Lighter valve train

[teamddr]

Hi all, i have located a cam lifter that are 30 grams less ( 53g total weight). I’m thinking of running less valve seat pressure to free up more horse power. These motors (spec formula single seaters) i build are flat fours with a lifter, push,rocker assembly. My seat pressure is 68 lb as these thing only spin up to 6,300 max ( carb and cam restrictions)but with circuit racing and mechanical downshift could be higher. The cam is a very mild asymmetrical profile.

[David Redszus]

Hi all, i have located a cam lifter that are 30 grams less ( 53g total weight). I’m thinking of running less valve seat pressure to free up more horse power. These motors (spec formula single seaters) i build are flat fours with a lifter, push,rocker assembly. My seat pressure is 68 lb as these thing only spin up to 6,300 max ( carb and cam restrictions)but with circuit racing and mechanical downshift could be higher. The cam is a very mild asymmetrical profile.

Valve spring seat force has nothing to do with engine rpm.

The open spring force is not determined by spring seat force. Seat force is determined by the spring rate and installed height.
Once the valve opens, the spring force is determined by spring rate and lift.

To determine the correct open spring force, we need to determine the mass of the entire valve train
and the rate of acceleration at the intended operating speed.

[LSP]

Hi all, i have located a cam lifter that are 30 grams less ( 53g total weight). I’m thinking of running less valve seat pressure to free up more horse power. These motors (spec formula single seaters) i build are flat fours with a lifter, push,rocker assembly. My seat pressure is 68 lb as these thing only spin up to 6,300 max ( carb and cam restrictions)but with circuit racing and mechanical downshift could be higher. The cam is a very mild asymmetrical profile.

I've never seen a mass reduction on the pushrod side of the rocker equate to a performance advantage, concentrate on the valve side of the rocker.

[PackardV8]

I've never seen a mass reduction on the pushrod side of the rocker equate to a performance advantage, concentrate on the valve side of the rocker.

For true. Can you go to smaller diameter valve stems and/or beehive springs?

[Geoff2]

Valve spring seat force does affect rpm. If the force is too low, valve bounce can occur, which can impact the HP the engine can make & reduce rpm potential.

[hoffman900]

It’s all just a guessing game without a Spintron.

Expensive, but if you regularly pay for engine dyno time, then it’s not much more than that.

[LSP]

Spin data is great, up until the engine is put in the vehicle and encounters g forces.

It's just like any other tool, take it with a grain of salt.

Observing valve train wear will point you in the direction you want to go.

[Walter R. Malik]

Valve spring seat force has nothing to do with engine rpm.

It can ... if valve bounce is an issue; as pointed out earlier.

[Walter R. Malik]

Hi all, i have located a cam lifter that are 30 grams less ( 53g total weight). I’m thinking of running less valve seat pressure to free up more horse power. These motors (spec formula single seaters) i build are flat fours with a lifter, push,rocker assembly. My seat pressure is 68 lb as these thing only spin up to 6,300 max ( carb and cam restrictions)but with circuit racing and mechanical downshift could be higher. The cam is a very mild asymmetrical profile.

I've never seen a mass reduction on the pushrod side of the rocker equate to a performance advantage, concentrate on the valve side of the rocker.

The lifter.pushrod side DOES make a small difference although, it is much less than the valve side, which should be addressed first.

[David Redszus]

Valve spring seat force has nothing to do with engine rpm.

It can ... if valve bounce is an issue; as pointed out earlier.

I'm afraid the problem is a bit more complex. Ignoring valve train mass and engine speed for the moment.

Using a constant lift, spring rate, and spring free length, the seat force can be set to a number of values,
by adjusting the installed height.

A shorter installed height will alter (raise) both seat and open forces.

But a shorter installed height with a softer spring can produce the same seat force and a lower open force.

Seat force alone does not determine open force since the valve on the seat is not moving and cannot change
open force.

The best way to understand is to do the math for various combinations of springs and installed heights
and then graph them.

To control valve float and bounce, we need to know at which angle maximum acceleration will occur
and evaluate the open force needed that specific valve lift point. We are dealing with two positive
acceleration spikes and one negative value over the cam nose.

[LSP]

Hi all, i have located a cam lifter that are 30 grams less ( 53g total weight). I’m thinking of running less valve seat pressure to free up more horse power. These motors (spec formula single seaters) i build are flat fours with a lifter, push,rocker assembly. My seat pressure is 68 lb as these thing only spin up to 6,300 max ( carb and cam restrictions)but with circuit racing and mechanical downshift could be higher. The cam is a very mild asymmetrical profile.

I've never seen a mass reduction on the pushrod side of the rocker equate to a performance advantage, concentrate on the valve side of the rocker.

The lifter.pushrod side DOES make a small difference although, it is much less than the valve side, which should be addressed first.

"I've" never seen it, when I have it'll be the first time.

[Walter R. Malik]

Valve spring seat force has nothing to do with engine rpm.

It can ... if valve bounce is an issue; as pointed out earlier.

I'm afraid the problem is a bit more complex. Ignoring valve train mass and engine speed for the moment.

Using a constant lift, spring rate, and spring free length, the seat force can be set to a number of values,
by adjusting the installed height.

A shorter installed height will alter (raise) both seat and open forces.

But a shorter installed height with a softer spring can produce the same seat force and a lower open force.

Seat force alone does not determine open force since the valve on the seat is not moving and cannot change
open force.

The best way to understand is to do the math for various combinations of springs and installed heights
and then graph them.

To control valve float and bounce, we need to know at which angle maximum acceleration will occur
and evaluate the open force needed that specific valve lift point. We are dealing with two positive
acceleration spikes and one negative value over the cam nose.

I understand it full well ... I have viewed a number of Spintron tests and if the valve is not floating but, it is bouncing upon closing ... a different rate spring is probably necessary however, using the same spring and raising the seat pressure does help raise the RPM level, (in those cases), even though spring force was not needed to be increased at max valve lift.
This kind of thing seems to happen quite a bit with very aggressive flat tappet stuff.

Lots of people just don't realize what is happening or actually needed.

[mag2555]

If your valve train is not showing any signs of problems as in these photos from this Manly catalog then your likely fine for the rpm the motor is spinning and have good valve train control within that rpm range and any time spent on lightening up the valve side of the rocket arm will only provide you longer spring life and a extremely small performance gain.

In a limited class of racing where your fighting tooth & nail for greater performance then faster acceleration by means of lighter weight parts should have you looking into the recipercating parts as well for potential weight reductions if your valve train is good.

Even though you have a box motor a simple tight fitting crank scraper for example could be worth 5 hp at 6500 rpm if the motor has a small crankcase area.

[digger]

Valve spring seat force has nothing to do with engine rpm.

It can ... if valve bounce is an issue; as pointed out earlier.

I'm afraid the problem is a bit more complex. Ignoring valve train mass and engine speed for the moment.

Using a constant lift, spring rate, and spring free length, the seat force can be set to a number of values,
by adjusting the installed height.

A shorter installed height will alter (raise) both seat and open forces.

But a shorter installed height with a softer spring can produce the same seat force and a lower open force.

Seat force alone does not determine open force since the valve on the seat is not moving and cannot change
open force.

The best way to understand is to do the math for various combinations of springs and installed heights
and then graph them.

To control valve float and bounce, we need to know at which angle maximum acceleration will occur
and evaluate the open force needed that specific valve lift point. We are dealing with two positive
acceleration spikes and one negative value over the cam nose.

imagine a case where you have adequate force "over the nose" but the spring is installed without any preload and therefore the force on the seat is effectively zero. How well do you think that will work for preventing bounce?

now ask your self what is the theoretical acceleration of the valve as it comes off the closing ramp (assume constant velocity) onto the seat ? assume parts are ideal and perfectly rigid

[David Redszus]

imagine a case where you have adequate force "over the nose" but the spring is installed without any preload and therefore the force on the seat is effectively zero. How well do you think that will work for preventing bounce?

The parameters that affect valve spring selection and tuning are:
spring free length
spring installed height
spring rate
valve lift & acceleration

If installed height is equal to free length, then seat force will be zero. But a nose force will still
be present resulting from spring compression due to valve lift. Whether it will be adequate will
depend on valve acceleration and valve train mass.

If installed height is reduced by use of shims, then seat force will be increased and nose force will be
increased by the same amount.

now ask your self what is the theoretical acceleration of the valve as it comes off the closing ramp (assume constant velocity) onto the seat ? assume parts are ideal and perfectly rigid

Valve acceleration numbers are typically as follows:
base circle to ramp = +0.25 in/deg^2
cam flank = +2.0 in/deg^2
over the nose = -1.0 in/deg^2

While the term "nose force" is commonly (and incorrectly) used, it's force is greater than ramp acceleration
but not as demanding as flank acceleration. Maximum spring force to control valve float and consequent
valve bounce is necessary during flank acceleration. Unfortunately, spring forces produced by spring rate
and valve lift are sometimes inadequate to control valve launch. Increasing the seat spring force, and the
nose force by reducing the installed height is a more tunable approach.

Since maximum valve spring force needed to control valve launch occurs about 30 degs after/before the
ramp, higher seat forces are needed which may also result in excessive nose force.