Pelican Parts Forums - Piston pin offset

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Fred, So on your 3.3SS pistons, 3 must have been labeled from Mahle to use on the 1-2-3 side and the others 4-5-6 side, Right? Seems like that's the only way to end up having the arrows all pointing in the same direction and keeping the intake pockets up. I always though the arrow was to point toward the flywheel.

Piston orientation

I asked Mahle who said arrow points to pulley. And yes, arrows on 1-3 are different from 4-6.

Thanks Fred, good info.

Dave - I think it would be hard to persuade a Formula Vee engine builder, with a dyno at his disposal, that wrist pin offset can't be used to add useful horsepower.

To me the complete oddity of all this is Porsche producing asymmetric crowned pistons which all have the same offset, small though it might be. To date no one has suggested a reason for doing that. If you don't want right and left side pistons, why not just put the pin in the center? While the success of these engines does back up the notion that the effect is minimal, why have the offset to begin with.

My supposition is that engineers started contemplating where, relative to the wheel center, to locate the cylinder early on in steam locomotive design, and figured all that out for their purposes. All that engineering was well developed when the internal combustion engine came along.

Some of the latest Porsche engines do finally have asymmetric pistons which means they will be quieter when cold.

One of the other benefits of pin offset is that the inertial and shock loading's are reduced due to the pin offset and the direction of the offset doesn't matter in this case.

This allows the use of lighter pistons or the use a higher revs. The reduced piston slap noise may be less significant.

By making pistons with a wider pin tower it may be relatively cost effective to make asymmetric pistons as you would only need a single set of tooling but the piston may be heavier than absolutely necessary.

Chris - are you saying that there is a positive benefit from pin offset even though it is offset one way on one bank, and the other way (so to speak) on the other bank? In other words, only one piston variety?

So perhaps Porsche might have had this in mind? But if this allows lighter/higher revs, why wouldn't all race pistons do this?

Quote:

Originally Posted by Walt Fricke (Post 9547597)

I believe that this is the case.

The piston and half of the connecting rod stops twice per crankshaft revolution, even though the crankshaft continues to turn.

This means the piston and top of the rod accelerates from a stationary position twice per revolution whilst the crank and the bottom of the rod rotate continuously.

This acceleration creates a stress on all three of the parts, and these stresses increase with crankshaft rpm.

If the piston is mounted to the connecting rod with a small offset then the piston reaches top dead centre at a different time to the connecting rod, effectively spreading the shock loading over a greater number of crankshaft degrees.

I think aftermarket pistons are normally made with a zero offset as only a limited number of forgings are available to cover a wide range of pistons and it has just a simple matter of cost.

JE have recently introduced a range of asymmetric pistons an would probably have a much deeper understanding of the influence of pin offset.

Quote:

Originally Posted by Walt Fricke (Post 3168829)

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.

The thing is with the old VW/356 the valves are on the bottom side of the combustion chamber and the spark plugs more towards the top so the flame front travel is not as symmetrical as say the 911 motor which has a Hemi head with the plug in the middle so the push down on the piston is way more even going down. There is not such a thrust side to the event that you would have in the older pushrod engines so Porsche probably did not care about aligning this for production engines (on most cause to expensive to make a LH and RH piston due to production tooling required). I would think that you might have noticed they did had you and your friend measured pistons out of their race engines but I can't say that for sure!

The Symmetrical push for suck, squeeze, bang, blow!

Regarding Suck, Squeeze, Bang (and we are talking about the Bang part here), Blow...The thing is with the old VW/356 the valves are on the bottom side of the combustion chamber and the spark plugs more towards the top so the flame front travel is not as symmetrical as say the 911 motor which has a Hemi head with the plug in the middle so the push down on the piston is way more even going down. There is not such a thrust side to the event that you would have in the older pushrod engines so Porsche probably did not care so much about aligning this for production engines (on most cause to expensive to make a LH and RH piston due to production tooling required). I would think that you might have noticed they did had you and your friend measured pistons out of their race engines but I can't say that for sure! As some mentioned it was used on some of the higher power variants so I just wanted to add that to my response. Speaking about the Bang part here not the crank angles. Most diagrams show the push being the same across the pistons surface during combustion which is not so in practice.

My experience with VW rods is the pin has no offset, it is the piston that has the offset.
They also have an offset on the big end, that centers the small end in the piston, the rods have a bump on the beam that must face up.

Walt Fricke;10255245]Thunder BayP I don't have a VW head handy so all I remember for sure is the ports are top and side, not top and bottom.

But while the air cooled 911 heads are hemispherical, the plug I nowhere near in the middle of the chamber if one takes middle to be the center of a circle. The water cooled later engines have the plug there.

If the intake is 0 degrees looking up at the head, the plug enters at about 270 degrees, which is pretty much in line with the crank center line and the wrist pin at TDC and BDC. I would expect the slap noise would be pivoting around the wrist pin, and thus in an up and down motion. So I guess flame spread would not produce differential effects on rocking the piston? Or doesn't it matter - since most of the torque is created after TDC, there would be no rocking on a centered rod boss, but it would rock one way or the other with an offset boss?

Chris Seven gives his take a bit earlier in this discussion - reducing acceleration stresses.

The pin offset, machined into the piston not the rod, rocks the piston at TDC to soften skirt slap.
Two reasons I can think of why not on a /6 is the flat six is a naturally balanced engine and runs tighter tolerances on the skirts than a T1.

This is a pretty old thread, but as I am studying up for my build it's not really been answered. As I am not building a high power engine, but hoping for a reliable engine with bolt on mods to about 330 hp I may turn the pistons so all are set up for reduced slap.
Anyone have the final answer?

Reclino - My advice would be either to get pistons with no offset, or get whatever the piston maker you are using says you should use.

Reliable and 330 HP on an air cooled engine of less than 3.6 liters is kind of a contradiction in terms, isn't it? The gist of this discussion is that this just doesn't matter for either racing, or street purposes when you get to modifying stock engines. They are going to make more noise unless you put on significant mufflers (like, say, stock matched to a stock engine from the factory). The several technical responses (not mine) from engineers, and Steve Weiner's response as an engine builder of considerable experience, contain the best discussion you are going to get. There isn't any such thing as a final answer.

"reliable engine with bolt on mods to about 330 hp "

I just chuckled and moved along!

330hp? why 330? not even close with bolt on mods
Look into building a turbo or LS conversion.

My car is a 1986 930, putting in SC cams, I have a billy Boat intercooler and rarly8 muffler, it seems the limit of the US fuel head is about 340-350. My thought process was with a wide band O2 I could turn boost up just a bit and with normal tuning I could get around 330 hp. Please feel free to explain what I am missing.
David

Reclino - you didn't say you had a turbo. You do realize that there is a separate forum for turbos? Not that it is a major sin to post here instead of there, but explaining would have been helpful.

However, fiddling around with piston offset doesn't look like it is worthwhile, and the discussion gives you what you need to decide.

Reclino, the simple answer is that for mildly built turbo engines there is no benefit. When optimizing production engines where every 1/4hp will give you a leg up on the competition (55hp Formula VW), well worth it. On a turbo engine adjusting the boost up .02 bar is going to give you a better result than trying to optimize pin offset. If you really want to improve your engines performance, you might want to get yourself a set of 8-1 pistons. JE also has asymetrical skirts available (not sure they have them for Porsche yet) that would do as much for hp as piston pin offset would. There have been endless discussions for all types of engines as far as pin offset, rod ratio and all kinds of other "trick stuff" that always end up discarded. A 330-350hp turbo engine will see no quantifiable benefit.

I was not proposing doing this for a hp benefit. If reducing slap reduces the shock load on the pistons by a small amount. Then why not reverse 3 pistons so all of them see the benefit. I have had the stock cylinders re-plated, the rings are total seal with Napier profile on the 2nd ring. The stock pistons at 68k miles had about .0005" of wear on the skirts. They are going to have to wear into their "new" cylinders anyway, what would it hurt putting 3 in backwards.