70 for lifters.

    • Here is a picture of a flat tappet cam and lifters in operation. You can see the convexity of the bottom of the lifters, greatly exaggerated for clarity.

  • Solid flat tappet (aka mechanical flat tappet)

    • This is the same idea as the hydraulic flat tappet, except the lifters are solid, rather then filled with oil. This means that your RPM range can be wider, and you don’t have to worry about valve float at higher RPM. This style was factory for a few high performance cars in the late ‘60s, but that’s basically it. Generally there will be a sharper ramp rate. These cams require periodic maintenance, as the valve adjustment is not self adjusting like with a hydraulic lifter, budget re-lashing your valves every oil change.

    • Cost is roughly $140 for the cam, $85 for lifters.

  • Hydraulic roller (aka, roller cam)

    • This is your factory style cam from 1987 or so onwards. The camshaft has steeper ramp rates, giving you more lift for a given duration. The lifters have a small roller on the underside, which makes it more forgiving to harsh ramp rates. These cams tend to last longer, and have less friction, therefore giving more power, and are overall better than a flat tappet cam.

    • Cost is roughly $250 for the cam, $240 for lifters.

  • Retrofit roller

    • This is how a person goes about putting a roller camshaft, in an older, flat tappet style block. A roller block (1987 and on), will have tapped holes in the lifter valley, whereas the flat tappet block will not. That’s your dead give away. If you want to use a roller cam in your flat tappet block, you need to buy a Retrofit roller camshaft, and retrofit roller lifters. More on this in the definitions section.

    • Cost is roughly $250 for the cam, $450 for the lifters

  • Solid roller (aka mechanical roller cam)

    • This style was never used from factory. This is generally a race only type of camshaft. The same rules apply here as going from hydraulic flat tappet, to solid flat tappet. Due to the roller lifters, and solid lifter style, very large lift can be achieved, .700” lift is not uncommon. Solid roller lifters do not have provisions to oil the roller axles, this is not a problem during racing, as splash oiling will suffice, but if you wanted to use a solid roller on the street, special grooved rollers can be used, or you can groove your lifter bores (Comp cams make a tool for this).

    • Cost is roughly $250 for the cam, $350 for the lifters

  • General flat tappet notes:

    • All flat tappet cams require a break in procedure. Check with your cam manufacturer for more information.

    • Flat tappet camshaft lobes are ground with a slight angle, such that the camshaft is automatically held in place while running. No cam button is required.

    • This is a picture of run of the mill hydraulic flat tappets. The rightmost tappet (lifter) shows it’s bottom surface, this is the part that rides on the camshaft.

    • After break in, a flat tappet cam is married to its lifters. You cannot move lifters between different bores, and you cannot use those lifters on a different cam. The lifters either stay with that lobe, or get thrown out, period.

    • The following picture shows a flat tappet camshaft in the front, and a roller camshaft in the rear.

  • General roller cam notes:

    • A roller cam needs no break in procedure

    • Roller lifters are NOT designed to rotate in their bores while the motor is running. All roller lifters must have some provision to keep them from rotating. Common methods are; dog-bone & spiders as the factory style, with link bars being the common aftermarket “Retrofit” style.

    • A factory spider and dog bones are shown here.

    • Roller lifters are re-useable with different camshafts, assuming they aren’t visually damaged.

    • When upgrading to a roller cam, using a flat tappet block, you must have a method of keeping the camshaft from walking forward, commonly a cam button, with a strengthened timing chain cover. Also a method to keep the roller lifters from rotating. Pushrods on flat tappet, and roller motors are also a different length

    • This picture shows a grooved roller lifting resting on a roller camshaft lobe.

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:

Competition cams

Crane Cams

Lunati

Summit

Erson

Edelbrock

Iskenderian

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:

Intake Duration @ .050Recommended CR
2108.5-9.5:1
210-2209-9.5:1
220-23010:1
230-24010.5-11:1
240 and up11:1 or greater

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.

 
Navigation


Advertisement



Who's online
There are currently 53 users and 557 guests online.


   
Syndicate
Syndicate content