SERIOUS crankcase breathing...or not?

I was wondering...

Back in my car drag racing days we used crankcase pressure evacuation systems that consisted of a tube brazed into each exhaust collector at a 45 degree angle. The tubes had a high vacuum "pull" due to the venturi effect from the exhaust flow. Hoses were routed from the engine rocker covers to these tubes with standard PCV valves incorporated in the hoses. The vacuum in the crankcase was so high that after shut down, even with the engine just running at idle, you could hear the hissing as the pressure in the crankcase built back up to atmospheric. On a dyno this was worth around 30 HP on a Mopar SS/A hemi motor.

So..I've been wondering if it would be the ticket for a Commando? It wouldn't be at all hard to fabricate and there wouldn't really be much plumbing involved. Just need to drill holes/mount the tubes in each pipe just short of the muffler, run the breather hose through a single PCV valve and then Y it to both fittings on the exh pipes.

Anybody know if this, or something like it, has been tried on a Commando motor?

Mike,

Why the 45 degree angle between the scavenge tube and the exhaust pipe axis? Wouldn't a 90 degree arrangement work as well? It would seem to be easier to fab.

"Wouldn't a 90 degree arrangement work as well?"

Rick, I'm no gas flow expert so I can't answer that - a 90 may work fine. But the tube was always brazed at a 45 to the axis and the "business" end of the tube was cut on a 45 so that the tube's opening was essentially parallel to the axis. Maybe there's a more effective way with some other angle. But I can vouch for the 30HP gain on the dyno with a Mopar 426 Hemi in SS/A trim (producing around 700 HP at the time - now close to 1000HP)!

mike
I build transmission's for a couple of mopar drag racers in the winter months. nothing fancy. 440 wedge runs 10:20's reverse manual with trans brake.
one of the regulars told me that the record now with a ss/a hemi is 8:29 @160 + MPH
sure is hard to believe 440 CID limit 3,000 + LBS, 2, 4 bbl carbs on gasoline.

MexicoMike said:

I was wondering...

Anybody know if this, or something like it, has been tried on a Commando motor?

Click to expand...

No one wants to cut up the pretty pipes.

bill said:

mike
I build transmission's for a couple of mopar drag racers in the winter months. nothing fancy. 440 wedge runs 10:20's reverse manual with trans brake.
one of the regulars told me that the record now with a ss/a hemi is 8:29 @160 + MPH
sure is hard to believe 440 CID limit 3,000 + LBS, 2, 4 bbl carbs on gasoline.

Click to expand...

In the late '60's we set a national record for B/D with an Ed Pink built, Hilborn injected 426 Hemi backed by a "clutch-flite" 727 in a 2100lb rear engine car at 8.61 seconds. It blows my mind to think that today's door slammers could blow that car in the weeds!

Sorry for the hi-jack, back to Nortons.

I've done this very trick to 441 BSA powered Cheney I ride in vintage MX. The engine vents to the primary drive side like a BSA B50, then through a one way reed valve made from a BMW crank case breather, to the exhaust pipe that is at 90 deg. to the exhaust pipe, the ID of this system is 5/8" This system is very effective as I had to restrict the opening at the exhaust pipe as it would suck oil out of the primary. Once I figured out the size of the restriction it has worked very well and the engine does not leak. It's gotten a few eye balls at tech, I tell them it's my California polution control device.

Ken G.

motoracer8 said:

I've done this very trick to 441 BSA powered Cheney I ride in vintage MX. The engine vents to the primary drive side like a BSA B50, then through a one way reed valve made from a BMW crank case breather, to the exhaust pipe that is at 90 deg. to the exhaust pipe, the ID of this system is 5/8" This system is very effective as I had to restrict the opening at the exhaust pipe as it would suck oil out of the primary. Once I figured out the size of the restriction it has worked very well and the engine does not leak. It's gotten a few eye balls at tech, I tell them it's my California polution control device.

Ken G.

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Do you bring that bike up to Chehalis?

Hi,
In the early eighties Kawasaki and more recently late model Bonnevilles use exhaust port vacuum to pull air into the exhaust flow in order to facilitate an afterburn, supposedly reducing emissions. I don't know what kind of vacuum strength (mmHg) these systems produce, but I suppose it could be easily measured. Both systems also use use metal reed valves to keep the flow going one way. One problem with such a system on a Commando would be lack of an air/oil seperator. Having no place for oil to collect and run back to the engine it would be likely that you would end up pulling too much oil mist into the exhaust pipe.
GB

Hmmmmmmm...

I've still got all the AI stuff from my wyfe's new Bonnie, maybe I'll check out the reed valve...

"One problem with such a system on a Commando would be lack of an air/oil seperator. Having no place for oil to collect and run back to the engine it would be likely that you would end up pulling too much oil mist into the exhaust pipe."

I agree and was thinking about that when I was looking at a pair of old pipes I have that could easily have some tubes added. The breather on the 850 is just aft and above the passage where the rocker box oil drains into the timing case. Seems like it could easily pull that into the exhaust. Then again, I have no idea how much vacuum would be generated - these are rather small engines compared to SS/A Hemis (!) so it might not be too much. Only way to tell, of course, is to try it.

Re that, A few days ago I disconnected the regular crankcase breather plumbing to the oil tank/air filter, plugged the oil tank fitting, and ran the breather hose out the back of the bike, exiting just below the taillight, between the light and the rear fender (the top side of the fender). Since it's a fastback you can't see the hose. I ran the bike doing a lot of WOT, coast down, WOT runs up to around 70MPH in different gears. When I got back I checked the breather tube. There was no oil dampness at all; the tube was completely dry. Of course, that's a lot different than actually applying vacuum to the breather...

I not sure a exhaust assisted vacuum would really make much difference to a Commando motor. On a V8 you have 8 pistons rising and falling in various dispositions. There is always a number of pistons exhausting and the crankcase pressure does not fluctuate much. in a V8 exhaust assisted vacuum operates on every cycle. On the Commando both pistons go up and down together. One piston is exhausting but both pistons are compressing case pressure. By venting that case pressure to a one way outlet that's about as far as you can go. Adding an exhaust assisted outlet wouldn't make any differnce. Making an exhaust assisted outlet the sole means would possibly ingest a lot of oil mist and make smoke? Could be more trouble than its worth. Interesting idea though. On the late airhead BMW's and modern Bonnevilles' the air injection into the exhaust ports is a fuel emission dilution charge. You got to see the whole schematic to understand what they're doing.

Mick

Swoosdave, It's been to the premier/classic event in September several times, this last September I rode a Honda. It's been to most of the AHRMA events on the west coast at one time or another in the past ten years. I have a BSA B25 that I ride as well, it has the same device on it. It may come with me this year if I come up. My riding no. is 8X stop by and say hello.

Ken G.

Try a draft tube. Up to about 1960 cars had about a one inch draft tube that vented the engine by sticking down into the air stream under the car. The end was cut at a 45 degree angle with the open side facing rearward. The airflow caused a low pressure area on the tube opening and therefore drafted fumes and vapors out of the engine. I do the same on my bike by re routing the vent tube from the oil tank to the air cleaner. Plug the air cleaner fitting and route the tube to the under side of the bike. Cut a 45 degree angle on the tube and presto, you have a free draft system.

I thought about that but I didn't want the pipe/hose to exit anywhere that oil or oil mist (if any occurred) could be deposited on the tire so I ran it to the back of the bike. It is not in the airstream as you describe, like the old cars, so it is probably not as effective.
Do you have any problems with oil being deposited on the tire (or on the tyre)?

aceaceca said:

Try a draft tube. Up to about 1960 cars had about a one inch draft tube that vented the engine by sticking down into the air stream under the car. The end was cut at a 45 degree angle with the open side facing rearward. The airflow caused a low pressure area on the tube opening and therefore drafted fumes and vapors out of the engine. I do the same on my bike by re routing the vent tube from the oil tank to the air cleaner. Plug the air cleaner fitting and route the tube to the under side of the bike. Cut a 45 degree angle on the tube and presto, you have a free draft system.

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There was one of these on my TR4.

swooshdave said:

aceaceca said:

Try a draft tube. Up to about 1960 cars had about a one inch draft tube that vented the engine by sticking down into the air stream under the car. The end was cut at a 45 degree angle with the open side facing rearward. The airflow caused a low pressure area on the tube opening and therefore drafted fumes and vapors out of the engine. I do the same on my bike by re routing the vent tube from the oil tank to the air cleaner. Plug the air cleaner fitting and route the tube to the under side of the bike. Cut a 45 degree angle on the tube and presto, you have a free draft system.

Click to expand...

There was one of these on my TR4.

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Draft tubes were common on Meriden Triumphs well into the '60s. Usually ran along the curve of the rear fender.

"Draft tubes were common on Meriden Triumphs well into the '60s. Usually ran along the curve of the rear fender"

Hey, exactly where mine runs now!