Bent valve from miss shift

[santee]

I was wondering if anyone would explain how or why you can bend a valve from missing a gear. I was searching past posts on engines that start running rough and came accross the possibility of bending a valve when missing a gear.

I am courious...

Thanks!

-Stephen

[sammyg2]

The valves occupy the same space as the piston crowns, just as different times.
The valve springs close the valves just before the piston gets there.
If you miss a shift and let the revs climb way too high, the valve springs will not close the valves fast enough (valve float) and the piston will hit the valve while it is partially open. $$$$$

[santee]

When you say high RPM's, how high are you talking? 3500 or 5500?

[sammyg2]

Exceeding redline substantially.
The engine is designed to run at redline all day long.
If you get up in the 9000 range your valve springs prolly aint gonna keep up. anything close to 10,000 is gonna get real expensive. When you miss a shift under power the revs can climb so fast the tach can't even keep up so it is nearly impossible to accurately determine exactly how fast it spun up unless you have a laptop hooked up or something.

Condition of the springs, whether they are stock or aftermarket, cam profile, and compression ratio all factor in to the equation.
Every engine is different. There's only one way to determine how fast is too fast but I ainst gonna be the lab rat on this one

[santee]

Thanks for the info Sam!

-Stephen

[sammyg2]

A compression check or leak down test are good ways to eliminate the possibility of bent valves.

[911pcars]

How do valves bend? Momentum. At 8000+ rpm, the valve spring cannot control the weight of the valve train (rocker arm, valve retainer, locks and valve). During valve opening, momentum (or inertia) causes the valve to open further than designed and it hits the piston.

Sherwood

[Grady Clay]

Sam & Sherwood, good posts.

Stephen, the reason the valve gets bent is because the piston contacts it only on one side in most street 911s. Many very high compression racing engines have pistons that contact the full circle of the valve. If (when) the valve contacts the piston as Sam describes (9,000 RPM), the piston just helps the valve close. In these RPM ranges there are also other major issues.

In a 911 engine with a rev-limiter it is difficult to bend a valve just free-revving. The usual problem is downshifting into a lower gear when you intend to up-shift at redline. Expensive maneuver.

Best,
Grady

[Series900]

the money shift

[126coupe]

I missed a shift at the CA speedway 3 years ago. Instead of 3rd to 4th gear I upshifted to 2nd $$$$$$$$$$$ I bent all 12 valves, probably reved to 11,000 +

Since then I installed a fabcar shifter, have not missed a shift since

[Joe Bob]

Valves come in at an angle in relation to the stroke of the piston. When the valve and the piston occupy the same space due to the excessive rpms overwhelming the tension of the valve springs, the weakest link bends..... on many occasions the valve head will break off and cause more havoc inside the cylinder.

Not a pretty sight unless you LIKE carnage....

[Randy Webb]

"The valves occupy the same space as the piston crowns, just as different times."

Sammy - that is a perfect sentence.

[sammyg2]

I can't even believe I speeeled it wright