Dual Profile Asymetrical Cam

Explanation
• The intake lobe appears symmetrical but plots out asymmetrical.
• The exhaust lobe appears asymmetrical but plots out symmetrical.

Confused? Read On!

If you look at the BMW intake and exhaust cam lobes, they will appear to be a mirror image of one another and ground with an asymmetrical profile. When you plot out the lift curve on a piece of graph paper, you'll discover that the asymmetrical BMW lobe looks like a symmetrical bell shaped curve. The reason this phenomena occurs is because as the nose of the cam wipes across the curved foot of the rocker arm, the rocker arm ratio changes from a low of 0.9:1 to a high of 1.6:1 (with an average of 1.25:1). So, if we combine a fairly symmetrical lobe with BMW's variable rocker arm ratio we end up generating an asymmetrical cam map.

• TDC - Top Dead Center
• ATDC - After Top Dead Center
• BDC - Bottom Dead Center
• ABDC - After Bottom Dead Center

• TDC
  At top dead center, during the overlap cycle, the lift of the intake and exhaust valves can only be marginally greater than stock (1 mm) before the idle degrades and emissions increase.
  iii. Intake Events:
• 75° ATDC
  The piston is at maximum speed and the intake charge velocity will be very high. At this point Metric Mechanic's Asymmetrical Cam curve takes a radical departure from the stock cam curve.
• 110° ATDC
  The stock cam has hit maximum lift for a short period of time because the nose of the cam is rather narrow. Metric Mechanic's Asymmetrical Cam hits maximum lift at about 120° ATDC and the valve lingers at high lift because of the broad nose on the cam.
• BDC
  With the stock cam the intake valve is only now half open. The Metric Mechanic’s Asymmetrical design allows the intake valve to stay 3/4 of the way open. Remember at bottom dead center cylinder volume is at its maximum. At 20° BBDC to 20°ABDC the piston has only moved 1.5 mm (.060"). Obviously, keeping the intake valve open at higher lifts while the piston lingers at BDC can greatly enhance cylinder filling.
• ABDC
  After bottom dead center the stock valve drops at a descent rate of about .0085"/degree. The Metric Mechanic's Asymmetrical Cam takes a fairly fast descent rate of 0105"/degree. This faster closing rate helps keep the duration down to 275° versus 264° (measured at .011" lift) for a stock cam. To help cope with his faster descent rate from mid-lift on down, we have developed a special valve spring to be used with this cam (see Metric Mechanic's Valve Spring).
  Once the intake valve closes, the engine goes through its compression and power cycle, until the opening of the exhaust valve.
• Note:
  Metric Mechanic's Sport cam is an involute profile cam. An involute profile cam takes up valve lash and valve spring tension very slowly in about the first .020 lift. After the initial load of the valve train is taken up, the valve is accelerated to full lift very quickly.
  This slow initial take-up and closing becomes important in reducing shock load to the valve train. At high RPM, if the valve hits the valve seat too quickly on closing, it will cause the valve to bounce. This is commonly referred to as "valve float". An abrupt closing of the red hot exhaust valve against its seat will eventually beat out the valve face and seat.

For durability, our sport cams are hardened by nitriding to a hardness of Rockwell 60 - 65 on the "C" scale. This is a good bit harder than a factory chill hardened cam.

Introduction
These special double wound valve springs were developed to handle the faster descent rate of the asymmetrical cam from mid lift 5 mm (.200") on down to the seat. At 5 mm (.200") seat pressure is 5% greater than stock. This increases to over 10% at 2.5 mm (.100") valve lift. Seat pressure has been increased by over 18%. When both valve springs are rated at 10 mm (.400") the stock spring is 3% stiffer than the asymmetrical spring. This helps reduce the likelihood of having wear problems at the nose of the cam due to high spring pressures at full lift. To knock out spring harmonies, a great deal of dampening action was designed into the spring.

This was done in two ways:

1. By making the outer spring progressively wound
2. By adding an inner spring dampener.

We have carried this spring design over for use on our engines that use stock cams. These springs have about 10% more seat pressure than stock and have about 7.5% less pressure than stock at 10 mm lift. This type of design helps to reduce cam nose and rocker arm wear.