Engine breathers

[yorkie_chris]

It can't be good for power to be blowing loads of hot, oily crankcase gas into the airbox.

Anybody routed them to a proper catch can? Any results other than "well it 'felt' faster"?

[embee]

Blowby flow rates can vary a lot, but a very rough guide would be somewhere round 20 lt/min per 100kW in a modern engine, so in a SV650 I'd expect around 10 lt/min for 50kW (ish).

Assume 100% vol efficiency at 10k rpm, and that's 3250 lt/min of airflow, so you're talking less than 0.5% of airflow (and thus power). In theory any oil will reduce the effective RON of the fuel, but this effect is pretty trivial unless oil carry-over is high.

Of course if the engine is worse for blowby than this, the percentage will be greater.

Legally, manufacturers have to make the engine "consume" any crankcase gases.

Feeding anything other than clean, cold air into the airbox will be detrimental to performance. When I fabricated a proper airbox with a performance filter for my MGB with ducted cold air it gave an extra 8 bhp at the wheels with no other mods.

[yorkie_chris]

Originally Posted by embee

Blowby flow rates can vary a lot, but a very rough guide would be somewhere round 20 lt/min per 100kW in a modern engine, so in a SV650 I'd expect around 10 lt/min for 50kW (ish).

Assume 100% vol efficiency at 10k rpm, and that's 3250 lt/min of airflow, so you're talking less than 0.5% of airflow (and thus power). In theory any oil will reduce the effective RON of the fuel, but this effect is pretty trivial unless oil carry-over is high.

Of course if the engine is worse for blowby than this, the percentage will be greater.

Legally, manufacturers have to make the engine "consume" any crankcase gases.

But there is also the imbalance of crankcase volume to consider, so engine will breathe a couple 100cc with every stroke, some degree of mixing is bound to occur between airbox air and the hot vapour in the crankcase.

I think any change in effective octane rating will be negligible, otherwise oil consumption would be huge.
And also, legally bike must conform to drive by noise tests and other "stuff" which is just a damned inconvenience in real terms...

I know a few of the other minitwinners have blanking plates made up for their pairs system. Apparantly it does make difference.

However my last dyno at the circuit check vehicle said that I make a true 71.6bhp at the rear wheel and as the class limit is only 72bhp there isnt much point me doing the mod.

Oh and thats 71.6 on a "dyna-pro." The last "dyno-jet" run I did showed 76.9 but there is always a difference between the two dyno systems.

[yorkie_chris]

I'm not on about the PAIRs system, curvy doesn't have that

[ophic]

Can't you just pull the breather hose off and let the crankcase vent to the atmosphere? Just for a test to see what it feels like?

Originally Posted by ophic

Can't you just pull the breather hose off and let the crankcase vent to the atmosphere? Just for a test to see what it feels like?

squirts oil everywhere, as per the original post

Ah gotcha, my bad I know the ones you mean. Well funnily enough they have to be routed into a seperate catch tank anyway. And I just bung the holes up.

[embee]

Originally Posted by yorkie_chris

But there is also the imbalance of crankcase volume to consider, so engine will breathe a couple 100cc with every stroke, some degree of mixing is bound to occur between airbox air and the hot vapour in the crankcase.

...

But any backflow/mixing will simply dilute the gas, the net outflow of "non-air" from the engine will be the actual blowby, can't be anything else.