Valve Train Mass

[petert 587]

Some time ago, Dave mentioned he had a calculator for estimating maximum RPM of the valve train. I have gathered all the required info for a 16V solid lifter top end I'm building. Would you mind running the numbers through your program?

 

Std. Inlet

valve 60g

spring 50g

locks 2g

retainer 16g

bucket 62g

 

36.5mm Inlet + solid bucket

valve 60g

spring 38g

locks 2g

retainer 8g

bucket 38g

lash cap 1g

 

I'm not sure about the resolution of my scales to read less than 10g, but I think the order of magnitude is ok.

[Owain1602 0]

the maximum rpm you can take the valve train with solid lifters depends on the strength of valve springs

[petert 587]

But the seat/open pressure depends on the mass of what your holding.

[PumaRacing 2]

I think your scales are out. I have weights from my digital scales for standard components about 10% higher than your figures. e.g. 65g for an inlet valve.

 

For a simple valve train with no rocker and all the mass acting directly on the cam lobe you can work out the answers you want very easily without a computer. The effective valve train mass is all the oscillating components plus 50% of the spring mass. The required full lift spring force at a given rpm will be proportional to this so if you reduce the valve train mass by 20% the required spring force at a given rpm will also reduce by 20%.

 

The other thing you need to know is that required spring force varies with the square of rpm. So again if you reduce mass by 20% the valve float rpm will rise by the square root of 1.2 times the original valve float rpm. i.e. about 10%.

 

All of these calculations depend on the accelerations, or more precisely the decelerations on the closing side of the valve motion, produced by the camshaft and that will vary from cam to cam so there are no exact answers without also knowing that information. Generally it's easier to do an empirical test of the valve float rpm with a given set of components and then work back from that for any new components.

 

As to seat pressure there are no definitive rules that I'm aware of but I know from experience that lack of seat pressure will cap an engine's ability to rev like hitting a brick wall. I had a rather nasty experience many years ago with a set of Piper springs that were not only 20% weaker than specified in their catalogue but also lost 3mm of their free length almost immediately due to not being properly heat treated. The combined result of these factors was a seat pressure barely half of what the engine needed and it wouldn't rev past 5,500 rpm.

 

I therefore tend to design springs where the seat pressure increases in at least the same proportion as the full lift pressure. In fact one of my spring systems, the CVH one, has very little extra full lift load but a lot more seat pressure. With the standard components this increases the rev limit from about 6800 to over 7500 so clearly it's seat pressure that limits that engine rather than full lift load.

 

It's not easy to know whether an engine is being rev limited due to valve float where the full lift pressure is insufficient to keep the valve train in contact with the cam or valve bounce where the valve comes back off the seat after closing. I think the latter is actually more common than the former but often not properly diagnosed.

[petert 587]

I suspected it wasn't weighing accurately. I'll check it with some water.

 

What about a difference in seat pressures between a solid and a hydraulic lifter?

[PumaRacing 2]

I'm not aware of any specific difference to the above calculations.

[petert 587]

I was talking with a valve guru the other day who suggested hydraulic lifters should have more seat pressure than a solid (for a valve train of the same mass). ie if you take an Mi16, 80-85 lbs for hydraulic, 65-70 for solid. Certainly smaller twin cams with solids run much lower seat pressures, eg Toyota and BDA.

[B1ack_Mi16 67]

I was talking with a valve guru the other day who suggested hydraulic lifters should have more seat pressure than a solid (for a valve train of the same mass). ie if you take an Mi16, 80-85 lbs for hydraulic, 65-70 for solid. Certainly smaller twin cams with solids run much lower seat pressures, eg Toyota and BDA.

 

Good to know, seem like I'll have 76lbs seat pressure on mine

[Mandic 0]

With a hydraulic cam the valve spring must exert enough pressure against the valve lifter (or lash adjuster) plunger to keep it centered in its travel to prevent "lifter pump-up". When pump-up occurs the valve is held slightly off its seat resulting in a significant loss of power and possibly a misfire. It is this loss of power and misfire that is often misdiagnosed as a fuel system or ignition system problem.

 

High oil pressures and high viscosity oils aggravate "lifter pump-up" in hydraulic lifters. When either oil pressure or oil viscosity is going to be increased beyond the manufacturer's recommendation, a corresponding increase in spring seat pressure is necessary to prevent "pump-up" (even with an "anti-pump-up" lifter). Since oil viscosity in no way relates to the oil's film strength, and the scuffing protection provided by the film strength, Crane Cams recommends following the OE manufacturer's recommendation with respect to engine oil.

 

Hope this is of any help...

 

Cheers

 

Ziga

[petert 587]

That certainly supports the theory suggested.

 

A friend of mine recently rebuilt a BDA. It pulls 9200 RPM in 5th with 60lbs. on the seat. I guess it all depends on the mass.

[Mandic 0]

Common Misconception:

Many people mistakenly think that using higher seat pressures causes a reduction in the horsepower delivered to the flywheel because higher seat pressures (and also higher spring rates required for high performance) require horsepower to compress the springs. This thinking is simply incomplete! For every valve that is opening and its valve spring being compressed, another valve is closing and its valve spring is expanding. This expansion returns the energy to the valve train and the engine. This results in a net power loss of "0" hp. Many engineering texts refer to this as the "regenerative characteristic" of the valve train. Recent tests at Crane have shown no horsepower loss on a hydraulic roller equipped engine when changing the seat pressure from 135# to 165#. Power actually improved significantly at top end, probably due to better control of the relatively heavy valves in the engine.

 

In Summary:

Always run enough seat pressure to control the valve action as it returns to the seat. Heavier valves require more seat pressure. Strong, lightweight valves require less seat pressure. When in doubt, run slightly more seat pressure . . . not less.