Ever wanted to see what your valves are doing?

[dipshit]

Due to the action of the conrod the piston has a controlled rate of acceleration and decelleration. Does anyone know a ballpark g figure?

I heard one of the hardest things on conrods and pistons is slamming the throttle shut suddenly from high rpm. Especially on V-twins with relatively bigger and heavier pistons. The piston coming up no longer has a air/fuel mix to compress and cushion itself against at the top of the stroke... instead there is now a vacuum in the combustion chamber and no cushion. This is when a conrod can stretch fractional more than usual and damage the piston.

[bogan]

Due to the action of the conrod the piston has a controlled rate of acceleration and decelleration. Does anyone know a ballpark g figure?

hmmm, peak acceleration is when the velocity is zero so get dV/dTheta then:

V = RPM / 60 * 2 * sin(Theta) * Stroke(in m) /2

dV/dTheta = RPM / 60 * 2 *cos(Theta) * stroke(in m) /2

so peak acceleration is same number as peak velocity, just 90 degrees later, and g force is that figure over 9.818

[Pixie]

Due to the action of the conrod the piston has a controlled rate of acceleration and decelleration. Does anyone know a ballpark g figure?

A high value for an F1 engine: at 19,000 rpm, the maximum piston acceleration in a BMW P83 of formula 1 is 10'000 g .

But road engines are more like 1000 g

[peasea]

As a spring compresses one of the seating faces rotates relative to the other (tip - fit a thrust washer underneath one end of your shock spring to reduce stiction). The wind direction of the coil dictates which way it twists. I see what you are saying about the valve spring being a 'free' object but invariably there is some friction and the tiny force would add to the rottional force. The tip grinding alone couldn't cause ALL of the valves to rotate in the same direction. Does the mating face inside the bucket have any shape or is it flat?

To the best of my knowledge (coz OHC engines were never really my thing) it's flat. The outer face of the bucket that contacts the cam lobe should be convex though so that it rotates.

Sure, valve springs rotate (which is what I meant by 'doing the whatoosie) but it's a by-product not an intent. If the springs are kept under control then, yes, they are (or should be) in contact with the retainer and cylinder head/shim.

What I find interesting in modern, high rpm engines is the ability of the valve springs to survive as long as they do. Springs will exert a given force at a given length when new but there's always an initial 'fall-off' and in racing circumstances the springs need to be shimmed to regain that initial seat pressure. So many of today's engines run for ages without that need and can still attain giddy rpm limits.

Things have moved on from Morrie Minors and AJS single-bangers.

[peasea]

I heard one of the hardest things on conrods and pistons is slamming the throttle shut suddenly from high rpm. Especially on V-twins with relatively bigger and heavier pistons. The piston coming up no longer has a air/fuel mix to compress and cushion itself against at the top of the stroke... instead there is now a vacuum in the combustion chamber and no cushion. This is when a conrod can stretch fractional more than usual and damage the piston.

Quite right. In class racing, where we had to run a flat-top piston, the deck height was machined to a bare minimum. We dyno'd the engine and got what wanted etc, with no apparent difficulty in the piston-cylinder head department. However, in the real world, when the car went through the traps and the throttle was released rapidly (unlike the gentle wind-down on the dyno) the pistons started slapping the heads, only just, but they did.

Easy fix in the lathe, just a lesson learnt.

[peasea]

hmmm, peak acceleration is when the velocity is zero so get dV/dTheta then:

V = RPM / 60 * 2 * sin(Theta) * Stroke(in m) /2

dV/dTheta = RPM / 60 * 2 *cos(Theta) * stroke(in m) /2

so peak acceleration is same number as peak velocity, just 90 degrees later, and g force is that figure over 9.818

Are you a dentist?

[bogan]

A high value for an F1 engine: at 19,000 rpm, the maximum piston acceleration in a BMW P83 of formula 1 is 10'000 g .

But road engines are more like 1000 g

how did you get 1000g? I only got 3.

[Madmax]

try rotary valves