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Old 10-01-2012, 11:58 AM   #1
Brian Rupnow
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Default Increase in Compression

All 3 of the i.c. engines I have built have undergone a remarkable increase in compression after being ran for about 2 to 3 hours. I realize that this is a result of the internal components "wearing in". They all have Viton o-rings instead of cast iron rings, and they all have 01 steel valves with brass valve cages. I don't know how much a Viton o-ring has to "wear in" but there must be some factor involved. I think that what happens with the valves is that the steel valves, being a lot "harder" than the brass seats, must actually "pound" their individual profile into the brass seats until they are a perfect match at the microscopic level.---this as a result of heat and the pressure of the valve spring seating the valves after each firing stroke, and the pressure on the valves created by the combustion itself. I know the engines rotate more freely as a result of the camshaft and crankshaft bushings wearing in, but this is a totally different issue than the increase in compression. Each of my 3 engines have different material in the cylinder. The Webster is mild steel, with aluminum piston, the Kerzel has a 316 stainless cylinder barrel with an aluminum piston, and the Atkinson has a grey cast iron cylinder and piston. Lets hear opinions on this phenomenon of increasing compression after engines have ran for a few hours!!!---

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Old 10-04-2012, 01:23 AM   #2
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I have seen the same on my few engines. I came to the same conclusion as you, the valves must be sealing better. Perhaps carbon buildup helps?


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Old 10-04-2012, 02:09 AM   #3
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The only thing it can be is the valves. I would think that with the flexibility of the 'O' ring that it would seal any mismatch right from the get-go. Usually an engine's compression increases as the rings seat in because being made from metal they wear to match any minute irregularity in the cylinder wall.
When you say "remarkable" what exactly does that mean? The engine would need adequate compression to even begin to run so how much more has it gained with running?
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Old 10-04-2012, 04:11 AM   #4
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I don’t have much experience with small engine valves. I can say however that larger engine valves (i.e. automotive gasoline and diesel engines) will properly seal from the moment they’re installed - provided the valve guides are perpendicular with the seats, and that the valve & seats are properly ground and lapped in. Normally compression issues during the break-in period are a result of ring seating.

One thing is for sure – the air is blowing by somewhere prior to break-in, and is doing so less afterwards.

Mathematically, a perfectly round O-ring will have an undefined surface area against the cylinder wall (a pure tangential line of action). Pressure is equal to force per unit area – thus if the O-ring is round, it will have no defined area against the cylinder wall, and be incapable of holding back any pressure. Obviously there is a slight interference fit, which causes the O-ring to deflect into the cylinder wall creating a real sealing surface. Nonetheless, perhaps the O-ring even better conforms to the cylinder wall after a break-in period? Either by stress-relieving and conforming from the heat cycle(s), or maybe the O-ring develops a small flat-spot against the cylinder from friction?

How smooth is the bore? I.E. maybe the O-ring is somehow polishing the cylinder?

I’ll be following this post with interest!

Last edited by Entropy455; 10-04-2012 at 04:16 AM.
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Old 10-04-2012, 11:52 AM   #5
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I think that there is also a microscopic (sometimes a lot )amount of residues from the combustion in the engine surfaces that help to do a closest fit.

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