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91 Hatch rally car
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Discussion Starter #1
I'm starting an build and am looking for a good source for info about choosing piston to wall clearance and ring end gap when adding boost and when going NA. Is there a link or a book that I should be looking at here?
 

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91 Hatch rally car
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Discussion Starter #3
Thanks. I did the google thing, but got too many differing recommendations. The site you suggested gives recommendations for power adders, but the descriptions are pretty vague. How do I determine ring gap for my particular build? Or is it not that precise?
  • Modified or Nitrous Oxide - 0.005in x 4.00in bore = 0.020 inch ring gap
  • High Performance Racing - .0055in x 4.00in bore = 0.022 inch ring gap
  • Racing with Nitrous/Turbo - 0.006in x 4.00in bore = 0.024 inch ring gap
  • Racing Blower/Supercharger - 0.007 x 4.00in bore = 0.028 inch ring gap
 

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93 Civic HB SI
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It's all about heat and piston material used. The piston manufacturers have done the R&D to determine acceptable piston to wall and ring gaps for 95% of folks buying parts and slapping them into their builds. The other 5% have extremely specific needs and/or experience to deviate from the recommendation.

The piston to wall clearance is usually posted on a paper that comes with pistons. This is a rule of thumb, and will vary a little with the intended application. The piston manufacturers base P2W off the known engine bore size, the block material makeup, cooling system type, and known coefficient of thermal expansion for the material makeup of the piston.

The forged 4032 aluminum alloy characteristics of my Wiseco's actually expand more than stock cast aluminum does, therefore requires more P2W clearance, hence the recommendations from Wiseco. Stock cast piston P2W clearance for my Y8 is 0.0004" - 0.0016". Wiseco states the following, 0.001" bigger than the stock max:

20200726_200902.jpg



The smaller the piston, the less P2W you need, because there is less material for heat to have to absorb into and have to travel to exhaust through oil or cylinder via the rings. Therefore, larger pistons need more P2W because they will expand more due to the reasons above.

Luckily, we fall into the small piston category, so the 0.0026 spec from Wiseco with 4032 aluminum is an excellent rule of thumb in our engines. With 4032 forged aluminum pistons in our blocks from CP, Wiseco, Arias, etc., unless you are planning on going higher than 400WHP, 0.0030" is a good P2W clearance that will give very little warm up noise characteristics, and no chances of interference. That's where I asked my machinist to place my P2W clearance on the Y8.

Anything over 400WHP, and you want to start entering into the 0.003-0.004" realm. You could get away with another 0.0005"-0.001" every 100WHP above 400WHP, so that would put you between 0.004-0.005" at 600WHP.

Here are some CP rules they provide with 4032 forged aluminum pistons, to get an idea that P2W clearance follows the piston material type and intended bore size more than anything:

4032-instructions-je-pistons.jpg



Same thing with ring gap. There are general clearance numbers associated with them as well, and have more to do with heat and choice of material, just like the piston numbers. The rings themselves are conduits for piston heat transfer to the cylinder, and expand more with more heat, therefore gaps need to widen when more heat is added, so the ends don't touch each other:

20200726_203934.jpg
 

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Discussion Starter #5
Wow. This is exactly the info I was looking for. Your comments about the second ring gap being larger than the first ring gap is directly opposite from what the suggested site recommends.

Below is from the site. Notice that the gap for the first ring for modified/nos is smaller than the gap for the second ring, but for the rest of the categories, the first ring gap is bigger than the second ring gap.

For high performance engines, the multiplier changes to add more clearance, but the math stays the same:

  • Modified or Nitrous Oxide - 0.005in x 4.00in bore = 0.020 inch ring gap
  • High Performance Racing - .0055in x 4.00in bore = 0.022 inch ring gap
  • Racing with Nitrous/Turbo - 0.006in x 4.00in bore = 0.024 inch ring gap
  • Racing Blower/Supercharger - 0.007 x 4.00in bore = 0.028 inch ring gap
For the second ring, the process is the same, but with a slightly different gap, based on application:

  • Street - 0.005in x bore size
  • Modified or Nitrous Oxide - 0.0055in x bore size
  • High Performance Racing - 0.0053in x bore size
  • Racing with Nitrous/Turbo - 0.0057in x bore size
  • Racing Blower/Supercharger - 0.0063in x bore size
 

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Yeah, I do see that about the second ring gap being smaller on what you posted. That is typical for a lot of OE type replacement piston and ring specs, as the second ring sees less heat overall, and the ring pack is completely engineered for a specific max heat output. The second ring also tries to combat oil consumption. Again, this is OE applications, where goals are much different than the performance aftermarket. In some cases even in the performance aftermarket, depending on the application, a tighter second ring is beneficial, especially when naturally aspirated.

In performance applications using forced induction, you almost always want the second ring a bit wider than the top.

Reason being, is that during normal engine operation compression gasses leak between the first and second ring. The wider gap in the second ring allows gasses that have built up between the two rings and grooves/lands to exhaust into the crankcase easier. You don't want there to be a significant buildup of gasses and pressure between the rings, having a larger gap on the second gets rid of any significant pressure build up and allows for easier exhaustion.

Read this article that explains all about it. It's one of my favorites:



Bottom line, the instructions that come with a reputable kit will tell you what is best. When in doubt, gap the second wider than the top. A little blowby is better than toasted engine :)
 

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Discussion Starter #7
Yeah, I do see that about the second ring gap being smaller on what you posted. That is typical for a lot of OE type replacement piston and ring specs, as the second ring sees less heat overall, and the ring pack is completely engineered for a specific max heat output. The second ring also tries to combat oil consumption. Again, this is OE applications, where goals are much different than the performance aftermarket. In some cases even in the performance aftermarket, depending on the application, a tighter second ring is beneficial, especially when naturally aspirated.

In performance applications using forced induction, you almost always want the second ring a bit wider than the top.

Reason being, is that during normal engine operation compression gasses leak between the first and second ring. The wider gap in the second ring allows gasses that have built up between the two rings and grooves/lands to exhaust into the crankcase easier. You don't want there to be a significant buildup of gasses and pressure between the rings, having a larger gap on the second gets rid of any significant pressure build up and allows for easier exhaustion.

Read this article that explains all about it. It's one of my favorites:



Bottom line, the instructions that come with a reputable kit will tell you what is best. When in doubt, gap the second wider than the top. A little blowby is better than toasted engine :)
Thanks again for the explanation. I saw the different 1st/2nd ring gap stuff on different sites and didn't know enough to sort it out. I think I have what I was looking for in terms rings and piston clearances.
 
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