mark kibort

There is an offset on the 85 US 4 valve pistons that seems to not matter how they are installed. the pistons are all the same, yet, they are installed with the exhaust cuts pointing down. this means, the 2mm offset actually is correct on one cylinder bank and opposite for the other. Ive been told the offset is to couteract piston slap. but, if the offset is the wrong way on one cylinder bank, as it would be on a V8, is there any problem? Most say, it doesnt matter. interesting, as it shouldnt as we pulled out 85 pistons out of a engine and they are all the same.

I guess with the s4 pistons, the ones with the equal cuts for exhuast and intake, there is an arrow pointing to the front of the engine. this means the pistons are installed with the offset correct as the opposite side of the engine, the pistons will be reversed to have the arrow point to the front.

my question is, is this something to think about when doing piston valve pocket cuts, where you have a choice to cut them so the offset is correct for each cylinder bank, or is it such a non issue, that you shouldnt even worry about it?
this is what we are doing with Scot's engine as he is using 85 pistons on a bored out 4.7 block. we are almost ready to do the valve pocket cuts.

Im wondering where the offset came from in the first place. Was it a 944 thing, and they just used the same type of pistons and allowed one side of the engine to have different piston offsets than the other when used in a V8?

thanks for any comments

Mk

Vilhuer

Bump for old thread. Did some offset measurements today and it seem '87 S4, GTS, 944 S4 and 968 pistons all have this offset. Its in region of 1.5 to 2.5 mm if my measurements are correct. Different piston types have different offsets. Will do more thorough measurements later to find out if its always same within single piston type or not.

What is right way anyway? Looking from the front of the engine offset clockwise or anticlockwise or towards exhaust on both sides or towards intake on both sides? In another thread C.F. says Mopar has them anticlockwise but to gain power clockwise would be better as I understand it. Is it anticlockwise for low wear and clockwise for more power?

Other intersting thing about this is that there is only one part number for each piston tolerance group. Did Porsche do S4 pistons on random or are all spare part pistons right on one bank and wrong on another?

How about 104mm 944 S2 and 968 pistons? Will they be right on one back on 928 engine and wrong on another? Do they have same size valve relief cuts? If not, both intake and exhaust would need to be cut to intake size to be able to install them in way one wants.

918-S

interesting topic.

mark kibort

On the S4, where there is an arrow pointing to the front of the engine, the offset is correct on both sides, and pistons become different on each side.

On the 2 valvers, or the pre-s4, where there are exhaust valve reliefs that are different, and arrows that point the piston toward the exhaust valves, both sides of the engines will have different piston offset orientation and one side will be incorrect, but porsche didnt see a big problem with it so its probably not a big deal.

where you have an arrow that points to the exhaust side, (4 valve), or a piston that has 2 valves on one side of the engine that orients one way, you have an offset issue.

when you cut the smilie faces on different sides on the piston for a two valve conversion, so each bank has the correct offset, you solve the issue.

I guess the goal is to optimize rod to piston pin angle on the power stroke. the driver side is correct, the passenger side has to be flipped around, and that has been said to add 10hp on race engines. probably less on our 928s.

Mk

Vilhuer

Still, there is only one part number for replacement piston in each tolerance group.

mark kibort

yes, BUT, if you notice, the piston valve pockets are the same size top and bottom. THIS means that the pistons have an arrow for the front of the engine. if you think about it, this means that the offset will be the same on both sides (correct) where on the euro S2 and S3 5 liter, this is not the case. one side is right and the other is wrong. (ie differnt) . maybe this is one of the reasons the S4 makes more power than its valve size would indicate it should make. after all the 85 has the better cams (ie GT like) and just a tad lower compression.

Mk

GregBBRD

Mark:

I thought that I had explained this in detail to you and it was solved....why are you asking this question?

gb

Vilhuer

Oops, thats right. On S4 one part works on both sides. Do 944 S2 and 968 pistons have same size valve cuts? If no-one knows I quess its up to me to check it out.

Mark asked this long time ago and I dug his question up now. Can you either point to your answer or write short explanation again please. Viewing from the front of the engine, is it anticlockwise for low wear and clockwise for more power?

PorKen

Pistons with Off-center Piston Pin Bores (Engines M 28/43, 44)

The 100 mm diameter pistons have piston pin bores which are off-centered by 1 mm, in order to avoid tilting piston noise.
Valve pockets for the valves are now of the same size on both sides, so that the same pistons can be used on both cylinder banks with the same location of off-center to the pressure side of pistons.
An arrow shows the correct installed direction (forward direction).

mark kibort

I was trying to explain it to the other folks that still seemingly have questions

great information by the way!

Lets see if it makes any more hp on Scots 5 liter part euro engine

thanks

MK

mark kibort

Still kind of a grey area for me. But, if you think about it, the crank rotates clockwise. so, on the power stroke, the rod to piston angle is tightest on the right side of the engine and has more angle at the bottom of the stroke. if you offset the pin to the right on the pistons (as viewed from the front of the engine) you can see that that angle would improve. This, im told, gives a little more power. There are a couple of more factors that i dont totally understand as well, and that being the benefits of the pistons staying up top longer, and the piston "slap" reduction. there is something also called the "Jacoby" effect that is addressed with piston offset. With scots engine, we use the offset like the S4 has on both sides. i think this gives more power, but if it gives less wear, i guess thats good too. 1-4 gets the change, while the offset on 5-8 stays as it comes from the factory on the euro S2 or US 85s.

mk