67King

The concern was the ring lands, which is the part of the piston directly above the rings.

A couple of other things to consider. Getting slightly into the ring land won't hurt you much, it happpens pretty frequently on modern engines because of variable cam timing. You could consider notching the turbo pistons. If you take away any volume from the chamber by notching turbo pistons, you could recover it by decking the head.

I'm not certain if you have additional motives beyond just creating a non-interference engine or not. If that is all there is to it, I suppose you aren't as concerned with power of efficiency or whatever. But, your surface area to volume ratio on your chamer will have an impact on your efficiency, which will affect both power and fuel economy. If you start with a dished piston, the notches won't be that pronounced, which will give you a better chamber (even with decking the head). If you are concerned with that, remember that the ideal chamber shape is a perfect sphere (with the spark plug in the middle). You can squash it into an egg shape (which is kind of how the turbo chamber is) and it isn't that bad. When you start adding features to the inside of the sphere, that is when it starts getting worse. That is, of course, only the relationship between surface area and volume. If you need to reduce volume, then doing it however you need to do it is fine. Just saying there are different ways to do it, and they are not all created equally.

V2Rocket

flycutting valve reliefs is the first thing i will do next time my motor comes apart.

shaving the head will make up for any compression lost to the cuts.

id probably start with 951 pistons though.

MAGK944

Cutting valve reliefs AND shaving the head will get you back to where you started and the valves will still hit the pistons. That is why I am looking at forged na pistons. They start off at a higher compression 9.5:1 or 10.6:1 (Euro), cut reliefs to lower the compression and provide a non-interference fit, no need to shave the head which would only serve to lower the valves and you are back to an interference engine again.

Rogue_Ant

MAGK944, have you seen this thread:

https://rennlist.com/forums/944-turb...ce-engine.html

67King

Not a 1 for 1 trade. You'll reduce five times teh volume from decking the head as you will gain from notching both valves.

MAGK944

Thanks, yes I read through that post and one person asked about using na pistons but no answer. It mainly taked about fly cutting the 951 piston and went into technical jargon about hot spots and efficiency. No definative answers. Fly cutting/valve reliefing is nothing new, been done for years without issues.

Good point, it gives us a lot of flexible opitions to reduce na piston compression to 951 stock levels after fly cutting the valves to non-interferance - headgasket thinckness, head shaving, etc

MAGK944

Here we have the three pistons used in the 2.5L. All have valve reliefs already so there should be no issues with hot spots, volumetric efficeincy, etc as they are designed with reliefs anyway. If we fly cut the 951 piston reliefs further we lower compression which can probably be recovered by head shaving, but not confirmed that we could get it back to stock. If we use the na pistons we have a lot more flexibility to enlarge the reliefs to non-interferance and get down to the 951 stock compression of 8.0:1.

FYI: This is something I pulled from the internet about fly cutting the valve reliefs:

Fly cutting pistons gets its name from the type of tooling, or cutter used to mill reliefs in the tops of pistons. When aftermarket cams are used that have higher lift or a longer duration, or when the head is skimmed to raise compression, or to accomodate bigger valves. the valves sometimes can come in contact with the pistons. This usually happens from ten degrees before, to ten degrees after TDC. Most engine builders will temporarily install the pistons, then set the head on with no valves. Then with a long center punch made out of an old valve, drop it down through the valve guide to make a small mark on the piston top. Then with the pistons disassembled, and mounted in a special holding fixture, the cuts are made in the pistons, centered on the marks made with the punch. Then final checked again to assure that they have the required clearence. Safe clearence for valve to pistons is generally .060 for the intake and .090 for the exhaust. This operation is sometimes referred to as Bridgeport cutting for valve to piston clearence, because a universal head Bridgeport milling machine is commonly used.

67King

Have you done any calculations? Intake valves are 4.5cm. They cover an area of about 16 square CM. Figuring you use at most 1/4 of that area, if you notch the valve even another 1.0mm, you are talking less than a half CC. Chamber volume is what, somewhere in the mid 70's? If all you are trying to do is get back to stock CR, you will have no problem, and the volume you add may be insignificant. Just look at how big the dish in the turbo piston is to get where it is.