How valid would this be
Turbo Mini Forums

After reading about rocker ratios and cams for turbo use, I got to thinking how I might find a way to make a decision on what might be the optimum valve lift.
What i did is as follows.
1) Make an adaptor plate to fit over one inlet port to accept an airline.
2)Remove the valve spring from one valve
3)Fix a graduated scale at the side of the springless valve.
4)Tip the head on it's side(to negate the weight of the valve)
5)Blow air into the port at manifold absolute pressure (14.7 + boost) and measure the lift.
Can any of you technical whizz kids see why this should not give a good indication of optimum valve lift?
Regards
Dave

Sounds like a decent idea, theres probably some complicated mathematical equation to work it out.

1/4 Mile 14.3secs 96Mph Terminal 10psi of boost.


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Usually the formula Valve diameter/4 is quoted as the optimum, but for a forced induction this may not apply.
Regards
Dave

The valve will lift right out of the way due to the airflow before the pressure is reached.

You would have to close the valve to create the pressure, hence negating the experiment.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

The optimum valve lift is the lift at which you cannot achieve any more air flow if you lift it higher.

Hence, the air flow is restricted by the port rather than the valve and seat.

Flow is induced by differential pressure between the port and the cylinder.

The factors that affect flow through the port are surface friction and changes in direction.

The factor that affects flow through the valve is mainly velocity.

The main change in a boosted application is the density of the air entering the cylinder.

The density change has a similar affect on surface friction and head loss due to change in direction and velocity.

Hence, I think, optimum valve lift on a boosted engine is the same as NA.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

PS I'm sitting here running some complex hydraulic modelling software, so I'm in da groove this afternoon!

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Run as much as your cam, follewers, pushrods and springs can reliably handle.

But then, its also worth considering....do you have a turbo large enough, to warrant huge valve lift ?
If you want more airflow, fit a larger turbocharger, or use more boost.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

remember to rig up the exhausts the wrong way around. They are not inlets!

I have heard of a guy using a "floating ball" method of calculating airflow and using a modded vacuum cleaner.

Imagine this, consider valve diameter and cam profile/lift. Do your maths and see how the lift relates to area for flow. You will see that having a bigger valve is akin to having a faster opening cam profile, it would also give less of a pulse for a given pressure. I wrote something about this in my supercharger blurb. I reckoned that it was better to use a high lift, shorter duration cam with a bigger exhaust valve, etc, etc.

Depends what you want to do. Get the most out of an engine or get a good manners powerful engine.

What your idea will come up with is some sort of curve which will give you very little indication other than "as much as possible" which is best for your engine.

Do some excel jobbies, consider cam profiles/lifts against valve size/lifts, plot the area available for flow against crank angle and then consider the gradients. You want to carefully consider the overlap/projected residual boost, possible flow is.

Sadly, nothing other than trying it in a car can tell you how it will behave in an engine...... and then you have port sizes to consider and the inertia of gas for a given density.....

Bugger off, I'm getting there.