Piston pin offset

[304065]

I was reading the spec book the other day (in the smallest room in my house) and noticed that the early 2 liter engines have the piston pin offset .8mm. The pin is moved off-center, toward the exhaust side. Turns out this is a fairly common design feature among engines, insofar as it minimizes piston slap when the piston reverses direction at TDC. Also, offsetting the piston to the intake side will decrease rod angularity on the power and induction strokes and increase it on the compression and exhaust strokes.

Does anyone know any more about this? Has anyone specified an amount greater than .8mm when custom JEs are ordered? Or less? I note that many of the technical drawings on the JE web site do not have an entry for pin offset.

edited to change the word "crank" to "piston" in the first paragraph and to change the offset to the exhaust side.

[ChrisBennet]

Does this mean the early 2 liter had 2 flavors of pistons - one offset for the left bank and the other offset for the right bank?
-Chris

[euro911sc]

Very interesting... I always thought it was all supposed to be right down the middle of everything...

I guess it depends on where the offset is done. If it is in the piston and they are not symmetrical about the axis of the wrist pin then, yes you have two different pistons if the offset is to be in the same direction on both banks. If the offset is done in the rod, then you have six rods that are the same, but directionally dependent in terms of installation.

The interesting thing is that PET shows QTY of 6 in both pistons and rods so if there is an offset it would be in the rod. I do not have a factory manual for 66-68... what does it say about installing?

Best regards,

Michael

[racing97]

Offset always goes to the opposite side of thrust. Mahle has been known to confuse their markings on some pistons.
That would be down on 1-3 and up on 1-4.

regards

[psalt]

Very interesting... I always thought it was all supposed to be right down the middle of everything

If you are interested.......There are several engines with offset bored cylinders. Take a look at the Prius engine, which has just about all the known efficiency tricks in one bundle. I found a discussion on this topic in "The Modern Gasoline Automobile" (1916 ed), they compared it to a bicycle crank, with more of the force used in turning the crank around TDC, but the jury was still undecided about the benefits. I guess at 8 cents a gallon, some ideas have to wait awhile.

Paul

[Walt Fricke]

You are inquiring about one of the enduring mysteries of life (or at least of pre-964 911 engines). I once quizzed Bruce Anderson about this, and he said he had noted these things, but even with his factory contacts couldn't make sense of it.

It starts like this: on the plain old VW boxer, the wrist pins are offset. There is only one flavor of piston, with an arrow stamped on its top. The arrow points to the flywheel. Since these are flat topped pistons with no valve pockets, it doesn't mess anything up in the combustion chamber whichever way you install them. So the offsets are always the same relative to the crank rotation when the arrows are pointed the same way. In essence you are taking the same piston and rotating it up over the top of the engine to the other side as if it were connected to the crank. What was the top side of the crown is not the bottom.

The VWs had the offset aligned to produce "anti-slap." Formula V guys promptly reversed this to gain a few more ponies by improving the rod angularity when it counts, at the expense of more noise (and, one supposes, more piston skirt wear).

Per Smokey Yunick, Chevy did this for a couple of years, but they had piston tops that were not symetrical. So they had to have right and left side pistons. Clever tuners learned to reverse the installation. Eventually Chevy tired of this, and moved the pin bore back to the middle.

Which is where you would expect it to be on a Porsche 911. You will look in vain (at least I have) for any mention in the factory manual, or any secondary source like Frere, for mention of how to know if you have right side or left side pistons. A buddy said something like this was involved in 356s, but that's out of my realm of knowledge at this level of detail. The only reference I found was in one of the update pages for volumes 1 or 2 of the workshop manual, stating a change in the offset.

When I pulled the Alusil pistons out of my 2.7, just for the heck of it I measured the offset. I did it quantitatively, as I didn't have the tools (or perhaps the interest) to do an accurate measurement. But it wasn't hard to tell that they were not centered. But all six were cast exactly the same. Mentioning this to an engine builder buddy (who was puzzled) led to his measuring some pistons he had around. Some years/models had offset, some did not.

But it appears to be fact that Porsche had offset, but only one flavor of asymetrical piston top/offset combination, so our stock 911 engines have anti-slap on one side, and more ooomph on the other. We can't keep it the same because the piston tops are not symetrical, and we'd end up with the exhaust area of the piston up where the intake valve needs room to be.

There are no markings that I know of indicating which side a piston should be on for Mahle or KS pistons (as opposed to the VWs, where every reference to assembling an engine stresses getting this right and there are the nice arrows).

This all seems unfathomable. Old Dr. Porsche designed the VW, and certainly Porsche knew what VW was up to, maybe used offset in some 356s, and understood (way better than I do) the values and vices of wrist pin offsets. And Paul amazes us (me, anyway) by pointing out a 1916 treatise on bicycle cranks (I love these discussions). So this wasn't some new, not widely known to engineers, effect.

So what gives?

No one (including on this forum) has ever been able to answer this question concerning 911s. But if you have a variety of types of stock pistons lying around, take some measurements to see what has what offsets. Then you'll be as puzzled as I am.

No doubt you could have J&E cut you right side and left side pistons (and install them to make the most noise, of course). And you'd want to stamp an arrow or something in the crowns so you wouldn't get mixed up. Otherwise, naturally J&E leaves the pin in the middle. I contemplated this when having Cosworth make some 2.3s for me. I had 8 made, allowing for disasters (which happened), but decided that I was better off being able to use each spare on either side so kept the pin in the middle.

If, in fact, all this was hit and miss, say for the CIS motors, due to some miscommunication, and Mahle or KS made some sets offset toward the intake side, and some toward the exhaust, a guy could maybe get a leg up by rounding up 3 good used ones of each flavor, and building an optimized motor. All of us racing these engines in stock classes would love a few extra and legal HP, at least until everyone caught on.

Walt Fricke

p.s. - I don't understand how rod design could affect any of this - is not the force going to travel from wrist pin centerline to crank throw centerline no matter what the rod looks like? We are not talking about cylinder spigot bores which are not centered on the crank throws end to end. But since I can never remember which is anti-slap and which is more power, I know I can always stand to be enlightened on one thing or another.

[dion914-6]

Originally posted by Walt Fricke
If, in fact, all this was hit and miss, say for the CIS motors, due to some miscommunication, and Mahle or KS made some sets offset toward the intake side, and some toward the exhaust, a guy could maybe get a leg up by rounding up 3 good used ones of each flavor, and building an optimized motor. All of us racing these engines in stock classes would love a few extra and legal HP, at least until everyone caught on.

Nice analysis Walt. And this is exactly why I stopped racing Formula Vee. All of this adds up to $8K, 55 hp engines. But it does remind me of the goodol' days of yanking the engine between races for a quick bearing change and throwing it back in for another run.

Dion

[psalt]

This all seems unfathomable. Old Dr. Porsche designed the VW, and certainly Porsche knew what VW was up to,
So what gives?

Hello Walt,

It is probably just the next better idea on top of the first better idea. There is another reason to offset the piston pin. It alters the time that the piston and rod end each reach TDC and spreads the loads. I think a similar asymetrical situation exists for Subaru engines and the hassle and expense of handed pistons is not worth the slight gain in performance or noise.

I don't think Dr. Porsche had anything to do with it, didn't Karl Rabe design the VW engine ? Like most things that make a 911 a 911, rear engine, horizontal cylinders, forced fan cooling, aerodynamic monocoque body, torsion bars, etc. none of them are Porsche ideas. When you get past the journalistic fluff and national pride and study the actual history, all these ideas were in the hopper way before the VW. There are only three guys that invented everything, and the best "ideas" are the copies without the errors.

Paul

[304065]

Well isn't that interesting. So the major thrust surface ends up on the other side of the piston depending on which side of the crank it's on. Guys like me love a diagram, so here is the one I made.



Now, looking back at the images of the various pistons, you can see that even the 911T pistons had different valve reliefs. And the spec book claims the offset for the S, which we KNOW had huge differences in the size of the valve reliefs.



So I'm with Walt: unless we hear about right-side and left-side pistons, I've got to assume that they were quiet on one side and noisy on the other.

[304065]

Turns out the MAESTRO wrote about this issue too!

http://www.hcpresearch.com/quiz19.html

[304065]

One more. Now I get it. With the pin offset to the exhaust side, the piston rotates around the pin to preload the (new) major thrust surface against the cylinder wall. If it were offset to the intake side, it would preload the opposite side, but the rod would be straighter on the power stroke. Corrected my first post, I think Porsche's offset was toward the exhaust to quiet things down, this being an aircooled engine.

[304065]

One of my slugs. Can your eyes detect .8mm?

[psalt]

So, all we need to do to get that unfair advantage is install three pistons upside down, flip the heads on that bank, run one exhaust out the engine cover, maybe three Miller updrafts under the car......Should be good for around 1.602 hp (corrected). Best damn list in town.

[304065]

Ah Paul, but the scrutineers would be able to disqualify you just by listening to the car!

[wrljet]

Originally posted by psalt



I don't think Dr. Porsche had anything to do with it, didn't Karl Rabe design the VW engine ?

Paul

I believe Franz Reimspiess designed the VW motor (as well as the VW logo).

Bill

[psalt]

I believe Franz Reimspiess designed the VW motor

I can't tell from what little I have on the subject, but it looks like Josef Kales designed the air cooled flat four for the Type 32 and Reimspiess added the ducted forced fan cooling used in the Tatra T11 (1921). VW settled the lawsuit with Tatra for 3,000,000 after WWII, but the details are sketchy.

Paul

[Walt Fricke]

Paul offers the first explanation for the way Porsche dealt with offset (one way on one bank, the opposite on the other) on the 911s that I have seen: spreading loads by keeping the TDCs (slightly) apart. If the effect of a small offset on slap/power is small, then I can see how the power unbalance side to side could be overmatched by this other gain. Since I don't have the skills to quantify any of this, I'll stop being so puzzled and assume this is so.

Walt

[namasgt]

So looking at the drawing, the offset should be on the exhaust for the right bank (looking from the pulley side) and on the left bank the offset should be on the intake side. having the offset on the exhaust on both sides is wrong. apparently they did the same with the 928 engines up to 87 and up they fixed it on the 928. However, the pistons have arrows that should point to the front pulleys, this will make the pin offset to be on the intake side for the driver side bank (looking from front of the car) and on the exhaust side for the passenger side. This means the offset is towards the Major Trust surface of the piston (those red marks on the drawing above). But, like I said on the beginning of this paragraph some are saying that the offset should be opposite the major thrust surface of the piston. The 87 + 928s have the same exhaust and intake valve pockets (same size cuts), so turning them around shouldn't be a problem.
So which one is right offset towards the Major Thrust Surface or offset towards the Minor Thrust surface ?
Will the life of the engine decrease if I put the offset on the minor trust surface, opposite of what the factory did?

[Chris Galtress]

The 3.6s (963 and 993) have a 0.9 mm offset per the Porsche Technical Manual.

[Chris Galtress]

woops...typo...964