ptuomov

How heavy are the stock top, second, and oil ring?

Also, what's the horizontal width for them rings?

Lizard928

If I was at home I could help you out with that. I hope you get someone to answer that question for you though.

Rob Edwards

I've got a GTS piston here with the rings on it. Let me get all the oil sludge off of it this afternoon and I'll weigh 'em on the fine balance at work tomorrow.

EDIT: Jiminy Christmas, I thought I was kidding about the sludge- check the grunge on the oil control rings:



And the slightly used piston from where they came, showing the famous non-drilled drain holes:


And a pointless pic of the top and 2nd rings:

ptuomov

Thanks, guys! Rob, you are becoming the community's main source on 928 measurements!

For the reference, I weighed my aftermarket rings for Nikasil bore as follows:

Gas nitrided steel top ring: 1.5mm x 3.5mm, 12.0g
Ferrox coated 2nd ring: 1.5mm x 4.5mm, 13.8g
Oil ring: 3.0mm x 2.85mm, 7.6g

I just weighed all eight and divided the result by eight to get a decently accurate measurement on a 1g resolution scale.

The purpose of this exercise is to figure out if these new rings make ring flutter come on earlier or later than the stock rings. Heavier rings, lower cylinder pressure, longer stroke -> lower safe redline. Based on the rules of thumb from various textbooks, 1.5mm top ring should usually be ok up to 8000 rpm in a four stroke engine. This is of course a very crude rule, since it ignores the stroke and rod length which determine the piston accelerations.

dr bob

It also ignores the piston fit. pin height and offset, actual bore size, and the lengths of the skirts, all of which affect how square the piston stays in the bore. Lots of things to think about.


Rob, looks like the lower oil control rings didn't.



Back to work...

ptuomov

Yes, a lot to think about it. I've been trying to think about this for a couple of weeks but haven't made much progress.

I understand that the geometry that affects the piston acceleration from about the middle of the compression stroke to about the middle of the combustion stroke is relevant. F=MA, so the force pushing the rings up is proportional to the ring mass and the piston acceleration. On the other side, there's the gas pressure, which presses with a force that is equal to the ring area viewed from the top times the cylinder pressure. The force down better be bigger than the force up. This all makes sense to me.

When it starts getting too complicated for me is the piston rocking or the ring twisting. I tried to read some papers on the web on the topic, but pretty much learned that I'd have to get some simulation software to get further than basic rules of thumb.

This paper claims that a bigger top ring end gap reduces oil consumption. http://ricardo.com/Documents/Downloa...onsumption.pdf. It's relevant for flutter because their graphs 8 shows the simulated positions of the rings. This software would be nice to have, but I bet it's big bucks.

Coming a full circle, now I am just figuring out if my aftermarket rings are likely to make things better or worse relative to the stock rings. The ring mass is one thing there, the horizontal width would be another.

namasgt

I have the new OEM rings (951 rings )

from top to bottom they are :

12 gr
15 gr
14 gr (oil ring)

ptuomov

Thanks. What are the horizontal widths for the rings?

Also, you mentioned some months ago that you have access to Ricardo software at school. Do you have access to Ricardo's RingPak?

Rob Edwards

Top ring is 4.11 mm wide and 1.72 mm thick

2nd ring is 4.18 mm wide and 1.98 mm thick

Oil control rings are 2.77 mm wide and 0.50 mm thick. The sandwich of three oil control rings is about 3.80-3.82 mm thick.

AO

Tuomo-

Not sure what bore you're planning on using, but if possible, you'll want to use 951 rings vs. S4. I was told by a reputible engine builder that the 951 rings are far more flexible than the S4 rings. He said that under large amounts of boost that the S4 rungs can break up because of how stiff/brittle they are.

He also said that the GT's seem to have a more flexible ring than the S4, eventhough Porsche does not list a separate part number for the GT's rings. They do, however, list a part number for the different tolerance group pistons. My GT has Group 1 pistons, but I think the S4 I'm working on right now does too. SO the tolerance group is most likely for dimentional reasons.

Just passing along what I've heard.

ptuomov

I am planning to run these on Nikasil. The 951 rings or stock 928 rings are not ideal on Nikasil, I believe. If I were to use stock pistons, I'd probably use the 930 rings since they are designed to be run under boost and on Nikasil. I haven't investigated the 930 rings in detail, though.

The rings I have for these custom pistons are thinner than the stock piston rings. The stock piston rings according to Rob's above measurements are 1.75mm x 2mm x 4mm. From one of the catalogs I see 1.5mm x 1.75 mm x 3mm, so I am confused. According to Wikipedia, the rings were changed somehow in 1993. The NPR rings that I have are 1.5mm x 1.5 mm x 3mm, and the custom pistons are grooved for those rings.

The material is also different. The NPR top ring is gas nitrided steel. I suspect (but do not know) that the stock rings are iron of some sort. Would explain the apparent density difference between the stock and NPR rings.

I am using a 100mm bore in this planned short block.

Trying to learn more about rings, but it's slow.

UKKid35

Is the top ring tapered or in some other way handed?

Rob Edwards

The top ring I'm looking at has a ~0.8mm square notch in the upper inner aspect of its cross section, while the 2nd ring has a 45o bevel on the upper inner edge.

FWIW, the outer circumference of the top ring has a fully chromed surface on it, while the second ring has a chromed finish on the lower half of the ring while the upper half has a more matte finish- can't tell if it's wear or a different kind of coating.

As far as weights go, I get the following (I ran these thru the parts washer so they're more or less clean) on the fine balance in the lab:

Top ring: 12.91635 grams

2nd ring: 16.90590 g

Oil ring #1: 2.89365 g

spacer ring 2.84359 g

Oil ring #2: 2.88099 g

ptuomov

Thanks Rob. Valuable as always.

Based on this, I am basically concluding that the ring flutter rpm limit of my NPR rings is about the same as that of the stock rings, despite the the NPR rings having narrower vertical width. No signifiant improvement there.

The friction of the NPR rings will be lower, because they are narrower and the top ring is barrell faced. The gas nitrided steel is stronger under boost and will match better with Nikasil than the chromed stock ring. So they are somewhat of an improvement on other scores.

Does anyone have any idea whent he stock rings start fluttering in some of the racier NA engines? The flutter being detected as power loss and a spike in the crank case pressure after a certain rpm threshold is exceeded.

Thanks to everyone for help! If I've misunderstood something or somehow else not making sense, please correct me. Caveat emptor when reading my musings about engines. Never done this stuff before, and just trying to learn.