Guzzi "Bunn Breather" Installations

[geodoc]

I intrigued by the insight and work behind them for Rex Bunn's "Bunn Breathers". The breather set-up that I concocted for my Guzzi didn't really produce the effect I was hoping for, so this guy's set-up looked worth investigating.

His basic premise is that for a variety of reasons (which he enumerates on his web site), just evacuating the crankcase is not as desirable as producing an air flow through it while at the same time tuning the flow to maintain a negative pressure in the crankcase. Check it out:

http://bunnbreather.bigblog.com.au/index.do

I corresponded with him by email and while he is aware that there are at least a few people that have installed his set-up on Guzzis, he hasn't gotten any specific details on what plumbing arrangements they tried and what they ended up settling on. There are a variety of ways to plumb the system so I'm wondering if anybody on the board here has fooled around with a Bunn-Breathered Guzzi and how it went.

G

 

[GuzziMoto]

Not familar with his stuff but in the US it is an old and proven hot rod trick to create a vacum in the crank cases to increase hp. I don't see how maintaining airflow THRU the crank case would matter but it couldn't hurt and it could make it easier to maintain the correct negative pressure. Too little and it doesn't help and too much and it hurts power (and has been known too suck the oil pan in shrink wrap style around the crank according to old wives tales).
I will have to read further about his stuff. Thanks for the link.

 

[geodoc]

Heh GM,

yeah, the reducing-crankcase-pressure idea is indeed dated from way back. I take it that some race cars with a super abundance of power actually run vacuum pumps for the purpose. reading through his blog you'll get the gist of his theory which sounds reasonable.

G

Not familar with his stuff but in the US it is an old and proven hot rod trick to create a vacum in the crank cases to increase hp. I don't see how maintaining airflow THRU the crank case would matter but it couldn't hurt and it could make it easier to maintain the correct negative pressure. Too little and it doesn't help and too much and it hurts power (and has been known too suck the oil pan in shrink wrap style around the crank according to old wives tales).
I will have to read further about his stuff. Thanks for the link.

 

[Series4]

I've not heard of dedicated vacuum pumps but in race engines dry-sump systems can create negative pressure in a crankcase. The point of the dry sump being to reduce windage, and thereby release some power. I've read all the bumph but still can't see how the Bunn unit gets the benefits it claims. BTW I hope the Bunn breather is different to the bun breather, which can leave a bad smell...

 

[GuzziMoto]

They also have been known to use "free" vacum created from the exhaust flow to create negative pressure in the cases. Rumor has it they tried using intake flow first and gained power but as soon as they shut the throttle it caused a huge spike in vacum, deforming the oil pan. The key is to create the negative pressure without consuming power the way a turbo creates boost without consuming power, that way you gain more power then you would with a vacum pump or a supercharger.

 

[Series4]

Re: Guzzi "Bunn Breather" Installations - slightly OT

Yes, you are correct, you've reminded me of someone who tried it back many years ago but abandoned it because the power gain wasn't enough to offset the extra complexity.

 

[Christofero]

Hi George,

Read Grumpy Jenkin's How to Race the Small Block Chevy for info. If you look at JEGS they do a kit but it is little more than a air injector valve and some piping.

I did this on my lemans - it is not really very complex. The basic concept is two stage - when there is little exhaust flow get vacuum from the intake and then as the exhaust starts to flow get vacuum there. I found that connecting to the manifold upsets the carburetion and cannot be easily dialed out so disconnected that. Anyway there is less advantage to be had there as the engine is not making much power. The part plumbed into the exhaust does work and does produce negative vacuum - though only a few inches of water. That presumably is enough to make the rings work better.

I just used a standard north american emissions air injection valve as bought at crappy tire and welding a bit of pipe with a tip that goes into the exhaust stream at 45 degrees. The tip is bevelled parallel to the pipe. If you do this you have to either get rid of the 'oil return' pipe or shorten it so it is high above oil level as it will suck the engine dry. A large breather box filled with stainless steel wool is in order.

It is illegal for the track (except maybe drag racing where I believe it is standard practice on non-dry sump engines) as it makes a good track lubricator if things go wrong.

Does it make more power? Don't know, never tried it on the dyno and to be honest I cannot tell by the seat of my pants! My one at least did not spit oil I am glad to say.

 

[Christofero]

I looked deeper into the 'Bunn breather' and I think the inventor is overstating things. Yes blowby is hot and somewhat corrosive but the hot acidic gas is being feed into a mist of PH basic oil - you change the oil typically well before it gets neutral let alone acidic. Crossflow of fresh air as some early engine makers thought was a good move is not really helpful.

I also don't see what is so magical about the valve that he selects - not that it doesn't work.

If you have a good negative vacuum in the crankcase then the rings work better and you get less blowby which should always be the goal. It also helps to reduce parasitic losses as less oil is wrapped around the crank.

If you want to go about this scientifically I would plumb a vacuum and rotameter into the crankcase and evaluate PCV valves and maybe an exhaust extractor system as I described before. If you have a dyno and a vacuum pump you can pretty much quantify what the power increase will be.

This is all pretty much understood in the racing engine world - as someone said above with drysumps you get lots of negative pressure which is why you have typically an extra pump or two on the scavenge side. The main problem there is the scavenged oil contains a lot of air - Cosworth developed special pumps specifically to remove the air as well as making a proper 'swirl' tank.

Ta,

Chris