Lift is a huge part of the equation. If you noticed lower torque in an area where you wanted to gain, then intake duration is usually too big for the application.
When is it needed? That is relative to the build and what you are trying to accomplish. IF you are trying to get a blip on the screen at the local dyno for 600hp or 700hp, you can do that with a stock cam no doubt. If that is what you are worried about, an aftermarket cam is not needed.
There are many aspects that change from cam to cam depending on what you are trying to accomplish. On the intake side of things, the duration pretty much dictates at what rpm you move the most air. The exhaust dictates how long the pressure acts on the piston, how big of a pressure pulse hits the turbine wheel and, how long the exhaust is open after TDC(overlap) which can have some low rpm and high rpm affects. Lift affects how effective that cam is for a given duration. The better your head is, the more benfit that can be gained from a given duration with more lift. All that being said, two cams with the same duration @ .050" and the same lift can act totally different, based on total open time and duration at .200".
The benefits of a cam on a street truck are as follows.
Due to changed intake placement and shape
-more clean air into the cylinder on every intake event if correct duration is chosen
-lower egt from more air
-better spool from more trapped air
-less smoke from more trapped air
-more trapped air over over awider rpm creating a wider power curve
-more trapped air creates a higher dynamc compression and more oxygen these two things in tandem, create higher compression pressures, higher compression temperature, creates an environment which decreases the time from start of injection to start of combustion. This ends the combusion process sooner, the same effect as increasing rate of injection. It is actually better than increasing timing in that it shows a lot of the benefits without the increased negative torque associated with more timing.
- higher pressure at TDC allows for a greater expansion ratio
- a greater expansion ratio can mean two things, more work done on the piston via pressure acting longer on the piston or more work done on the turbine wheel by opening at the same time as the stock cam. Opening much sooner on a performance cam done correctly (*for a street truck) is wasting pressure that should be acting on the piston to instead create excess drive pressure.
Benefits from a correctly placed improved shape exhaust
-because of more lift and better shape of exhaust lobe you will experience less pumping losses. This will allow the energy that was pushing back on the piston trying to exit the cylinder to instead be placed on the turbing wheel........ "free"energy to help spool the turbo
-later opening on a conservative cam can help increase efficiency by allowing the pressure to act on the piston longer, effectively lengthening the power stroke.
-earlier opening wastes some of the energy acting on the piston to spool up that large single or to create huge sums of drive pressure if installed with an incorrectly sized turbo. On street trucks this is usually not wanted as it gives up efficiency, but if you are not able to spool your large single, then it is a good tradeoff
-less in cylinder EGR effect when teamed with the correctly sized turbine wheel and turbine housing
-In nitrous applications, huge gains in power can be had from retarding the exhaust opening and letting the pressure act on the piston longer and then adding a lot of lift to allow the exhaust to exit without excessive pumping losses.
Gains from overlap
-Higher volumetric efficiency can be gained from more overlap if the correct cam is chosen for the correct setup. To take advantage of this, (at higher rpm) you need to have very well matched turbo with near 1:1 drive/boost pressure ratio. it can also be amplified with a long runner style exhaust header. With a long tube header, the exhaust gasses that are leaving the cylinder creater a lot of inertia, if the exhaust closes after the intake opens in the correct spot, the exiting exhaust inertia will creat a bit of a low pressure area in the cylinder. This low pressure area creates a larger pressure differential between the intake charge and the in cylinder pressure. As a result more air can be brought into the cylinder. Too much overlap and much fresh air can be wasted out the exhaust. Too much overlap with excessive drive pressure and you create an EGR effect which lengthens the time from start of injection to start of combustion. You will also lengthen the total time of combustion which can allow fuel to still be burning as the exhaust valve opens.
If all of the fuel that you are injecting is being burned and your egt's are in check, then adding more air will show little gains in power. In this instance, the gains in power that are shown will be from lower pumping losses. IF this is the case, then more fuel needs to be injected to see gains.
If you are at the limits of your turbos flow potential, then opening the intake valve longer and higher will net you little, because the turbo cannot effectively push more air into the cylinder to take advantage of the longer opening.
If you are injecting fuel very late after TDC (longer Pulse Width Duration) allowing more air to enter the cylinder will help very little with clearing the smoke or making more power. This is an instance where the cam will help little in clearing smoke or lowering EGT. There is not enough time to completely burn all of the fuel by the time the exhaust valve opens.
