Valve Spring Flutter

[macka'sxr6t]

"More of a concern is the issue with valve springs, as the standard XR6T springs have been found to cause valve flutter with near standard engines"

Can some please explain how say 6 - 10 psi of boost can make a 100lb valve spring flutter

macka

[Guest]

Ask Tur botom(turbotom)

vik

[turbotom]

There seems to be quite a bit of variance in the quality of the standard XR6T valve springs.

There have been several threads and posts regarding concerns that members have had after modifying their T's... problems such as perceived misfiring at higher rpm and loss of power issues. They have been identified as due to the inability of the valve springs to maintain adequate sealing at higher rpm/boost levels.

The remedy is to replace the valve springs with the XR8 springs which provide a bit more seat pressure.

Macka wrote: "Can some please explain how say 6 - 10 psi of boost can make a 100lb valve spring flutter"

I don't know the exact cause. I do NOT have actual data of manifold pressures... of both inlet and exhaust.

But I offer a hypothesis......

My understanding is, that the problem is NOT with the inlet valves, but with the exhaust valves.

The exhaust gases pulsate into the exhaust system, creating pressure peaks.

As you increase rpm, the peak pressures become greater. As you increase the turbo boost, you amplify the peak exhaust pressures in the exhaust even more, as greater volume of exhaust gas has to be expelled in the same duration.

As I said, I do NOT have the factual data, but visualise the following....

With a standard exhaust system, on a standard T, there is a nominal (average) pressure developed in the pipes in expelling the gasses out the rear.

For this explanation, lets just say this figure may be quite low as an average value (lets say 5 psi of back pressure at 5000 rpm and 5psi boost)

But that is the average pressure. The peak pressure pulse at the exhaust port when the valve opens will be much higher, due to the expanded gas volume (due to heat) coming out at high velocity hitting the relatively slower moving gas in the exhaust system. A pressure wave is created, and just like a stone dropping in the water, waves occur. These pressure waves have peaks and troughs, and the manufacturer selects the appropriate valve springs to ensure that the valve operates correctly under worst case conditions.

When we modify our T's and increase turbo boost, the volume of exhaust gas increases significantly, and so too does the peak pressure in the exhaust ports.

If 100 psi valve springs are fluttering with 6~10 psi boost, I would suspect that the peak pressures in the exhaust are greater than 100 psi.....

One solution is fitting valve springs with more seat pressure, though there is a limit with how much stress can be tolerated by the valve actuating components too.

Reducing the back pressure (restriction) in the exhaust is also a good idea when chasing more power with higher boost as it too reduces the peak pressures which occur in the exhaust ports.

This is just my opinion and I welcome feedback.

cheers

tom

[turb06]

got nothin. Seems quite logical to me.

Dazza.

[dswiders]

Turbotom,

Great explaination, my T is standard and I get Loss of Power at times. Does this mean that my Valves aren't too crash hot ?

I have taken it into Ford, but they say all is ok.

Mmmmmmm, maybe I should go for the complete package.... Strengthened engine components, Phase 2 or 3 APS kit, Modded SSS and big muva brakes. One small, miniscule problem........ funds.

Daniel

[macka'sxr6t]

Thanks turbotom,

I understand what your saying, so if it's the exhaust valves fluttering why are people changing a full set of valve springs. Why don't they just change the exhaust ones.

If most people are running 12 - 15 psi boost I would think that a 100lb valve spring should be able to coupe with that as ther is no backpressure on the intake valves

macka

[ktford]

  dswiders said:

Turbotom,

Great explaination, my T is standard and I get Loss of Power at times. Does this mean that my Valves aren't too crash hot ?

I have taken it into Ford, but they say all is ok.

Mmmmmmm, maybe I should go for the complete package.... Strengthened engine components, Phase 2 or 3 APS kit, Modded SSS and big muva brakes. One small, miniscule problem........ funds.

Daniel

It means your springs aren't that hot, you aren't the 1st stocker to get flutter.

Scotty

[ktford]

  macka said:

why are people changing a full set of valve springs. Why don't they just change the exhaust ones.

Personally I changed the lot for two reasons, suspect quality of the standards & because they only me cost $221 for a full set.

Scotty

[turbotom]

  macka said:

Thanks turbotom,

I understand what your saying, so if it's the exhaust valves fluttering why are people changing a full set of valve springs. Why don't they just change the exhaust ones.

If most people are running 12 - 15 psi boost I would think that a 100lb valve spring should be able to coupe with that as ther is no backpressure on the intake valves

macka

I have only stated what I think is happening.

most people will replace components in an engine as a set.

looking at your photos of the shaker scoop (good job) I would say you don't do things in half measures either......?

cheers

tom

[Pretzel]

Turbotom,

I have a couple of questions after reading your explanation (But I have no idea about the problem myself, so bear with me if the questions are not relevant !)

So if we were to say have a 'perfect' exhaust with no backpressure, this problem wouldn't exist ?

Or is it not a function of backpressure, and only the shockwave you mention ?

What is the cause of the wave ? Is it purely the exhaust pulses, (I would have thought to get a wave of some sort coming back towards the valve there would have to be something to stop the exhaust flow and generating it ?)

The turbo itself is spinning at the same speed as the exhaust gasflow, so I assume not causing much of a restriction itself ?

Thanks

Pretzel