[ap72]

What is the difference between a hyper piston with coated skirts and a fully machined hyper piston with coated skirts?

[ap72]

anyone????

[automotive breath]

Inexpensive pistions have the valve reliefs cast into the compression face.
A fully machined piston cost a little more and has machined valve reliefs.

[Jmark]

Originally Posted by automotive breath

Inexpensive pistions have the valve reliefs cast into the compression face.
A fully machined piston cost a little more and has machined valve reliefs.

Is one better than the other? Or is it a wash?

[automotive breath]

The cast reliefs are normally small and lack the accuracy of machining. This is
not an issue when piston to valve clearance is not a concern like with small
valve diameters and a mild street type cam. With large valves and or an
aggressive cam this becomes more important.

in addition a piston that has four reliefs like this will result in a lower compression
than a machined piston that has two relies.

The second picture shows how the machined piston is more accurate, with
larger reliefs.

[ap72]

So since I'm planning on getting a speed pro hype coated piston with 2Vr should I just get the unmachined version? I'm running a fairly large solid cam but I'm not worried about valve interference, and it seems that the cast VR's would be less prone to detonation, especilly if I polished them up a bit.

[GenYnNC]

I would always double check piston to valve clearance on anything over .500 lift, might be overkill but it's best to check... if you read the new CHP magazine, they ran into piston to valve clearance with a 2.08 valve and around .550 lift/250ish duration on a 350. That's a big solid cam in my mind so not sure what your cam specs are but better be safe than sorry...anyways, I think detonation is a very scary word when talking cast or hypereutectic pistons. They shatter like glass when detonation occurs at WOT pulling some rpm's and prone to breaking ring lands, so do make sure of your compression ratio is 'within reason.'

[ap72]

LOL, reason... I had that once, threw it out the window when I decided to run a circle track cam on a street car though. I will be checking P2V clearance though, the valves are only 2.02"s though so nothing major. I may end up going with forged pistons, but the hypers are looking very good with tighter clearances, lower mass, and half the price.

The build I'm doing is very simmilar to one I've seen posted on the net, the main differences are professionally ported heads (I did mine and imagine I'll be down a few cfm), AirGap manifold instead of Super Vic, and the carb. I imagine I'll end up being down about 15 hp and up about 15 ftlb. But it ran on 91 octane and I'll be feeding mine 93.