piston rings & sealing against cylinder wall

[gmax137]

I had someone telling me about drilling small holes, down from the piston crown into the small space "behind" the top piston ring, purportedly to allow the combustion pressure to get into that space and force the ring out against the cylinder wall. The theory is that the gap between the top face of the ring and the adjacent land is too restrictive to permit this pressurization from occuring fully/fast enough. Anyone here heard of that, tried it? Does this really work? This is in the context of race engines. I have difficulty believing that blow-by between the ring & the cylinder is a big concern (that would rob power) in a race engine, which has fresh rings, good cylinder walls, etc.

Thanks

 

[evelrod]

The practice of "piston porting" is claimed to work and is common practice among several prominent (really big name) engine builders.
In my experience, on small displacement road race engines, the small holes tend to carbon up. At least on the one engine that I used them on. I'm sure there is a performance advantage to match the additional expense on some of the big bore drag race engines, I just did not see any difference in performance on mine.

Perhaps Pat P. or someone closer to drag race engines would be a better authority on "piston porting".

Rod



Rod

 

[ivymike]

yes, I've heard of it, and it is reportedly effective (especially in engines with particularly tight ring side clearances).

The big driver, as I understand it, is not an excess of non-conforming areas between the ring face and the cylinder wall, but rather radial "flutter" of the piston ring under conditions of rapidly changing cylinder pressure. With four bolts per cylinder, the bore distortion will tend to have fourth-order and lower components, to which most compression rings could readily conform. The concern is that cylinder pressure will rise much more rapidly than top groove pressure due to restricted flow between the top land of the piston and top groove. When the pressure acting on the exposed portion of the ring face is sufficiently high compared to the pressure in the groove, the ring can lift off of the cylinder wall and allow large amounts of gas to bypass. This problem is compounded if the piston top land distorts significantly due to temperature and/or firing pressure, further reducing ring side clearance.

 

[Compositepro]

I find such a claim really hard to believe from a technical perspective.

 

[evelrod]

I don't like to think of myself as an expert, but I have had a bit of experience with many of the less than traditional building practices. Examples of piston porting, gapless rings, exotic coatings, etc. I've done a few myself and been around a couple really big name engine builders that use all these practices in their race winning engines. Certainly enough evidence that the practices work...at least in limited applications.

Now, Compositepro...You have been on this forum for enough years to know who we are and you certainly must know we are not going to let you off with such a flippant post---Please explain yourself, sir!

Rod

 

[gmax137]

Once the piston is beyond TDC (ie, on its way down) don't the drilled ports actually bypass the ring? Provide a path from above the piston to the underside of the ring (since the ring is hard up against the top of the groove). Maybe I don't have a clear picture of the geometry involved. Do the ports connect to the grove at the back, 'inside' the ring, or do they come in at the top of the groove, such that the ring closes them off on the downstroke?

 

[ivymike]

on the firing stroke, the ring is firmly pressed against the bottom of the groove, due to gas pressure above the ring which significantly exceeds the combined effects of pressure below the ring, friction on the ring face, and ring inertia (the latter interesting only near TDC).

 

[Compositepro]

I didn't really want to get too detailed but I can't imagine that small holes drilled in the piston can even come close to to the amount of flow that can occur through the annular gap between the piston and cylinder above the piston ring. This gap can be as wide as desired since sealing occurs below the ring. I just immediately think of many other approaches that seem superior. I would say that drilling holes won't hurt anything (except cause stress risers in the piston).

Another major consideration I had in my comment is that this technique is something I have not heard of and is not used commercially to my knowledge. In a business as competitive and mature as auto racing, when I hear about secret performance enhancing techniques that are discussed in public forums my reaction is "snake oil". I don't mean to be insulting to anyone but I'll believe it when I see some data. People can easily be made to believe almost anything.

Of course drilling holes will also lighten the piston and therefore improve performance. If the piston and rings are really poorly designed to begin with then maybe drilling holes will help. But, again, I see better approaches.

 

[ivymike]

The gap cannot be "as wide as desired," because excessive side clearance will lead to increased wear and ring breakage. Excessive side clearance is also commonly believed to result in degraded sealing performance in the short-term, and as a result race engine builders will often select axial clearances as tight as practicable. A drag race engine's piston will operate almost exclusively in a state of transient thermal loading. The shape of the top land and top groove will change significantly from the start of the race to the end of it. Axial clearance at the groove OD will likely decrease significantly as the piston heats up (assuming there is room), and it is not uncommon (at least in development of automotive pistons for production applications) for "ring pinching" to occur during transients. Under pinching conditions, the axial clearance at the OD of the groove goes to zero... it seems to me completely reasonable that having "ported" pistons would provide a better seal under those conditions.

About 10 years ago I was asked to evaluate such a design for a racing team sponsored by a major OEM.

Ported pistons couldn't be used in "production" applications because of the carboning concern Rod mentions (and the additional cost of multiple drilling operations).

What superior approaches do you have in mind?

 

[patprimmer]

As far as I know, ported pistons are only used in drag racing where tear down and inspection occurs every few miles or less than 1 hours operation, so carbon is not an issue.

It is mainly used on low tension rings.

I agree with all that ivymike says.

I also think that rings distort or rock a little in the groves, and to limit this, the axial clearance above the top ring needs to be minimal to avoid flutter and twisting and breaking at very high engine speeds. Serious drag racers run piston speeds higher than F1 as they do not need to last nearly s long. A race might be 700 or 800 turns of the engine. Most other forms of motor sport involve tens of thousands of turns and street motors do several million turns of the engine.

Only high budget teams tend to bother with ported pistons and it is done 2 ways. One is to bore about 8 or 10 holes from the piston top to the back of the ring grove. The other is to mill notches in the edge of the top land so you have areas where there is much more clearance between the piston and the bore, but still substantial areas to support the top outside area of the ring.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[gmax137]

Well one thing I have learned from this thread is that I don't know much about rings. Can you guys point me to a good treatment of ring dynamics? I was surprised to learn that the ring is forced against the bottom of the groove while the piston is descending. In my mind's eye, I saw the piston "dragging" the ring along for the ride; apparently it is the other way around. Like I said, I don't know much. But I'm interested to find out more.

Thanks to all who responded, this is a great forum.

Gregg

 

[automatic2]

This practice was not all that uncommon during the big hp era of the past generation. As mentioned, these engines where tore down after every event to diagnose wear and tear.

It was also mandatory to align bore with a torque plate mounted to simulate mounted heads.

Was the practice of directing combustion pressure to the backside of the rings effective? One must first delve into the 'black magic' of hotrodding to understand that every theory in the book was at some point applied in the quest for response. Radiused rings where a companion to the pressure assist.

 

[Tmoose]

gas ported pistons

sometimes down thru the dome to intersect the back of the ring groove.
Alternatively, a series of grooves milled into the top (non sealing) face of the ring groove

Bill "Grumpy" Jenkins talks about them a bit in his "Chevrolet Racing Engine" book.

http://g-ecx.images-amazon.com/images/G/01/ciu/da/59/4c5cc060ada0431d4a6cb110.L._AA240_.jpg

They were hot high rpm drag race technology in the mid/late 70s when combined with tight ring side clearance, reduced back clearance and crankcase evacuation. The shop I worked in drilled and milled a lot of tiny holes in pistons for folks who asked. Even pistons with standard ring clearance.
It was almost like those stainless steel mufflers with large openings are today.

AIR in some piston or ring tech article Kevin Cameron mentioned the late new England cam grinder Domenic Ravesi was developing an engine and among other things tested very "tight" ring side clearance. I believe the result at some tighter than stock value was power shutting off at high rpm.

 

[Tmoose]

Looks like Wiseco still gets a few questions about them.

http://www.wiseco.com/FAQs.aspx

number 14

 

[larrylcoyle]

Gas ported pistons are still used today by some of the professional engine builders that build custom engines for the higher rated competition classes. I.e. comp eliminator and Pro Stock. We also used to drill lateral holes in pistons for the same effect with Super Stock engines as it was illegal to drill from the top. They still worked on the same principle though.

Larry Coyle

 

[anotherME]

Hi, i am trying to do some research on piston porting. ivymike (as well as some others) seem to have an idea of whats going on here. I was wondering if there was any research done on this. The gains expected and such from dyno's? I ask bc i am curious and bc i need to come up with a senior design project and i think that building a couple of engines and running a couple tested and maybe some FEA, matlab, or other software models would be interesting as well. (have less than a year left of school.) Thanks for any tips or direction towards some references you might have.

 

[Lou Scannon]

Tmoose, "Bill "Grumpy" Jenkins talks about them a bit in his "Chevrolet Racing Engine" book.",
you beat me to it!
I have no theory or first hand experience to offer, but my observation is that this seems to remain a serious topic of discussion since over 30 years ago.

Changing the subject slightly, Grumpy also propounded the reflective paint (or ceramic?) coating on the piston crown as a barrier against heat transfer, for increased performance. So far I haven't seen this migrate into commercial engines, any thoughts from the "experts"?

 

[patprimmer]

The Toyota 7MGTE, an inline 6 cylinder 3 litre twin cam 24 valve turbo engine used ceramic coated piston tops in production.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Pud]

Aren't "Dykes" rings much the same thing for the same purpose.

Does any oem still use them on 2 strokes? (Does anyone make 2 stokes, come to think of it!)

Harry

http://www.tynevalleyplastics.co.uk

 

[ivymike]

check SAE papers (sae.org). There should be a few oldies-but-goodies from MIT, Caterpillar, Ricardo, and others.