DJ
6th June 2002, 08:13 AM
Allen Lofland recently posted this technical discussion of camshafts from Guy Croft on the Yahoo Scorpion forum. It was apparently a response from Guy to someone's question about camshaft timing figures. I thought it was a good explanation that anyone could understand and thought it would be a great thing to have in the Lancisti archives as well.
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The quoted timing figures them only tell part of the story, so one cannot tell much about the performance from them.
Using your typical examples here's what the figures do tell you: 35/75, 75/35 - This is a pair of symetrical cams, duration 290 deg (sum of 35 +
75 + 180 deg). They could be billet or regrind, there is no way of telling just from the figures.
Inlet cam starts to open 35 crank deg before top dead centre (at the end of the exhaust stroke), achieves full lift 110 deg AFTER top dead centre [(35+180+75)/2]-35 deg, closes 75 deg after bottom dead centre.
Ex cam starts to open 75 deg after top dead centre (on the firing stroke), achieves full lift 110 deg BEFORE top dead centre and closes 35 deg after top dead centre (on the intake stroke).
The period for which both cams are open at the end of the exhaust stroke and the comencement of the intake stroke is 70 deg (35+35 deg).
The timing points outlined above will depend on the running clearance, so one might reasonably ask - are the figures of 37/75 75/35 quoted with clearance or without? If they are without, the true open and close points and overlap will be markedly different, although the most critical point, the full lift posiiton, will remain unchanged. This issue also affects the cam full lift.
Now - going to the nub of your question, what the timing figures above don't tell you is what the important part of the cam profile looks like. It might, to put it most simply, be fat - or skinny. If the full profile is mapped out, measuring the lift at each degree, then the shape of the profile can be plotted graphically and compared with another cam of the same quoted characterisitcs. the one with more area under the curve will
tend to generate more flow and thus a higher volumetric efficiency, particularly if shows a higher rate of inlet valve lift in the opening region.
The period of dwell at full lift is another thing you cannot predict without mapping the cams. Inlet dwell at full lift is important because it tells you that the cam is held open for a few critical degrees (usually not more than 6-10 crank degrees) at full lift, thus enhancing the airflow at the time when the valve intrudes least into the airstream.
It is perfectly possible for a cam of ostensibly shorter duration (say 290 deg, 37/75) to have stronger mean (average) airflow characteristics than a cam of longer duration (eg 300 deg 40/80) by virtue of the fact that the shorter duration cam has a fatter profile (and thus a higher mean lift if summed across the whole profile).
At the end of the day you really have to put two cams side by side and compare them, or better still map them and dyno them. This is why I always tell people to but only from reputable sources, giving preference to manufacturers who have bothered to invest in dynoing and track testing their own cams, and never trust just what the figures say.
All engines respond differently, for example the cam characterisitcs required by supercharged or turbocharged cams are radically different from those of naturally aspirated units.
Putting cams in an engine should never be undertaken without evaluating the whole engine spec. With luck, the engine will light up a treat, at worst it will be a mismatch and give no more power and be unreliable. The valve event, CR, and particularly the fuel and exhaust systems must match the cam characteristics. High back pressure for example, (ie: more than 4 psi) in the exhaust system, between the header and the first silencer (muffler) will wipe out any gains from a cam swap instantly.
If anyone wants me to pursue this topic further, please say!
Best wishes
Guy
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The quoted timing figures them only tell part of the story, so one cannot tell much about the performance from them.
Using your typical examples here's what the figures do tell you: 35/75, 75/35 - This is a pair of symetrical cams, duration 290 deg (sum of 35 +
75 + 180 deg). They could be billet or regrind, there is no way of telling just from the figures.
Inlet cam starts to open 35 crank deg before top dead centre (at the end of the exhaust stroke), achieves full lift 110 deg AFTER top dead centre [(35+180+75)/2]-35 deg, closes 75 deg after bottom dead centre.
Ex cam starts to open 75 deg after top dead centre (on the firing stroke), achieves full lift 110 deg BEFORE top dead centre and closes 35 deg after top dead centre (on the intake stroke).
The period for which both cams are open at the end of the exhaust stroke and the comencement of the intake stroke is 70 deg (35+35 deg).
The timing points outlined above will depend on the running clearance, so one might reasonably ask - are the figures of 37/75 75/35 quoted with clearance or without? If they are without, the true open and close points and overlap will be markedly different, although the most critical point, the full lift posiiton, will remain unchanged. This issue also affects the cam full lift.
Now - going to the nub of your question, what the timing figures above don't tell you is what the important part of the cam profile looks like. It might, to put it most simply, be fat - or skinny. If the full profile is mapped out, measuring the lift at each degree, then the shape of the profile can be plotted graphically and compared with another cam of the same quoted characterisitcs. the one with more area under the curve will
tend to generate more flow and thus a higher volumetric efficiency, particularly if shows a higher rate of inlet valve lift in the opening region.
The period of dwell at full lift is another thing you cannot predict without mapping the cams. Inlet dwell at full lift is important because it tells you that the cam is held open for a few critical degrees (usually not more than 6-10 crank degrees) at full lift, thus enhancing the airflow at the time when the valve intrudes least into the airstream.
It is perfectly possible for a cam of ostensibly shorter duration (say 290 deg, 37/75) to have stronger mean (average) airflow characteristics than a cam of longer duration (eg 300 deg 40/80) by virtue of the fact that the shorter duration cam has a fatter profile (and thus a higher mean lift if summed across the whole profile).
At the end of the day you really have to put two cams side by side and compare them, or better still map them and dyno them. This is why I always tell people to but only from reputable sources, giving preference to manufacturers who have bothered to invest in dynoing and track testing their own cams, and never trust just what the figures say.
All engines respond differently, for example the cam characterisitcs required by supercharged or turbocharged cams are radically different from those of naturally aspirated units.
Putting cams in an engine should never be undertaken without evaluating the whole engine spec. With luck, the engine will light up a treat, at worst it will be a mismatch and give no more power and be unreliable. The valve event, CR, and particularly the fuel and exhaust systems must match the cam characteristics. High back pressure for example, (ie: more than 4 psi) in the exhaust system, between the header and the first silencer (muffler) will wipe out any gains from a cam swap instantly.
If anyone wants me to pursue this topic further, please say!
Best wishes
Guy