Cam design/Duration/Lift/Valve Sizes
Discussion
OK, with you there. Just to recap, I assume phasing is relating to the harmonic content of the lobe shape?
My key curiosity was how there is a current trend to high lift/less duration with performance engines whilst the long cams have dropped in favour.
The engine I was thinking about was the venerable old A series (Which has proved one of the most thoroughly developed/tested engines going). In the old days, due to classes, restrictions there seemed to be more interest in short stroke screamers, now longer stroke, bigger capacities are in favour BUT as a general rule the overlap has come down. As you say exhaust tuning has fallen from favour quite a bit. I was wondering about how bore/stroke play a part in the equation and if this influences rough cam choice.
I started (but didn't finish) a mathematical treatment of what was actually happening. It got rather complicated, but the area which suggested a relationship was "Area for flow" vs crank position for different bore/stroke configuration. Assuming the port velocities have not been topped, I figure that enlarging the valve has an identical bearing on the area for flow as increasing lift. (Ignoring the speed/direction/weight of the gas for the moment).
I am convinced that bore/stroke are critical factors but will chuck in a thought for good measure. Harmonic Profiles. I suppose in the recent years, a lot of work has been done regarding these profiles instead of using symmetric, simple patterns. So, perhaps a modern short duration cam actually works better!?!?! Still not convinced.
I completely understand the basics of torque curve behaviour vs duration and how you fine tune that for the size of your engine with lift/valve sizes. But not quantitatively.
I suppose then there is also the "high octane fuel" problem of recent years and general trend of declining compression ratios.
What made me think about this was the lack of information available for selecting a supercharger cam. Since my engine is going to be about 8-12PSI of boost and will be a unique large capacity. The proprietry cams are designed for N/A or turbo engines of a different capacity. I immediately saw the need for about another 15 degrees of exhaust duration and getting the valves as big as possible as well as ports. But since I am considering alternative bore/stroke ratios I wondered how these would tweak the final figures.
Since I wasn't 100% sure of the right answer, I thought I'd better understand the principles of how bore and stroke couple with "area available for flow" as I will call it.
I expect quite a bit is to do with the available fuel, the resulting compression ratio and how that is achieved and the revs/duration which favours it. Also I expect that high lift rockers have taken the place of vicious ramp rates in the interest of life. But what I have not considered are the properties of the moving gas. This has to be critical in the equation and relating that to piston speed has to give a massive insight.
What do you reckon?
(By the way, my background is Chemistry, not Physics and my Maths is about good A level standard..Don't get too complicated too quick
)
My key curiosity was how there is a current trend to high lift/less duration with performance engines whilst the long cams have dropped in favour.
The engine I was thinking about was the venerable old A series (Which has proved one of the most thoroughly developed/tested engines going). In the old days, due to classes, restrictions there seemed to be more interest in short stroke screamers, now longer stroke, bigger capacities are in favour BUT as a general rule the overlap has come down. As you say exhaust tuning has fallen from favour quite a bit. I was wondering about how bore/stroke play a part in the equation and if this influences rough cam choice.
I started (but didn't finish) a mathematical treatment of what was actually happening. It got rather complicated, but the area which suggested a relationship was "Area for flow" vs crank position for different bore/stroke configuration. Assuming the port velocities have not been topped, I figure that enlarging the valve has an identical bearing on the area for flow as increasing lift. (Ignoring the speed/direction/weight of the gas for the moment).
I am convinced that bore/stroke are critical factors but will chuck in a thought for good measure. Harmonic Profiles. I suppose in the recent years, a lot of work has been done regarding these profiles instead of using symmetric, simple patterns. So, perhaps a modern short duration cam actually works better!?!?! Still not convinced.
I completely understand the basics of torque curve behaviour vs duration and how you fine tune that for the size of your engine with lift/valve sizes. But not quantitatively.
I suppose then there is also the "high octane fuel" problem of recent years and general trend of declining compression ratios.
What made me think about this was the lack of information available for selecting a supercharger cam. Since my engine is going to be about 8-12PSI of boost and will be a unique large capacity. The proprietry cams are designed for N/A or turbo engines of a different capacity. I immediately saw the need for about another 15 degrees of exhaust duration and getting the valves as big as possible as well as ports. But since I am considering alternative bore/stroke ratios I wondered how these would tweak the final figures.
Since I wasn't 100% sure of the right answer, I thought I'd better understand the principles of how bore and stroke couple with "area available for flow" as I will call it.
I expect quite a bit is to do with the available fuel, the resulting compression ratio and how that is achieved and the revs/duration which favours it. Also I expect that high lift rockers have taken the place of vicious ramp rates in the interest of life. But what I have not considered are the properties of the moving gas. This has to be critical in the equation and relating that to piston speed has to give a massive insight.
What do you reckon?
(By the way, my background is Chemistry, not Physics and my Maths is about good A level standard..Don't get too complicated too quick
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