Boost vs. Valve Springs?

If you have a hydraulic roller cam with relatively low spring rates, and then add a bunch of boost to it, do you have to increase valve spring pressure so the valves close?

If supercharged I think less of an issue than turbocharged.

On intake cycle supercharged, positive pressure is filling the cylinder and nearing intake valve close I can't see a reason why there would be much higher pressure behind the valve than in the cylinder, at that point.

Then of course, the moment the valve seats the compression cycle starts and cylinder pressure is instantly much higher than boost pressure.

I would be more concerned with stability given an efficient supercharger system can increase the usable rpm range by, a lot. (especially centrifugal)


If turbocharged, how much more you need would depend on your drive to boost pressure ratio. If you have something on the small side as far as turbo(s) go, you can have double drive pressure to boost pressure

So, if 25psi boost and 50psi turbine drive pressure, the exhaust valve needs to close against 50psi and doesn't get offset much/at all by boost as the intake valve is opening during overlap.

Absolutely
You don't want the manifold pressure to hold the intake valve open making the lifters float and you don't want the exhaust valve pushrods to bend when opening either so make both a lot better

So if you run serious boost, you need to switch to a mechanical roller then?

Wouldn't the spring pressure need to be increased relative to boost pressure AND valve size ?

Not necessarily but wouldn't hurt depending on what you consider serious boost.

Still don't understand how increased manifold pressure holds an intake valve open.

I might be way off the mark here, but the way I think it works is: boost and valve surface area = extra pressure required. So 10 psi on the back of a 2.02" (pi x r squared) valve would require a valve spring with (roughly) 31lb more seat pressure than it's N/A counter part to work the same ?

My question is, why is that pressure only on the back side of the intake valve?

If you have say 15psi, it's filling the cylinder through the entire intake stroke and as the piston is coming up from bottom dead center chasing the intake valve closed, would it not be fair to say that boost pressure and pressure in the cylinder has equalized (or close) at that point?

Then as soon as the intake valve touches down you're on the compression stroke, far exceeding any boost pressure.

I see your point, but in my limited experience with boosted engines the manifold pressure seems to be more or less constant but dependent upon RPM. I don't know but I'm going to guess that the cylinder fill vacuum is greater than typical boost pressures seen on gasoline engines.
Again, I must reiterate, it's just conjecture here, anyone know for sure ??

I don't know what the secret sauce is but there is plenty of modern engines (LS and Gen III) making big power with hydraulic lifters, big RPM as well.

Yes, I've ran those at nearly 1400whp and 8000rpm hydraulic.

Not entirely sure what you mean. I'm just saying ithe boost, whatever it is, on the backside of the valve is filling the cylinder so there isn't going to be a differential across that valve equal to boost pressure by the time that valve is closing.

(It's not just 15psi boost behind valve and zero psi in the cylinder as the cylinder is being filled by that 15psi, for example)

Thank about what pressure is in the intake track and how much compression pressure is on both valves when closed
As far as boost on the back side of the intake valves on the intake stroke where else is the pressure suppose to push on
The last thing is think about the crankshaft degrees(720 degrees for each complete 4 stroke power cycle) and how long the blower is building boost compared to how long each intake valve is open during each power cycle
I'm a firm believer in having more valve spring pressure than needed to keep the valve train stable than having a tiny, tiny bit too little

Remember at peak tq which is near peak volumetric efficiency, the cylinder is full (or maybe even over full).


Since your on the compression stroke, the pressure in the cylinder is increasing..


So whatever pressure is on the backside of the valve is minimal, or at least very similar (offset) by the chamber pressure.



I've sucessfully run as high as 23psi with over 2:1 backpressure on a hydraulic roller without doing anything crazy.

I even tried shimming the plunger travel tight, and adding spring pressure searching for more RPM .... and it didnt do squat.

This was a 235@.050" .540 lift 6500 RPM combo.

Point is, when the intake valve opens it's not like someones hand comes down and is just pushing on the backside of the valve, the pressure at the valve is moving to the cylinder, equalizing pressure.

So if manifold pressure is a steady 15psi, go through the motions.

Intake valve opens, intake charge flows into cylinder and pressure starts equalizing behind the valve and after the valve (in the cylinder).

I would think it has near/same as manifold pressure in the cylinder at intake valve close at which point compression takes over and cylinder pressure multiples higher than boost pressure.

Plug fires, power, again, much higher pressure in cylinder than back of the valve.

Exhaust, ok, we will reach a point now where boost pressure will exceed cylinder pressure after blowdown but the valve has been closed for long enough now it certainly isn't getting blown open by whatever boost pressure is.

Now we start over at intake valve opens.

At no point in time do I see where boost pressure acting on the back of the valve isn't the same or higher on the combustion chamber side of the valve, at any point in time it would matter.


I'm not a fan of light spring pressure myself and would rather have some overhead boosted or not. I just don't subscribe to the theory of add X amount of psi for boost pressure acting on the area of the valve at seating surface minus valve stem for reasons stated above.

Yes.
Comp Cams sold me the wrong springs TWICE.
Had terrible valve float with only 5lbs. of boost

Just shooting from the hip here, but the spring is holding the valve shut during power stroke, the pressure will go anywhere it can, insufficient spring pressure would cause a leak back into the port?

I'm definitely with INTMD8 on this one, without getting into too many specifics I have run into valve float at high P3/turbine inlet pressures. It has not been an issue on supercharged stuff, just turbo applications.