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I'm not sure it does.
The same negative (or less positive) pressure that is "helping" pull the downward traveling pistons is also "hindering" the upward traveling pistons.
Would that not balance out?
Pan Vac is something commonly monitored by racers. Especially timing sensitive combos
Seeing steady pan vac is good. A fluttering/unsteady pan vac indicates you're rattling the rings. A pan vac that gets higher and higher each pass indicates you likely hurt something.
I'm told that one of the advantages of dry sump systems is that they keep the crankcase in vacuum. Less air in the case = less power lost pushing the air around (hard to believe that's a measurable loss) and better ring seal (no idea why).
Related question: what's a good sensor to use to measure crankcase pressure? I'm about to get a dry-sumped engine, and I've got an unused breather port on one of my heads, so...
I'm told that one of the advantages of dry sump systems is that they keep the crankcase in vacuum. Less air in the case = less power lost pushing the air around (hard to believe that's a measurable loss) and better ring seal (no idea why).
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Dry sump equals less windage, Beating oil droplets and oil mist does rob power. Dry sumps also creates crankcase vacuum that helps control the rings, Simplest way to describe it, You have combustion pressure pushing down and out on the rings creating a seal that holds cylinder pressure, Having pressure in the crankcase impedes ring seal by pushing against the rings from the bottom. With vacuum below the piston (crankcase vacuum), The suction created works with rather than against the cylinder pressure allowing a tighter ring seal and making more power.
There is less aerodynamic resistance in a vacuum but the vacuum created by the dry sump wouldn't be enough to make a worthwhile difference, It's all about keeping oil and oil mist away from the spinning crankshaft.
I'm told that one of the advantages of dry sump systems is that they keep the crankcase in vacuum. Less air in the case = less power lost pushing the air around (hard to believe that's a measurable loss)
Think of the crankcase as a balloon and as the pistons pump up and down, that balloon is forced to go back and forth. If the balloon has more pressure it's harder to push around, basically it's easier to compress a mostly empty balloon than a full one. Also as mentioned, the vacuum removes a lot of oil mist making the balloon filling less dense/solid.
Keep in mind there are EIGHT pistons going up and down in pairs, so the "pumping pressure" likely stays pretty level, leaning to positive from cylinder leakage past the rings.
After reading an article that showed some interesting pictures, I asked a superstock racer why he had holes in the webbing of his aftermarket SBC block.
Thought I read that the LS blocks later got some of these balance holes due to their large main cap/webbing. Pumping losses!
Lower crankcase vacuum helps in 2 ways. It reduces piston resistance, particularly on the power stroke where it makes the combustion pressure "seem" higher, and on the intake stroke where it helps balance the pressure below the piston with the low pressure above the piston as it's sucking air/fuel into the cylinder. The other way is that it improves ring seal, reducing blow-by.
Lower crankcase vacuum also reduces resistance to oil flow through the engine. It also helps reduce windage. The oil also stays cleaner for longer since blow-by gases, water dilution/evaporation, and evaporated light ends from fuel dilution are sucked out of the crankcase.
There is much discussion of pressure on the Piston Crown (BMEP)
How does LOW Crankcase Pressure effect engine performance ?
The pressure on the piston crown is IMEP.
I have seen a vacuum pump worth up to a 30 HP increase on the engine dyno.
Indicated mean effective pressure
(i.m,e.p.) is the uniform pressure that would be required throughout the power stroke of an engine to do the same amount of work as is done by the varying pressures that are in fact obtained during the stroke. Brake mean effective pressure is the proportion of the indicated mean effective pressure that is available to perform external work at the flywheel. The difference between i.m.e.p. and b.m.e.p. is equal to the mean pressure required to overcome engine friction and to perform the functions of filling and emptying the cylinder and of driving the auxiliaries such as the oil and water pumps, generator and so on. These are generally known as friction and pumping losses.
The indicated horsepower of an engine (i.h.p.) equals the brake horsepower (b.h.p./ plus the friction and pumping .losses.
11-03-2021, 03:41 PM
