It is fair to say that the development of the BRM H16
suffered due to numerous reliability issues. Vibration and resonance issue was
by far the biggest cause of these issues.
First engine test bed runs resulted in broken inlet
camshafts at the shoulder for the driving gear. Initially this failure was
attributed to the high loads in the camshaft. In order to try to cure the
problem, a temporary additional camshaft bearing was added for both inlet
camshaft drive gears to prevent the gear being over-hung. With this arrangement
the engine could be revved up to approximately 6000rpm, when it started to
vibrate violently, especially at the fuel injection units.
Further investigation by BRM highlighted that the 0.75”
diameter torsion shafts that drove the output gears were bright blue in their
centres. This unusual observation confirmed that the natural frequency of the
torsion shaft coincided with certain modes of vibration of the whole engine. A
torsion shaft with a diameter of 1.15” was required to increase its natural frequency
above the exciting range. Unfortunately packing space would not allow such
large shaft diameters; instead the maximum diameter allowed was 0.95” which
allowed the engine to rev up to 10,000rpm for short periods.
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| Output gear pack with steel 'spectacle' plate |
With this running, failure of the gears or their bearings
that connected the two crankshafts would then occur. In order to cure this
issue the diametrical pitch of the gears was changed (from 8 d.p. to 6 d.p.) as
were the arrangement of the bearings to increase stiffness (by four times).
This redesign would then allow an engine to run for up to 4 hours; just an
adequate duration for racing. With this design re-work the torsion shafts were
now further increased in diameter to 1.05”. The shafts were also made of maraging
steel to try to increase their working life. The next difficulty came due to
the bearings for the centre gear which connecting the output shafts working
loose in their magnesium case within about 2 hours. The bearing location was
therefore reinforced with steel ‘spectacle’ plates. By now, it was however
apparent that the vibration would need curing at the source.
A pair of eight-pin type crankshafts were manufactured. The
extra balance weights of this crankshaft caused the offset between cylinders to
increase by 0.4” to 1.05”. This in-turn meant new crankcase castings were
required. It was also necessary to increase the crankpin diameter to 1.875”
from 1.5”. Since this modification would take considerable time due to
re-tooling, the decision was made to prove off the theory by welding and
bolting steel rings onto the balance weights of the original design crankshaft.
It was possible to add only four additional weights could be added to each
crankshaft, but as each pair weighed 2lbs, there was still a substantial
increase in inertia. This crude modification proved successful to the extent
that the engine could now be used in a car. In fact for the majority of the
1966 racing season the H16 ran in this form; even winning the US Grand Prix in
a Lotus.
The downside of these modifications was a weight increase of
the engine, which now tipped the scales at 520lbs. When the mk2 engines were available
(with correct 8 pin cranks), problems with connecting rod failures resulted in
destruction of all the engines. The connecting rod failures were attributed to
a machining mistake which caused a stress concentration point in the con-rod
bolt bore.
BRM kept experimenting with the engine, for example chrome
plated steel and then chrome plated aluminium cylinders. The latter items were
reasonably successful, however an increase in oil consumption was noted with
their use. Camshafts and tappets by now were now made of case-hardened En-39
steel. This modification along with the new crankshafts allowed the removal of
an additional steady bearing on the engine which reduced mass by 10lbs.
The biggest gain in engine reliability came as a result of
BRM’s greater understanding of the working life of each component. BRM
introduced a “ruthless scrapping” policy, for example; output gear bearings
were changed every race, output gears, camshaft driving gear bolts and dowels
every 10 hours. Although the engine reliability was now far improved, the
vibration characteristics caused by it resulted in a spate of gearbox failures.
As well as the H16 being heavier than anticipated, low
engine speed torque was below what was expected. For example at 7000 rpm the
1.5 litre V8 produced 140 hp, whereas the 3 litre H16 produced only 240 hp. The
specific fuel consumption of the H16 was also high which further increased the
required car mass due to higher fuel loads. In order to improve both these
items, the intake ports were reduced in diameter by 0.1in and the valve lift to
0.35in. This resulted in 4-5% more power at 7500 rpm and a 3% reduction in fuel
consumption. Conversion to upstream fuel injectors also gave a considerable
improvement in power and fuel consumption at lower engine speeds.
So what could be concluded about the 2 valve 3 litre H16
engine. It was not the success that was hoped. This was due to initial
reliability issues stalling development, an engine that was both heavier and
with less low speed power performance than anticipated, an engine that required
high levels maintenance and the final death-nail was that even when engine
reliability was acceptable, gearbox reliability became a problem due to engine
vibration.
