Applying Crank Case Vacuum in single cylinder engines

[michaelwoodcoc]

Hey guys, so I've considered that V8s and even inline 4's get great benefits from vacuum pumps on their engines.

I would like to apply this to a single cylinder, here's some things I'm considering:
[ol 1]
[li]The engine may provide an adequate vacuum pump for half of the crank case revolution, being a single cylinder displaces lots of air[/li]
[li]I was just going to put a check valve on the breather line[/li]
[li]The engine already has a dry sump setup. The vacuum inside the crankcase may overwhelm the ability of the high pressure low volume pump to get the oil out of the crankcase and into the oil tain[/li]
[li]if not, just a check valve combined with the high volume low pressure pump may provide vacuum through the whole crankshaft revolution[/li]
[li]the vacuum, since the volume inside the engine is not always the same, will pull the piston down through at least half of the crankshaft revolution[/li]
[li]the vacuum amount in the crankcase may be hard to measure with cheap tools since it will probably change a lot[/li]
[li]the inconsistent vacuum may not provide all of the benefits[/li]
[/ol]

I'm still curious to try. I don't have a dyno, however, just GPS data logging (10 HZ) but if the difference is big enough I can measure it.

Has this been done to anybodies knowledge? and has it been good?

 

[michaelwoodcoc]

Rodrico,

that's a good way of putting it, my research has indicated that some vacuum pumps cost power down low in the rev range as well. If an engine spends it's time down there, there's likely no benefit to a vacuum pump. I think higher in the rev range that, how would you describe it, I guess the piston collides with air molecules in the crank case, and has to accelerate them very rapidly at high RPM. Also, vacuum at high RPM pulls the piston down on half of it's cycles, so this should balance out.

It's also important to note that many engines which are run under vacuum use a pump that draws vacuum and removes the oil, so the engine is effectively a dry sump. These reduce parasitic losses from collisions with an oil mist. Also in formula 1, it's necessary so that you have an oil source during periods of high lateral G's as an oil pan becomes unreliable at some point.

Honda put a dry sump on the ST1300 and they claimed it improved fuel efficiency and horsepower. I don't have access to these documents anymore, however.

in my case the engine is a single cylinder, not a multi cylinder like the ST1300. Like the ST1300 though, it is already a dry sump (KLX400) and it has a high volume low pressure pump, but I would also like to utilize free sources of vacuum as well.

I could have the crank case not under vacuum at idle, but the engine is not fuel injected, there's no ECU to pull a lot of data from and see the effects on idle. I don't really want to invest in a lot of sensors and monitoring equipment. I could just check the idle speed with the same exact carburetor settings, but what if there's not much of a difference?

I'm going to give this a shot one way or another. It I can't tell the difference at idle I'll just leave it under vacuum all the time.

 

[michaelwoodcoc]

ok here's something I found interesting:

https://www.hotrod.com/articles/dyno-tested-vacuum-pump-help-engine-make-power/

summary:
dry sump small vacuum: 751 peak horsepower
wet sump no vacuum: 741 peak HP
wet sump higher than dry sump vacuum: 760 HP

If you look closely at the graphs you will see the vacuum pump doesn't do much at low RPM, and doesn't so much better than the dry sump except at peak power.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1561146230/tips/20-vaccuum-pump-dry-sump-dyno-test-hpr-e1511374419296_shw4dl.webp[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1561146237/tips/21-vaccuum-pump-dry-sump-dyno-test-hpr-e1511374468346_n787wq.webp[/url]

 

[CWB1]

Main reasons for wanting vac in the crankcase are

emissions and sealing.

Fixed it for ya!