Does the closing rate of a cam effect valve springs(or any of the valvetrain) as much as opening ramp rate? IE will a cam with a mildly aggressive opening ramp that snaps shut quickly(aggressive closing ramp) effect valve springs like a cam with an aggressive opening ramp would?

More specifically, does closing rate effect valve float as much, more less than opening rate? What about lifter bounce?

 

It's valve bounce on the seat that can be your biggest headache. All valves bounce when they close, but minimizing the height and number of bounces is the goal. I suggest that what many call valve "float" is really aggravated valve "bounce". It may be much easier to control the valve over the nose where you imagine it "floats" than to contol bounce off the seat.

Everything effects the valvespring and the entire valvetrain. Isolating one area or one component may make it easier to think about, but it is the combination and how things work together throughout the rpm range that determines performance and durability. If it were easy, everyone would get it correct!

 

the nose is where it comes together with the rocker arm and seat is where the valve sits on the head(circular part), right?

 

The nose is the pointy end. When the lifter is there the valve is near/at max lift. When the lifter is on the base circle of the cam, the valve is on the seat.

Rich

 

So a cam with an aggressive closing rate could, in theory, furthur aggrevate valve "bounce" off the seat, correct?

Here's another question: Let's say a cam with an aggressive ramp rate spinning at high rpms. Could valve float be caused by the inability of the spring to rebound to its seat in time?

 

Yes and no. The spring never comes off the seat unless something is very wrong.

Rich

 

Old SStroker said all valves bounce on the seat to some degree when they close. Minimizing it is key.

 

Springs on some engines jump around enough to move with respect to the spring locator (seat) and damage it. Same with the retainer.

maro, the valve bounces because everything in the valvetrain is a "spring", including the stem of the valve and even the head. The rocker arms and their mountings are "springs", and of course the pushrod, which is the second weakest spring in the system, the lifters and even the camshaft are all "springs" with different spring rates.

Imagine hooking 8 or 9 springs with very different spring rates together in series and hanging a big weight on one end. Now start cycling the top spring up and down with your arm. You'll be amazed at what happens, especially as you increase or decrease the cycling rate.

Disclaimer: Don't try this in the spring aisle at Lowes or Home Depot. They'll haul you away to the funny farm. Buy the springs and try it at home.

 

good post

 

What I'm trying to do is relate the "control" of an aggressive lobed cam by a valvetrain. The XFI grinds, for example. I've heard with quality lifters(maybe Morel), hardened pushrods, good rocker arms, and titanium retainers, it's still very difficult to get a valvespring to contol those lobes in the 6000 rpm range. I suppose you can't just get an extremely "tight" spring, correct? That would put too much stress on the valvetrain at low rpms?? Any thoughts?

 

what springs in particular are you looking at using? Comp claims that the beehives work with them.

 

Right. The XFI lobes were designed using the beehive springs and Pro-Magnum rockers on the Spintron. You have to use the right valve train components with the XFI lobes.

 

What about Crane Gold RRs? Would they be able to be used with an XFI grind?

 

they will work, but they might flex a bit

 

1) There is no free lunch. Very aggressive lobes can give you lots of "area under the lift curve", but it's hard to maintain control past a certain rpm.

2) Just throwing spring load at a problem doesn't necessarily help.

3) Inertia loads (which result from the multiple accelerations the valvetrain goes thru every cycle) can be much higher than the spring loads at high rpm. With a roller lifter, low speed loads aren't harmful.

4) See 1) above.