Valve Spring Rates

[The American]

Supertech advertise two double spring rates and I am looking at the installed seat pressures.

Their ‘light’ doubles are 63lbs and their ‘heavy’ doubles are 74lbs.

For an engine that will see 25 to 30lbs of boost pressure, the valve springs on both intake and exhaust need to handle regular engine operations plus the additional pressure on the back of the valve from the manifold gases.

So, if there is a valve seat pressure of 74lbs, less 25lbs of manifold pressure, this leaves 49lbs of spring left for seat pressure and regular engine operations. This is less than Supertech single springs offer. This does not sound like enough.

1) Am I understanding this correctly?
2) (if yes) Does anyone know of a source for BP valve springs that are closer to the 80-90lb range?

[RS2000]

I might be wrong, but aren't you subtracting a pressure from a spring rate?
I would think that both would have to be converted to a force for it to make any sense.

[The American]

I might be wrong, but aren't you subtracting a pressure from a spring rate?
I would think that both would have to be converted to a force for it to make any sense.

The valve springs have a rate, but they are also described as having a seat pressure at the installed height which what I have subtracted the boost pressure from. I am assuming (perhaps incorrectly) that the seat pressure is what has to be overcome before the valve could float.

I suppose that boost pressure is pounds per square inch, but the seat pressure of the valve spring is pounds per...?

[RS2000]

Ok, but aren't the pressures acting on different areas?

[The American]

Ok, but aren't the pressures acting on different areas?

Yes I think so now you point it out. I don’t know how seat pressure relates to PSI

[Red_Bullet]

So, I'm certainly no expert. I'd imagine that for a given boost level:

Work out area of the back of the valve in square inches. Pi r squared.

Multiply area in square inches * maximum boost pressure. Add a good margin for safety. Of course I hasten to add that I have no idea what I'm talking about.

[plohl]

It's incorrect use of terminology.

This video is a perfect example:



The "pressure" measuring tool he is using is essentially a closed hydraulic system - the gauge is measuring pressure in PSI, but what they're actually measuring is the spring force.

Pressure = Force / Area

So the tester has an internal area of 1" squared... then

Force = Pressure * Area = PSI * 1" squared
.: Force = Pressure

So when old mate checks the spring, he compresses to 1", looks at the PSI and checks that spring is 200lb/inch, if the gauge says 200 or there abouts, happy days.

Using pressure cylinders (air or hydraulic) and a pressure sensor is a much cheaper way of measuring force, you just need to do some math to get the answer.

I actually emailed proform about this, I wanted to confirm that the tool actually has an area of 1" squared, but they never got back to me.

[StillIC]

If you have enough boost to open the intake valve then this is a good thing, not a problem. It means the cylinder is ready to accept the intake charge. But this is not normally something even worth considering or worrying about.

Valve springs exist to keep the valve/follower against the camshaft lobe so that the position of the valve follows the shape of the lobe. When engine speeds increase the spring force needs to be great enough to accelerate the valve to follow that cam on closing. If the valve can't accelerate fast enough the cam lobe gets away from the valve and valve float occurs. Bounce can also occur when it finally does contact the lobe again.

So, valve spring rates are a function of engine speed and camshaft lobe shape/lift.

[CrazyRacer]

Its only a pressure differential across the valve when its shut that will try to open it against the spring.

For the most part, when you're trying to hold the valve closed you would hope that the cylinder pressure is either close to manifold pressure (because you've just filled the cylinder), under compression or in a power stroke, both of which should see the cylinder pressure way in excess of the manifold pressure and thus assiting in keeping the valve shut.

The valve spring pressure is more about controlling the mass of the valve at your expected RPM.

If you go too high in the valve spring pressure you'll create parasitic drag on the valve train and rob yourself of power.

[StuwieP]

Not adding to the technical side but by way of illustration, I understand the SE actually uses the lightest spring pressure out of the 1.8s... even though they have boost.

The difference?

Redline was 6,500rpm, lower than the non-turbocharged 7,000rpm.

[CrazyRacer]

Using pressure cylinders (air or hydraulic) and a pressure sensor is a much cheaper way of measuring force, you just need to do some math to get the answer.

Could probably make something to measure valve spring rates using some cheap load cells from aliexpress and a Raspberry Pi or Arduino.

Sounds like a project though and I don't need another project at the moment.

[The American]

Thanks for the many replies. I’m none the wiser as to how to work out the ‘right’ valve springs for an application, only when ‘more’ may be better and that ‘just enough’ is ideal.

I decided to go with Supertech heavy doubles.

When I enquired about their rate options, they advised me that the 74lb of the ‘heavy double’ is now 89lb seat pressure. Not sure if the spec has changed or if they changed how they quote the seat pressure number.

[plohl]

Gary runs about 75 in the 2f engines and iirc, good for 8k. Mine are 85.
They were like 120!

[ManiacLachy]

I think the heavy doubles should see you through. They're what Supermiata use in their Whammy engines.

[The American]

I had an update from the engine builder who mentioned that the SUB setup with the Supertech valves and springs worked out nicely. The ‘heavy doubles’ assembled with a hair under 90lbs at the seat.