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TERMINOLOGY
Just to explain a few terminologies
Note that all reference to degrees is with respect to No.1 cylinder

ATDC - After Top Dead Centre (Top Dead Centre is when No. 1 Pistion is at the very top - corresponds to yellow mark on crank pulley in line with the pointer

BTDC - Before Top Dead Centre
ABDC - After Bottom Dead Centre.
BBDC - Before Bottom Dead Centre.

CAM OPENING AND CLOSING SEQUENCE
In a four stroke engine - the engine goes through two complete revolutions (360 x 2).

REVOLUTION 1 – The piston is at TDC. At the start of the first revolution the intake cam has just opened, and as the piston goes down, air/fuel is drawn in to the cylinder, the piston reaches the bottom (BDC and starts to come back up -then as the piston is coming up the intake valve closes. The compression of the air begins (as the exhaust valve is also closed at this time), and just before the piston get to the top the spark plugs fires, and the mixture starts burning ( this is usually about 15 BTDC ignition timing at idle).

REVOLUTION 2 - Now the second revolution starts, the air/fuel mixture is burning and this is the power stroke where the piston is being pushed down by combustion. Before piston reaches the bottom, the exhaust cam starts opening, once the piston reaches the bottom and starts coming back up with the exhaust valve open - the spent gases are pushed out of the cylinder. Now the cycle starts all over again – i.e Revolution 1 start over again at the end of Revolution 2. The exhaust cam only closes after Revolution 2 finishes, and after the start of Revolution 1, but the intake valve opens slightly before the end of Revolution 2, stays open through the beginning of Revolution 1. This is where cam overlap happens. Therefore for a short period of time the intake cam and the exhaust cam are simultabneously open.


INTAKE CAM TIMING
Cam intake centreline is where you will normally have the maximum lift (and it is generally halfway through the duration of the cam).

For example, the Tomei intake cam is of 260 duration, and if it is installed at the recommended intake cam centreline of 110ATDC then :-

The intake cam will start opening at approx: 110 ATDC - (260/2) = 110 ATDC - 130 degrees = 20 BTDC (Revolution 1)

Maxuimum lift will be at approx 110 ATDC – Revolution 1 (ie valve fully open)

Likewise this intake cam will close at 110ATDC + (260/2) = 110 +130 = 240 ATDC – Revolution 1 (or 60 ABDC)

Advancing the cam means that the cam centreline will be less than 110ATDC (i.e the cam starts opening sooner than recommended). E.g. if the intake cam was advanced 5 degress say, then intake cam centreline is now at 105 ATDC (cam will start opening at 25BTDC and will close at 235 ATDC).

Retarding the intake cam means vice versa. Cam centreline is now more than 110 ATDC (e.g 115 ATDC cam centreline if the cam was retarded 5 degrees, cam will start opening at 15BTDC and will close at 245 ATDC.

Advancing the intake cam means that the cam closes more open at TDC, hence there is a limit to which intake cam can be advanced before valve/piston interference occurs

EXHAUST CAM TIMING.

Likewise the Tomei exhaust cam is of 260 duration, and if it is installed at the recommended exhaust centreline of 64 ABDC (Revolution 2) then :-

The exhaust cam will start opening at approx: 64 ABDC - (260/2) = 66 BBDC (Revolution 2)
Max lift will be at 64 ABDC (Revolution 2)
The exhaust cam will close at approx: 64 ABDC + (260/2) = 14 ATDC (Revolution 1)

Advancing the exhaust cam will mean the exhaust cam centreline will now be less than 64 ABTC (e,g advancing by 5 degrees – 59 ABDC). Opening is now 61BBDC and closing 9 ATDC (Revolution 1)

Retarding the exhaust cam will mean that the exhaust cam centreline will now be more than 64 ABTD (e.g retarding by 5 degrees will make it 69 ABDC). Opening is now 71BBDC and closing 19 ATDC (Revolution 1)

Also note that when exhaust cam is retarded it closes at 19 ATDC (instead of 14 ATDC) – i.e the exhaust valve is now open more at TDC. If the exhaust cam is retarded too much (generally over 7-10 degrees then there is the chance of exhaust valve to piston contact.

CAM LOBE CENTRES AND CAM OVERLAP

Many cam manufactures quote the cam lobe centreline in addition to cam duration and max lift.

Cam lobe centres value indicate how many degrees separate the intake valve maximum opening to the exhaust valve maximum opening position. This is normally expressed as half the actual degrees.

For the Tomei cams, the cam lobe centreline can be calcalated as follows. Intake valve maximum opening (intake cam centreline) = 110 ATDC, Revolution 1. Exhaust valve maximum opening (exhaust cam centreline) = 64 ABDC, Revolution 2. The Separation angle = (180 –64) + 110 = 226 degrees. The cam lobe centre value is 226/2 = 113 degrees.

If the lobe centre value decreases then the cams are coming closer together (e.g 105 lobe centre means that the separation angle btw the intake and exhaust cam centrelines is now 210 degrees). As the lobe centre value decrease the overlap increases – as the cams are closer together, and vice versa

The cam overlap for the Tomei can also be worked out ( from the above recommended installed positions) The intake cam starts opening 20 BDTC (Revolution 2), and the exhaust cam only closes 14 ATDC (Revolution 1). Hence there is an overlap of 14 + 20 = 34 degrees.

POSIBLE CAM TIMING COMBINATIONS AND RESULTS

Normally for billet cams from reputed aftermarket companies, adjustable camgears are not strictly necessary as they have been designed to install in the recommended centrelines with the stock camgears. Adjustable cam gears are needed when the cams need to be advanced or retarded from the stock positions (to customise the set-up).

Idle quality is normally affected by overlap. Lesser the overlap the better the idle, but top end power is sacrificed. More overlap (within boundaries) will make for a rougher idle, but gives better top end power.

Generally if you advance the intake cam (say 3 degrees), and retard the exhaust cams ( again say 3 degrees)…then overlap between the cams will be increased , idle will be a bit rougher, but will give a fairly good mid range power, without much sacrifices in top end power.

Retarding both cams by 3-4 degress will give good top end power, but at the expense of bottom end. Not best for normal day to day driving

Advancing both cams by 3- 4 degrees, gives good low/midrange at the expense of top end – good for autocorsses, hill climb type events.