Camshafts

Things that go "Bump in the Night"

Cams are the secret ingredient in the internal combustion soup...bumpsticks instead of breadsticks! More dangerous sounding names have been assigned to cams than any other part you'll find in your motor, further compounding their mystery by giving them personalities. Who would dare to ask the "Dominatrix" that lifts your valves what her stats are? We've put together a calculator that will tell you what your cam's lobe centers and durations are to shed some light on this mysterious subject.

Open and Shut Case

When does that valve open anyway? Well, to determine this we have to put dial indicator on the valve to see when it lifts off of its closed position...for both the inlet and exhaust valves. We need a reference point for this and we'll choose the engine's Top Dead Center (TDC) on the particular cylinder we are measuring. TDC is the point were the piston is as close to the cylinder head as possible and the place in the engine's rotation where both inlet and exhaust valves are closed. You'll know they are closed if both pushrods, in the case of a 2 valve motor, are "loose" and not under tension.

Where the Hell is TDC ?

How do we know exactly where this TDC point is anyway? Well, to do this we need to put a circular degree wheel on the end of the crankshaft that has 360 marks, one for each degree of crankshaft movement as well as marks for TDC (zero degrees) and BDC (180 degrees). BDC is bottom dead center, where the piston is as far away from the cylinder head as possible. A piece of wire can be manipulated into place to align up with the TDC mark (zero degrees).

The best way to find the exact TDC is to use an adjustable mechanical stop that screws into the the spark plug hole. Put the motor at a point near to where you think TDC is i.e. both valves are closed and screw in the mechanical TDC stop. Gently screw in the adjustable portion of the stop till it contacts the top of the piston and lock it down. From this point you can gently turn the motor clockwise and counterclockwise until it gently hits this stop. Note these two degree figures, split the difference, and put the wire on the new TDC point (zero degrees). Rotate the engine cw and ccw again to verify that the mechanical stop hits the piston the same number of degrees before and after TDC (BTDC, ATDC). Adjust the wire you've arranged as a pointer, if necessary, to point to the TDC mark. You've now found TDC! Ground zero in your search for truth.

No Tower of Babel

To insure we all speak the same language we have to agree on the same starting point. In this case it's how far the valve lifts before we start writing down those degrees. The industry standard is when the valve lifts .050 inches (fifty thousandths) off it's seated position. The idea in this is that we will start at an agreed on point where there is "measurable" flow, the assumption being that there is no meaningful flow in the first .050" of valve lift. So, get the cylinder to TDC, making sure both valves are closed and set a dial indicator on either your inlet or exhaust valve. Set the indicator dial to zero and slowly rotate the engine and note the degrees at which the valve hits this magic .050" point. This will be .050" of lift after it opens and .050" before it closes.

TDC, BDC, BTDC, ATDC, BBDC, ABDC, Hike!

Since we all speak the same language on this monumental project we know that BTDC is "before top dead center" and ATDC is "after top dead center". BBDC is "before bottom dead center" and logically ABDC is "after bottom dead center".

When you write down the figures you have to add these acronyms to your degree figures. Inlet cams will generally open x degrees BTDC and close x degrees ABDC. Exhaust cams will generally open x degrees BBDC and close x degrees ATDC.

Once you've got your figures at .050" lift for both the inlet and exhaust valves you can plug these figures into the calculator to get information about your cam's duration and installed centerlines.

Sample of Harley-Davidson M8 Specs

 Manufacturer/Product Intake Open BTDC Intake Close ABDC Exhaust Open BBDC Exhaust Close ATDC Screaming Eagle SE8-447 0 7 42 5 Screaming Eagle SE8-462 7 24 53 2 Screaming Eagle SE8-498 4 33 60 7 Screaming Eagle SE8-515 2 45 61.5 5

Sample of Harley-Davidson TC Specs

 Manufacturer/Product Intake Open BTDC Intake Close ABDC Exhaust Open BBDC Exhaust Close ATDC Andrews TW37 12 42 48 12 Andrews TW64G 30 62 68 32 Crane HTC-290-2 18 42 46 22 Crane HTC-304-2 25 49 56 24 Leinweber RE-T3S 17 41 51 11 Leinweber RE-T5S 24 40 56 12 Red Shift 577TC 25 46 53 22 Red Shift 657TC 21 52 58 16 S&S 640G 25 60 65 25 S&S 675G 25 64 70 25 Screaming Eagle SE-264 24 60 60 22

Sample of Harley-Davidson EVO Specs

 Manufacturer/Product Intake Open BTDC Intake Close ABDC Exhaust Open BBDC Exhaust Close ATDC Andrews EV51 28 44 54 22 Andrews EV84 32 64 70 30 Crane H290 17 43 45 23 Crane H310 23 63 68 28 Leinweber E-4 49 74 76 39 Leinweber E-9 53 81 86 38 Red Shift 575 25 54 63 18 Red Shift 656 28 52 52 26 S&S 600 20 55 60 20 S&S 631 34 61 66 29 Screaming Eagle SE-60 26 56 61 24 S&S 640 25 60 65 20
Camshaft Lobe Center / Duration Calculator/Lobe Separation Angle
 Camshaft Name / Number : Intake Opens BTDC (ATDC is -) : (in degrees) Intake Closes ABDC : (in degrees) Exhaust Opens BBDC : (in degrees) Exhaust Closes ATDC (BTDC is -) : (in degrees)

RSR
Calculations

Overlap, Close the Barn Door

The amount of time, expressed in crankshaft degrees, that describes the window of time between the the Inlet Cam's opening point BTDC and the Exhaust Cam's closing point ATDC. This figure can vary between zero degrees on some stock cams to as much as 70 to 90 degrees on some race motors. In general most street engines will have 20 to 30 degrees of overlap and most performance cams will have 50 to 60 degrees of overlap. Increasing the degrees of overlap tends to move the powerband up the RPM band. Increasing the overlap can increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder. Decreasing the overlap tends to boost lower rpm performance.

Lobe Separation Angle (LSA)

This is the angle between the inltake and exhaust camshaft lobe peaks described in camshaft degrees. Generally speaking the majority of cams will fall between 98 and 120 degrees. This angle dictates two important events: the valve overlap around TDC, and intake or exhaust valve closure delay there is in the relevant stroke (inlet/exhaust). Tightening the lobe center angle produces more overlap around TDC and wider angles mean less overlap.

Lobe separation angles in turbochargers are used to be around 112-114 degrees but should be less with more modern faster spinning turbos that have less exhaust back pressure. With less exhaust backpressure LSA's of 109 dgrees are more the norm. The LSA for turbochargers is highly dependent on the exhaust backpressure present in the system design and the turbocharger chosen. More restrictive A/R ratios in the exhaust scroll of the turbo, with attendant increase in backpressure in the exhaust, calls for an increase in the LSA to prevent dilution of the inlet charge.

Valve Lift

A little appreciated consideration is the effect of valve lift on engine performance. As the engine speed increases there will be a need to increase valve lift to keep the inlet speeds from exceeding the Mach Index value of .6, beyound which volumetric efficiency falls off. This leads to the intriguing possibility of planning your valve lift in advance relative to your design or performance goals be they street or racing. Check out our Mach Index Inlet Valve Calculator.