Crank case breather

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crankcase breather

Jean, The 265.00 price tag on CNW's valve is for the valve and machining. Most of the price is in the machining. There is more to it than just making a hole in the back of the cases. I am the guy that spent a bunch of hours on the dyno with a absolute pressure sensor in the tach drive hole connected to a scope trying to figure out a breather system that actually works. Found out it can not be done through the stock breather hose at any speed much above an idle because of the damping of the pressure wave before it gets to the valve. At 3000 rpm a reed valve in the hose would open every few seconds and release the pressure and then close again untill the average pressure in the cases got high enough to open it again. With the volume of the crankcase reduced by blocking holes into the timing chest and the reed mounted directly to the case the reed opens every time the pistons go down up till about 3500 rpm and every couple of revolutions after that. The average pressure in the cases stays below zero. James Comstock
 
Re: crankcase breather

comnoz said:
Jean, The 265.00 price tag on CNW's valve is for the valve and machining. Most of the price is in the machining. There is more to it than just making a hole in the back of the cases. I am the guy that spent a bunch of hours on the dyno with a absolute pressure sensor in the tach drive hole connected to a scope trying to figure out a breather system that actually works. Found out it can not be done through the stock breather hose at any speed much above an idle because of the damping of the pressure wave before it gets to the valve. At 3000 rpm a reed valve in the hose would open every few seconds and release the pressure and then close again untill the average pressure in the cases got high enough to open it again. With the volume of the crankcase reduced by blocking holes into the timing chest and the reed mounted directly to the case the reed opens every time the pistons go down up till about 3500 rpm and every couple of revolutions after that. The average pressure in the cases stays below zero. James Comstock
Click to expand...

:oops: And it is truly well made, my apologies for the misleading information on the price tag, that is what my friend told me but obviously he did not understand the billing.

Many people have reported leaks stopping when fitting any kind of one way valve that is sensitive enough, did you find negative case pressure increases power while at the same time eliminating leaks?

Jean
 
Yes, the best part of the valve is how well it stops leaks. I have never before had a Norton that I could take on a 1500 mile trip at 80 mph and have the engine as clean when I got home as it was when I left. A little bit of added power is just a bonus but not something you would notice without a dyno. James
 
hi jean, i can,t see why your read valve wont work maybe jim can explain further,ie whats the difference between fitting it on the back of the crankcase or on the back of the timing chest ,in my view utilizeing the timing chest reduces primary compression so less pressure on the valve. how can jim say that the reeds will open every few seconds, a reed valve is a reed valve, ie pistons come down reeds open, pistons go up reeds close,no matter what the revs,thats why they work so well in 2strokes(cough)at much higher revs than our nortons,(in a 2 stroke its oposite ,pistons go up reeds open pistons come down reeds close)do the reeds in a 2stroke open every few seconds,i dont think so.its such an ingenios device i cant beleive the japs thought of it first,or maybe they just developed a theme of the g50 flap valve or the timed breather of earlier nortons and triumphs and bsa, either way reeds work on the same principal
 
Its the Volume!

By emininating, or minimizing the timing chest you eliminate the volume that gasses can compress in. The smaller the volume the more effective the reed valves can be (in the "old" days one of the prime methods for getting more power and throttle response out of the English two strokes was to "stuff" the cases---reduce volume).

I can't (and won't) speak for Jim, but I'd bet a hydraulic clutch system that I'm right, wink wink :wink:

RS
 
Reducing the volume of the crankcase makes the reed valve work better because the pressure change is higher. When you had an old 2-stroke and wanted to hot rod it one thing you did was stuff the crankcase by filling in any unoccupied space. That would raise the "compression ratio ' in the crankcase and make it pump mixture more efficiantly. When the reed is mounted to the timing case the extra volume plus the damping of the pressure wave that occures when the air has to travel through the holes into the timing chest and then back through more holes to the valve means there is not much pulse left to open a reed valve. Remember at 3000 rpm the pressure must travel from the crankcase to the reed valve 50 times per second. Believe me I tried a lot of simpler ways to get a reed to make a vacuum in the crankcase before I got to the point of mounting it on the crankcase. I had seen the benifit of a vacuum in the crankcase after I installed a belt driven vacuum pump on my Norton racebike years ago. The reed valve doesn't work as well as a pump but it's a lot better than anything else I have tried. James
 
jim a reed valve does,nt need pressure you can blow one open and suck one closed we not talkin about 2 stroke tuning thats a black art as far as im concerned i was just saying if a 2 stroke can open and close reeds then why cant a 4 stroke as i see it less primary compression on a norton is easier on the reeds
 
Well if we were trying to relieve static pressure in the cases a reed valve [or no valve at all] would do just fine on the end of a hundred foot of garden hose. But since we are dealing with pressure that changes from a positive value to a negative value up to 100 times per second at 6000 rpm then we need a valve that has the capability of opening for a fraction of a second when the piston is near BDC to relieve the small positive pressure and then close quickly as the piston starts moving back up so that a negative pressure will exist for most of a revolution. Air has inertia and by the time it has made it into the timing chest and began to raise the pressure there enough to open a reed valve the piston has already traveled back up and is then trying to draw the air back out of the timing chest. Pumping the air in and out of the timing chest through the holes takes power and dampens the pressure peak that is seen in the crankcase when those holes are closed up. A Jake brake on a truck works by holding the exhaust valve open a little bit. The work that is required to pump the air out and then back in through that partially open valve is a dramatic example of what is happening between the crankcase and timing chest on a Norton. James
 
Has anyone dyno tested any of these air valve conversions? I'm sceptical not of any breather device but the alleged benefits. My understanding of Phil Irving's work on a range of bikes, taking a single or parallel twin as an example, is that negative pressure on the up stroke is not beneficial as a 4 stroke will have extreme opposing positive pressure on the head side in 2 moments. Effectively the underside of the pistons are sucking a vacuum. Irving suggets that the displacement of air is best achieved through the largest orifice to permit the column to travel in and out as easily as possible. That obviously has some physical limitations on engine cases.
On my 850 Mk2 I run the stock breather hose into a 750cc catch bottle against the battery. This then outlets into the oil tank connector, breathers as per normal and then outlets into a larger bore exit hose to atmosphere. I also have a 1/4" hose off the inlet rocker cover. The combined volume of the hoses and catch space exceeds the that of the engine displacement. I have no leaks or vapour from the bike.

Mick
 
Jim,

Everything you say makes perfect sense and you did experiment different ways to do it. If you don't mind, I will copy your design (and make my own parts). From the looks of things, Norton was closer with the timed breather than with all the other breathers they tacked on the Commando except it did not have enough flow.

Jean

BTW I again asked my friend for pricing and unless Matt made a mistake on his invoice, the price of machining is $265 and the kit of parts is $220.
 
Hi Mick, I can say that with 30 years of racing. Ten of them on a Norton. And a Dynojet load control chassis dyno in my shop that has a Norton on it most of the time I have tested every combination of breather I could come up with. By the way, after this breather was designed I turned it over to CNW. So I am not trying to sell you something. James
 
The Bunn sounds a bit snakeoilish. I think ANY one way valve is better than none but I trust Jim's solution more than any other (if you ever saw what Jim does to a Norton, you would see he is not all talk :D )

Jean
 
Hi there everybody , nice and interesting, so just to add more questions : is the fact of adding any more breather will act better or counter-act the process, id est one breather on the inlet cover and/or exhaust cover (or both), plus the obvious reedvalve at the timing chest outlet ( after reading Jim , it seeems that it should be better to fit it near the outlet than near the oil tank , which means less travel for air), or may be I don't understand. More explanations about CNW mods may be could help ???
Somewhere in another forum , I had appreciated one guy (Beltdriveman to name him), who spoke about ending all breather into oiltank as usual, but as far I had understood , this oil tank kept pressure (only a tiny hole in the cap ).......??????? Keep on like this , I feel much clever everyday!!
 
Bonjour Marinatlas,

More breathers may be detrimental to the operation of the crankcase breather I would think. On my bike, the main breather will go to the oil tank and from there, I will run a small tube to the inlet manifold to insure there is no pressure in the oil tank, I will have to see how that works out, but that is the way it is done on cars with a PCV (Positive Crankcase Ventilation) valve.

Jean
 
Here is what I have learned with breathers on the dyno,
Any check valve breather was better than an open breather if the opening pressure was low.
To keep average crancase pressure low requires a breather with a very low opening pressure [the Bunn was lowest]
No changes in size of the breather exiting from the timing chest made a measurable difference in horsepower with or without a valve.
Drilling extra holes from the crankcase to the timing chest gave a very small power increase [theorectically any hole or combination of holes smaller that the area of the piston bores is a resriction to the air being moved by the pistons and consumes power]
Breathing only from the valve covers caused more oil to stay in the head because oil had difficulty draining down through the lifters when blowby air was trying to escape up through the lifters.
When I blocked off all holes from the timing chest to the crankcase except for one small hole down low for oil return I saw a larger power gain- 2HP- That suprised me.
None of the checkvalves I tried created a negative average pressure in the crankcase when it was connected to the timing chest hose or valve cover except at low speed. With the timing chest to crankcase seriously swiss cheesed and a reed valve on the chest I could maintain a negative pressure up to around 3000 rpm or a little more. That case became scrap after it cracked from the cam hole to the crank hole through the added holes between [If you reed the Bunn literature he states that the valve needs to be connected directly to the crankcase to be efficiant]
A small hole [1/8 inch] venting directly from the crankcase with no valve made more power than a 1/2 inch open hole. Obviously less air was being moved in and out so less power was wasted. The small hole can't be used because under certain conditions blowby will overwhelm it and cause high case pressure. [blowing oil out every seam]
My testing was carried out over the period of several years and therefore was not well controlled. My opinons are just that. Opinions. James
 
Hi , I had never red before the bunn's theories, so now it's done, he said that for Norton he put inlet valve at the timing chest and exhaust valves at the inlet rocker cover.............I had recently fitted the XS yam reed valve on my Commando 850 in the normal way (in the outlet breather pipe, as shown in that forum), note that I had no problem before , neither after!! On the other Commandos (mainly 750) I was used to make as per NOC recommandations , idest making holes in the timing cases, and either keep the other timed breather or blanked it, I even made the old brits mods on a post 2OO.OOO engine of a friend of mine, but no feedback as we don't yet start this engine.......but for my next project may be the last one , that will be 850 cases , with Jim Schmidt rods and pistons, 380 Johnson cams, that's why I am so much interested into your stories about theese venting/breathing mods!!! enjoy your day , old chaps.
 
Hello James, Did you design the breather for the Combat cases? I read on the CNW website you can add the unit without doing anything to the cases. I have a motor that has yet to be started that has had the oil pickup mod. done at Old Britts. Will this unit still work with this setup? The bike is about ready to go with the inline type reed valve in it off of the timing chest but I would like to do better if possible. Thanks for your time, Chuck Horton.
 
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