Why not more lift?

[Kenny]

Just curious - anyone know if the relatively low lift values on most aftermarket cams is just a rocker arm durability issue, or if there are piston-to-valve clearance or other rocker arm geometry issues as well?

For example, 11mm of lift on a 47mm intake valve is not actually lifting it enough.

I'm putting steel rockers in my (B35) head so they aren't a limitation so I'm curious if a custom grind might be worth considering.

[Coldswede]

I'm hoping Paul Burke will chime in on this.

Basically the entire valve train operation is way too complex and interwoven to be reduced to saying "More valve lift is good and I need it"

[Kenny]

It's not complex. The principle of valve curtain area are relatively universal, actually.

The basic concept is that a valve of a given diameter need to be lifted off of the seat a certain amount before what is known as the "curtain area" becomes great enough that the valve is effectively "out of the way" of the flow. This is independent of what is actually actuating the valve.

Flow bench data seems to support that these heads pick up flow right up to almost 12mm of lift...

This lift equates to around 25% of the valve's diameter, or about 11.75mm in the case of a 47mm intake valve. Pretty much no one makes and off the shelf cam with that much lift, for some reason.

The B35 port volume is way too large as it is, especially on the intake side. To oversize the port and then not even lift the valve all the way out of the flow is typically considered... odd...

Maybe the ramp angles get too steep (and/or valve accel gets too high) for other components to be able to deal with other than just the rockers. Or clearance becomes an issues (rare on a head without canted valves though).

So I'll rephrase, because to be frank, I'm not looking for a lesson in flow dynamics or handwaving comments about it all being too complex, no offense.

Is there any mechanical reason, other than the rockers breaking, that restricts the valve lift on an M30B35 head to under 11.5mm?

[mooseheadm5]

My guess is valve acceleration. The springs are already dual springs on a B35. You can profile the cam to reduce acceleration, but you still need to get it pretty far open pretty quickly at high RPM to get to that lift, and the rockers and valves have a good bit of inertia.

[paul burke]

It's not complex. The principle of valve curtain area are relatively universal, actually.

The basic concept is that a valve of a given diameter need to be lifted off of the seat a certain amount before what is known as the "curtain area" becomes great enough that the valve is effectively "out of the way" of the flow. This is independent of what is actually actuating the valve.

Flow bench data seems to support that these heads pick up flow right up to almost 12mm of lift...

This lift equates to around 25% of the valve's diameter, or about 11.75mm in the case of a 47mm intake valve. Pretty much no one makes and off the shelf cam with that much lift, for some reason.

The B35 port volume is way too large as it is, especially on the intake side. To oversize the port and then not even lift the valve all the way out of the flow is typically considered... odd...

Maybe the ramp angles get too steep (and/or valve accel gets too high) for other components to be able to deal with other than just the rockers. Or clearance becomes an issues (rare on a head without canted valves though).

So I'll rephrase, because to be frank, I'm not looking for a lesson in flow dynamics or handwaving comments about it all being too complex, no offense.

Is there any mechanical reason, other than the rockers breaking, that restricts the valve lift on an M30B35 head to under 11.5mm?

I will make this short because I just don't have the time to sit here and type all day.

ALL non roller type followers offer limitations to rate of motion. The contact point between lobe and lifter/follower is determined by the radius, diameter and or length of the follower which in turn dictates the rate of lift/timing.


I do have shelf camshafts up to .480 lift and yes you can go higher but the M30/M10 type valve train in its production form limits lifts mostly due to spring stacking, rocker geometry (fulcrum point) and the ability to install the cam into the cylinder head (large lobes with the correct size base circles will inter fear with journals during assembly).


Hope this helps a little.

Paul

[mooseheadm5]

So I guess the next questions for PB:
If a roller rocker could be engineered for the M30, would it be worth it?
Could you bore the journals out enough to accept a higher lift cam, or would it be better to attempt to move the fulcrum point?

Obviously those would be questions for another day.

[Jeremy]

If a roller rocker could be engineered for the M30, would it be worth it?

That's not an "if", a little bird told me Paul's already done it. Whether the cost is worth it is up to the individual customer I suppose, I honestly have no idea what Paul charges for that type of work.

[shocka]

If a roller rocker could be engineered for the M30, would it be worth it?

That's not an "if", a little bird told me Paul's already done it. Whether the cost is worth it is up to the individual customer I suppose, I honestly have no idea what Paul charges for that type of work.

I like where this is going.