M8 crank breathers are crap
#31
08-16-2018, 09:16 AM
Seasoned HDF Member
The oil bag and crank case are connected via an air port to provide a shared space (volume) to compress air when then pistons come down.
Larger that space, less pressure, less air out the breathers. Smaller that space, higher the pressure and thus more air out the breathers.
When you put more oil in the oil bag you take up more of that air space making the available air volume smaller... thus higher case pressure, more air out the breathers.
Science.
Furthermore, an air filter designed to be oiled (K&N) filters better when oiled, but an oil filter designed to be dry (HD) will clog choking the engine and staving it of air.
A clogged air filter will pull more air out of the breathers.
Larger that space, less pressure, less air out the breathers. Smaller that space, higher the pressure and thus more air out the breathers.
When you put more oil in the oil bag you take up more of that air space making the available air volume smaller... thus higher case pressure, more air out the breathers.
Science.
Furthermore, an air filter designed to be oiled (K&N) filters better when oiled, but an oil filter designed to be dry (HD) will clog choking the engine and staving it of air.
A clogged air filter will pull more air out of the breathers.
If your air filter was clogged, there would less air in. However, an oil wet filter is not going to restriction air in a street Harley. Sure, even I will admit at WOT it could.
I personally would rather have better filtered air over some small amount of HP at WOT.
FI systems know how to handle proper fuel rations.
For just the sake of argument, when the pistons go up, they actually create a vaccum in the crankcase and that is what the poppet valves are for. Smaller case are would create a higher vacuum. However, I think either way, that smaller area sounds like something a tin man said.
Last edited by Jackie Paper; 08-16-2018 at 09:45 AM.
#32
01-16-2021, 10:00 PM
Stellar HDF Member
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