Stock Cummins 12v Valve Size?

NickTF

NickTF

Single turbo turd.
Joined
Jun 4, 2007
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What valve head diameter and valve stem diameter are stock cummins 12v valves?
 
If a 1.76" diameter and an 8mm stem are correct one needs ~130 lbs on the seat to keep the valve closed with 55psi of boost and ~188 lbs on the seat to keep the valve closed at 80 psi. Reasonable conclusion?

Does anyone know what lift the stock cam is?
 
Did you account for the overlap between the valve and the valve seat? There might only be 1.6" diameter of the backside of the valve exposed to boost pressure.
 
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Big Blue24 said:
Did you account for the overlap between the valve and the valve seat? There might only be 1.6" diameter of the backside of the valve exposed to boost pressure.
Click to expand...

That thought has crossed my mind, trying to be conservative however.
 
If there is only 1.6" exposed and you deduct the 0.0779" of area from the 8mm valve stem, I come up with 1.933 sq inches of surface area for the boost pressure to work against.

130# of seat pressure would be equal to 67# of boost.

155# of seat pressure would be equal to 80# of boost.
 
^^^ I get the same numbers. I suppose that is the area exposed then?

What boggles my mind is the numbers reported for the 60lb springs. They are reported to have 155ish on the seat yet people indicate they blow open over 35psi. Something doesn't seem to add up.
 
NickTF said:
^^^ I get the same numbers. I suppose that is the area exposed then?

What boggles my mind is the numbers reported for the 60lb springs. They are reported to have 155ish on the seat yet people indicate they blow open over 35psi. Something doesn't seem to add up.
Click to expand...

I'm not 100% knowledgeable when it comes to valve timing vs TDC, BDC, etc. However, when I think about it, it seems like there will always be a little bit of pressure remaining inside the cylinder pushing up against the intake valve. Plus, the pressure inside has 2.433 sq inches to push against, not 1.933. so it has a 25% area advantage.
 
^^^ yeah, again all good points! The only time in a mild 12v cummins I would think there could be any chance of little pressure in the cylinder compared to what exists in the intake is ABDC as the intake valve is closing and even then in a perfect world the pressures should be near equal or somewhere close to I would wager to guess.

With the turbine(s) in the way and the horrible scavenging taking place due to short runner manifolds don't think there is ever any kind of depression again compared to the intake manifold right up to exhaust closing and intake opening.

Obviously compression and power strokes are conditions where there is far more force on the cylinder side of the valve than the intake.

Call this a think out loud thread lol.
 
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