by Michael Decipha Ponthieux
This write up pertains to typical Ford four stroke internal combustion engines.
Last Updated: 2018-11-13
The camshaft is the most important factor in an internal combustion engine's operation. The camshaft profile actuates the valves to directly effect idle stability,
engine performance, and overall emissions compliance. Camshafts are the most determining factor in an engines behavior. The camshaft(s) can be recut (re-ground)
or replaced to alter the engines operating characteristics. A camshaft is characterized or "profiled' by its lobe design. The camshaft lobe is the journal that the lifter rests on.
During engine operation, the camshaft is spun at half the crankshaft speed (for four stroke engines), therefore,
the CRANKSHAFT makes 2 (two) rotations for every 1 (one) rotation of the camshaft.
There are 360 degrees in 1 (one) complete rotation.
Since the CRANKSHAFT makes 2 (two) complete rotations for the 1 (one) rotation of the CAMSHAFT, the camshaft is using 720 degrees of CRANKSHAFT rotation. [360 degrees of the first rotation and 360 of the second rotation, 360+360=720 complete degrees of rotation of the CRANKSHAFT for each rotation of the CAMSHAFT (since the camshaft moves at half the speed of the crank)
Since there are 4 (four) cycles to the engine, and the CRANKSHAFT moves a total of 720 degrees to complete a full rotation of the CAMSHAFT, 720 / 4 = 180 degrees per cycle.
0 - 180 is the first cycle of the engine called the POWER stroke, (stroke is the technical name for cycle) power stroke = power cycle.
The piston starts at TDC (top dead center) and is pushed down by the air and fuel combusting.
180 - 360 is the second cycle or "stroke" known as the EXHAUST stroke.
The piston is all the way down at BDC (bottom dead center) and blows the EXHAUST out as it rises to the top (and the exhaust valve opens).
360 - 540 is the third stroke known as the INTAKE stroke this is where your injector timing comes into play.
The piston is now at the top again TDC, however, for maximum efficiency the intake valve is opened BTDC (before top dead center) to start sucking air as soon as possible (scavenging).
540 - 720 is the fourth and last stroke known as the COMPRESSION stroke/cycle.
The piston is at the bottom BDC and as it comes up it is compressing the air and fuel it has sucked in during the intake stroke.
Lobe / Cam Specifications
Cam card information for camshaft's usually only go up to 360 degrees because they are referencing the location of the piston (crankshaft).
So on the intake BTDC means you subtract those degrees from when the piston reaches TDC at 360.
and ABDC means you add that many degrees to 540 which is when the piston reaches the bottom.
All specifications of the camshaft profile are cut/ground into the lobe. There are three (3) primary specifications of a camshaft, all measured at the lobe;
- lobe height - maximum lift of the lobe as compared to the base circle, measured at the center of the lobe peak, defines the peak valve lift.
- ramp rate - angle of the peak lobe height from the base circle, determines how quickly the valves open/close.
- duration - the length of the lobe height from open to close, defines camshaft overlap.
Cam manufacturer's will typically give you a .006" lift number as well as a 0.050" lift number.
These numbers are referring to valve lift.
The .006" numbers are most significant for low airflow situations such as idle and decel.
The .050" numbers are most significant for high airflow situations such as WOT.
Lobe Seperation Angle
The lobe seperation angle is the offset of the lobe height of the intake valve as compared to the exhaust valve measured in crank degrees. This value has little overall
effect on the camshaft and is typically thought of as "extra information." It is not a critical factor in camshaft selection.
- Hydraulic Roller
- Solid Roller
- Flat Tappet
Most current engines are equipped with one of the following types of camshaft types:
- In Block - Single camshaft installed underneath the cylinder heads in the central section of the engine block, this is typical of pushrod ford engines.
- SOHC - Single OverHead Camshaft, where one camshaft is installed atop each head for a total of 2 camshafts, this is typical of earlier style ford modular engines.
- DOHC - Dual OverHead Camshaft, where two camshafts are installed atop each head for a total of 4 camshafts, this is typical of SVT Cobra engines.
Camshafts and Boost
Boosted engines are not very responsive to camshaft changes. In a boosted engine camshafts generally have VERY LITTLE effect on performance while in boost.
I recommend all boosted engine owners to select a camshaft that gives them the desired idle they prefer with minimal overlap. It is not uncommon to use a stock
camshaft or a camshaft near stock specs for boosted applications.
Turbocharged engine specific camshafts typically lean on the exhaust (hold it closed) to create higher exhaust temperatures and pressures.
Since turbochargers spool on heat, this can be advantageous in reducing spool up time. As a result, you CANNOT run nitrous on a turbo specific cam.
Typically supercharged engine specific cams will open the exhaust earlier to get hot exhaust gases out quicker. This is similar to
nitrous profile cams as well.
Nitrous specific cams are similar to that of supercharged cams. On larger nitrous shots where you have to get the heat out of the engine
quickly you will often open the exhaust valves sooner than a non-nitrous engine. It is typical to get nitrous specific cams
on dedicated nitrous engines or when spraying more than a 175 n2o shot.
Injector timing is not to be confused with ignition timing. Injector timing has very little if any affect on horsepower output.
The injector timing is the CRANKSHAFT degrees where the injector stops firing (this assumes INJREF is set to falling edge).
Everyone has their own theories on injector timing.
Me personally, I ALWAYS set the injectors to fire late, I usually set the injector timing to 500 in all of my tunes.
Doing so will get rid of any fuel odors at idle every time.
Some people prefer to fire the injectors much earlier, I have yet to hear of any valid reasons why though.
If you fire the injector too early the fuel will puddle on the closed intake valve before it has opened.
If you fire the injector too late, the fuel will puddle on the closed intake valve after it has closed.
If you set it to finish firing at 500 crank degrees you can never go wrong.
Injector timing makes very little difference to engine performance, however, cold engines do often run more stable from firing the injector a
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