so from the last drawing, that is for quiet, and for more power, do the opposite. I forgot, its been so long since I did Scots motor.

 

Doing opposite you mean the piston arrows will now point to the flywheel on both sides, right? and this is okay on the S4 + pistons since they have the same valve pockets for intake and exhaust.

 

It has been a while, but opposite from the drawings would be the narrow side would be on the driver side of each piston. (opposite of what is shown on the drawings) as far as the arrows, yes, they point to the front of the engine, and from what Ive seen and what greg brown has said, this sets the pistons up normally with the narrow side of the wristpin to the piston edege to be on the driver side. this gives LESS rod to piston angle as the piston goes down on the power stroke, thus giving a little more power. (also less side forces on the piston to the cylinder wall)

is that true ? Im not sure, its been a long time as I said.

 

From what I read Porsche used to put the offset on the exhaust side for both sides, they did it with the 911s and the 928s. apparently this will make one side correct in therms of the offset and one side wrong. (In one forum someone was saying that this will make one side wear more than the other side, I don't know if this is true).

From 87 + for 928s, the offset was corrected by having it on the Major Thrust side of the piston. That is, looking at a "V" engine from the front of the car (front belts) the Major Thrust surface is on the intake side (the valley side) for the driver side and on the exhaust side for passenger side, which is what the drawing shows on page 3, The "Narrows" are where the Major Thrust side of the piston is. The "Wides" are the Minor Thrust surface of the piston. Doing this offset like the drawing will reduce piston slap, that is (I think) the major and minor thrust surfaces swapping between each other.

Now would this type of offset towards the Major Thrust surface increase piston wear ? doesn't the Major Thrust surface see less oil compared to the Minor Thrust surface (with the crank turning clockwise on the drawing).

If it does increase wear would turning the pistons 180 degrees in their bore (this will make the piston arrows point to the flywheel, that is the pin offset (Narrow) is now on the Minor Thrust Surface) reduce this wear ?

Some are saying that having the offset like the drawing on page 3 (the way the factory intended) is wrong and the offset should be on the Minor Thrust surface. This is supposed to give a little more power. But I want to know if the wear will increase or decrease ??? (Some are actually saying it increases)

 

Just to make sure we are talking the same language. the early cars had the pistons oriented with an arrow to the exhaust side. this meant since all the pistons were the same, that one side would be correct and the other would not be. with the S4 pistons with symetrical valve reliefs, the pistons could be oriented correctly, buy using the arrow pointed to the front of the engine. this meant that the offset would follow the piston vs rotation orientiation. point the arrow to the front, and the narrow side was to the clockwise side of rotation (or toward the driver side) so that on the power stroke, the rod angle was less. It also put less side load on the piston pushing on the cylinder wall. so, more power, less wear. The arrow facing rear ward, could be done to increase the angle, and side load, and make the engine quieter. (with more wear potentially) This is what I get from the comments greg brown made, and I hope I am understanding it correctly.

mk

 

I think this says it all.

case closed?

 

on the early cars the arrow pointing to the exhaust would make the narrow side on the exhaust side for both sides, correct ? okay
now on the S4 + cars the arrow is supposed to point to the front of the engine that is the front belts as the WSM says. This is shown on the drawing on page 3, exactly like what factory is saying to do.

Now your saying the Narrow side towards the driver side, Clock Wise crank motion, Which is opposite of what the drawing is showing on page 3, therefore opposite of what Porsche is suggesting to do. Narrow side on exhaust side for the driver side and on the intake side for the passenger side, therefore the arrows will now be pointing to the flywheel.

 

For greater power I believe you want the piston set up to give the greater rod angle. by increasing the rod angle at the begining of the power stroke it gets the rod closer to perpendicular to the crank throw earlier in the stroke while the cylinder pressure is still high. the cylinder pressure will give the greatest torque when the rod and crank throw are perpendicular. the negative effect is that you increase the side loding on the piston and therefore piston/cylinder wear.

By offsetting the rod to reduce rod angle during the power stroke you will minimize piston/cylinder wear, by reducing the side loading on the piston, of course this is at an expense to torque.

 

well, lets confirm this first. when I built Scot 5 liter, using 5 liter 86 pistons, i only changed the orientation of the diver side pistons. so, whatever was on passenger side, exhaust side, as far as piston orientation, was now on the intake side on the driver side. (because we flipped the pistons on the driver side to have the same offset). Greg brown said that the orientation of the S4 was incorrect, and with the early cars, mixed side to side. I believe we measured the passenger side to show that the exhuaust side of the piston on the early cars , was the wide side. can you confirm?

mk

 

this seems to be contrary to what GB is saying.

can you confirm?

 

Mark This is quoted from you
"Thats it, you win!

So, normally all the pistons will face with the exhaust cuts (the smaller cuts) facing down, right? right. However, there are piston offsets (the pin is closer to the exhaust side of the 4 valve piston, or the 2 valve cut side of the piston.
this means the offset is correct for the driver side bank and totally wrong for the passenger side. However, this obviousy wasnt a big problem, or porsche wouldnt have done it. Yet, on the S4, notice there is an arrow toward the front of the engine, and the valve cuts are the same , both intake and exhaust. SO, this means the S4 cured the problem and gives slightly more power. (in theory)

I was told that this gives a slightly better angle of the rod to piston during the power stroke.

so, the reason the #2 and #6 piston is different, is because now , they are the same!

the odd "dent" on the #6 piston is a shadow or jpeg artifact. Ill check! that surely would be a bummer if it had a dent like that!

Mk "

from this thread
https://rennlist.com/forums/928-foru...ground-up.html


So the narrow side should be on the exhaust side for the driver side and on intake for the passenger side. reverse of the drawing on page 3 of this thread and reverse of what Porsche wants us to do. (the drawing is how all the S4 + engines are assembled from the factory)

 

I remember! I also contradicted myself in this thread too. I dont think the driver side is correct, as you can see from the pics, i reversed the piston orientation on the driver side pistons and not the passenger side. (#5-8)

I have to fall back to GB's comments who does this all the time. from the drawing posted on this thread, it looks like the better rod angle is to have the wider side of the offset of the piston on the driver side of the piston.(or exhaust side on the driver side).

I have a 2 valve piston in the garage. Ill measure the offset and orientation and get back to you. I really dont remember, but the pictures do tell a lot.

mk

 

I just did some more reading, and I think the factory way for the S4+ cars (like the drawing on page 3) is fine. apparently some racers used to do the reverse of what is shown on the page 3 diagram to get a little more torque and power in expense of more wear. They did not care about piston slap, since they took their engines apart between races to replace worn out parts. Almost every car forum I read about reversing whats shown on that picture on page 3 says that wear will increase.

originally I thought that the narrow side should be on the exhaust for both sides, due to some misunderstanding I got talking to the machine shop guy that was doing the balancing for my engine. So I got confused during measuring and assembly, but I found this thread and realized it should be like the diagram shown on the last page.

 

Ok, just measured the 85 euro pistons . Yes, you are right. the narrow side is on the exhaust pipe side. its about a 3mm difference . I would think the 85 4valve pistons have the same orientation. As seen by our build of scots 5 liter 2 valve, ( https://rennlist.com/forums/6573613-post17.html ) This picture shows the DRIVER side in place, with no change to piston orientation.

we swapped the orientation on the passenger side, so that the narrow side was to the right.

So there is the proof that we did the change based on Greg Browns advice to get better rod angles on the pistons for both sides. the drawing that you refer to is opposite how you want the pistons to be oriented.

mk

 

^^^^^ you did it right for better torque and power on that engine.
Thats what its said at least on the first page and from information from the web.