I have been doing a little research on the matter. Always the reference to valve bounce is its influence on the exhaust timing. Is the exhaust the first to start bouncing; I would think the intake would bounce first and need more seat pressure being that it is the heavier valve.

 

i've never had valves on a scale but i'd bet the exhust is heavier, even if it's diameter is smaller. the exhaust is a lot more massive than the intake.

 

You might want to put some on a scale then. I have and I know you lose that bet. A better bet would be to bet that a 1.84 is heavier that a 1.94. That one you can win.

 

I have been giving this a little more thought and I think the reason for the emphasis on the exhaust might be that intake bounce is not as critical as exhaust bounce. Knowing the intakes are heavier valves I still would think they bounce first. Is there something I am unaware of as to why the exhaust might bounce first?

 

could you define "bounce" for me?


are you talking about when, at high RPMs, the valve closes so fast that it slams agienst the face, and opens up again?

 

Possibly we're talking about a dual pattern cam??
I could see it in that case, where's there's simply more lobe on the exh side.
But with a single pattern cam whatever reasons there might be would have to be related to differences in harmonics between the two designs of valves, and how that is effected by operating temperature. The exh components do get significantly hotter.

 

That is the bounce I am talking about, valve bounce, not spring surge.

 

Valve bounce? I've heard of it as valve float.

 

Ive heard it called bounce too. I guess in flat out terms thats whats happening.

 

It's two different things. Valve bounce occurs after a valve closes. Valve float is when the lifter doesn't follow the cam lobe after a certain RPM point, and happens when you don't have enough spring pressure for a given application. Valve bounce is brought on by numerous factors, but is most prevelant on high RPM, race mech. roller cam applications (very steep ramp profiles). I recently had a conversation with a Super Stock engine builder who had trouble figuring out why one of his customer's engines was way down on power. To make a long story short, he replaced the harmonic balancer with an ATI unit (that he typically uses), and the engine immediately gained back the 30 HP it was down on. It was perceived that valve bounce was the culprit, but without actual lab type testing, it really couldn't be confirmed. I personally have worked in valvetrain dynamics labs, and every given combination has it's own critical frequency speed, where typically problems like valve bounce will occur.

 

I weighed a pair of valves from a Bcar LT1 engine, 1.94" intake diameter and 1.50" exh diameter. IIRC the intake weighed 112 grams and the exhaust weighed 91 grams. Those valves didn't look any different than the ones I pulled out of an LO3 head, with other than the intake valve being 1.84". I haven't weighed that one yet but it's probably going to be around 100 grams.

Valve bounce is what happens when the valve motion is no longer controleld by the profile of the cam. If the valve accelerates (during opening) too much (i.e. the inertia force, m*a, exceeds the spring force k*x) then the valve stem will not be in contact with the rocker cup. Eventually the valve stops opening (when it apexes at max lift --- that's valve float btw) and then it begins to close from the spring force still acting on it. The valve will actually slam into the seat and bounce (like dropping a steel ball bearing on a steel plate), and that's bad for the valve seat and even worse for the valve.

Spring surge, and "bad valvetrain harmonics" are both unfortunate terms used by auto writers who never took an engineering class. More correctly it's described as resonance modes of vibration of a valve spring. When a valve spring resonates, the spring windings move MUCH more than they would and that causes the forces acting on the valve to be less than what they should be --- so the valve is no longer being controlled by the cam profile.

All of this has to do with the valve motion not being controlled by the profile on the cam. The dirty fix is using a stiffer valve spring, or a combination of 2 or 3 valve springs. The downside is that the wear on the rest of the valvetrain is accelerated and the springs also don't last as long (they lose stiffness more quickly with time). The better fix involves some combination of lighter valvetrain parts and better springs (beehive-type for example).

Good post Motor City Mike.

 

The way people use the term "valve float" they could be talking about a number of different conditions: valve bounce, over the nose problems, pump-up issues. I am sure people are thinking 'valve float' kind of describes any of these, the valve is not where it should be at any given moment. It is out floating opend where it should not be.

It is my understanding that bounce occurs mostly because of incorrect seat pressure. And logically, to me anyway, the one that is bouncing first would need more seat pressure than the other. I was just wondering if anyone has heard of setting up heads with different seat pressures?