Who wants to teach me about Cams? (camshafts) - Page 2 - Nissan 350Z Forum, Nissan 370Z Tech Forums
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post #11 of 69 (permalink) Old 03-15-2006, 10:05 AM
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Unless you're running cams with a wide enough duration to worry about valves and pistons colliding, what's the downside of running cams with a big lift?
There really isn't a downside. There's just no upside either. Once you open the intake valve to about .200 inches of lift or so, there's little to be gained by opening it further. Due to laminar airflow patterns, the intake charge will take on a "tulip" like shape as it rushes into the combustion chamber. As the valve opens further, the charge will maintain this same pattern. It's like opening a can of soda... most of PSHHHHT!!! occurs just as you crack the can open, you don't gain any more airflow opening it up the rest of the way.

In short you want to keep and eye on lift/duration to insure you don't ram the piston into an open valve. This is, as they say, bad.

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post #12 of 69 (permalink) Old 03-15-2006, 10:13 AM Thread Starter
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There really isn't a downside. There's just no upside either. Once you open the intake valve to about .200 inches of lift or so, there's little to be gained by opening it further. Due to laminar airflow patterns, the intake charge will take on a "tulip" like shape as it rushes into the combustion chamber. As the valve opens further, the charge will maintain this same pattern. It's like opening a can of soda... most of PSHHHHT!!! occurs just as you crack the can open, you don't gain any more airflow opening it up the rest of the way.

In short you want to keep and eye on lift/duration to insure you don't ram the piston into an open valve. This is, as they say, bad.

Regards,
Ronin Z
Would you think me, as a consumer, even need to worry about that? Are there any Cams for the Z that would be designed to be harmful?

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post #13 of 69 (permalink) Old 03-15-2006, 12:46 PM
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Would you think me, as a consumer, even need to worry about that? Are there any Cams for the Z that would be designed to be harmful?
No, for the most part. There is nothing to worry about as long as you are running the stock (or lower) compression ratio, and the cams are installed "straight up" as in 0 degrees (no initial retard or advance). Things get a little sketchy when you get into high compression ratios, custom spec cams, and/or more than about 6-8 degrees of initial retard/advance*.

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*Installing a cam with an intial amount of retard/advance is often referred to as "degreeing" a cam shaft. This technique is most commonly used in vehicles that do not have a system of variable valve timing. It is used to shift the torque/power peak up or down in the rev range. On motors with DOHC, the cams can be degreed individually. The VQ motor has the ability to change cam phasing (within a narrow range of degrees) on the fly. All aftermarket cams should be installed per the vendor's instructions.


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post #14 of 69 (permalink) Old 03-15-2006, 08:00 PM
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There really isn't a downside. There's just no upside either. Once you open the intake valve to about .200 inches of lift or so, there's little to be gained by opening it further. Due to laminar airflow patterns, the intake charge will take on a "tulip" like shape as it rushes into the combustion chamber. As the valve opens further, the charge will maintain this same pattern. It's like opening a can of soda... most of PSHHHHT!!! occurs just as you crack the can open, you don't gain any more airflow opening it up the rest of the way.

But that's because the air in the can is pressurised but with a finite amount of gas, so releasing it gives the most violent release, but then it peters out because you're running out of gas, so opening the can the rest of the way doesn't give as much flow.

In an engine where the intake gas is "unlimited", I understand that the opening of the valve releases the pressure and gives a sudden inrush of air. But if I open the valve more won't that permit more airflow as there is less backpressure from the valve blocking the hole?

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post #15 of 69 (permalink) Old 03-15-2006, 08:23 PM
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In an engine where the intake gas is "unlimited", I understand that the opening of the valve releases the pressure and gives a sudden inrush of air. But if I open the valve more won't that permit more airflow as there is less backpressure from the valve blocking the hole?
When the inlet valve first cracks open, the intake charge “sees” the pressure differential between the intake manifold and the combustion chamber and begins to flow. At first the air molecules take a relatively random path through the valve opening. This initial rush is fairly turbulent and creates a high pressure region around the valve opening. The air molecules closest to the valve and valve seat begin to pile up on one another and the pressure rises. As the valve gap widens, an area of lower pressure develops in the middle of this region and molecules begin to squeeze their way through. Soon air is flowing readily in this area. Air loves to take the path of least resistance. It becomes easier for the air molecules to follow its buddy in front of them than to find their own path. This is when laminar flow begins.

Now if all this was happening in slow motion, then of course the wider we opened the valve… then more air would get through. But we’re talking milliseconds here. And air, like any other object, has mass and mass means inertia. It takes time for the air to get moving. Takes times to begin laminar flow. And takes still more time to adapt to an ever changing orifice size.

It’s because of these tendencies that engine designers tend to focus on increasing valve diameter and cam duration when they want to increase flow. Not lift.

Regards,
Ronin Z

P.S. Sorry about the soda can analogy, but it’s a simple way to picture what’s going on. But you are right, it’s not completely accurate.



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post #16 of 69 (permalink) Old 03-15-2006, 11:18 PM
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note to self....Kenny, stay away from this thread, you don't need cams, although learning about them would be interesting.
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I'm very curious.

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Honestly? if that's the case, I would suggest reading that How Stuff Works link (or similar material). I can clarify the points you don't get, or would like more info on.

-RZ
That's where I first 'learned' about cams while reading up on adjustable cams. Good stuff.

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post #17 of 69 (permalink) Old 03-16-2006, 02:21 AM
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It’s because of these tendencies that engine designers tend to focus on increasing valve diameter and cam duration when they want to increase flow. Not lift
On an FI application would you go for lift?

You don't want to run big duration on a high boost motor, but you want as much of that pressurised air to get into the cylinder as possible.

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post #18 of 69 (permalink) Old 03-16-2006, 02:59 AM
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Ronin Z = the win

I told ya he'd fit right in.

Good job, Ronin-san. Hopefully soon I can return and join in on this discussion.


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post #19 of 69 (permalink) Old 03-16-2006, 09:52 AM
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On an FI application would you go for lift?

You don't want to run big duration on a high boost motor, but you want as much of that pressurised air to get into the cylinder as possible.
Actually FI has no problem with duration, it's the overlap that is the killer. For those that don't know overlap is when both intake and exhaust valves are open at the same time. Overlap is great for high revving NA motors because it helps scavange the cylinders during the exhaust stroke. Plus the flow-through of the intake charge helps with cylinder filling at higher RPMs. Though it does cause charge contamination and inlet reversion at lower RPMs. This is why NA motors with big cams idle roughly.

Since you have to keep overlap to a minimum (SC is not as upset with overlap as a turbo) this obviously cuts into the max duration you can run. So how do we get more air in with an FI motor? If possible we increase the size of the valves. If we can't do that, we can increase lift... but it doesn't help a whole lot.

Hrrrmmm.... we can't increase the volume of air entering the cylinder.... but wen can increase the density! That's why we're running forced induction in the first place. Get a bigger intercooler and turn up the boost.

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Ronin Z


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post #20 of 69 (permalink) Old 03-16-2006, 01:24 PM
 
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Awesome info, Ronin! I think all the details are interesting (note: NO sarcasm! I read it and it could be taken that way.)

Ok. So how do I use this new wealth of knowledge to choose between cam manufactureres? Here's what I found from web shopping:
  • JWT Cams: 260 DEG DUR. / .428"(10.9MM) LIFT, INT. ADVANCED 2 DEGREES.
  • Nismo Cams: 262 Deg Dur. / .426" Lift, Cold Lash - .010" to .013" (WTF is "Cold Lash"?)
  • Tomei Cams (1): 256 duration with 10.2 mm lift
  • Tomei Cams (2): 264 duration with 10.22 mm lift
  • Tomei Cams (3): 268 duration with 10.5 mm lift
So we've got everthing from 256 to 268 duration and 10.2 mm to 10.9 mm of lift, with Nismo listing the "cold lash" -- whatever that means. There must be a reason for having different setups. If I don't care about gas milage and am simply looking for the best over-all power increase, which would you recommend? I'm assuming there's plusses and minuses for all.
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