Piston pin offset
 

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.

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

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

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

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

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.

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

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

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.

http://forums.pelicanparts.com/uploa...1174427253.jpg

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.

http://forums.pelicanparts.com/uploa...1174429035.jpg

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.

Turns out the MAESTRO wrote about this issue too!

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

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.

http://forums.pelicanparts.com/uploa...1174432031.jpg

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

http://forums.pelicanparts.com/uploa...1174440702.jpg

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.

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

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

Bill

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

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

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?

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

woops...typo...964

Sorry for bringing an old thread up but one of Bruce Anderson's comments in the latest Excellence mag (April) caught my attention:-
I wasn't aware of this. He was talking about JE's but did all Mahle's (Birals, Nikasils) in all sizes have an offset wrist pin? Even the new ones from Andial?

In American V8's, turning the piston 180 degrees will increase hp/tq due to offsetting the pin for better rod angle.

Blue

Read my post on page 1. Some did, some didn't. It is a pretty strange deal, since the pistons don't come in right and left side versions, the way Chevy did it until they decided it wasn't worth the extra inventory.

Birals are a kind of cylinder. You may mean Alusils.

If you are planning to use J&Es, are you really concerned about a little rattle?

Walt

I do not know about the 911 pistons but the 930 turbo flat tops are clearly offset as stated/shown in the factory manual. Here is the quote from the factory manual.

"Because the piston pin bore is offset, the piston
must be installed correctly. The installation mark
faces the clutch -see figure."

ZC - makes sense - if you have flat top pistons without different sized valve pockets, they will not have interference issues whichever way they are installed. This was exactly the case with the VWs. This means that the offset will be in the same orientation to crank rotation on both sides, so you will get its benefits (whichever you are after) on both sides.

The mystery involves non-symetrical piston tops, where you can't do this because of valve interference (think 2.0S pistons) or messing up the air flow (CIS pistons, if the valves would still clear). Why would there be any offset on them? But on at least some there is.

No, I didn't mean Alusils. When I was asking about Birals and Nikasils, I was referring to pre-1973 meaning that original p+c's were Birals (except 2.7RS) and new Mahles today (2.0, 2.2 and 2.4) are Nikasils. Do the new Mahles have 2 flavors of pistons?



Any idea exactly which did and which didn't? What about the racing versions - 86.7mm, 87.5mm, 89mm and 92mm?



No, I'm not going to use J&Es. Are you saying that Mahles, even those without an offset wristpin, don't rattle?

Blue72

I don't think I wrote down which (other than the 2.7s of mine) did or didn't. He probably measured 84s and 80s, though he raced a 2.8 so he might have had a race 92 on hand. I've got a Mahle racing 2.7 piston somewhere, and an Alusil 3.0, which I could check. I got the other information over the phone, I think, while we were musing over this.

It was of some importance to me when I was acquiring 85mm pistons for the 2.3 I built. I, thourgh a local shop owner, was negotiating with Malvern (rip) out on the East Coast to have Cosworth make up a batch. Which they did. Seemed easier to specify the pin in the middle - that meant having a couple of spares (I think I ordered 8 for myself) would work on either side. But having the "more power" offset was tempting, as you might imagine.

I've never heard a rattle that I associated with piston slap. But my mufflers have never been very quiet.

I read that pistons should be installed with the arrow towards the clutch. My pistons did not have an arrow, but were stamped with the letter "E". According to the Bentley book, this E should "always install toward the intake port." Curiously, Porsche chose the letter E, I am assuming the abbreviation for the German word for "intake" starts with an E. I wonder how many mechanics in America thought the USA car should have the pistons with the letter E point toward the exhaust?

If this is true we have offsets oriented (for noise? or for power?) in the flat top 965 piston engines, of which all six are the same. Stamping the orientation in the "top" means that the pistons are not simply swung over the top of the engine to keep the arrow pointing toward the clutch like other Porsche engines. Instead, the pistons are turned end for end when going from one bank to the other to keep the "E" up ^.

Would this affect the way the offset is oriented? It would seem that if the offset is toward the exhaust side on stock pistons, turning the pistons end for end (keeping the E up) would accomplish that. But if we want the optimum condition, i.e. with the offset on the exhaust for cylinders 1-3 and the offset on the intake for cylinders 4-6, then the E should point down on the 4-6 bank.

Or is it the other way around? OMG, half of my pistons are upside down!

Mark
http://forums.pelicanparts.com/uploa...1312731119.jpg

oops-just saw this
 

I too have the "E" on top of the pistons for both banks, which doesnt seem to make mechanical sense. I assumed that someone had been in here before me and installed them improperly, but as my car only has 60,000 miles, that seemed unlikely.

I'm confused.

I think intake is "Einlass" and exhaust is "Auspuff", so yes, the E should always be on the intake, top side.

"E" is up
 

See a picture from a Porsche manual and if fact, the E should be up on BOTH sides. Interestinghttp://forums.pelicanparts.com/uploa...1312845234.jpg

Thanks for all the help and this has also been verified by a gold miester mechanic.

The 3.6 pistons are all made with the same offsets. This means that 3 of them are correctly offset and 3 of them are not.

Been that way for years.

In some cases, we've machined the exhaust pockets to clear the intake valves and installed the left side pistons with the correct offset to reduct thrust wear and reduce drag. Naturally, those machined pistons do not have the exact same CR as the right side and thats simply a tradeoff.

Couldn't you just machine the other side's pistons as well to even out the compression ratios? Then you have even offsets and even compression, just a little lower, but you could compensate for that in other areas.

Yes, one could do that, however in high-performance engines, I hate to lower CR any further than I have to.

Like many things in life, its just another compromise.

So, Steve in our flat-topped turbo 3.3 and 3.6 cars we could turn over the pistons "E" to the bottom (exhaust) side on cylinders 1-3 and take advantage of a piston pin offset to our advantage?

Probably would not do that in worn-in piston/cylinder sets, but if the pistons/cylinders are new...

Mark

Hi.

I have a question for my recondition porsche 911 T 1968 my piston just one flat surface piston left picture :) :

http://forums.pelicanparts.com/uploa...1174429035.jpg

And my question is : flat surface orientation intake or exhaust valve cylinder ?

Thx for all :)

Moderator – a great question and a subject that I might be able to contribute to. Wrist pin offset is done in an effort to reduce piston noise. The small amount of offset (almost always less than 2 mm) does absolutely nothing for the power produced and has negligible effects on changing bore/piston wear. It is strictly done to minimize piston “slap”, the racket produced when the piston uniformly takes up its running clearance by moving from one side of the bore to the other. The mechanism causing this is comprised of 3 factors working simultaneously:

1) Gas pressure acting on the piston dome, with resultant force centered on the piston bore axis

2) Center of gravity (CG) of the piston mass, rarely coinciding with the piston bore axis

3) Angularity of the connecting rod, contributing the lateral force that drives the piston side to side

Laterally moving the rod off-center by offsetting the wrist pin displaces the line of action of the rod (at the dead center positions) with respect to the piston bore axis. When gas pressure is applied during the compression/power stroke and the rod goes through TDC, the offset in centerlines tends to rock or “cock” the piston in the bore, versus “slamming” the piston from one side of the bore to the other as the con rod angularity changes over TDC. Conversely, on the exhaust/inlet stroke when inertia forces are prevalent, having the rod offset from the piston CG also allows the piston the same rocking versus slamming effect. This greatly reduces noise especially prevalent in air-cooled engines having lots of cooling fins as noise radiators. Fortunately in the 911 engine using similar aluminum alloys for piston and cylinder materials, pistons can be fitted with minimal clearance so noise is not really an issue. But in the days of the VW iron cylinder engines, pin offsets were more of a necessity due to the larger working clearances required.

What has been stated is a simplistic description. The forces exerted on the piston are complex during a 720 degree rotational cycle and a computer simulation is necessary to fully understand the piston’s rocking motion in the bore at a given engine speed and power setting.

Wrist pin offset is commonly defined as being toward the direction of crank rotation, or opposite crank rotation. Offsets toward crank rotation reduce rod angularity on the power stroke, while offsets opposite crank rotation increase rod angularity. In actuality, an offset of 2 mm or less has only miniscule changes in piston position vs. crank angle, side force difference on the piston skirt during the power stroke, valve or ignition timing variations bank-to-bank, etc. This is easily proven mathematically by studying the con rod/crank throw triangle, or by constructing a graphical solution. It will show crank position varies by less than 1 crankangle degree for practical offset values.

So, bottom line, it makes no difference how the offset is oriented in the 911 engine. The important thing is to orient the pistons so the domes match the heads on that particular bank and consistency is achieved.

The offset can be grossly exaggerated by offsetting the entire cylinder axis relative to the crankshaft rotational axis. This is known as a Desaxe’ engine, obviously a French invention (at least so they claim). This was a somewhat popular way to build an in-line engine in the early days; the British particularly liked this design and quite a few auto engines were constructed in this form but has now dropped from favor. By using severe offsets, all in the direction of crank rotation, con rod angularity was substantially reduced on the power stroke with the claim that bore wear was reduced. The fact is that bore wear is greatest in the ring travel area and offsets contribute nothing here. There is a reduction in piston side loads and resulting friction on the power stroke, but this is somewhat offset by higher side loads on the compression stroke due to the higher degree of rod angularity at this point in the cycle. The Toyota engine is a Desaxe’ configuration (at least to my understanding) but I think the benefits to mechanical efficiency are, in reality, minimal. The big claims as to being more efficient sound more like marketing hype from the sales department.

To conclude, wrist pin offsets are done for noise reduction. There are no discernable effects on engine performance, efficiency, or wear reduction.

The flat indentation is for intake valve clearance.

Left and Right
 

The pistons that I used in my 3.3SS are Mahle pistons. Here is a picture of one showing the orientation marking. The arrow points towards the crank pulley end of the engine. I don't recall there being any offset on these pistons, but .032" is pretty hard to see with the eyeball! These are 10.1:1, 100mm pistons in LN Engineering slip fit cylinders.

http://forums.pelicanparts.com/uploa...1491796074.jpg

Thx steve :)