23-11-2018, 11:21 AM
I run a 2 bearing engine with a Phoenix crank, and changed from a LC head to a '37 HC head some years ago. Plug fouling only happened once, during a freezing cold Easter week on Guernsey when the poor thing never got properly warm. A change to NGK plugs effected a cure. If your ignition system is weak, a slightly oiled or carbon fouled plug will begin to stop sparking and you enter a vicious circle of increased misfiring and worse fouling. I suspect its more to do with the smaller size of 14 mm plugs and the reduced gap between insulator nose and body, rather than the positioning in the head. So the message is, keep the ignition system in A1 shape, and keep the engine hot by driving technique or in cold weather by slackening the fanbelt or muffling the radiator.
One factor which doesn't ever seem to get a mention is that the later head has a "squish" design. The flat part of the area above the piston has a clearance at TDC of maybe only 20 thou, which induces useful turbulence in the compressing mixture as it is forced sideways at the last part of the upward stroke. In theory at least this is one factor which should reduce the need for ignition advance.
The HC head definitely gave a harsher feel, presumably because of the greater mean and peak cylinder pressure. The latter is increased further if too much advance is used, and I suspect that the extra noise and more "spiky" peak pressure isn't in fact accompanied by much useful increase in mean pressure, i.e. torque. Above about 2500 RPM as torque falls due to restricted breathing (untuned engines) and the general engine noise rises, the harshness is less apparent. For a lightweight special where the revs rise relatively quickly through the gears harshness is probably less of an issue than say in a heavier saloon that may spend more time around the 2000 RPM mark.
For my engine, apart from an increase in mid-range torque due to higher CR, I also found the 37 head improved power above about 3000 RPM presumably due to better breathing. A sensible maximum change-up point from 3rd to 4th gear was increased from about 3000 RPM (27 mph) to 4000 RPM (35 mph). This drops the revs back to 2400 RPM in 4th, which on all but steep hills allows one to carry on up in 4th rather than being stuck in 3rd. For me, improved hill-climbing ability is probably the biggest plus point of the later head.
One factor which doesn't ever seem to get a mention is that the later head has a "squish" design. The flat part of the area above the piston has a clearance at TDC of maybe only 20 thou, which induces useful turbulence in the compressing mixture as it is forced sideways at the last part of the upward stroke. In theory at least this is one factor which should reduce the need for ignition advance.
The HC head definitely gave a harsher feel, presumably because of the greater mean and peak cylinder pressure. The latter is increased further if too much advance is used, and I suspect that the extra noise and more "spiky" peak pressure isn't in fact accompanied by much useful increase in mean pressure, i.e. torque. Above about 2500 RPM as torque falls due to restricted breathing (untuned engines) and the general engine noise rises, the harshness is less apparent. For a lightweight special where the revs rise relatively quickly through the gears harshness is probably less of an issue than say in a heavier saloon that may spend more time around the 2000 RPM mark.
For my engine, apart from an increase in mid-range torque due to higher CR, I also found the 37 head improved power above about 3000 RPM presumably due to better breathing. A sensible maximum change-up point from 3rd to 4th gear was increased from about 3000 RPM (27 mph) to 4000 RPM (35 mph). This drops the revs back to 2400 RPM in 4th, which on all but steep hills allows one to carry on up in 4th rather than being stuck in 3rd. For me, improved hill-climbing ability is probably the biggest plus point of the later head.