mark kibort

What do you disagree with? Yes, by all means, use the arrows, it makes sure the offset is correct. If you have Asymetrical pistons like the S3 and it seems the 968 from what was said, then one side will be optimal and the other will be wrong. With the 2 valve conversion, we can correct this by spinning one side of the pistons around to be correct for the offset. IN otherwords, if the arrows point to the front of the engine, you are taken care of. if it points to the exhaust side, you have engine side wrong, unless there is no offset in the piston set up. But in that case, there is nothing you can do about it, except cut new valve reliefs. (and maybe do the other side too to keep the compression ratios identical )

The S4 has symetrical pistons, so there is no difference in the valve pocket depth, and the front engine pointing arrow that forces the pistons to spun around opposite on each side. Again, with the S3 and the 2 valve conversion, why wouldnt you take advantage of the chance to make it right?

Now, which way you orient the pistons, that was advised by Greg and I forgot which way is optimal. sounds like the greater angle on the powerstroke gives better power, which would put the wide side toward the exhaust on the driver side as looking at the front of the engine.



mk

mark kibort

So I guess my initial post was right then recapping what Greg had mentioned. So, if the narrow side faces the exhaust side on the 5-8 cylinders, this means there is a straighter angle of the rod and piston. does this induce more power but a chance for piston slap? I dont know, but Scots was built this way, and all the 2 valvers have one piston bank "wrong" and Ive never heard piston slap in any of them. So, the drawing is opposite is how they should be installed. BUT, I seemed to remember that one of the sides was correct on the Scots. So, did we do it backward? Now I'm confused. Here is a picture of what we did. Driver side (as shown) was installed in the way it would normally be in the S3 engine. Passenger side was done the opposite to match the offset. From Greg, " Install the piston so that the rod angle is straightest acting on the crank for most power". This would mean the 5-8 pistons are correct with the narrow side toward the exhaust side, and the driver side pistons need to be spun around.

Greg????

GregBBRD

Like I said...too much brain damage.

ptuomov

If this really is an important issue, why not design the block in a way that the crankshaft is offset and producing a shallow rod angle during the power stroke? A V engine would look a little like left-tilted L engine, where the left bank is taller and the crankshaft is offset to left? Assuming the crankshaft rotates clockwise.

GregBBRD

Say what?

Vilhuer

OK. Wrong way round they will go then.

jon928se

My brain hurts

Garth S

Hey Erkka,
To avoid any future brain strain, would you post a follow up pic of installation orientation - both banks please .

ptuomov

I am talking about having the crankshaft main bearings moved to the left. This is an extreme version drawn just to illustrate the point:

6.0-928S

Hammer

GregBBRD

Look carefully at this picture...does this give you the worst possible angle to the crankshaft, when the power stroke occurs, or the best?

GregBBRD

Hmmm. Need to think about that.

Vilhuer

I don't know anything anymore.

That's how arrow was in one '88 MY S4 piston I checked. Is that good or bad way to install them?

ptuomov

My understanding of this offset business is limited to what I've read on the internet. Caveat emptor.

I am planning to install the pistons with the shorter side to the direction of rotation. I am believer in keeping the rod angle shallow on the power stroke. That and $1.89 will get you a venti cup of coffee at starbux.

I think (but do not know) that reversing the pistons will alter a couple of things:

If the crankshaft is not offset, then zero-offset piston pin will bring the piston as high as possible. Any offset will lower the max travel height of the piston, no matter which way. Because of symmetry, the max height doesn't change when reversing the piston with zero crank offset.

If the crankshaft is offset, however, the piston pin offset of same magnitude in the same direction will bring the piston as high as possible. Thus, if the crankshaft is offset, the deck height, compression ratio, etc. will change when piston is reversed. Many things are affected.

Can someone confirm that 928's 90-degree V8 has zero crank offset? [Edit: I think it has to be, since the intake ports are level between banks and the bare left and right heads are interchangeable.]

Even if the crankshaft is not offset, reversing pistons will alter timing because it alters the rod angles. Suppose the engine rotates clockwise. If the short side is on the left, the piston will come up to TDC earlier in the crank angles. If the short side is on the right, the piston will come up to TDC later in the crank angles. If the cam timing is not adjusted, in principle the valves could hit the pistons when pistons are reversed even on zero crank offset engine. It seems to me, and don't believe this without thinking it thru yourself, that if one reverses the pistons from having the short side on the left to having the short side on the right, then one should also retard the camshaft timing.

Does this even make any sense?

If the above about cam timing makes sense, it would be interesting to figure out whether the relative cam timing of the left and right banks was adjusted when Porsche decided to stop installing the pistons in the different offset directions and to start installing them in the same offset directions between banks.

---

Here's an interesting calculator:
http://www.wfu.edu/~rollins/piston/offset/offset.xls

Some simulation results that seem to say that there's a tradeoff:
http://202.117.208.15/CMDNETUpload/book/33.pdf

Reversing pistons:
http://books.google.com/books?id=sZH...esult&resnum=1

m42racer

""Porsche screwed this up for so many years and on so many cars...it's just stupid."""

Without a doubt, this is the most real comment I have ever read on Rennlist!!!

And they still screw it up. On all 911 pistons, a set will give you 1/2 right and the other 1/2 wrong.