Stiffer Valve Springs Rob Power???

[spdu4ea]

I keep seeing the same guys on various BMW forums saying "stiffer valve springs rob power" everytime someone builds a head and I can't help but wonder if anyone has any actual proof to support their theory. When questioned why they believe that to be true, I received three distinct replies:

A) It is simple physics
B) Duh, the cam has to push down harder on the spring
C) A simple google search will tell you

(all wrong answers imo -- even if the underlying theory is right).

My thinking: Energy is conserved... If the spring was perfect, the amount of additional energy required to push down on the spring (above what is needed to prevent float) would be offset by the energy restored as the cam lobe passes BDC whereupon the spring's force reduces the effort needed to turn the cam.


But real springs are not perfect for 2 main reasons:

1) bending metal creates heat (energy loss)
2) the spring has mass (it's motion requires energy)


To point 1) I say that the metal is being bent the same amount no matter what the spring stiffness, so I would not automatically assume the stiffer spring created more heat. But it is possible. Still I don't see 5hp worth of energy being wasted by 24 valve springs that are 25% too stiff... That much extra heat would raise the temperature of 1 kg of water 1*C every second!

To point 2) I say that a modern quality aftermarket valve spring will be lighter than a 20 year-old OEM BMW spring -- something that may make the new springs more efficient -- not less.




So with that all said, I welcome seeing evidence to the contrary.

[PencilGeek]

I can tell you from personal experience that my former company's aftermarket valve springs did seem to create additional heat. We saw engine oil temperatures rise about 7-10 degrees C. That may have also been due to the valve spring design itself. The factory spring was a dual coil design where the inner coil floated inside the outer coil. Our springs were also a dual spring design. However, the inner coil was a few thousanths larger diameter than the ID of the outer coil. So the coils were interference fit. I always thought the extra friction of the interference fit coils was the cause of the increased oil temp.

Aftermarket springs are now available for the S85 and S65. Here's a picture of the OEM spring on the left, and aftermarket spring on the right.

[black bnr32]

new thread on bfc...read up

[Sticky]

To point 2) I say that a modern quality aftermarket valve spring will be lighter than a 20 year-old OEM BMW spring -- something that may make the new springs more efficient -- not less.

Are you basing the majority of your opinion regarding efficiency on the weight of the spring?

And to your main point, is there any real support for stiffer springs robbing power?

[GG///M3]

Are you basing the majority of your opinion regarding efficiency on the weight of the spring?

And to your main point, is there any real support for stiffer springs robbing power?

He lacks much Sticky. Asking him anything will result in wasteful time and bandwidth. Avoid at all costs.

[Sticky]

He lacks much Sticky. Asking him anything will result in wasteful time and bandwidth. Avoid at all costs.

Actually, he is a very knowledgeable guy who presents a good amount of data.

I think he just missed following up on this.

[GG///M3]

Actually, he is a very knowledgeable guy who presents a good amount of data.

I think he just missed following up on this.

Ok I will kindly disagree.