boost camshafts ?

[miafori]

what is the best alternative for cams in blown engines ? keeping stock one or installing any with higher duration ?

Do performance cams lower combustion chamber pressure or inscrease air flow ? which one ?

thanks

[mpowered325i]

airflow depends on durations and timing, if your valves stay open longer, you get more air

[Active Alpine]

I'm not going to pretend I know what I'm talking about, but I've heard that you want the highest duration cams without going into overlap. In NA cars its a good thing because it causes more air/fuel to enter, but on FI cars, this isnt needed and actually causes air/fuel to be pushed out the exhaust, defeating the FI.

[miafori]

yep, I heard same thing but want to be sure. I don't know any performance camshaft for turbo engines. Is the stock one best ? any other idea ?

[bemw 1]

Active Alpine is partially correct. All camshafts have overlap since this refers to the period when the inlet & exhaust valves are open at the same time. All camshafts open the inlet valve at some point before TDC on the exhaust stroke & close the exhaust valve at some point after TDC on the inlet stroke. By opening valves earlier & closing them later ie: bigger duration cams, we can take advantage of airflow momentum to try & fill the engine cylinders with more air/fuel in order to get closer the the elusive 100% cylinder fill or 100% volumetric efficiency & therefore get more power. All this happens only when everthing is in harmony, this is when we say the engine is "on the cam". The bigger the duration then the higher the RPM before it happens, usually with the result of a loss of power &/or responsiveness below the cams' ideal RPM, a bit like "turbo lag" for want of a better description. Also to support "bigger cams" we need an increased compression ratio or the previous mentioned downside will be even worse.
With forced induction the air is being pumped into the engine & therfore exceeding 100% VE. So we don't need bigger duration to acheive this. Also depending on how much boost is being used the the compression ratio of the engine may have been lowered which is what we don't want with a big duration cam! If the forced induction is via a turbo then there will already be some turbo lag, so you can now see that a bigger cam with a low comp. ratio will add to that lag.
However with a positive displacement supercharger it's a little different. Increasing the duration can move the power band farther up the rev range if this is desired, true for turbo also. There is also a school of thought that increaseing the overlap period can cause a type of blow through effect in the combustion chamber under certain conditions which has a cooling effect on pistons & valves etc.
Bently publishers put out a good book called Maximum Boost (turbocharging)which touches on this & there is a sequel book on supercharging, I can't rember the title even though I have it on the shelf at work? Both are worth a read. Hope this helps, sorry it's a bit long winded.