Thanks Keith you've expressed what i meant better than i could have!
Besides i won't despise any mis-informed 33ist, some boy racers learn and evolve, too.
So i'm going to try an explanation of the lube principles.
When an engine gets designed, considering all the clearances for crank and rod bearings, tappets and bores, camshafts bearing, even turbocharger bearings get summed up and the equivalent cross-section for oil flow is obtained. How much oil flow the pump shall provide is a requirement that comes from engine design.
Then the pump is designed to provide sufficient flow to ensure proper lubrification in worst conditions, that is engine & oil hot and low revs.
When engine is cold, oil is too viscous and can't get through bearings at a sufficient rate. The pressure would increase in excess of 20 bar and destroy filters and seals. That's the reason why there is a relief valve in the pump, which opens around 4 bar. When oil is cold, the valve opens and limits pressure. Oil flow to the bearings in this condition is minimal and stressing/revving a cold engine will not do much good to the bearings.
When engine is hot, oil is hot, the relief valve closes. Oil pressure increases then whith rpm, from 0.5 bar at idle to 4 bar at around 4000 rpm. Above 4000rpm, maximum oil flow is reached and there is no need to increase pressure further. The relief valve opens above 4000 rpm to keep the pressure at 4bar.
Increasing relief pressure by shimming the spring will have two effects : engine cold, pressure will be at 5 bar instead of 4 ; engine hot, pressure will reach 5 bar at 5000rpm.
But oil flow and pressure will be exactly the same between idle and 4000rpm when hot.
Above 4000rpm the pressure increase will have little or no effect on the oil flow.
What do serious modders do? Race engine have increased bearing clearances (precision job!) to reduce friction hence losses and improve efficiency. As a result they require more oil flow. Remember GTA/GTAm oil pumps? gears were ~ 1cm higher than standard ones to improve chamber volume and increase flow, even at low rpm. Race engine oil pressure, when hot, need not even be in excess of 4 bar.
For those wanting to do the math, the pump is a flow generator (volumetric pump) whose flow is proportionnal to the rpm. The circuit can be modelled as a restrictor whose pressure vs flow characteristic will determine the operating condition of the engine.
Now when using thinner oil like 0W30, when hot the oil flows more freely through the bearings, hence pressure is lower and flow increases. The trouble is that more flow is required because the oil won't stay much in the bearings. Breaking the oil film will cause wear and thrown rods. Higher viscosity is a preferrable solution, like 15W50 or 20W60.
For boxer engines, i'd say bottlenecks in lube system are
- filter : most aftermarket cartridges do not have the bypass valve, which opens when engine is cold and allows for the oil flow to reach bearings, unfiltered. Standard filters deprived of this feature restrict cold oil flow and most of the time, cylinder number 2, the farthest from the oil pump, is worn more than other. Guess what? Cold oil starvation. Won't happen with oem filters
- oil : engines fitted with hydraulic lifters require proper oil grade, preventing the use of heavy viscosity and racing oils. frequent (5,000 kms) oil changes won't hurt the engine, mine is in excess of 700,000kms now thanks to such care. However, most of the time the previous owner was a cowboy, not replacing oil in five years...
- sump : baffling is not sufficient for heavy cornering and numerous rods have paid the price of a bad compromise between good handling and poor sump design.
- bottom end : block design lacks of rigidity, when it comes to very heavy use, and the block has a propension to twist itself, distorting bearing clearances and causing the oil flow to escape. I've seen a block explode (literally) due to this problem.
In my opinion, there are very few mods liable to provide significant improvements in terms of reliability or performance without much money or much work. If shimming of the pump was liable to increase engine reliability, the factory would have done it for sure, at no extra cost. Some improvements can be made to an engine by carefully adjusting and matching the parts, to ensure optimal clearances, minimal losses, and good balancing. But they all require care and efforts to obtain minimal gain. Further optimization requires expensive performance parts and further hassle to assemble everything right. Just have a look at Adie Hawkin's engines to convince yourself of the work quality level required to achieve significant performance.