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Cylinder head - Suitability basics |
1 May 2001 |
Whatever else owners do to their Minis by way
of interior/exterior modifications or none at all, a very large proportion desire
and search for an increase in engine performance. Now this may not be
super-sonic speed-inducing power, it could just as well be for maximum economy.
In either case, it’s the engines volumetric efficiency (VE) under scrutiny - it’s ability to breath in as much correctly proportioned
air/fuel mix as possible in any given situation. Although the induction/exhaust
system plays an important part, there are effectively two major factors that
dictate performance increases - the camshaft and cylinder head. The induction
and exhaust elements (stage one kits, exhaust systems, etc.) and camshafts are
covered in various other articles in my 'Corner'. So here we’ll have a look at
cylinder heads and dispel some of the myths and legends surrounding them.
It is a fact that almost any engine will give
better performance from a suitably and - in particular - a well-modified head.
The key items to note here are ‘suitably’ and ‘well’. There are quite a number
of modified heads on the market that will not improve performance at all. The
main problem is identifying a good one from a bad one - almost impossible to
the untrained eye; particularly as devious would be 'head-sperts'
(am I allowed to say that?) employ the ‘sparkly baubles’ (dazzle with
brilliance? Or, as an esteemed colleague and friend puts it, 'eye candy')
approach, i.e. much polishing and careful painting in bright colours, to
attract the eye of the unwary. Techno-babble is added into the equation (baffle
with bullsh*t approach), and an endless list of
tuning levels (stage 1, 2, 3, 4, 5, etc.etc.). So let
us have a methodical look at what goes into a head, modifications employed,
applications, and directly associated ancillaries to try and decided what is
suitable and beneficial to your particular application.
VALVE
GUIDES
Myths
and legends item 1. There are no such things
as steel guides. They are actually cast iron. If they were steel the valves
would very likely seize in them unless elaborate and expensive materials and heat
treatments were employed. And contrary to popular opinion, they are more than
capable of dealing with a vast majority of fast road engines with high lift
cams and/or rockers, providing chrome stemmed valves are used. They are not
suitable for use with stainless steel valves as used in race heads, although I
have been playing with the latest breed on nitro-carburised stainless steel
valves in iron guides. Jury's still 'out' on this at the moment.
There are three designs of cast iron guide.
That fitted to the small bore head is basically a straight tube. Next is a
slightly longer version with a groove near the top to hold a top-hat type valve
stem seal used on large-bore motors, and lastly one with a slightly tapered
nose (known as bullet shaped) but otherwise the same as the previous
offering. At approximately half the
price of the more (falsely) popular ‘bronze’ guides, they make for a reasonable
cost saving.
‘Bronze’ guides have become the ones to
have/use basically because of ‘old’ technology spilling into modern life.
Originally they were made out of stuff called ‘Navy Bronze’, a phosphor-bronze
mix that held oil well (kind of self lubricating), were very orangey/copper
coloured, and were used in the early days to avoid valve seizure at high rpm when
used with 'standard' spec valves before stainless steel came into use. Then
they were needed when the first nimonic steels were
used (as in AEG163 MK1 S heads). They enjoyed a very long period of use but
suffered severely from rapid wear. Technological advances have seen a number of
alternatives used, mainly of silicone/brass or manganese/brass bases. There is
one type with a high aluminium content (sort of silver-gold looking), then ‘HT20’
which is specially manufactured to a
Myths
and legends item 2 - they don’t last longer
than iron guides when used with standard material valves. Wear rate is totally
down to cam lift and rocker ratio/geometry.
For those fitting their own guides, or having them
done by a local machine shop, the correct finished reamed size is 9/32”
(0.2812”). There isn’t an exact metric equivalent so don’t let ‘em tell you there is! The smaller 7.10mm metric one is
0.0017” smaller (0.2795"), the larger 7.2mm metric one 0.0023” larger
(0.2835"). So when 0.0003” (that’s 3/10’s of a thou)
makes a difference, you can see one’s too tight, the other too slack!! The
valve stem diameter is 0.2795". Of course this is ignoring specially
made/sized reamers…having said all that - there are one or two head modifiers
who are either using worn out or specially made reamers to achieve tighter
running clearances. Apparently to achieve less oil
contamination of the incoming charge where unleaded fuels must be used.
I have only seen this cause problems.
VALVES
Although there are several material
specifications for the standard valve range, it isn’t important to know all the
gory details. They are all of reasonable quality, the later the valve type
(i.e. as for A+) the better they generally are. Most of which
come with chrome plated stems as standard - consequently being a little more
expensive. The stems are chromed to vastly increase stem and guide life,
particularly when unleaded fuel is used - more a prerequisite these days. The
later valves tend to have more flow-efficient shapes too. Particularly
if they are further tweaked to improve flow across the valve backs, which
generally takes the simple form of a 30-degree back-cut to reduce valve seat
width to a flow and longevity maximising potential of 0.060".
You may have heard the term ‘two piece valve’ -
especially when some vendor is engaged in flogging a race component-equipped
head for road use. Basically the stem and valve head are two separate pieces,
spun at great speed in opposite directions then pressed together. The friction
generated causes immense heat which sticks them together - ‘fusion welding’. It
is the 'joint' created that those crafty vendors work on.
Myths
and legends item 3 - They say that this
weld/joint is a weak point and can cause the valve head to fall off, then
divulge into expansive description of the catastrophic and cripplingly
expensive damage this would cause. The point is correct, but they don’t give
any performance levels at which this will happen. Continual, extended high
engine rpm combined with high lift and very strong valve springs and missed
gear changes are the killers - as can be experienced when racing. An
outstandingly small number of folk using their cars purely on the road would
ever get near the ‘red zone’ that causes this failure, so investing in race
quality valves is a waste of money. If you don’t run the engine at or in excess
of 6,500rpm constantly and for protracted periods and aren’t using a cam with
vertical opening ramps (up to a
‘Race valves’ are basically - and should be -
one piece to vastly improve strength, although there are some cheaply
manufactured two-piece ones about so beware! Rods of stainless steel (now all
EN214N steel) are drop forged on one end to form a mushroom shape. These are
then machined to the desired and hopefully flow-efficient shape. The all over
machining means the valve can be cut to enhance maximum flow as well as minimum
weight. Invariably they have a stellite tip (very
hard) to increase durability. There are a variety of valve shapes around. The
most common and generally effective inlet valves are the ‘penny on a stick’
types (see drawing below for profile comparison). A similar story exists for
the exhausts, although the latest flow bench developed, super-efficient ones
retain a near standard profile so are worth tracking down. Bronze-type guides
MUST be used unless the stems are chromed - but....
 |
Click to show enlarged picture |
Chroming the stems was all the 'go' a good few
years ago, mainly to allow their use in cast iron guides. The shortcoming of
this was the heads used to fall off!! The chroming process leaves a ‘fatigue
line’ where it stops - generally near the valve head - so needs very careful
(expensive) heat treatment to relieve this. If not done absolutely correctly,
the stem fractures. Instant two-piece valve syndrome and all the horrendous
damage it does.
‘Rimflo’ valves
became very popular some years ago, developed by Paul Ivey of REC (Specialised
Valves), and much flaunted by David Vizard. They are
in essence race valves, but with specific head designs. They were described as
‘anti-reversion’ valves, inferring they flowed excellently in the right direct,
whilst inhibiting flow in the wrong direction. And at the time, they certainly
did what they were designed to do, particularly when installed in an otherwise
standard head. Chromed stems allow their
use in cast iron guides. Advances in head modifications and better
understanding of the A-series peculiarities for camshaft design has seen the
overall effectiveness of these valves much reduced. They are also quite a bit
heavier at the valve head and more expensive than other options.
Almost all current manufactured valves, be they
standard or race, are suitable for unleaded fuel. If standard type valves are
to be used, go for those with chromed stems. Stainless steel ones should be
nitro-carburised for longevity. As for valve sizes, don’t get obsessed with
size. Refer to the chart below for general guidance in this area - although
this is by no means the definitive of what to use where!
CYLINDER
HEAD SPECS AND APPLICATION CHART
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VALVE DIA IN / EX
-MM. |
PORT SIZING |
COMPONENT SPECS & APPLICATION |
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LARGE
BORE |
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33.3/29.4 |
Cleaned up, near standard with reworked guide
bosses. |
Std valves, iron guides, 140lb single
springs. Use with cams under 255 deg duration for economy-orientated engine.
Not cost effective as similar economy can be had with more power with same
spec but with 35.6mm inlet valve. |
|
|
35.6/29.4 |
Inlets opened up to approx. 1.23” high x
1.12” wide. Exhausts roof raised. Reworked guide
bosses. |
Std valves, preferable A+ type for good
shape/ flow and re-worked for max efficiency. Iron guides. Dual springs, up
to 180lb. All road use up to 290 deg duration cams, best with dual phase type
(more ex duration than inlet). Rev limit 6,500 continuous. For higher rpm use s/steel
valves, silicone/brass guides. |
|
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35.6/30 or 36/30 |
Port work
slightly larger than above Guide bosses removed. Detailed valve throat work. |
S/steel valves, bronze guides, 180lb dual
springs. Cams up to race spec. Super road head, hill climb/sprint/rally.
Under 1330cc race except short stroke 73.5mm. Works well on engines using
twin SU carbs i.e. historic race. |
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37.2/29.5 |
Large ports. Guide bosses removed exhaust
ports need to be ultra-efficient to get benefits of bigger inlet. |
S/steel valves, bronze guides. 180lb dual
springs road/rally, 240lb race. Really needs dual phase cam to work well.
Good with ‘649’ cam though! |
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37/30 |
Large ports. Guide bosses removed. Acute
valve throat work needed. |
S/steel valves, bronze guides. 180lb dual
springs for road/rally, 240lb race. Biggest valve combination that can be
made totally effective but port/valve throat work must be up to it. |
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37/31 or 37.5/31 |
Large ports. Offset exhaust valves. Guide
bosses removed. Acute detailing at valve throats. |
S/steel valves, silicone/brass guides. 240lb
dual springs. Serious race use only. Reliability suspect due to little
material between valves. Very difficult to maximise valve size potential.
Best application on short stroke 73.5/74mm bore circuit racer (1300cc class
limit). |
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SMALL
BORE |
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31/26.4 |
Larger ports, modified guide bosses. |
Biggest
power gains for your money. Beware those fitted with 1.0625” ex valve - it’ll
hit the block unless min. re-bore of 0.040” has been done. Best results when
done on 12G202 casting. |
|
Utilising
valve sizes bigger than this is complete over kill - it’s impossible to make
them flow effectively although some claim it does work. Only one small-bore
head represented as it is the only one currently readily available off the
shelf.
VALVE
STEM SEALS
Three types have been used over the years. The
small ‘O’ ring type as fitted to all small-bore heads, and two ‘top-hat’ types
used on the large-bore engines. One of the later A+ items has a tensioner
spring around the valve stem orifice to make the seal more effective and
durable, so is consequently the better one. The small-bore ‘O’ ring type has me
at a loss to understand how it works - other than possibly squashing away oil
accumulated at the guide tops as the valve is opened and reaches full lift. Pretty useless. The top-hat seals can be used on small-bore
heads even if the guides don’t have a retainer ring. I've done this hundreds of
times and not had any come away from the valve guide. It certainly makes a
better fist of things than those crappy 'O'-rings.
It has become customary to fit seals to all
valves instead of just the inlets. Fitted to the inlets they reduce oil
consumption and intake charge contamination (effectively causing power loss and
possible piston damage through detonation).
BUT if this is done to the exhaust valves extra guide-to-valve stem
clearance is necessary to prevent sticking or seizure, especially when high
lift cams/rocker gear/rpm are used. Personally I prefer to run correct clearances
and seals on the inlets only.
SPRINGS
AND CAPS
Not really much to say here. The springs should
be suitable for the cam/use to which the car is being put. But be warned -
excessive spring pressure will cause premature valve train wear and unnecessary
power loss. For road use it is highly unlikely that springs over 180lb maximum
will be needed. Dual springs work better than single springs where high poundages/rates are needed; they retain more control over
valve attitude and help damp some of the horrible harmonics created in the
A-series valve train. As with everything there are dodgy springs about, but
getting them from a main supplier should solve that problem.
Myths
and legends item 4. The Iskenderian
(‘Isky’) springs that became ‘vogue’ and hailed as
the be-all-and-end-all aren’t. They weren’t even made for the A-series engine.
They were originally produced for a Ford 100E race engine, needing special or
re-machined spring/top caps, as they are smaller in diameter than Mini springs.
This also gives the valves mechanical advantage over the smaller diameter
spring, especially where seriously big valves are used (37mm+) - so are far
from ideal. They were used as they performed better than anything else at the
time - and we are talking over 20 years ago now - when mega-lift cams and high
lift rockers appeared on the scene. R&D and current technology has seen the
rise of decent springs at sensible prices that fit with the standard caps.
Those sold by MSC/MM are no exception.
Talking of which, the standard caps are pretty
good and certainly good enough for all but full race use or where lightness of
the valve train is sought. In which case alloy, or titanium if a large bank
balance is possessed, caps are the way to go. The alloy ones must be hard anodised
- not just colour anodised - to make them resilient enough to withstand the
battering. The ultimate steel items are those as fitted to the Cooper S -
referred to as ‘W’ caps as this letter was stamped into them. They are very
durable and highly recommended for all serious competition use. They really
don't give much away in weight to their counter-parts and are crucially much
stronger, but are generally passed over because 'fashion' and 'cred' err towards their lighter/prettier cousins.
PORTS
AND CHAMBERS
Perhaps the most controversial parts about
modified heads discussed. Mostly incorrectly. And how
many times have you heard/read the phrase ' modified, fully polished, ported
and gas-flowed'. Talk about advertising hype/blurb! Gas-flowing IS porting and
modifying.
Massive ports usually flow a lot of air, but
velocity will suffer. Velocity on road
going engines is far more important to give half decent drivability. Not to the
point of being overly small, but certainly don’t need to resemble the channel
tunnel! Correctly shaped ports (from the valve seat to manifold face) of a
reasonable size are far more effective than gopping
huge ones of any shape. Not at all helped by many head modifiers using the
manifold gaskets available as port size/shape templates!
Myths
and legends item 5 - biggest is not best.
Unfortunately many head modifiers believe this and don’t spend time grinding
where it counts. Big ports are quicker and easier to achieve than effective
valve seat areas. Much of the head's performance depends on the work done
around the valve seat/throat area. Detailing is critical. Done incorrectly it
will negate the modified port’s effectiveness - no matter how big it’s made.
The standard combustion chamber shape of the
A-series is actually very good. Really good heads tend to maintain this basic
shape. Wildly altered shapes rarely improve performance unless used with
specific, dyno developed piston crown configurations - and these are RARE.
Three angle valve seats are superior to single angle ones, but they need to be
the correct angles and the seat the right width. In fact having flow-efficient
valves on single-angle seats improves airflow more than 3-angle seats in the
head with single-angle seat valves!
Myths
and legends item 6 - pencil-line-thin valve seats don't give the best over-all results. Avoid heads with
anything less than a 0.050” inlet seat, 0.060" exhaust seat for road use.
Even on race heads, thinner seats than these rarely prove more effective unless
accompanied by very specific valve throat detailing. Apart from being
detrimental to performance, they’ll wear out damn quick - especially with
unleaded fuel use.
Surface finish is another bone of contention. And subject to the 'eye candy' alluded to earlier. Some head
modifiers spend serious amounts of time, effort and resources polishing the
crap out of port and chamber surfaces. Might look good - but adds to the
production costs and can seriously hurt over-all road performance.
Myths
and legends item 7 - Mirror-polished ports achieves absolutely nothing except - well - extreme
shiny-ness. And you can’t see it once it’s bolted on. Very smooth is good. The
only parts worth polishing to some degree if deemed necessary are the
combustion chambers and exhaust valve throats. A fine, stone ground or abrasive
tape finish on the ports (inlets particularly) is ideal for road use as it
helps maximise fuel vaporisation. Very smooth will assists correctly shaped
ports to maximise power outputs - particularly at higher rpm. However,
mirror-polished inlet ports can cause fuel to stick to the port walls at engine
speeds generally experienced on the road - which compromises vaporisation. This
reduces comparative power outputs and increases fuel consumption.
UNLEADED
COMPATIBILITY
Anybody buying a head now, whether it’s
modified or just a recon, should be getting one suitably modified to use
unleaded fuel. All this entails is special exhaust valve seat inserts pressed
in to the head. See article 'Cylinder head - Unleaded use' for further details.
CONCLUSION
Many head modifiers are producing heads to suit
the whims of customer demand rather than practical, cost-effective
modifications to maximise performance for the intended application. Here we are
talking whopping port sizes, mirror finish polishing, huge valves and 'race
quality' components. All of which bloat the cost to the customer. And that’s
YOU. You’ve done it to yourselves here folks - because it was what was
necessary earlier in the life of automotive performance, it's what you - the
customer - became to expect/demand. So that's what the modifiers did. Think
hard about what you are REALLY going to use the car for most then get a head
that suits your application done by a competent head grinder. It will save you
loads of beer vouchers, and probably give better performance!
Useful part numbers:
Note - All modified cylinder heads now come
converted for lead free as standard. For full spec on heads, see inventory.
Specific application heads can be done to special order.
|
MSE3 |
Mintec
pre-'92 large-bore road-rocket spec head, race quality components,
35.6mm/29.5mm dia. valves. |
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MSE4 |
Mintec
post-'92 large-bore road-rocket spec head, race quality components,
35.6mm/29.5mm dia. valves. |
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MSE7 |
Mintec
TPi road-rocket spec head, race quality Components,
35.6mm/29.5mm dia. valves. |
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C-AHT135 |
Pre-'92
large bore road-rocket spec head, A+ standard quality components,
35.6mm/29.5mm dia. valves. |
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C-AHT465 |
Longman
GT7 head, race quality components, 35.6mm/29.3mm dia. valves. |
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C-AHT221 |
Longman
GT14 head, race quality components, 37.2mm/29.3mm dia. valves. |
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C-AHT221A |
Longman
GT17 head, race quality components, 37.2mm/31mm dia. off-set valves. |
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C-AHT88 |
Min
Tec Stage 3 small-bore head, race quality components, 31mm/26.4mm
dia. valves. |
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C-AHT55 |
1.464"/37.2mm
S/Steel inlet valve, large-bore. |
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C-AEG544 |
1.401"/35.6mm
S/Steel inlet valve, large-bore.
|
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C-AEG569 |
1.311"/33.3mm
S/Steel inlet valve, large-bore. |
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C-AEG588 |
1.218"/30.9mm
S/Steel inlet valve, small-bore. |
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C-AEG107 |
1.220"/31.0mm
S/Steel exhaust valve, large-bore. |
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C-AEG106 |
1.161"/29.5mm
S/Steel exhaust valve, large-bore. |
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C-AEG587 |
1.040"/26.4mm
S/Steel exhaust valve, small-bore. |
|
C-AEA526 |
Nominal
180 psi dual valve springs, road/rally use up to 0.495" lift/7,500 rpm
continual use. |
|
C-AEA527 |
Nominal
240 psi dual valve springs, rally/race use up to 0.570" lift. |
|
C-AEA528 |
Alloy
top cap set, hard anodised, very light, single groove collet only. |
|
AEA653 |
Steel
'W' type top caps, single groove collet only, need 8. |
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C-AEA654 |
Competition
inner spring locator (no step). |
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AEA403 |
Standard
lower inner spring locator, need 8. |
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SS4 |
Set
of 4 spring spacer-shims, 0.080" thick. |
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88G459 |
Single
groove collet, need 8 (come in pairs). |
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ADU4905 |
Valve stem seal, late 'sprung' type. |
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AEG327 |
Valve
stem seal 'S' type without spring |
.