Engine and Transmission Tech

If you only consider port runner area and volume, at what point does the intake valve become too large? Should be the same whether or not supercharged ?

As a theoretical question, that's a good one Larry.
It would seem to me that based upon the port only.
Any intake valve that flows more than the port would be a waste. But not a detriment to performance.
Actually, thinking about it, a big valve, with the right cam, could provide more push ( harder pull) on the port at a lower lift.
This would be particularly true in a forced induction scenario. Building more torque at low revs. And less stress on the valve train due to the lower lift levels.
In a NA scenario it might be an advantage to be slightly small to increase the pressure drop across the valve.
This would allow a longer duration intake before significant reversion becomes a problem.
Once initiated, port flow could continue longer before interference from cylinder pressure became a problem.
Just thinking out loud.
What do you think?
Cheers
Fred

JMHO, cfm vs port velocity are inseperable. Bigger intake valve, often includes dimenishing returns. Unless the application is splayed valves, larger valve will have more shrouding, and require larger valve notch. Heavier valve needs stronger spring = more frictional loss. It's a fine line between bigger is better. This is why air flow guru's consider ALL of the combo. Valve/seat profiles are critical and specific to a particular combo. It has been said, best flow improvement occurs , by first working 1/4" above and below valve seat areas. CFM is only part of the complete picture. Cheers, roverman.

Of course there is far more to the picture than just the port and valve relationship.
But the question was "port related to valve" only.
It is interesting to contemplate what would happen if all the other constraints were removed.

Cheers
Fred

> If you only consider port runner area and volume, at what point does the intake valve become too large?

The quick answer is when the peak intake velocity port drops below 0.55 Mach or
so. Port volume doesn't really enter into as its more of a cross-sectional area
property.

Larger ports mean slower velocity and slower intake flow port flow is more
susceptible to reversion. Momentum is the product of mass and velocity. If the
velocity is low, the momentum is low and may not be able to overcome the rise
in pressure that occurs when the intake valve closes which allows reversion
flow (from the cylinder back into the intake tract). A low air speed also
implies a weak pressure wave action (and subsequently weak resonant tuning).

There are waves that travel the length of the intake tract. The frequency
of these waves are a function of the runner length and cross-sectional area.
Note that these waves move within the intake (and exhaust) flow. Depending
upon their direction, they can either aid or hinder flow motion. One
characteristic of an finite amplitude wave is that when it hits an abrupt
area change (such as a runner opening into the plenum in an intake manifold
or a primary pipe ending in a header collector), it will change direction.
So an expansion wave moving up the intake tract will change direction when
it comes to the plenum opening changes and become a compression wave heading
towards the intake valve. By timing (via the valve opening events, intake
runner length and cross-sectional area, header lengths and diameters) the
waves can aid in cylinder filling on the intake side and cylinder evacuation
on the exhaust side. Get these events right and you can certainly have
volumetric efficiencies of over 100%, especially near peak torque but also
near peak power.

For a given engine configuration, it is reasonable to assume there is an
optimal timing of the wave harmonics which would also determine an optimal
runner cross-sectional area and length which imply an optimal velocity.
Areas larger or smaller than the optimal, all else being equal, will generally
produce less average power. This cross-sectional area would only be optimal
for one RPM and perhaps its harmonics so picking the area needs is a compromise
for the RPM range desired.

It has been noticed that when the intake tract flow velocity exceeds 0.6 Mach
or so, engine efficiency drops and an increase in port area to lower the
velocity will provide an increase in average power. Note this increase in
port area is usually associated by higher steady state (i.e. flow bench) flow.
Likewise, if intake tract velocity is lower than 0.55 Mach or so, reducing the
port cross-sectional area can yield better average power.

I generally use PipeMax and/or Dynomation to simulate a given engine and plot
the intake tract velocities or predict the desired cross-sectional area but
there are various formulae around to calculate the required minimum port area.
Here is one:

MCSA = (0.00353*RPM*S*B*B)/690

where:
S = stroke (inches)
B = bore (inches)
MCSA = minimum port cross sectional area (square inches)
RPM = peak power RPM

Note the 690 in the denominator is simply Mach 0.6 expressed in feet/second.
The formula is the minimum area to keep the Mach number under 0.6. One implicit
assumption is an ideal valve discharge. In practice, the area needs to be
somewhat larger (factored by the valve discharge coefficient). An inefficient
head can have a discharge coefficient considerably below 1.

As a practical matter, the Rover V8, especially larger displacement versions,
suffers from too small ports and valves for higher performance applications.

> Should be the same whether or not supercharged?

No. I have an old B&M supercharger catalog that has a warning about running
one of their positive displacement superchargers on a Windsor Ford V8 unless
you port the heads and install larger valves. They claim the blower will
overheat due to increase backpressure with the stock heads. The small block
Ford suffers from the same problem as the Rover for the same reason. The SBF
started life with 221 cubic inches and grew to 302 cubes and 351 cubes in a
taller deck version that shares the same heads. Valves and ports that were
marginal on the original displacement are way to small on larger displacement
and/or higher performance engines. With turbos you have to worry about surge
and choke.

> I am using the Rover 3.9-4.0 heads and I am looking at intake valve sizes.

What bore size and displacement is your engine?

> The 225 V6 valves are similar, but larger. 3.1 GM valves are much larger,
> but require larger seats. How much is too much?

The port mcsa needs to increase along with the size of the valve, so limitations
on the port size need to factor in. I used 1.775" diameter Buick Stage 1 Buick
V6 valves for the intake and 1.5" diameter valves for the exhaust in my Buick
300 heads but those heads have larger ports to begin with. They required larger
seats but I'm not sure they will fit or have shrouding problems in Rover heads
in a smaller bore.

Dan Jones

Do you have flowbench info on the stock landrover 4.0L heads? Looks like in order to do the little engine right, I need to build a flowbench, set up an anodizing process, and a nitriding operation. Lots of fun, little time.

Larry and clan, Unless your moving the valve centerlines,(much work), 1.77 and 1.5", are bout IT. Much easier flow results, are obtained from the 64' 300 Buick head, by virtue of port cross sections-mainly. These heads would be pretty useless on a 3.5L, and more of a racing head, on a 4L. I must ask how anodizing and nitriding, play into this exercise ? roverman.

"t would seem to me that based upon the port only.
Any intake valve that flows more than the port would be a waste. "

This statement appears to assume that the valve instantaneously snaps to full lift, then instantaneously snaps full closed. Obviously this is not the case. Valve diameter is less important that curtain area (the cylindrical area between the open valve head and the seat), and more importantly, the value of integrated curtain area over time is what governs flow. Obviously a larger valve diameter provides more curtain area at low valve lifts, which improves overall cylinder filling.

I always try try to consider curtain area vs increased shrouding, by installing larger valves. More is not always better/dimishing returns. roverman.

Art, how much room between the edge of the intake valve and the edge of the chamber/cylinder would you consider minimum?

Larry, I wouldn't, too many variables. The flowbench, could be a good guide-here. Valve/seat angles and widths are crucial to maximum flow, especially when shrouding the intake valve. Exhaust is slightly more tolerant, of shrouding. Valve centerlines are 1.660". IF valves were centered in the bore, math would be easy, they aren't. Good Luck, roverman.