Under Pressure

[Wow, how time flies when you are busy living life and working for a living.  There have been an untold number of things I have had the urge to write about over the last several months, but could not seem to justify taking the time.  Some of those topics I consider very important, some interesting but not very important, and some perhaps just somewhere in between. This post is definitely not important (in the grand scheme of things), and probably only interesting to a limited audience, but you know what?  I am going to go ahead and write it anyway, if for no other reason than to see if anyone is still paying attention.]

A few weeks ago I had Billy's Iron Head vintage flat track bike here for a little maintenance.  It had been smoking pretty good the last couple times out, and that coupled with the fact that one of the recent races featured extended time spent at 8500 RPM meant it might be due for more than just checking the valve adjustment.  As it turned out everything was in pretty decent shape with only a fresh set of rings required.

Given that I would not have to spend very much time freshening things up, I took the opportunity to fabricate a custom breather system for the bike, something that has been on my "to do" list for some time.

As with most early Sportster engined flat track bikes, Billy's has a magneto and runs without a generator.  Now, having my "druthers" I would love to see the mag mounted in the generator position so it would be out of the way of the pushrods and carb, but that's probably not going to happen any time soon.  So with the generator gone, a gaping hole is left at the front of the motor; easy enough to seal off with a flat plate, but of course that leaves the breather system in a state of chaos.  In stock form, the large diameter washer which resides on the inboard end of the generator serves as as an oil separator allowing excess crankcase pressure to be relieved by way of a tube attached to the cam cover. Without the washer, the breather tube would tend to do a credible impression of the Exxon Valdez (if you youngsters are scratching your head, click on the link for a little history).  Okay, that may be a bit of an exaggeration, but without the washer you will get at least some oil out of the breather tube, something most tracks take a dim view of.

But of course you can't then plug off the breather tube unless you're willing to give up a bunch of horsepower and initiate oil leaks from every conceivable joint.  Okay, you got me.  Another slight exaggeration, but only concerning leaks from every joint; that plugging the breather will result in loss of horsepower is a given.  The temporary bandaid that went onto Billy's bike when he showed up with an rebuilt stock replacement engine last year was as simple as it was marginal.  An elbow in the generator block off plate with about  18 inches of 3/8" hose pointing up, along with an in-line PCV (Positive Crankcase Ventilation) type valve topped with a generic plastic automotive fuel filter. In retrospect, perhaps the filter should have been put in place between the fitting and the one way PCV which would have allowed the oil seperated by the filter to drain back into the engine, but its a moot point now.

The actual breather that I came up with was to a large extent  dictated by the scrap aluminum material that I had on hand (keeping in mind mine is an engine shop, not a fabrication shop). What I came up with is shown below.  Keep in mind that I stressed the part about material on hand.  With more to work with, I probably would have built it with the oil separating baffle before the umbrella valves to keep more of the oil in the cam cover rather than transferring to the catch can.  But, ...oh well, the first prototype of anything is seldom the best or final version.There is a drain plug on the bottom of the canister for easy draining, so it shouldn't be a problem anyway.

1st piece -machined a flat plate for a recess and drilled holes for umbrella valves

Umbrella valves installed in plate

Engine side of plate - center hole of each group mounts umbrella valve - other 4 holes for air transfer

The umbrella valves are from a late 80" Evo engine, which uses only two.  I calculated the cross sectional area of the transfer holes to insure that added together they exceed the area of the new breather outlet. Consider this: due to the common crankpin design of the 45 degree V-Twin engine there will be 90 degrees of crankshaft rotation where both pistons are descending in their bores at the same time.  That means that without a "breather" to alleviate the potential pressure, the air/oil mist in the crankcase will need to be compressed, at the price of the horsepower it takes to do the compressing.  As mentioned earlier, pressure will also tend to seek a way out, usually at whatever gasket has the least resistance.

Now, if one is willing to take the time, they might calculate the amount of air displaced by the downward motion of the pistons that needs to be moved through the breather system, and then factor in the amount of time available to do that at, say 7500 RPM and apply that to estimate the needed cross sectional area of the choke point (smallest orifice) in the breather system.  No, I'm not going to do it either, but barring that, it is at least a good idea to increase the size over stock.  The I.D. of a standard 1/8" pipe x 3/8" hose barb fitting is only .265" which calculates to only .055 square inches. That seems awfully small.  The outlet pipe on my completed breather assembly has a 1/2" O.D. (to take a breather filter) with a .380" I.D. for a cross section of .113 square inches, or double the stock size.  Still seems a bit marginal when just looking at the cross section, but it has to be an improvement.  Oh, and by the way, I did calculate the cumulative area of both the baffle(oil separator) holes and the holes at the umbrella valves to be sure they would not be a "choke" point.

Finished assembly

Orientation of umbrella valve plate when assembled

Tack welded in place oil separator inside canister

Installed on bike - breather filter not installed.

Before I go, one other think to consider.  Some of the breather fittings that go onto a Twin Cam motor are ridiculously small.  First of all, the heads are only tapped for a 3/8" - 16 tpi (thread per inch) bolt so there is little to work with (even the smaller cubic inch Evo motors were tapped 1/2" - 13 tpi) but some of the breather fitting that go into those holes have a through diameter of only .145" which works out to a cross sectional area of only .017 square inches per head or .034 for what are often 103 cubic inch or larger engines.  Its a testament to how well all the gaskets and o-ring in the engine work to keep (what I assume to be) all that excess crankcase pressure on the inside.