HIGHBOOST

Alright,

I was reading in a turbo book of mine, and they recomend upgrading your valve springs when planning to run high boost levels on a constant basis.

I understand why to upgrade the springs when increasing the RPM limit/range, but why should they become a weak link with high boost applications?

Thanks for the insight!

Silver Rose

Boost is actually pushing down on the top of the valve fighting the spring. The higher the boost the lower you are making your effective valve rate. Increasing the valve spring rate is just counteracting the effectis of the boost pushing down on the valve.

The real debate is at what boost levels do 951's need higher rate springs? My guess is well over 20psi but that is just a guess.

gt37vgt

so how much pressure do you imagine is on the underside of the valve ??
and why would it not be increased at the same rate ??
its stupid that no one really bother's to calculate the inertial load's of the valve chain witch are a function of valve chain mass cam ramps and rpm .
excessive valve spring rates is giving away free horse power...

can we please see a first hand account of valve bounce please ??? at what spring rate what rpm etc

barks944

I guess theres a difference due to volumetric efficiency. So at high RPM where the pressure in the cylinder is less than the manifold the pressure differential might effect the closing of the valve.

Chris White

If you drop a valve from any height over 2' it will bounce....

The only reason to upgrade your valve springs is age or a significant increase in RPM usage.

An intake valve is aprox 2 sq in - so increase of intake manifold pressure of 5psi will add 10 psi of pressure against the valve spring. If the stock ones are in good shape this is OK, if you are adding 10psi of boost (20 psi total) then you might be on the limit.

Willard Bridgham 3

Valves and valve train runs in a hot environment. Run hot, cool down........ This is heat cycling and spring materials we use loose their strength with heat cycling.

Increase in pressure with heat cycling means your existing springs (especially if they are original) won't do the job at higher pressures.

Silver Rose

The heat cycling argument is a good one for replacing old valve springs, but does not answer the question as to why have stiffer springs.

The increase in combustion preasure countering the effects of boost on top of the valve is present for about 3/4 of the compreshion strokeand the 3/4 the power stroke. This leaves the the valve possibly opening in other times during the other strokes.

I don't think people would argue that the high boost we use, when applied to the top of the valve would actually open the valve on its own, but more over, it might not allow the valve to close when you want it to. If you do not shut the valve in time you loose some of the effeciancy of the compreshion stroke in the engine and power accordingly.

Chris White

Yes, after skiing in 0 degree weather yesterday I was very fatigued by the heat cycling!

Jeremy Himsel

I used to believe the same thing until I read a test article done by one of the cam companies a few years ago. What the test showed was while it does take more HP to open a single valve spring, the regenerative characteristic of the valve train cancel out HP losses associated with higher spring rates provided all springs are equal. For every valve spring that is being compressed, another is expanding and returns the energy to the valve train and the engine. Higher rate springs do however expedite the wear process on other valve train components.

TurboTommy

Yes, this is it.
But, unless the valve spring is extremely weak, I think the valve would still eventually close on time at the correct crankshaft angle (in the way of making power). The issue is it will be SLOW to close, not following the cam lobe, hammering down and wiping out your cam and lifters in a very short time.

95ONE

I don't feel like this is one of those "must do" upgrades. sure, its added sscurity, but if they're in stock tolerance, I see no worries to 18-20psi.

But I'm somewhat cheap, so grain of salt.

gt37vgt

Check your compression fluids and plugs then wind the boost up ..from many accounts it seems that standard springs are often a bit down on tension so if the head is being done they should be checked .

barks944

Is there a recommended mileage to replace valve spring?

333pg333

So these uprated Ti springs, retainers are obviously available. At what point do we decide that we need them?
If it's based on boost, what about if we chose to run a big cam but at relatively 'normal' boost levels?

gt37vgt

I'll say it again.... if the pressure has not increased on the inside of the valve what was the point of winding the boost up ??
Cam profiles are 1000 more important than boost when selecting valve springs . remember the V is squared and the cam and RPM determines the magnitude of V .

and once more if the pressure doesn't increase inside the valve with boost what did the boost increase possibly achieve ??