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Piston Ring Sealing Pressure

  1. May 25, 2009 #1
    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. 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.

  2. jcsd
  3. May 28, 2009 #2


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    While it's true that a good amount of combustion pressure is needed to help the top ring seat (particularly straight after a rebuild before everything's bedded in), this sounds extremely dodgy to me. The radial clearance between the top land and the bore should be plenty to ensure gas pressure can seat the top ring properly. To drill would add stress raisers just where you don't need them, and potentially protrusions for localised hot spots to develop. Leave them be!
  4. May 29, 2009 #3

    Ranger Mike

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    * Gas porting: consists of drilling small holes through the tops of the pistons, which allow cylinder pressure access to the back of the top ring to force it out making it seal more tightly to the cylinder wall. They are most effective with tight ring grooves and high ring positioning. I do not recommend vertical gas porting (holes drilled through the top of pistons) for street applications. First, the engine probably doesn't develop enough cylinder pressure to unseat the rings and second vertical ports plug up with carbon in a street engine after a short time. which will be a potential source for pre-ignition. Also, you most definitely do not want one or two to plug up and apply unequal force circumferentially around the piston.

    As a result vertical gas ports are most often use for drag engines and horizontal ports (drilled through the side of the pistons) are used on circle track and road race engines (and on street engines...sometimes). Unless you have 600 hp or more, do not to worry about gas porting.

    Gas porting is needed in race engines, but it’s done because of the type of rings used and the absolute need for total ring seal at high rpm. Titanium and Chrome nitrite top rings are typically the type used, and in a 1.2 mm size. Both of which last very well, but are difficult to seal. Gas porting gets them to seal. Molly seals very well, but will not last in a race engine for very long. Also, machining the gas ports is a trick best left to the manufacture. JE happens to be my favorite and I’ve never had an issue with them, ever. Wiesco is a back up. Leave it up to them to determine placement, size and number of ports for the ring type, ring size and how long the race is. I’d also recommend that if you do go this route, coat the bottom of the top ring land, or the bottom of the ring itself with a dry film molly lubricant to keep the ring from welding in the grove. Make sure you use a good Indian stone to de-bur the groves too. Just hit the edge lightly. You’ll also want to take an exacto knife to the ports to make sure that there are no bur’s that will promote carbon build up, or hot spots.

    Make sure you do all this port and land prep before you mock up the assembly. Bur’s throw off your measurements. And don’t forget to polish the piss out the pistons with a light scotch bright buffing wheel.

    The deal with gas ports is they allow tighter ring land clearance since pressurization is accomplished with the ports. This tighter clearance is how flutter is controlled, this can be worth 20 to 30hp since it allows higher rpm with out ring seal loss.This also adds power by taking less piston movement to start building pressure,similar to moving top ring up on piston.Lateral gas ports are more street friendly since they wont carbon up as quickly as vertical ports.
    Ifin you still want to do this..get a gas port drill kit from Goodson..they make all kinds of trick stuff for race car engine builders...
  5. May 29, 2009 #4
    The material is not the only reason why gas ports are used. The gas ports are also used to seal a very low tension ring at high piston speeds.
    Is this suggesting polishing the piston crown or piston face? If piston face, I would have to disagree here. Forcing carbon to tightly bond with the piston can act as a natural coating/thermal barrier.
    I was curious as to who made a kit for gas ports. Thanks for the source!
  6. May 30, 2009 #5

    Ranger Mike

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    piston should always be inspected for nicks and burrs made during manufacturing process. these will tear up a piston ring so it is vital to deburr and polish..especaily if yo udtart knarling on um when putting in gas ports...think about it..the piston is going to make contact with the cylinder wall regardless,,,the ring groove needs the extra attention
  7. May 30, 2009 #6
    Understood. I did not disagree with what you said about ring land/groove prep. I was only in disagreement if you were instructing that polishing of the top of the piston was in some way beneficial.
    The perfect engine will have a piston that never makes contact with the cylinder wall. The only contact will be the "bridge" of oil film that lines the cylinder wall. Granted, nothing is ever perfect, and sloppy machining practices are all too commonly used in "race" engines. HTH
    Last edited: May 30, 2009
  8. May 31, 2009 #7

    Ranger Mike

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    good point..I failed to clarify my point of finishing the top of the piston. Pistons are manufactured and are "roughed" to an estimated compression ratio. And it is rough. Even the custom ones are rough as a cob. The ACTUAL comp ratio depends on actual Combustion Chamber volume, dome volume, head gasket volume , dome height. These should be measured and actual CR calculated so every cylinder is the same. This I done by CCing the chamber, dome , measure deck height after assembly, calculating gasket volume. BTW, this applies to flat tops as well as domed pistons. You must finish the piston top with a die grinder and hand polish with fine emery paper. The point of reworking the dome is not to remove material but to round off the sharp edges, left over from machining, where heat will collect an form a hot spot that can cause detonation in the cylinder.
    Even flat top slugs have sharp edges ..scrutinze them all. Finally, before assembly, you need to glass peen the piston..BUT NOT THE RING GROVES. Then clean, CLEAN, C L E A N um.
    BTW, Detonation is what grenades engines so prevention is THE biggest factor in building a race engine.
  9. May 31, 2009 #8
    You are correct in preparation of the piston before it is ran. I understand the concepts of compression ratio. I develop performance cylinder heads and engines locally. The preparation of piston face/shape and chamber shaping in very important but irreverent to the original question.
    Maybe we can discuss via private message?
  10. Jun 1, 2009 #9

    Ranger Mike

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    great..always want to learn as much as I can..thanks
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