High zink oil for built high boost cummins?

[NVRENUFF]

I need to find a good oil for my 12v cummins. It’s fully built, 16:1 compression, roughly 65psi boost, 140# over stock valve springs with hardened pushrods. Originally built for a drag truck, now it’s in what I want to be a daily driver and tow vehicle. Turned the power down to about 550-600, so nothing crazy. Just need some oil recommendations to help longevity with all the extra stress in the valve train.

 

[redneck47441]

On competition diesel they seem to be big on Brad Penn. I like Amsoil Marine seems to have alot of zinc.

 

[Linctex]

Might want to go with softer valvesprings.

 

[A_Harman]

Originally Posted By: Linctex

Might want to go with softer valvesprings.

I agree. If you're not turning the engine over 3200 rpm, stock valve springs are fine. (As a matter of fact, the stock valve spring on a Cummins is designed with enough load margin to control the valve up to 50% above rated power rpm.) Extended idling and low rpm operation on the street puts a lot of contact stress on the cam.

 

[NVRENUFF]

I was turning it to 4,800rpm, but doubt I’ll go over 4,000 now. Would like to keep the ability to play around a little. Lol.

 

[BrocLuno]

Yup, high spring pressures are there only to control ramp contact and prevent bounce or valve float. If operating in normal RPM range, I'd cut back on spring pressure too. Once the load is reduced, any premium HDEO with around 1,200 PPM ZDDP will be fine

 

[Langanobob]

Diesel engine builders have a deeply held belief that stronger valve springs are needed, not to prevent high RPM valve float, but to prevent high turbo induced manifold pressure from lifting valves off their seats at much lower than redline RPM’s. I’m not a Diesel engine builder, just tossing this concept out there for discussion.

 

[440Magnum]

Originally Posted By: Langanobob

Diesel engine builders have a deeply held belief that stronger valve springs are needed, not to prevent high RPM valve float, but to prevent high turbo induced manifold pressure from lifting valves off their seats at much lower than redline RPM’s. I’m not a Diesel engine builder, just tossing this concept out there for discussion.

That's an easy calculation. Manifold pressure in PSI multiplied by intake valve area in sq. inches is the amount by which the spring seat pressure in pounds is reduced due to pressure on the back of the valve.

 

[Langanobob]

Yeah but...the pressure on the combustion chamber side of the valves is never zero so to some extent there should be a compensating effect. That seems to be the opinion of automotive engineers who don't subscribe to turbo pressure lifting valves.

 

[BrocLuno]

You have to be making serious pressure to even be a factor in valve lifting. If you are making 2 atmospheres (30 PSI) on 1.5 sq in of valve surface and you have 120# of seat pressure, you still have over 50# of seat pressure which is all some engines have to start with. And I have always had a devil of a time keeping head gaskets in engines with 30 PSI I'm more worried about clamping pressure, than a valve wandering off the seat ...

 

[A_Harman]

Originally Posted By: Langanobob

Yeah but...the pressure on the combustion chamber side of the valves is never zero so to some extent there should be a compensating effect. That seems to be the opinion of automotive engineers who don't subscribe to turbo pressure lifting valves.

Let's think about the 4-stroke cycle, and when the intake valve(s) could be blown off their seat(s). 1. During the power stroke, cylinder pressure is obviously much higher than intake manifold pressure, peaking at ~2400 psi for a fairly modern diesel engine, and decreasing to ~180 psi at exhaust valve opening. So there is no way the intakes will be blown open. 2. During the exhaust stroke, after cylinder pressure has blown down to match exhaust manifold pressure, is the most likely time the intakes can be blown open. If exhaust manifold pressure is equal to intake manifold pressure, there is no net pressure force acting on the valve, so it remains seated with the net force of the valve spring closed load. In most turbodiesels, exhaust manifold pressure will be higher than intake manifold pressure due to the small turbine housing inlet area. But I expect in high performance applications where turbo lag is tolerated, intake manifold pressure can be higher than exhaust manifold pressure, so there may be a net pressure tending to blow open, but this would only be on the order of ~10 psi (as a guess). 3. During the intake stroke, the valves are open, so the issue is moot. 4. During the compression stroke, the valve closes, trapping the fresh charge air at some pressure close to intake manifold pressure. Then the ascending piston causes cylinder pressure to rise above intake manifold pressure, making it impossible for the valves to blow open. On the other side of engine breathing, it is a real concern for exhaust valves blowing open in applications where exhaust brakes are used. Cummins used to have different exhaust valve springs for those applications, that had high seat loads. I recollect that 60 psi was the design pressure for those springs.

 

[ofelas]

The white stripe "60psi" marine springs are 150psi at the seat. They are rated at 60psi above the stock 90psi springs. Good to use 10 degree keepers & larger chrome moly locks with beehive springs, and doesn't hurt to use these with the "60psi" springs at higher rpms either.