Story by Liam Quirk Pics by Scott Stoneman

head porting explained
Port
Of
Call
Let’s look at the physics behind
the VN V8 cylinder heads, and how
to work them to maximum effect
Story by Liam Quirk Pics by Scott Stoneman
Head Work 101
The cast-iron cylinder heads fitted to the top
of the VN 5L V8 are commonly regarded as
the best heads Holden ever fitted to its V8s
for a number of reasons.
The airflow figures from the factory
and potential for increased flow with
minor work, the size and shape of the
factory ports and the sheer amount of
material in the heads all point to one thing
– a great pair of metal chunks to have
bolted to the top of your V8.
George uses VN heads atop his boosted
stroker in TUFUTE and has worked them
to flow phenomenal figures. Well-known
car owner Sean Fardell of MR HDT fame
says that he has seen the heads flow up to
595hp, and that’s obviously before exploring
the alloy heads where figures of 700hp and
above can be obtained.
So desirable is the design of the VN
cylinder head that Yella Terra has based its
Holden V8 alloy head on it. When it comes
to heads, the VN items are the bee’s knees.
“The VN heads are pretty accurate
except for where the pushrods holes are,”
Sean from Allports says of the late-’80s
cylinder heads.
“The HQ heads were average. They
got into the pollution thing and were crap.
Once the VL ones came around, they were
good. The VN one on the whole is a very
high-quality piece of equipment. Holden
did its job well with those heads … The
VN heads are a very good casting.”
Before we go any further, let’s clear
one thing up. Unlike most things in life
(cubic capacity, for example), bigger
isn’t always better when it comes to
porting cylinder heads.
Take too much metal from the walls
or floors of the ports and you will either
break through and cause structural
issues, or end up with an oddly sized
equation. Then, you come to the sad
realisation that there’s physically no way
you can make them all the one size.
If you’ve taken the heads off your
bog-stock 5L to port them, you don’t need
to take masses of material out and prep
the heads as if they were being strapped
to MR HDT. Let’s face it, your standard
motor simply doesn’t need that much air,
and providing it may very well cause it to
run worse.
Sean is quick to point out that too
many people become obsessed with getting their heads to flow huge horsepower
figures. In fact, this figure accounts for
little more than bragging rights at the pub.
Instead of focusing on flashy numbers
and the heads’ potential for flow, concentrate on the quality of the work done to
the ports and appropriately sized valves.
Above all, look at the quantity of air your
motor requires.
The theory behind expanding the
ports in the cylinder head is simple. On
a small scale, porting tidies up the dags
and mess left around by the casting
process, and it equalises the sizes of the
ports. The factory margin for error and
variances in port sizing mean that no
Here you can see the work Sean has
done through the roof of the port.
“Normally they have a distinctive
channel in the roof area through the
injector notch. You can see the area
I’ve worked on, and my goal was to
reduce that notch because, as far as
I’m concerned, it upsets things. I like
things to be as smooth as possible,”
Sean explains
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head porting explained
two factory heads flow the same, even
when untouched.
The first thing to do is bring all ports out
to the same size to ensure a solid base to
work on. Slight overhangs and irregularities
inside the ports can disrupt airflow, and as
is known, turbulent air is harder to move
than air flowing on a smooth surface.
Once the inside of the ports have
been tidied and worked to the same
specs, we can focus on extensive porting with performance in mind. The basic
idea goes further than simply moving
air efficiently; it’s about moving bulk
amounts of air as well.
We can all appreciate that a bigger
person is going to be able to fit through
a bigger door, and with small ports on an
angry motor, it’s like trying to walk a horse
through a cat door – you’re just making life
harder than it really needs to be.
The Facts
For all the info, we talked to Sean. He is
the man behind Allports and the artist
responsible for the spectacular work done
to the VN heads for Phoenix. He kindly
took us through, in detail, the process of
working the specific heads that will be
strapped to the motor. He explained to us
his techniques, and why these heads are
going to help us make bulk grunt.
“Firstly, you need to know what you’re
doing with the engine, which means the
basic combo,” Sean begins. “What the
intended purpose and use of the engine
is, what vehicle it’s going into and what
the owner expects from it.”
In the case of Phoenix, we know that the
heads will be above a rebuilt 304ci motor
and below the Yella Terra supercharger. We
know that with the manual box, there will
be the potential to wind the revs out a little
higher, but what we didn’t know was the
style of driving it would be subjected to.
“From there, you work off a rev
range. Where’s the engine going to be
used from, and what is the peak engine
speed you’re going to be running? You
can convert this into feet per second
and calculate the port sizes.
“I like to plot my numbers out first and
work to a plan. In the case of the Phoenix,
being supercharged, I could work on using
larger ports than are normally found in an
NA 304ci engine. I’m not talking stroker in
this case as that’d be different again.
“So, I set the intake mean gas velocity average at 272fps at 6000rpm, 294fps
at 6500rpm and 317fps at 7000rpm.
Then, I worked it out with the camshaft,
which is why it’s good to know cam
specs at the start.
“It’s good to work around the cam
instead of working the heads and then
getting a cam to suit. The guy working the
heads might calculate the air speed at a
This shot allows us to appreciate the work done to
the exhaust port and to the
bowl area. The bowl itself
hasn’t been enlarged, or
‘hogged out’. The exhaust
port has been extensively
smoothed to calm airflow.
This shot also highlights
the three-angle-machined
valve seats
Here is Sean in action. He is
working the exhaust port using an oval-style cutter to gain
the desired shape inside the
port. The aim here is to give
the area a greater radius
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Here is the work being done to the power wall. One of the first things to go is
the mini pushrod lump that is cast in there from the factory. “I like to remove
that, as it sets up the straight-line shot into the bowl. It’s one of the first areas
I work on,” Sean says
certain amount for a specific lift, and then
the cam might not give that lift or will give
more. It upsets the apple cart.
“I like to have things set in concrete
before doing anything,” Sean says.
From here, we talk with Sean about
porting. “I don’t agree with people flowing
the exhaust and increasing that flow. It’s
like piercing a pressure vessel, and that’s
what an engine is like, a pressure vessel.
“The instant you crack a tinny, the
pressure is gone, and an engine is no
different. It doesn’t extend its pressure. When it’s gone, it is gone and it
happens instantaneously.”
Back pressure, no matter what the
circumstances, is imperative. It just
depends on how much back pressure
in question. “I try to keep the exhaust
as small as possible, but granted some
form of flow bias is needed.”
Qualifying this, Sean explains. “Everything starts at the exhaust with the engine. Some people prefer to talk about the
inlet first; I like to talk about the exhaust.
Without that action, there isn’t the induction. The exhaust exiting the chamber at
speed, if it is maintained, helps pull the
intake that charges or fills the cylinder.”
So where’s the best place to start?
“On the inlet side, keep things as straight
as possible. Some heads can’t do this,
but the VN heads can. With the particular
heads we’re talking about, if you look at
the port entrance to the valve face, it is
almost a straight line on the active side of
the port,” Sean says.
This straight wall he’s talking about
will also be referred to as the power
wall, and it is an extremely integral part
of the head design.
“Assuming that the heads are already
cleaned and the guides are done, I prefer to do my valve seat work first.” This
work is heavily affected by the spring
rate of the chosen valve springs and the
height of said springs.
“It’s important to have all the parts on
hand. The collets, the retainers; have everything there so that you’re not working blind
and there is a chartered course. Therefore,
you don’t finish a pair of heads and then
have to fight to get compression, or fight
with springs and shims,” Sean says.
The Ports
Port sizing is very important. At the end of
the day, it’s not about going as big as possible, it’s about controlling air speed.
“Go in and measure them with some dividers, work out the square-inch area of the
port and convert that into a feet-per-second
(FPS) air speed. This is the mean average
gas velocity. Depending on the engine,
it can run from 280fps at peak power to
350fps in some cases, though that number
tends to be a little bit high,” Sean explains.
But what of Phoenix? Where does it
lie on the spectrum? “An engine like the
VN-headed one we’re doing is calculated
at around 300fps–320fps peak speed,”
Sean says. With the numbers plotted, pull
out the tools.
“When working on the port, I make
measurements and decide whether or
not to increase the port size. I use the
verniers and go through the valve opening area or bowl to take a measurement
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head porting explained
from the floor of the port to the deck of
the cylinder head.
“I record that reading and go through
all of them. Whichever is the lowest, I’ll
probably start on first because I can always
bring the others down to suit,” Sean says.
One of the first things to do to the ports
involves working the pushrod choke area,
which is where the highest inlet speed is
needed. Air should be entering this part of
the head at around 320fps.
From here, the walls of the ports can
be straightened up (keeping in mind that
removal of too much material will mean a
broken pushrod wall). “Sometimes ports
are that far out in their casting that you can
go through. That’s the only area of the VN
head that can be ticklish,” Sean says.
“Some only have 20thou’ worth
of material there, and as soon as it is
touched, it’s gone.”
After this, the air needs to begin
decelerating. “You don’t have a truck
making a turn without slowing down. Air
is the same as a truck, it has to change
speed. If we’re at 320fps, I like to try to
slow that down to 270–280fps by opening up that area,” Sean says.
The particular area opened is the
window above the short turn. By making
this area larger physically, it takes more
air to fill. Thus, it slows the air down
and allows it to turn into the port at a
reasonable rate.
“In the case of the VN head, about
80thou’ has already been taken off
the floor material. This is roughly 2mm
measuring from both sides through the
seat and through the outside of the
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port. I record the measurement and go
from there,” Sean says.
One of the token phrases when you
ask someone if they have had head work
done is ‘yeah mate, fully ported and
polished’. The latter would be quite detrimental in the quest for power.
“Smooth is not an option [for the
ports]. Currently, the myth has probably
been dispelled about polishing ports – it’s
the last thing you want to do. It’s the
surest way to have a slow engine as far as
I’m concerned – one that’s hard to tune
and doesn’t burn fuel properly.
“Generally, a rough grinded finish is
left on, or a sanding sleeve is used to
create a coarser surface than a polished
finish. To look at it, it might be shiny,
but in effect it’s putting sanding lines in
the port and leaving the fuel and air in
suspension,” Sean says.
While the ports may not need to be
silky smooth, some areas of the head
should be polished. The short turn area
into the bowl and the radius from the wall
(as it makes its way into the bowl and seat
area) can be polished, Sean admits.
“The most bump-free smooth thing
you can have on the cylinder head is the
short turn. Grind a short turn as close as
possible to a smooth area, and then use
some inch-wide emery paper and polish
the hell out of it.
“That area receives a lot of attention
as it’s basically free power. You polish it
up because the air is moving generally in the high side of the port as it’s
coming around the short turn area. For
this reason, that is where most of the
Sean goes to work on the pushrod
wall. The cutter being used is wedge
shaped, and this makes the material
easier to remove. It also allows Sean
to resize the area as he sees fit
Sean then moves all his attention to
the ‘short turn’ area. The wedgeshaped cutter comes out again so
Sean can remove material from the
floor and the base of the port, plus
smooth the area out
work is done. The short turn is an area
where a lot of power can be made,”
Sean explains.
it to travel. “It isn’t ever perfectly straight,
but you can almost see from the port to the
bowl how straight the line is,” Sean says.
Port Potential?
Flow Bias
One thing Sean was kind enough to explain
to us was ‘port potential’. For those unfamiliar with the term, it’s a measure of how
much flow one port is good for with the
major restriction (the valve) removed. The
test is done on a flow bench with the valve
out and a finger blocking the guide.
“Your 0.800in [at 10in] reading is
160.1CFM. Without the valve in the hole,
it’s only 160.2CFM. In many heads, you
will pick up lots of CFM, but it didn’t pick
up any. Only 0.1 of a CFM isn’t even worth
talking about. From 600thou’ and up, it
was basically the same,” Sean says.
Port potential could also be called a
flow coefficient, and it is a good way to
measure how efficiently a port has been
made to work.
“In this case, it is 95-percent efficient at
600thou’. At 700thou’ it’s over 98-percent
efficient and at 800thou’ it is 99.93-percent
efficient. This is all bearing in mind that it’s
an L34-sized valve we are flowing. It’s not a
2.02, 2.05 or 2.08,” Sean concludes.
So, what is this notion of flow bias? Flow
bias is yet another way of controlling the
way air enters the head and can be used
to control direction, speed and a number
of other things.
Sean talked with us mainly about biasing the exhaust valve. He said, “If you’re
talking flow bias, a lot of people are using
75–80 percent. I don’t like to work to those
numbers. It changes if you’ve got a street
exhaust system, if you’re open piped, if
you’re short piped… I try to work an area
to get the speed high.”
So, while biasing a port to flow to meet
one criterion, the minute those parameters change, the maths has been thrown
out. If Sean has taught us one thing, it’s
that when working cylinder heads, everything needs to add up.
The Power Wall
The power wall is the longer side of the
port and is called so because it directs the
majority of the air down the port and into
the large, cavernous bowl.
The most important thing to do here,
according to Sean, is straighten the wall
right out. That way, the path the air takes is
quick and direct – the most efficient way for
One of Sean’s most important tools is a pair of dividers and a digital
vernier, which allows him to take a lot of important measurements.
Maths is at the base of the power game in the heads, and sizing all
the ports equally is imperative. “It’ll do nothing but help the engine as
long as you can make everything as even as possible,” Sean says
Valve Seats And
Hogging The Bowl
Sean’s first step, after cleaning the heads, is
to cut the valve seats and tidy up the bowls.
“I cut valve seats and do bowl hogging before I start the cylinder heads.
I like to work to a known area rather
than porting the heads and then cutting
seats. Otherwise, there will be anomalies, undercutting etc. I don’t believe it’s
a good way to work, and I prefer to work
with the seats cut.”
After the seats have been properly
worked, it’s on to the bowls where the
“You work the power wall and get it
as straight as you possibly can as it
exits into the bowl. You’ll find that
you make nothing but power.” – Sean
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head porting explained
Behind the exhaust valve, you can see just
how smooth Sean has made the port. Cast in
from the factory is a sharp step area – one of
the first things Sean removes. “The step can
sometimes hurt things. You want it all to be
gentle, not abrupt. Smoothness is everything
when dealing with a cylinder head,” Sean says
hogging process begins. For those that
think hogging is a matter of stuffing some
kind of swine into the heads, it is simply
a matter of making the bowls bigger by
removing material.
Here, it’s all a matter of keeping things
uniform. “When starting on the bowls,
work on the ports first. Port it all the way
to the 60° cut under the valve seat, which
leaves 30–40thou’ material before the 60°
cut” Sean says.
“You can change the characteristics
of a cylinder head and how it works
by the way you port it and the way the
seats are set up. The seats are everything,” Sean matter-of-factly concludes.
“I like to keep the valve seats on the
inlet as thin as possible, within reason.
If they are too thin, they just chop the
valve and pound the seat out. But, it’s
good to have a thin seat because it
also stops carbon build-up, or helps to
prevent carbon build-up. You’re never
going to stop it.
“Back cutting valves is another option [putting a 20–30° chamfer on the
back side of the valve above the seat
area]. But, I didn’t back cut the valves
on this engine because it is going to be a
supercharged induction system, and I’d
prefer to leave some of the integrity in
the valve instead of machining it away.
“When dealing with back flow or restricting back flow, then only a small back
cut is used. If restricting it is not the issue,
use the biggest back cut possible.”
Chambers
This is the finish Sean was talking about that helps keep the air/fuel mixture
in suspension. “It looks shiny and smooth, and it is but it’s not mirror finish.
It’s actually a little bit coarse,” Sean explains
Sean had this to say about the VN chamber.
“It’s a fairly efficient chamber. A lot of
people gouge things out and grind things
off, like the square area around the sparkplug. Personally, I don’t like to do that.
“I like to scribe it out with the bore line
in most cases. Basically, nothing incredible, just de-dag all the casting and smooth
it up. I’m not one for absolutely polishing
it, but a lot of people around do.”
Other Tricks
Number Crunching
For those interested, Sean gave us the flow figures for the heads to be bolted
to the Sam’s-built short motor going into Phoenix. He provided us with figures
for the heads flowed at both 10in and at 28in.
Flow at 10in
(CFM)
Intake
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
Exhaust
39.4
63.3
78.1
86.7
91.3
95.2
96.2
98
54.3
86
112
132.3
144.1
151.9
158.2
160.2
Flow at 28in
(CFM)
Intake
0.100
0.200
0.300
0.400
0.500
0.600
0.700
40.68
143.62
187.04
220.77
290.64
253.67
264.19
Exhaust
65.79
105.71
130.42
144.78
152.47
158.98
161.32
0.800
267.36
163.66
At 28in, the port potential was 267.53cfm, so under full load you can see that
the heads on Phoenix are working at maximum efficiency.
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There are numerous other secrets to
making power inside a head – secrets that
can only come from years of experience
and pushing the boundary. For instance,
there is a hump in the floor of the cylinder
head from factory. When Sean ports a VN
head, that hump is ground out.
Another area where Sean uses a bit of
trickery and experience is in working the
guide boss area.
“A lot of people like to narrow them
and thin them right down. I’ve used that
on many occasions, but with a lot of the
racing stuff, I’ve never seen much of a
correlation between performance and
thinning it. I think the easiest way is to
just drop the top of the guide down,”
Sean says.
The final method could best be
described as over-engineering, or rather
aptly, over-flowing. “Why I keep flowing a
head beyond where it’s going to be used
is because it is a very good yardstick.
“If you’ve got 500 lift and the head falls
over at 550, in many cases the engine will
only make mediocre power. Especially if
the head falls off a long way – some heads
don’t, some heads do.
“There might be people around that
would dispute that, but that’s what
I work to,” Sean says. In essence, by
pushing the heads that little bit further,
making them efficient even beyond their
intended use, the results trickle down
into the lower ranges for a head that
performs across the board.”
Back Flow
One form of testing that is not often heard
of is back flowing. As Sean explains, at
low lift (0.100–0.300in) the head as a
whole (but more specifically the intake
valve) is still suffering the aftershock from
the force of the piston propelling towards
the top of its cycle.
It has to deal with the resultant explosive forces, heat and other phenomena.
“The more flow that is inhibited from
going back up the port as the valve is taking off, the better. This means the valve is
at 100–200thou’ on the overlap before the
piston is accelerating away from the combustion chamber and pulling on the port.
“Here an edge is put on the valve. As
it’s coming up, if a valve has a deflector
angle, especially around the 200thou’
region, it is better. Reverse flow is cut by
at least 30 percent and anywhere up to
50 percent.
“The idea is to keep the charge in the
cylinder, and it does tend to make the engine run a little smoother, which helps when
during street driving. As far as feel goes, it is
fairly minimal. It takes a lot of power change
in an engine to feel something.
“On the exhaust side, there’s not a lot
you can do. Suffice to say, if the exhaust
is over ported, it will hurt power more so
than make power. In the case of a carby
engine, it will be less responsive to tune.”
Valves
“If the engine can run an oversized valve,
buy an oversized valve. This doesn’t necessarily mean the biggest oversized valve,
as bigger isn’t always better. It helps, but
it’s not always the best way.
“Consider that the valve is a variable
venturi. It starts from zero and opens to
anywhere between 450–700thou’ and
even higher. Whatever the valve lift is, it
has an organ effect on the inlet tract.
“The speed will, albeit very momentarily,
increase and decrease,” our expert Sean
says. In the case of Phoenix’s heads, Pep
Pro L34-spec stainless steel valves were
chosen. The seats of the valves have been
three-angle-ground – a 20–30° top cut, a
45° seat and a 60–70° drop into bowl area.
With that in-depth exploration, hopefully you guys understand what’s going
on inside the head a little better. Knowing
the major restrictions inside those chunks
of metal puts you in a better position to
work in search of performance.
We have to thank Sean of Allports for
the work he did on the heads and the
time he took to explain the process to us.
Not to mention divulging trade secrets
and his personal techniques, which would
have taken years behind the cutting tool
to develop and finetune. Thanks also goes
to Thornleigh Cylinder Heads for allowing
Sean to use their shop. sc
Contact
Allports
Sean - 0404 780 630