M30B34 Head: One or two valve springs per valve??

[Aldo525]

In the next few weeks I am going to change the cylinder head on an M30B34 engine (85' e24). The new cylinder head comes complete, assembled but with a difference from the current/original one: the old one only uses one spring per valve and the new two springs per valve (as in the images). Looking at RealOem, the cylinder head is the same, the camshaft is the same, the valves are the same, everything is the same except the springs.
Any idea why the two springs were used later (since 06/86) instead of one???....maybe it's a resistance issue, two springs are better over time than one so I'm not sure if I leave everything as original (one spring) or leave the cylinder head with the two springs. Will the resistance opposed by one spring be the same as that of the two springs?

My 87' 525i (built 01/87) M30B25 uses the same two spring configuration, but if it had been an 86 model it would have had one spring.

Thoughts??

One spring setup





Two springs setup
 


 

[kojo96]

Just an updated version of the same cylinder head. BMW must have had their reasons, but the single springs don't really fall short on performance/reliability. 

[turbodan]

You don't want any more valve spring than you need. It will increase the RPM at which the valves begin to float but it also increases the load across the nose of the cam lobes.

The double valve springs in the M20 rev over 7200 RPM without float. If the M30 is similar it's fair to say you don't need that much valve spring in a motor that usually stops building power by 5000 RPM. 

[Aldo525]

Thanks for your comments. It is very likely that the upgrade to two springs was due to the "float" issue at high rpm, which will not be the case for me (weekend use, not in races), but I also suppose that BMW engineers thought about not wearing out the cam lobes beyond what a single spring did.

[Tiit]

High comp (10:1) b34 has more aggressive cam and double valve springs. Low comp(8:1) has milder cam and single valve springs. B35 has double valve springs and aggressive cam profile. 

[Mike W.]

High comp (10:1) b34 has more aggressive cam and double valve springs. Low comp(8:1) has milder cam and single valve springs. B35 has double valve springs and aggressive cam profile. 

The high compression B34's I've been into have only had single springs, same as low compression.

One thing I suspect people are overlooking is the reason they went to dual springs may, and probably is more complicated than just more spring tension. It might have been to give a non linear curve to the spring tension to lower wear. Since I don't have the spring specs, I can't say for sure, but something to consider.

[Aldo525]

High comp (10:1) b34 has more aggressive cam and double valve springs. Low comp(8:1) has milder cam and single valve springs. B35 has double valve springs and aggressive cam profile. 

The high compression B34's I've been into have only had single springs, same as low compression.

One thing I suspect people are overlooking is the reason they went to dual springs may, and probably is more complicated than just more spring tension. It might have been to give a non linear curve to the spring tension to lower wear. Since I don't have the spring specs, I can't say for sure, but something to consider.

 

 

 

 

Good point....Verified all the numbers on pre 06/86 (single spring) and 07/86 to 06/87 (two springs), all the parts are the same: engine block, crankshaft, camshaft, inlet and exhaust valves, rockers arms, cylinder head, gaskets, pistons,  etc, etc.....just the dual valves difference. I'm still waiting the head arrival so I have time to consider if I'll go for the one spring or the two springs config.....

[turbodan]

High comp (10:1) b34 has more aggressive cam and double valve springs. Low comp(8:1) has milder cam and single valve springs. B35 has double valve springs and aggressive cam profile. 

The high compression B34's I've been into have only had single springs, same as low compression.

One thing I suspect people are overlooking is the reason they went to dual springs may, and probably is more complicated than just more spring tension. It might have been to give a non linear curve to the spring tension to lower wear. Since I don't have the spring specs, I can't say for sure, but something to consider.

I believe it may help dampen resonance too. Those springs can start doing unpredictable stuff at certain frequencies.

[Mike W.]

The high compression B34's I've been into have only had single springs, same as low compression.

One thing I suspect people are overlooking is the reason they went to dual springs may, and probably is more complicated than just more spring tension. It might have been to give a non linear curve to the spring tension to lower wear. Since I don't have the spring specs, I can't say for sure, but something to consider.

I believe it may help dampen resonance too. Those springs can start doing unpredictable stuff at certain frequencies.

That sort of thing is what I mean. I know enough to know it can be much more complicated than it seems. Not always, but it can be.

[Aldo525]

I believe it may help dampen resonance too. Those springs can start doing unpredictable stuff at certain frequencies.

That sort of thing is what I mean. I know enough to know it can be much more complicated than it seems. Not always, but it can be.

BMW...what else????......Things like that are the reason why people (like me) loves (and frequently hate, but not too much) these cars...

[Shawn D.]

BMW isn't unusual in having a spring damper arrangement.  For example, Small Block Chevy V-8s had springs with dampers since the beginning in 1955 (here's an aftermarket one with the same type of damper as from 1955).  The difference is that dampers like this SBC one don't contribute much in spring rate.

FWIW, I suspect the ETK drawing is somewhat erroneous, as I've never seen dual springs wound in the same direction.

[Aldo525]

BMW isn't unusual in having a spring damper arrangement.  For example, Small Block Chevy V-8s had springs with dampers since the beginning in 1955 (here's an aftermarket one with the same type of damper as from 1955).  The difference is that dampers like this SBC one don't contribute much in spring rate.

FWIW, I suspect the ETK drawing is somewhat erroneous, as I've never seen dual springs wound in the same direction.

Right !!...check some pictures of the springs: xxx767 (inner) coil counterclockwise and xxx768 (outer) coil clockwise.