Main Motorcycle: T140 cafe NRE900cc
Join Date: Jan 2012
Other Motorcycle: Yes
Extra Motorcycle: Of course
I did not think that such a casual throwaway remark would create so much interest, I should have known that showing any hint of contempt for Edward Turners much loved lubricator was asking for trouble. Now I know how Epimetheus felt.😊
The first reason that I like to keep above 3,000 rpm on a Meriden engine is partly due to the design and partly due to the pump.
If we look at a couple of popular contemporary engines by other manufacturers I believe they have an advantage over the Triumph:
The BSA twin engine has one camshaft, the lobes are quite close together, but the camshaft is situated at the rear of the engine benefiting from oil being directly flung up from the flywheel, the cams also dip into an oil trough that is fed from the oil pump.
The Norton engine is also single camshaft, but is placed at the front of the engine, so does not benefit much from direct oil splash from the flywheel as most will be flung off at the rear of the engine, but it has its cam lobes quite wide apart bringing them more in line with the big end bearings, so oil escaping from the big end helps lubricate the cams.
The Meriden Triumph is twin cam with the lobes right in the centre so are not in line with the big ends and are almost completely masked by the flywheel . The inlet cam benefits from oil flung up from the flywheel, but the exhaust cam lobes only get a residual amount of oil as most is thrown off at the rear of the engine.
I would want as much oil as possible being thrown about in the crankcases due to the position of the exhaust cam, but the barely adequate (please note I did not say inadequate) oil pump does not deliver very much oil at low rpm. Flywheel splash is not going to be that great.
The answer might have been to pressure feed the cam followers, and Triumph did this. However, because the oil pump has so little reserve capacity it was necessary to fit a restrictor jet, or use timed pulses for the tappet feed to reduce the flow of oil to the cams, so that the big end bearings are not at risk from oil starvation. The pressurised cam followers are rendered pretty much useless.
Most of us have been inside the engine and have seen the cams pretty much always have score marks on them, I know the small tappets create a lot of pressure at the cam lobes, but I believe lack of lubrication plays a major part in this.These are after all nitrided camshafts and
I try to choose a gear to keep the engine at over 3,000 rpm as much as possible, knowing the oil is not at reduced pressure reassures me that there will be enough flow to splash feed the cam lobes (a least a little bit).
Given the Triumphs propensity for detonation it might have been advantageous to direct oil jets to the underside of the pistons, so they are cooled, just like they do on the Norton Commando with drillings in the con rods; again the oil pump has so little reserve capacity that the brief venture into this in 66 was soon reverted back.
It also might have been nice to pressure feed the rocker gear, rather than rely on the 0.3psi generated by the oil return system.
Apart from all roller bearing engines, dropping to 20psi at tickover seems ridiculous to me, the lack of reserve capacity is worrying. That said Meriden Triumph twins are not renowned for wearing out big ends, thankfully.
I often see on this forum, the opinion that the oil in frame tank with itís reduced capacity adequately cools the oil, in fact over cools it as the oil does not get very hot. I am not convinced it is the tank over cooling the oil, but instead it is the lack of oil flow that does not draw enough heat from the engine at lower rpm.
If I am just running about town or riding in the countryside the engine gets pretty hot, but the oil remains relatively cool.
If on the other hand I hit the motorway for any length of time at an RPM that means there is a reasonable oil flow, for example 4500 rpm, not a crazy speed by any stretch of the imagination, if I look in the oil tank the oil is very hot with the consistency of water.
These two contrasting scenarios might be an argument to take the Trident T160 approach, high oil flow with thermostat controlled oil cooler. The Triples seem happy with sustained motorway speeds, where as the Meriden twins just do not seem happy at all.
Enough conjecture from me.
The function of man is to live, not to exist. (Jack London)