|
Cylinder head - What can easily be
achieved |
28 August 2002 |
The engine build I recently covered in previous Mini
Magazine issues developed into a bit of an ongoing 'project'. During a
discourse with the editorial staff at Mini Mag, it
was decided the build feature presented an ideal opportunity to demonstrate
just exactly what such an engine build is capable of in days where it's
generally believed you have to have an all-singing, all-dancing 1380cc engine
to have an enjoyable road burner - leaving those with very limited budgets a
little depressed.
So the idea was to finish the engine off using a
relatively 'mild' specification to maximise drivability. The camshaft used was
the fantastically versatile Swiftune Racing SW5 profile that provides drive
from nowhere up to 7,000rpm - depending on the finished spec - with minimal
hydrocarbons for emissions testing. We're looking at the most used rpm band of
1,500-5,500rpm. Thrashing an engine to death to achieve any progress doesn't
make for a pleasant driving experience on the road except for the masochistic.
To complement the camshaft, a suitable cylinder head
was needed since - providing the engine has been decently assembled - an
engine's power output is mainly developed from the camshaft and cylinder head.
Volunteering my services to carve a head out for this engine instead of blagging one from another supplier, I took the opportunity
to deal with a number of questions about cylinder heads I hadn't yet covered
'in public'. These centred around questions such as 'which head is best of the
standard ones?' and 'I have an old standard S head - is it worth fitting on my
MG Metro engine' and 'I've been told 3-angle seats cut into the head
dramatically improve flow - is this true, and is it worth doing on a standard
head?' and so on.
The plan then was to start out by flow-testing a
bog-standard MG Metro head then working through a number of practical and
often-used modifications and re-flow testing at each stage to gauge effects.
The stages would be -
As outlined above, I selected the MG head as a
starting point because it is the most efficient factory-fitted head used on the
A-series and nobody - to my knowledge - has produced any useful comparative
data. A consequence being folk still believe the old Cooper S head is the one to
have. This simply isn't so. Leyland/Rover went to great lengths to ensure the
MG Metro produced plenty of useable power since it's opposition at the time in
the 'hot hatch pack' were running away with the performance crown. Partly
through the seriously sporty (for Leyland) camshaft profile used - a cross
between the 997 Cooper inlet profile and the old 731 fast road exhaust profile
- and partly through very careful preparation and machining of the cylinder
head; especially the inlet valve throats.
All previous A-series production cylinder heads have
the valve throats simply bored out and the valve seat cut in with some small
consideration made to the chamber side of the seat by way of a 'levelling' radiused edge cut around the seats' perimeter. The MG head,
however, have the throats machined using a tool that cut the valve throat up to
the underside of the valve seat to a special profile - a smooth concave curve
terminating in a shallow convex ring around the inner seat edge. Thus forming a
slight 'choke' effect and eradicating any sharp edges that inhibit good
airflow. This choke effect is used in all manner of high performance engines; I
have to say I was surprised to find this in an A-series engine the very first
time I stripped one of these heads down. I won't go into a dissertation on how
and why this choke effect works since space is limited; suffice to say it works
in certain circumstances, and the A-series MG engine is one.
Of immediate interest is the fact that direct
comparisons across the entire test reveal seemingly very small gains. This is
something I alluded to earlier - the gains made by modifying cylinder heads
aren't in the 'stupendous' region many folk seem to believe. An average gain of
10% is considered extremely good. I say 'average' here because the actual gains
at each measurement increment vary and it's the net result that's important,
not a huge increase at one particular point. Having said that there are flow
figures achieved at certain lift points that indicate a good head for certain
applications.
A decently modified cylinder head can make a big
difference to performance, but it isn't an easy thing to do. The main factor
that makes this somewhat difficult is getting the air to perform at it's best
at all speeds. Air doesn't much like flowing in any other direction than
straight lines. To further complicate matters this single-minded approach by
air to direction changes gets stronger the faster the air is moving. When a head is modified, it needs to be done
in such a way that the air is persuaded to deviate from its single-mindedness
at all speeds. A head for full-race applications will have slightly different
needs to a road engine and whilst it's reasonably easy for a skilled head
modifier to achieve decent results for either one, the most skilled are those
that can combine both to achieve perhaps the pinnacle of results. Improving the
flow in as many areas as possible is critical in developing more power since
higher engine rpm means higher air speed, consequently the more difficult it is
to persuade the air to flow from the port, round the right-angle bend onto the
back of the valve and eventually out into the chamber. Inexperienced or clumsy
modifications - particularly in the port throat and that 90-degree bend - can
loose airflow instead of gaining it. It is also interesting to note that no
matter what the modifications the improvements don't really start showing until
the valve is open more than 0.100"/2.54mm. Also of note is the fact that
both exhaust and inlet valves account for a similar obstruction value - around
4-5cfm.
When scrutinising the results very carefully and
comparing each improvement against the base data of the standard MG head, it's
interesting to note that it's the exhaust port flow that sees the biggest overall
gains. The gain on the inlets alone doesn't explain the sort of power increases
achieved with a modified head. It would suggest the more efficient, and
therefore less restrictive, exhaust ports have more of an effect through
allowing better breathing via more complete blow-down of the cylinder. In other
words, a greater percentage of the burnt, power producing fuel/air charge is
evacuated causing the cylinder fill with a greater volume of fresh in-coming
fuel/air charge - improving the engine's volumetric efficiency (it's capability
to fill the whole cylinder with fresh fuel/air charge). The greater efficiency
also reduces power loss caused by the piston having to physically push the
exhaust gas out.
|
Valve lift |
Test 1 |
Test 2 |
Test 3 |
Test 4 |
Test 5 |
Test 6 |
Test 7 |
Orig S Head |
|
(in.) |
|
|
|
|
|
|
|
|
|
Inlet |
|
|
|
|
|
|
|
|
|
0.050 |
23.8 |
24.7 |
24.0 |
24.5 |
24.8 |
24.7 |
24.4 |
19.5 |
|
0.100 |
42.1 |
43.0 |
42.2 |
43.3 |
43.0 |
42.7 |
45.7 |
43.0 |
|
0.150 |
58.3 |
63.2 |
63.2 |
59.1 |
63.6 |
63.4 |
67.3 |
53.1 |
|
0.200 |
75.3 |
80.6 |
81.6 |
75.2 |
81.6 |
81.6 |
87.2 |
69.1 |
|
0.250 |
89.8 |
94.7 |
96.2 |
87.9 |
95.3 |
95.6 |
102.1 |
85.5 |
|
0.300 |
99.5 |
103.5 |
103.2 |
97.8 |
104.8 |
104.6 |
113.8 |
93.4 |
|
0.350 |
105.4 |
107.8 |
108.1 |
105.2 |
108.0 |
108.2 |
119.4 |
102.1 |
|
0.400 |
108.7 |
109.5 |
109.1 |
108.5 |
109.4 |
109.2 |
123.5 |
106.8 |
|
0.450 |
109.8 |
109.1 |
109.3 |
108.8 |
108.9 |
108.9 |
127.4 |
109.0 |
|
0.500 |
110.1 |
110.3 |
110.2 |
109.4 |
110.0 |
109.9 |
128.8 |
109.0 |
|
0.550 |
111.4 |
110.2 |
110.8 |
110.0 |
111.1 |
111.0 |
129.0 |
108.0 |
|
Exhaust |
|
|
|
|
|
|
|
|
|
0.050 |
19.1 |
20.6 |
21.2 |
19.0 |
21.0 |
20.0 |
18.8 |
14.3 |
|
0.100 |
33.2 |
37.2 |
37.0 |
33.4 |
39.1 |
38.2 |
33.0 |
32.8 |
|
0.150 |
47.8 |
51.3 |
50.3 |
43.9 |
51.7 |
50.0 |
62.9 |
40.4 |
|
0.200 |
56.3 |
59.3 |
58.1 |
55.4 |
59.6 |
57.7 |
63.9 |
49.4 |
|
0.250 |
63.6 |
66.0 |
64.3 |
63.2 |
66.2 |
64.8 |
71.9 |
58.1 |
|
0.300 |
68.9 |
70.7 |
69.2 |
68.6 |
71.2 |
69.9 |
78.0 |
62.4 |
|
0.350 |
72.7 |
74.1 |
73.0 |
72.5 |
74.2 |
73.8 |
83.2 |
66.0 |
|
0.400 |
75.0 |
75.8 |
75.4 |
74.2 |
75.8 |
75.8 |
87.5 |
68.0 |
|
0.450 |
76.1 |
76.6 |
76.8 |
75.0 |
76.3 |
76.9 |
90.6 |
68.0 |
|
0.500 |
76.8 |
77.2 |
77.5 |
75.5 |
76.8 |
77.0 |
93.1 |
67.0 |
|
0.550 |
77.0 |
77.6 |
77.9 |