Nobody I ask seems to know. The most common flat top pistons have 4 releifs for a head with only 2 valves. Why is this?  

RIVERRAT77:
Nobody I ask seems to know. The most common flat top pistons have 4 releifs for a head with only 2 valves. Why is this?

that way they can be use on the right or left side on the engine. a true flat piston with only 2 valve reliefs uses a dedicated right and left casting, therefore costing more to make(usually only done with a forged or hypereutectic piston).  

Actually there must be some other reason, (that I can only guess at).

The mid priced and most popular flat top pistons from companies such as Mahle and SRP and Probe, and ROSS come with only 2 valve reliefs.  They are not left and right pistons.  The 1,3,5,7 pistons are installed with the valve reliefs toward the intake manifold and the #2,4,6,8 are turned 180deg and installed so that the valve reliefs are also pointed toward the intake manifold.

(the true left and right pistons will have a large valve relief and a small valve relief and the right piston goes in #1,5, 4,8 hole and left piston goes in the 3,7,2,6 holes).

I think you see aftermarket pistons with 4 valve reliefs offered mostly because that allows the piston to be run in a "stock" eliminator motor, (it looks stock).  Thats probably the true and simple answer.

Possibly the OEM designer does it because:
The piston is weighted a tick one way or the other for less bore wear?
The stock OEM Mahle piston for GENII (LT1) small block Chevy motors must be installed with the arrow on the piston top facing forward therefore it must have the 4 valve reliefs because 1,3,5,7 face forward the exact same direction as 2,4,6,8.  Possibly that's a pin boss geometry deal?  These are wild guesses.  But it's not for ease of assembly I know that because you can mix up the piston/rod assembly and get them pointed in the wrong direction if you are not follwing the book.

I reserve the right to change all of my opinions after learning more or finding out I'm just totally out in left field.  

Karl Ellwein  

Why do you need valve reliefs?To create space between
pistons and valves when pistons are at top dead center and valves opened.(overlap)So in a engine with two valves per cil.you need only 2 valve relief right?
Ok so why four?Because you can use this pistons in any side of the engine(left or right bank).Economic reasons.
You cant just turn 180 degree the piston to face reliefs to intake manifold because piston pins are not in the center line of piston circunference for offset reasons.
You must respect the arrow on pistons top,or you will have a noisy engine.(pistons claping)  

alexfreddi17:
Why do you need valve reliefs?To create space between
pistons and valves when pistons are at top dead center and valves opened.(overlap)So in a engine with two valves per cil.you need only 2 valve relief right?
Ok so why four?Because you can use this pistons in any side of the engine(left or right bank).Economic reasons.
You cant just turn 180 degree the piston to face reliefs to intake manifold because piston pins are not in the center line of piston circunference for offset reasons.
You must respect the arrow on pistons top,or you will have a noisy engine.(pistons claping)

..................You must respect the arrow on pistons top,or you will have a noisy engine.(pistons claping)

True for stock pistons but on most aftermarket pistons there is no arrow.  And for most aftermarket 2-valve relief pistons they are turned 180deg to fit on the rods for the "other side".
So it must be the stock OEM piston only have pins boss locations offset?  
Karl  

What do you mean by 'aftermarket'.Every time you buy new pistons for an engine, doesnt matter for stock or high perfomance use, you have piston pins off center.
Some times it doesnt have a arrow but some kind of mark in the top or near the skirt.If there are pistons
for any modern engine(Any 4,6,8 cyl )without this design,probably is a cheap China made piston.  

Aftermarket means anything that is not OEM.

Here is a photo of a high quality USA made Mahle (and very typical) flat top piston which does not have an offcenter pin boss and is not to be installed like an OEM piston.  Left side of SBC engine.

Same piston rotated 180deg and installed [properly] on right side of engine.

It is not installed like an OEM piston with any arrow or mark pointing forward.
That is a very typical high quality aftermarket piston.

That same piston underside


Here is a high quality aftermarket ROSS piston that is truly a "Left/Right" piston. (left meaning intake valve relief on the left or right meaning intake valve relief on the right)


And the stock OEM SBC piston, (usually made by Mahle in Brazil).



Karl  

I am observing these pictures. I was wondering if 194/160 valve 350ci heads would work on a 305 chevy motor? unsure in tenn.  

This website must on central standard time. My post above was 5:04 est.  

Nice excuse to show off your high dollar aftermarket parts...

 

GibTG:
Nice excuse to show off your high dollar aftermarket parts...



every chance I get.........
For some reason my girlfriend doesn't want to see any of these nice new parts.
 

Those Mahle pistons look just like mine. I just never gave it any thought until I bought my pistons that have only 2 releifs. The only reason I went with 2 releifs is to get that last bit of compression to suit my motor. Also I think that the sharper edges on the releifs can (in some cases) cause that destructive naughty sound that is so bad I cant even say it. Get rid of 2 out of 4 releifs and cut the chance down 50%. I also had the pistons worked on and they did slightly round off the edges of the valve releifs. Having only 2 might have saved me a hour or 2 of labor.  

they will work, but you will lose a shitload of compression, unless you have pistons to compensate, i know this from experience from when i first started doing this stuff a few years ago. I didn't really know what i was doing back then, so dont make the same mistake i did.  

damn stupid internet...the above comment was for carefulmechanic..so uh, yea  

There is no reason why you shouldn't take a high-speed grinder and at least a cartridge roll to the sharp edges left by the machining process on the piston crown. This is not only important to preventing detonation but also to mixture motion. I think that higher dollar pistons use two valve reliefs because they make more power and it's cheaper! Not solely because it cuts down on the amount of sharp edges that may glow...

Valve reliefs rob power, plain and simple. It may be a very small amount of power but they take away from mixture motion and combustion turbulence by getting a "notch" in the way. If our engines didn't need valve reliefs, then the pistons wouldn't have them!

I've always thought of 4-valve relief setups as a way to reduce compression more without spending the money of cutting a full-scale dish in the piston top. Remember, the engineers making 4-relief pistons may not have outright power production first on their agenda, they may want to prevent any excessive compression surprises by re-using the tooling used to cut the "first" set of reliefs.