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Originally Posted by judgie
so is the cam thats needed more duration or more lift? i would guess you want a faster ramp to open them and less overlap but not more lift? faster ramp would mean better springs to control the lift/close speeds
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Yes, I would say you'd always want to have a ramp as fast and agressive as your lifter foot would support IF...and a big if...if the additional wear and extra needed spring wouldn't be an issue. However, it IS an issue and that means its always a compromise with what your particular valve train will support.
The other downfall of more duration is less torque down low.
If you can lessen both downsides of more duration, you optimize the engine's efficiency.
The first issue of spring control is 'easy' to manage by better parts. Lighter yet stronger is the goal.
The second issue is more difficult to control. A lot can be done here too though with cam timing: Most view duration as a "1-dimensional" property of a given single number of degrees duration. However, the duration is made up from both the opening AND closing of the valve. And that for both intake and exhaust valve which doesn't need to be the same either.
For example, you can have the same duration for intake and exhaust, yet still have different opening and closing timing for either. Most all cam makers have symmetrical opening and closing timing, its convenient, but that doesn't need to be.
Long story short, I have a (Pauter) cam here that has 20 degrees more duration, but its intake valve closes (that determines dynamic compression) at the same time as my current Webcam. That of course means it gets its extra duration from earlier opening resulting in more overlap. My take is that overlap can be managed by a better exhaust system, i.e. equal length exh. manifolds and even better: twin scroll design.
Guess what's on my car and for what reason?
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. from what i understand big lift is not needed on turbo motors, just the time the valve is open, the turbo is pushing the mix so you dont need the lift, but you want the valves open for as long as possable with the least amount of overlap.
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So, overlap is manageble as shown above and the myth that turbo engines don't need the (extra) lift is pretty much busted by my first turbo engine (stock 914 2.0) when I just (only change) put a set of Pauter ratio rockers on and made 40-50 more hp (222->270hp) at the same boost!
A fast ramp of course is also favourable as it gives you the lift as soon and long as possible. If lift wasn't much of a concern, a fast ramp wouldn't be necessary either, which contradicts your saying that lift isn't much of a concern with turbo even more.
If it all works out as I have thought it out, remains to be seen of course. Its difficult to do 'scientific' and therefore conclusive testing for a privateer (and even for a shop), but if I could do just a cam change and nothing else, that would be very interesting. Not sure if it comes to that in 2012 though...