BASICS

When the intake valve opens and the piston starts its intake stroke, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air/fuel is moving at a pretty high speed. If we were to slam the intake valve shut, all of that air/fuel would come to a stop and not enter the cylinder.

By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel continues to force air/fuel into the cylinder as the piston starts its compression stroke. So the faster the engine goes, the faster the air/fuel moves, and the longer we want the intake valve to stay open. We also want the valve to open wider at higher speeds -- this parameter, called valve lift, is governed by the cam lobe profile.

The Cam profile therefore is an essential variable factor in the state of tune of any engine. But there's more to it than just that.....

Any Camshaft in a standard engine is designed to run with the standard cylinder head flow and deliver the compromised power profile that most manufacturers require in road cars.

In addition, they work within the quite wide tolerances seen in production engines. The result is an engine which fits the purpose of a standard road car but is essentially compromised in terms of power, torque and delivery.

When you change factors like head flow, valve size, rod ratio, spring rates, bore and stroke etc... you change many of the parameters used to define the camshaft profile.

Its time to specify a custom made camshaft.

It is often overlooked fact that you should consider head flow characteristics before fitting a new camshaft. You may be able to obtain more power from a standard head but without knowing the flow profile and, subsequently, the camshaft profile which will suit that head, you are really working in the dark in terms of optimising that engine.

Look at the graph to the right (Click for a larger image). This is an image showing gas flow through our gas flowed Big Valve Ford Cosworth Cylinder head. You can see there is a massive 57% in inlet and 39% increase in exhaust flow over the standard head.

Not only is peak flow improved but also the profile of the flow as the valve opens is radically changed. If you were to use a standard camshaft with this head you wouldn't be gaining the full benefit from the improved flow.

With a custom designed camshaft we can match the camshaft profile to the head flow and deliver the exact power profile that the customer requires for the application.

The process behind specifying the correct camshaft requires a number of precise parameters which are themselves variable factors in the design of the engine.

• Application and max revs used
• Target BHP and torque and power profile required.
• Gas flow and flow profile during valve lift
• Piston speed and rod ratio
• Valve size
• Valve spring rates

The camshaft itself has several key variables which must be defined to match the engine:

• Theoretical Valve lift
• Actual Valve lift (Accounting for valve lash, valve train deflection, rocker arm ratio variation etc)
• Lobe Lift (i.e without rocker arm ratio multiplication)
• Duration including lobe clearance ramp.
• Overlap (between inlet & exhaust openings)
• Lobe center (the amount of camshaft degrees between the point of maximum lift on the intake lobe and the point of maximum lift on the exhaust side of a given pair of cam lobes.)
• Lobe centerline (should not be confused with lobe center or lobe displacement angle) The term "lobe centerline" refers to an imaginary line drawn through each respective lobe but does not combine the separation angle between the intake and exhaust lobes in a given pairing. Lobe centerline can be altered by advancing or retarding a camshaft, while lobe center is a figure that cannot be altered since it is incorporated into the camshaft when it is designed and manufactured.

Specifying a camshaft is no place for the amature or rulle of thumb. All the factors used as parameters in the calculation and the specifications of the camshafts themselves are extremely complex to calculate. Also, it is important to factor in the the customers requirements in terms of total power and power delivery.

In order to achieve this calculation and to predict the power characteristics of the engine we use simulation and measurement software (Port Flow Analyzer). This gives us a means to handle all the parameters and gives a predictive capability which we use to match the customers' requirement.

We also look carefully at the way the power is eventually delivered across the rev range.

There is no guesswork involved here.

No rule of thumb is used.

It's the right camshaft for the engine.


Custom made camshafts : Price £500 + vat

Call us on +44 (0)161 483 4810 or email on sales @ cncheads.co.uk . Check out our Frequently Asked Questions page for more information.