Valve lift vs. port flow and cam selection

[Krooser]

I'm trying to understand the relationship between valve lift and port flow in regards to cam selection.

Let's say the intake port flows 300 cfm @ 600 lift...exhaust flows @140. You rpm limit is 7000-7500.

When selecting a cam for an endurance engine do you spec a .600 lift or more lift or less lift.

I'm getting answers all over the spectrum from people I respect.

I have to add that this is a SBM that has the wonky valve train that some say will lose .20 of lift at the valve.

Your suggestions?

[englertracing]

Whats wonky about the valvetrain?
Doesnt it have shaft rockers?

In my opinion which isnt worth much,
You want to run as much valve lift as you can for the duration you end up with. Use good components to make it last. If your having issues loosing lift maybe you want to use 7/16, or even 1/2" pushrods.

[af2]

I think it is .020"

[superpursuit]

Whats wonky about the valvetrain?
Doesnt it have shaft rockers?

In my opinion which isnt worth much,
You want to run as much valve lift as you can for the duration you end up with. Use good components to make it last. If your having issues loosing lift maybe you want to use 7/16, or even 1/2" pushrods.

Lifter to pushrod angle.

[Krooser]

Correct...pushrod angle.

Two guys I respect who have built hundreds of engines say I need to keep the lift below the point where the port stalls... which is .600 in my case.

Others who run the same engine (drag) suggest the engine will gain more power with more lift despite the port slowing down or getting turbulence.

I'm finally ready to pull the trigger on this cam (SFT). I value everyone's input.

[KnightEngines]

More lift, always more lift, when you think you have enough give it more & it'll pick up.
Up to the limit of your chosen valve train components.
If you don't see a gain then you're losing control somewhere in the valve train.

[Orr89rocz]

Dont you only need enough lift to meet air demand? If engine only needs 280, then you dont need to lift much higher than 280, even if head goes to 300. You lift above the point where your required flow is to get the area needed ofcourse and then duration is picked for the rpm range, since you need time to fill the cylinder

[RevTheory]

Does it just quit flowing more air or does it back up? Does that even matter?

[Lizardracing]

I'm pretty sure Mike Jones covers this in a previous thread and his theory is about the area under the curve.
More lift, even if the port can't flow it, means the valve get too and stays at longer, the lift range that does matter the most.

Sort of like the port hitting peak flow twice, once on the way open, and another on the way closed.

[Stan Weiss]

Dont you only need enough lift to meet air demand? If engine only needs 280, then you dont need to lift much higher than 280, even if head goes to 300. You lift above the point where your required flow is to get the area needed ofcourse and then duration is picked for the rpm range, since you need time to fill the cylinder

Let think about that statement for a minute. If peak piston flow demand is @ around 75 ATDC and ICL is @ 108 ATDC, How much more lift do I need to get the head flowing peak piston flow demand @ around 75 ATDC?

Stan

[Orr89rocz]

I’m not talking meeting peak piston demand. I am talking cfm total to make the power desired at a certain rpm

[CamKing]

There's not one answer for your question. There's a lot of factors at play.

First, you have to know the difference from a port where the flow is just flattening out, and one where the port is going turbulent above a given lift.
If the port is going turbulent above a give lift, lifting the valve above that point can actually hurt the power. If the flow is just flattening out above a given lift, increasing flow beyond that point can pick up power, because you've increased the number of degrees you are at that max flow.

Another factor is, does the engine require more lift. That depends on the flow requirements for the given application, and is effected by CID, valve size, port flow, and rpm.
In your example, with the 7,500rpm limit, the engine may respond to running .800" lift. Now take that same engine, and lower the RPM limit to 5,000 RPM, and it might not make any more power, going beyond .600" lift.

Now you also have to factor in valvetrain stability, and diminishing returns. Maybe the engine makes 3 more hp, going from .700" to .800" lift,
but requires much more spring pressure, and wears out valvetrain parts, 3 times faster. Is that worth the 3hp ?

[Steve.k]

So if your flow starts to drop at .600 for example by adding lift to say .750 you would still net gains?

[CamKing]

So if your flow starts to drop at .600 for example by adding lift to say .750 you would still net gains?

No, If the port is going turbulent, and the flow is dropping off above a given point, you can lose power if you run more lift.

[RevTheory]

In his SpeedTalk interview, Darin Morgan talked about lifting the valve I believe .100 shy of a port going turbulent and it was still way down on power.

It would appear to me that if a port goes turbulent at say .650 on a bench at 28", you're probably not out of the woods lifting to .600 on a running engine. I think you'd be way better served getting the port calmed down even if you don't pick up flow beyond where it started getting fussy.

Just get the port to flatten out and run the lift you need and/or can afford.