Max valve lift questions

[Peter J Boncella]

1) if my cam has a intake valve lift of .500 with a rocker ratio of 1.5:1 , my actual valve lift is .750?

2) if the max valve lift of a cylinder head is .700 I would need to get a cam with around .450 lift with 1.5:1 rockers? or would a cam that has a valve lift of .650 work?

3) do they make 1:1 rocker arms?

4) what would be the consequences of having a cam with a valve lift that is more than the cylinder heads max valve lift?

I apologize if these are dumb questions, I am new to engine building and just trying to fully understand the rocker arm/valvetrain thank you.
(edit added question no.4)

 

[MomsDuster]

Most cam manufacturers show lift with 1.5 rocker ratio. So if it says .500 valve lift that’s what it is. If you look at cam specs it should show lobe lift, that is what you would multiply by rocker ratio. So for example a cam with .395 lobe lift would be .5925 valve lift with a 1.5 rocker.

 

[yellow rose]

1. Don’t confuse lobe lift with valve lift. They are not the same. Lobe lift times rocker ratio is your nominal valve lift.

2. How did you calculate what the head’s max lift should be? Sometimes you can’t get all the lift you want (or need) because the lobe required by the limited rocker ratios available would make the lobe so aggressive you couldn’t use it.

3. No. That would be an overhead cam arraignment with the cam riding on a bucket (a big upside down bucket looking piece of junk).

4. See number 1. If you can fit the lobe with a reasonable grind, and your valve gear can handle the lift, more lift is almost always better.

 

[dart_68]

2) if the max valve lift of a cylinder head is .700 I would need to get a cam with around .450 lift with 1.5:1 rockers? or would a cam that has a valve lift of .650 work?

Most manufacturers that sell complete heads will list the max valve lift that the SPRINGS can handle...not the max valve lift the heads can handle. However, heads can have a max lift that will see a flow increase and less or stagnant flow with more lift.

 

[furrystump]

No such thing as dumb questions. We all started where you are. Max valve lift depends on many things. Almost like the weakest link in a chain, but not weakness but what parts have the least clearance. Which could be the piston to valve clearance
Which could be the valve retainer to valve guide clearance.
Which could be the coil bind clearance on the spring.
Which could be the cam ramp too steep so the lifter can’t follow the ramp.
Which could be how far out or in of the lifter bore the lifter can travel before uncovering the oil band.
or, can the valve spring actually control the speed of the valve. More lift in the same time frame means you have to move the valve faster. That is harder to control.
Something is going to be the tightest clearance and will determine max lift.
However, don’t worry about max lift number. Let me draw you a picture. With a cast iron production head you have about 1.80”s of valve spring install height. That is measured from the spring seat to the bottom of the spring retainer. You take a spring that is 1.80 inches and compress it until there is no space between the coils. This is called coil bind. The spring will be a little over 1” tall. 1.1-1.3 really depends on the spring shape and wire dia. At this moment you are limited to .500-.600” of lift. Is that max lift? Not really. You can install longer valves and space the rocker shaft up. That should take care of your installed height limitations, but now you have valve cover clearance issues and flat tappets aren’t going to work, roller lifters only. My point is when you try and approach a “max” anything, the definition of “max” varies and out on the fringes , theybe dangerous waters there, a place where money goes to die.
The real question, what are you goals and end use for the motor. Max valve way down the list of important things. I run 11.80 116mph with .558 lift solid flat tappet. The car could run low 11’s with that lift if it were “normal”