Are you sort of refering to beehive or dual springs?
I've always assumed that the rates are different across the portions of the been hive springs. I also have always assumed that the two springs (for dual springs) are different rates.
I guess this is a good time to ask.

 

Dual Springs have a total rate that the valve sees.....

Beehives have a pretty constant rate thru most of the range, it's only really close to coil bind that the rate increases.

"Hardened Pushrods" in essence just mean they can be used with guideplates, doesn't mean they are good quality. One of my peeves is assuming these are good, when in reality most are junk and a 1 PIECE 4130 alloy pushrods should be used.

maro, What you are getting into with the cams is not all about the ramps, either opening or closing but how that ramp is related to the nose. If instability starts at one RPM on a portion of the lobe then most likely it's not going to stop till it has enough time to settle, which usually means the base circle at a much lower RPM. Even on the base circle the spring is surging all around.

Bret

 

the specific pushrods I am referring to are FMS hardened pushrods.

 

Ok that doesn't tell me much, if they are more than $50 they are probably good. 1 Piece 4130 is the key here.

 

Yeah, they're one piece chromoly.

 

ok, little different than just hardened. Those will work.

 

So with proper valvetrain components, the XFI grinds can be successfully spun past 6000?

 

Not that I have found.... 6200rpm is about the stable limit, 6700rpm or so they crash. They can go past that with lots of work, but you are better off just using another lobe. You can get hyd roller stuff to 7500rpm without tricks and 9000rpm with lots of money and a well built system.

Bret

 

Not on a stock bottom end I'm sure.

 

Actually I meant that EVERY part in the valvetrain (even in the universe ) has a spring rate and a natural frequency at which it likes to vibrate.

Let's say the valve spring has a rate of 400 lb/in. The rocker arm might have a rate or 40,000 lbs/inch, or 100 times as much, but under a load of say 400 lbs, it will deflect about .010 inch. A pushrod has a rate higher than the valve spring but lower than the rocker arm, let's say 4000 lb/in (I really don't have an actual rate, and it will vary a lot with pushrod diameter, wall thickness and length).

All of these parts, along with everything else I mentioned will deflect under load. When the load is removed they spring back or give back the deflection and most of the load they endured. If the frequency (rpm) that the valvetrain is cycling is close to the natural (resonant)l frequency or some fraction or multiple of it, some of the parts might continue to vibrate (go into resonance). You can see this in my multiple-springs-in-series demonstration as you vary the rate that you cycle your hand up and down.

It would be nice to design a valvetrain where the lowest resonant frequencies were below engine idle rpm and the highest were above max rpm. Often they are, but each part can interact with its neighbors so lots of complex motions and resonances can happen.

Each coil in a valvespring can act independently, and it they are identical they may all go into resonant vibration at the same rpm (frequency). The different coils in a beehive can have different resonant frequencies, so they may only get one coil in resonance at a time which gives a larger rpm range in which they can operate without severe resonance.

Let's say a given valvespring went into resonance at 50 Hz (cycles/second), that would be 3000 cycles per minute or 6000 rpm. When it went into resonance it would be dancing all around between the spring seat and the rertainer and not exerting the designed load on the valve, so as the intake valve closed there would be insufficient load to keep it on the valve seat and it might bounce .010-.015 off the seat a few times, letting the charge leak out during the bounces. This loss of charge kills power so you see a more rapid dropoff than you might expect.

Dual springs each have different resonant frequencies, but they may still have the problem of the each spring going nuts at one frequency. Usually there is some interference (rubbing) between the springs so one helps control (damp) the other.

Hope this helps. I think I've beat this to death now.

 

Interesting

 

And well put. Even my limited amount of brain cells actually understood your explaination. Never really thought about how the flexing of "all" the parts in the valvetrain could impact the harmonics of the springs. Thank you.

 

Well explained Dad.

Bret

 

Here is a link to a mass/spring/damper model of a pushrod system.
It's the second one from the top.


http://www.profblairandassociates.co...is_theory.html

I count more than 20 "springs" in this dual spring model.

 

There was a recent article in Chey Hi-Perf on this same subject. Valve seat bounce with spring resonent frequency, and, sharp decrease in power. A good read. Their solution was a behive spring (reduce the mass), or, increase the spring force. Move spring/mass resonence above the max operating rpm.