For me, the whole point of fitting a deep sump is to reduce oil LEVEL inside the crankcase. You only need sufficient oil to ensure a constant supply at the pickup tube under all driving conditions.
There’s no rule that states oil must be run to the top mark on the dipstick. On a stock engine (no deep sump), the dipstick marks indicate factory recommended capacity. Fitting a deep sump doesn’t necessarily mean you must increase overall capacity by the equivalent extra deep sump volume.
Extreme example… if it were practical to fit a sump that extended downward to 1mm above the road surface (maybe 10 litres extra?), I’d aim to run conservative capacity (3 to 4 litres) but would have succeeded in dropping the LEVEL way below the crank and cam. That would do all sorts of good things for the engine. In the real world we have to be practical and take into account ground clearance, potholes etc.
Drag racers can afford to go deeper and ideally keep all of the oil outside/below the crankcase. Some of the mods already discussed in this thread help reduce the amount of oil pointlessly flying around inside the case (wrapped around the crank, stuffed up inside the 3-4 rocker cover etc). If you can rely on that oil being efficiently returned to the sump, you can afford to further reduce oil capacity, which has the added knock on effect of reducing oil level.
For obvious reasons it’s better to err on the cautious side, so my general (cautious) rule is…
- 1.5 quart sump – set to lower mark on dipstick
- Larger sumps (3.5 & 4qt) – just touching the bottom of the stick
Never had a problem with that, street or strip.
Droppping oil level from the top mark on the dipstick makes more power! I’ve seen it plenty of times on the dyno. If you have oil to the top mark and test the motor at 6000rpm+ you generally see very aerated oil pumped out the breathers. Drop the oil level, repeat the test and the puking stops… or at least reduces – depends on the engine (oil mods, build quality, ring seal etc). Power gains may only be small but happens due to the crank not getting as wrapped up in oil. Which is easier…. running on land or wading waist deep through water? Note: I’m referring to engines fitted with a deep sump.
Anyone experimenting with even lower levels needs to make some oil control/breather mods if they want to avoid sucking air and frying bearings.
Application has to be considered too…. a circuit car throws oil around in a different manner to a drag car and has different requirements.
Some of the mods people have discovered tend not to get shared, especially amongst the circuit racers…. it probably cost them a few motors to find the tricks that work!