Piston/valve cutouts

[LandingSpot]

Why do some pistons have cutouts for the valves? I understand some are for non interference engines, but other shaped designs I'm not sure.

Like this JE forged piston:



or



whereas this one has none.



Can anybody enlighten me?

[GavinPearson]

It's because the valve(s) would hit the piston if the cutouts weren't there.

[LandingSpot]

I had guess that, but why? Why no make the valves shorter? Why not profile the cams differently so that they operate the valves at a shorter distance but stay open for longer?

[LandingSpot]

Is it because the valve opening would still be open on the compression stroke?

[ian_uk1975]

Valve reliefs allow for more flexibility in the cam. Most aftermarket pistons (even flat tops) have valve reliefs. In most cases, the reliefs aren't strictly needed as the engine will be spec'd with a small-ish cam. However, more lift and duration = more power (basically). Without valve reliefs, you'd be limited on lift and/or have to compromise valve timing ground into the cam. Either that, or you'd have to use other work-arounds like a shorter compression height (pin height) / taller deck height. On domed pistons, the dome is usually profiled to minimise any possible valve interference for the same reasons.

[spend]

LandingSpot said:

I had guess that, but why? Why no make the valves shorter? Why not profile the cams differently so that they operate the valves at a shorter distance but stay open for longer?

The valve ~ piston clearance problems are normally due to that duration rather than max lift, ie when the piston is around the top of the bore.

[Mr2Mike]

LandingSpot said:

I had guess that, but why? Why no make the valves shorter? Why not profile the cams differently so that they operate the valves at a shorter distance but stay open for longer?

Because this would all lead to big compromises in performance. Low lift gives less peak flow, longer duration reduces low speed torque, fuel economy and can make emissions worse.

[anonymous-user]

It's pretty much because of the geometric compramises required when you try to get both large, high lifting valve kinematics and a high static compression ratio into a combustion chamber design.


For optimum performance, you need a high CR to maximise thermal efficiency, you also want the fastest "burn" possible for your aircharge (so you can burn the fuel at the lowest cylinder volume and loose as little heat as possible to the cylinder walls) so you want as much "squish" as possible (where the piston gets v close to the head in places, and litterally squeezes the aircharge out of the gap, resulting in very high charge motion)

Generally, the higher the engine performance, the more pocketed the top of the piston will be (obviously, i'm mainly talking about pentroof 4 valvers here)