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5 March 2001 |
Terminology:
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MSC/MM |
Mini Spares Centre/Mini Mania |
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BBU |
Big Bore Unit (refers to all
1275cc-based units) |
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SBU |
Small Bore Unit (refers to all
sub-1275cc units, here the 998) |
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BHP |
Brake Horse Power |
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CR |
Compression Ratio |
Following on from the
easily bolted on performance enhancing components out-lined in the stage one section,
we need to consider where to go to get more power output.
To improve engine
out-put, you need to increase the engine's 'breathing' ability. The stage one
kit deals with all the easily bolted-on external parts, and represents the best
increase for investment. From here on in the power increases will cost
commensurately more money. Power production of any engine, once the 'breathing
apparatus' (induction/exhaust system) has been dealt with to cause no
restriction - or certainly minimised where use of relatively standard major
components such as the carb is concerned - is down to the cylinder head and
camshaft. Both play a vital role in improving the engine's ability to breathe. The head through improved airflow by way of modification or
replacement, the cam through increased valve-opening periods. How far
these modifications and changes should go are dependant on what the engine is
to be used for, or the power output required.
Books have been written
on these subjects; here we're dealing with sensibly priced options for
reasonable gain in engines that are used every day. Waxing lyrical about the
virtues of this or that cam, and so-and-so's or such-and-such's
cylinder heads are all well and good - but mean nothing without hard, practical
proof. Consequently the following will be based on recent tests carried out by
me to assess such changes.
Cylinder head.
After the stage one kit,
this is the next easiest option as far as fitment goes. It's almost a 'bolt-on'
part, but is a little more involved and a little more costly so is moved up a
category.
The standard cylinder
head is pretty restrictive where bigger power outputs are the main aim. To make
appreciable gains it needs a fair amount of work done. Simply carrying out 'stage one' port,
chamber and polish mods really don't do much at all
for your money. The only sensible option is a standard head modified to 'stage
three' spec, or a 12G295/12G206 Cooper head. Using a BBU 12G940 casting is an
option - but not terribly practical (see 'Engine - 998 tuning, fitting a 1275
head').
The much coveted
12G295/12G206 heads are all-but non-existent realistically now. There are some
about - most have been well used and abused, many cracked or modified into uselessness,
with just a few, rare, un-molested examples of the species. But even a good
example is likely to need complete refurbishment, particularly for the modern
world where unleaded fuel is to be used. Suitable exhaust valve seat inserts
will need fitting, new guides, seats re-cut, probably new valves, valve stem
seals, and in all probability new valve springs. And then there's the
machine-work needed to get it to work on your 998 - hefty re-facing to get CR
to a sensible level since the standard head has a capacity of 24.2cc, the
Cooper heads 28.4cc. That's a chunk to remove; charged by the cut it'll cost
plenty. And having gone through all that - reliability is a problem - they're
prone to cracking.
That leaves us with the
'stage three' spec modified standard head. MSC/MM sell heads modified by myself
(Min Tec). The valve sizing is as that of the Cooper heads with all necessary
port and chamber work carried out to give the best flow gains available within
limiting criteria and inserted for unleaded fuel use. The main limitation is
the combustion chamber - modifications are kept within sensible bounds so
suitable chamber size is obtainable without excessive cost. To improve the
chamber beyond that used would require large amounts of material to be removed
off the head face. This generally breaks in to the rocker gear oil feed gallery
necessitating this be brazed up, re-routed, and
re-faced to true. The small improvement available simply doesn't warrant the
extra costs incurred to do this. Even so, flow testing (results below) has
shown that this style of stage three head all-but matches the inlet flow of the
Cooper head, with better exhaust flow results so offers directly comparable
performance, are available 'off the shelf', and are far, far more reliable.
Particularly when comparing the figures below 0.350" lift as the
cam/rocker combinations used only reach 0.340" lift.
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STANDARD HEAD |
STAGE 3 HEAD |
COOPER 998 12G295 HEAD |
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VALVE LIFT (in) |
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INLET |
EX |
INLET |
EX |
INLET |
EX |
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|
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0.050 |
16.7 |
16.0 |
19.7 |
18.3 |
17.9 |
16.0
|
|
|
0.100 |
32.5 |
27.1 |
36.4 |
35.3 |
38.2 |
29.7 |
|
|
0.150 |
42.6 |
38.1 |
51.1 |
49.6 |
51.4 |
41.9 |
|
|
0.200 |
50.2 |
47.0 |
62.8 |
57.2 |
65.2 |
52.5 |
|
|
0.250 |
55.8 |
51.4 |
71.4 |
61.4 |
73.5 |
57.2 |
|
|
0.300 |
60.4 |
54.6 |
78.9 |
64.5 |
81.1 |
61.0 |
|
|
0.350 |
63.8 |
56.9 |
84.3 |
66.8 |
87.0 |
63.6 |
|
|
0.400 |
65.3 |
58.9 |
88.9 |
68.9 |
91.8 |
65.1 |
|
Superflow 600 fully computerized flow bench. Tested
at 25-in of water.
Flow test readings are
average of all ports tested on one of each cylinder head types.
Camshafts.
Off the mark, a camshaft
change to something more suitable will give you more 'bang for your buck'
compared to a cylinder head. Having said that, it will not give initially as
much power with the standard head as a stage three head will with a standard
cam.
There is a multitude of
camshaft options available, and most of them will have been tried by some one,
somewhere. The problem is, many of them will have been
tried in combinations that really didn't suit them - be this the wrong
combination of parts, or used for the wrong application. It's no good fitting a
camshaft that doesn’t start working until 5,000rpm shows on the tacho when you
spend almost all your time driving in traffic. Likewise, a mild road cam isn't
the best option for a circuit racer. Unfortunately, too many folk get lead down
the wrong path here, and end up with a completely unsuitable cam for their
particular situation. No more so than where road use is concerned.
I've run the gambit on
just about every size of Mini engine in almost every possible situation along
with a huge variety of camshafts. One thing stands tall above all the others -
I have nearly always gone back to a relatively mild profile for roadwork.
Something that gives it's best between 1,500-5,500rpm almost irrespective of
engine size. The reason being this is where you tend to spend most of your time
when driving on the road. Needing to run much over this means you either have
an unsuitable final drive ratio, or just use the engine for scaring the local
community at the weekend.
Considering the engine
size and the operating range, this really does cut down camshaft choice to
something in the 'mild' as opposed to 'wild' area. In my book the 997 Cooper
profile takes some beating and would be my first recommendation to anybody
wanting a decent road-burner. Or did until a few months ago -
but more of that in a moment. It gives the right spread of power without
being fussy or peaky. And the gain is pretty impressive over the standard item
in an otherwise completely standard engine. Not only that, but it will idle
smoothly and quietly and give emissions passing CO and HC readings. Something
many of the 'modern' breeds of camshafts simply can't do without much frigging
and fudging. With this in mind it was my first choice for the test I recently
carried out for Mini Magazine where the criteria given matched almost exactly
the aforementioned points.
Unfortunately I'd
already built and installed the engine before I got my hands on one of Swiftune
Racing's SW5 cams. This is the one I really would have liked to test, as it is
something new that has been giving unbelievable results in practically all
engines it's been tried in. I'm not allowed to give out specific details, what
I can say is it is a 'modern' version of the 997 Cooper cam - which goes to
prove that the old profile was pretty good in the first place and hard to beat
by later 'developments'.
Rocker Gear.
It's pretty widely known
that high-lift rockers don't give good results on 998s. The standard rocker
gear, however, is a little short on what's required. Despite being 'accepted'
as giving 1.25:1 ratio, they don't. You'd be lucky if the pressed steel versions
give 1.23:1, and wildly lucky if the later sintered type gave 1.22:1 (they
generally only manage a paltry 1.21:1). The 998 needs around 0.340" valve
lift to make good use of the head mods - but not much
more. The 997 camshaft has a lobe lift of 0.263". To get the required lift
means rockers with a real lift ratio of 1.3:1 are needed, so I used the MSC/MM
1.3 roller-tip versions for the SBU. These give the required lift within a few
thou.
As a footnote to this
section, using the SW5 camshaft means sticking to the standard rockers as it
develops more lift at the lobe.
Power Gains.
So what do we get after
all our deliberation and hard work? The tests were carried out on the same
rolling road I use for all my testing now that I live in the 'northern wilds' -
GRV at Littleborough, home of the British Vita Racing Cooper Ss run by Harry
Ratcliffe and Geoff Goodliffe (the 'G' & 'R' in the title). The exception
was the stage one kitted Mini with a stage three head fitted. This was done by
a good friend of mine, Steve Harris (ex-Downton man and well known Mini engine
builder). But this doesn't matter over-much as actual bhp readings are not what
we're looking for, as they are meaningless considering standard power outputs
vary so much. It's the percentage we're interested in for comparative purposes.
A good, low compression
998 gives 30bhp. A good stage one kit (mod1) boosts this to 36bhp. The 997 Cooper camshaft and standard head
(mod2) gave 42bhp and the 997 Cooper camshaft/stage three head/1.3 roller-tip
rockers combination (mod4) gave 48BHP. Using percentages is a more relevant way
of comparing results from different sources, so converting the aforementioned
we get the mod1 kitted motor giving 20% more, mod2 giving a 40% increase over
standard, and mod4 giving a whopping 60% over standard! Add into this equation
the 43% increase that Steve Harris got by adding a stage 3 head to the stage
one kit (mod3) on a standard engine and we've a full house of results -
Percentage power
increase with each step:
|
Stage one kit |
+20% |
|
Standard engine plus 997 Cooper cam |
+40% |
|
Stage one kit plus Stage 3 head |
+43% |
|
Stage one kit plus 997 Cooper cam,
stage 3 head and 1.3 rockers |
+60% |
A 60% increase is mighty
impressive - especially when you consider it still idles sweetly, gives very
low CO and HC readings at idle without any MOT/vehicle testing 'fore-play', and
actually gives more miles per gallon than standard! I largely suspect the
Swiftune SW5 cam would give a few more bhp, but more importantly an increase in
the over-all torque out-put - the most important factor for a street unit.
Useful part numbers:
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C-AHT88 |
Stage three, unleaded SBU cylinder head
assembly |
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C-AEG588 |
1.218" diameter nitrocarborised EN214N inlet valve |
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C-AEG589 |
1.040" diameter nitrocarborised EN214N exhaust valve |
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C-AJJ4037 |
Manganese-bronze valve guide - set of 8 |
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C-AEA526 |
Dual valve spring set - 180lb |
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TAM2068 |
Lead-free exhaust valve seat inserts |
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ADU4905 |
Stem seals - latest top-hat/sprung type |
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GUG702506HG |
SBU genuine Rover head gasket |
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KC567M |
A+-drive type 997 Cooper profile
camshaft |
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KC948 |
Pin drive type pp7 Cooper profile
camshaft |
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C-AHT405 |
1.3 ratio roller-tip rockers, SBU only |
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AJM601 |
Exhaust manifold gasket |
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GUG705009VC |
Rocker cover gasket |
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GTG101 |
Thermostat housing gasket |
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88G221 |
Heater tap gasket |