stefanst

iTrader: (1)

^ That

jmann

That ^ Subaru. That's why I think it has more to do with the metallurgy of these valves then the springs as a stock Mazda intake valve with a stock single spring has never had an issue on any head I've run.

TNTUBA

iTrader: (1)

I had issues with a bone stock 99-00 head with stock springs and stock valves, standard sized ST valves with Volvo singles, +1 ST valves with ST doubles and +1 ST valves with custom PAC springs with SUB lifters and Ti retainers.

The valves lasted the longest with the PAC springs.

So yea.

TNTUBA

iTrader: (1)

I just went through the excercise of adding up how much time I got out of the last set and it was roughly 6.3 hours most of which was betweem 6000 and 8500 rpm.

Alternative

Thats equivalent to less than 1000 miles of highway driving.

(Figuring roughly 3 Million revolutions of the engine and taking some very gracious liberties.)

TNTUBA

iTrader: (1)

You are a total mouth breather if you think 6.3 hours of full throttle operation in a race car is the equivalent of 1,000 "highway miles."

Alternative

It was a fun comparison.... Take a step back from the keyboard and breathe hotshot. I wouldn't even try to make the connection without knowing more specifics about your engine but there are not many scenarios where that would be considered a good lifespan.

jmann

I ran a stock head on a 02 turbo'd with a lot more then 6.3 hrs, like 4 yrs of numerous track days and 1 1/2 season with the 99 with no issues at all.

endura

Stock junkyard engine/14psi/34 hrs/ 7000rpm fuel cut/6 speed. 4.1 rear. Every shift within 200 rpm of redline.
This includes 2hrs W2W and Mosport GP track (andretti straight, 3/4 mile uphill, 25 sec./ vmax 128mph)

TNTUBA

iTrader: (1)

Yea. The stock fuel cut makes that data point a bit skewed. The harmonics that are causing these issues really only become rapidly destructive above 8,000 RPM. Stock heads in NA motors have been known to beat the seats up above 8200 ish

jmann

My car has a 7500 redline and very seldom sees that, as posted above it is usally around 6800 to 7000 shifts.

Leafy

iTrader: (1)

Hmmm another valvetrain problem that seems to be the bane of auto-x racers (even csp) and high powered road racers that less fast road racers and street people have never run into, and its happening to cars at a variety of revlimits. Starting to sound like its a 3rd order harmonic issue related to the rate of change of rpm more than the rpm itself.

jmann

You engineers lose me sometimes but that ^ actually makes sense

emilio700

iTrader: (3)

You are probably referring to CSP autocrossers that rave stock valve spring NB2's to 7800-8000. The valve springs reach a harmonic and things go bang. NB2 intake cam has a slow ramp that accelerate the valve late then drops it just as fast with relatively gradual ramp. NB1 has a faster ramp just off the seat so (valve) acceleration is more linear, thus no harmonic at that rpm, just destroyed seats.

Easy fix is stiffer valve springs, which at least check, still not allowed in CSP.

endura

Agreed. i burned a built head in under 20 hrs with 7200 rpm fuel cut. Went back to stock and all was well. Upped revs to 7500 and broke TB within 5 hrs. I don't wanna hear about valves/springs/porting/machinists any more. Stock head, 7000max, hood opens only to check fluids. Done.

stefanst

iTrader: (1)

What is the connection between 3rd order harmonics and rev change?

Leafy

iTrader: (1)

rate of change of revs, like the acceleration of the engine.
1st order harmonic is based on position, like if someone wasnt paying attention and made your valve springs coil bind right at peak lift but it wasnt enough to make the engine not cycle.
2nd order is speed, this is your rpm dependent valve float or anything else that happens as a specific rpm.
3rd order is acceleration. You see more engine acceleration in 2nd gear than 4th.

stefanst

iTrader: (1)

This is very interesting. So when talking engines, harmonics are defined in a matter different from traditional physics? In traditional physics the first order would be equivalent to crank rotation (i.e a force that happens every time the engine goes through 360deg- say the force caused by the inertia of the piston going up-and down in a one-cylinder engine). A 2nd order would be something at twice that frequency (i.e. a force that would occur every 180deg- say the forces caused by the unequal piston velocity in a 180deg two-cylinder engine). A 3rd order harmonic would then be something that occurs at three times that frequency, so every 120deg of crank rotation (I'd guess you probably get those in a 3-cylinder 120deg crank engine, but am not sure at all).

If memory serves me right (and it's been decades since I dealt with that kind of stuff) something that's caused by a change in rpm (drpm/dt), in traditional physics would typically be called a derivative of rpm. Why do engine designers always have to use their own secret language and confuse everybody?

Alternative

Thats a first for me too.....

JasonC SBB

Rate of change in RPM is very very slow compared to the motion of the valves. If there is some kind of "harmonic" issue related to RPM, it means you need to avoid certain RPMs because some resonant frequency is being hit. Big rotating machinery such as 500 kW steam turbine generators have "forbidden RPMs". In order to avoid those destructive RPMs, on startup/ power-down, you accelerate/decelerate as fast as possible through those RPMs. If you dwell at those RPMs, then things break.

What is more likely happening is that at some RPM the lifter loses contact with the cam as it is closing, so the cam isn't decelerating the valve for a soft landing on the seat. So the valve crashes down. Rapid wear. The fix is stiffer springs, or a lighter reciprocating mass.

This thread is tl;dr - is this problem less pronounced with a lightweight lifter?