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-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (

60Gunner 9th July 2019 21:10


Originally Posted by Hippysmack (Post 5759719)
The breather bolts in 91 Up engines have 2 different size holes through them.

04 Up

The MoCo manipulated crankcase pressure with them I believe.
The crankcase splash holes were restricted to keep more pressure in the crankcase.
They introduced piston squirters (feed pressure from the pump) into the crankcase.
Since more oil in the CC means higher pressure and more drag, what were the affects?

The scavenger gerotors were not redesigned larger until 2007.
So for 04-06 models, they got the same return rate (as previous models without the added oil).

So I feel that the smaller breather bolt holes were designed to move CC pressure out the breathers faster than previous models.
(instead of bottling up CC pressure)
The smaller hole creates higher pressure. Higher pressure equates to faster flow.

So wouldn't the smaller holes actually control CC positive and negative pressures better in the higher pressure environment?

So the aftermarket breather bolts I got from DK Customs for my 05 are the wrong ones? They have the bigger holes.

Hippysmack 9th July 2019 23:16

The holes in the 04 Up bolts are stepped.
They are smaller on the head side as above but bigger on the A/C side.
That creates more of a restriction than 91-03 models.
The restriction can do a couple things.
It will create more backpressure inside the engine until a stronger force is applied from inside.
That stronger force would be the air created on downstroke.
(edit: In a perfect world), The downstroke won't be impeded so the force won't slow down.
So the major result would be air moving out at a faster rate thru the smaller hole.

'Why' I'm still working on but that was by design.
I feel it was to move CC pressure out of the engine faster as that wouldn't really do a lot for keeping pressure in on an OEM engine.
They also added the piston squirters in the CC which adds more oil to the sump / creates more responsibility on the oil pump scavenge to remove that drain oil faster and keep it off the wheels.
(which they didn't upgrade the scavenge section until 07)

The results of using the bigger holed bolt using the above reasoning would be a slower rate of outflow.
But the timing of the upstroke is the same which then pulls a vacuum.
The umbrella will close down only when inside air pressure is lower than outside air pressure.
If the inside pressure can't get out fast enough before piston upstroke happens the umbrella may 'float' or not close all the way.
If the umbrella floats, X amount extra air is pulled in until it does close.
(on top of what didn't get completely out on the last downstroke = more air in the engine on the next downstoke = oil puking out the breather)

How much slower the bigger bolt hole yields, I don't know.
If it's a problem, I don't know.
I don't have any flow data which would be what is needed.
That's why I was reading bunny32's comments closely.

Since you've upgraded to bigger pistons, there will be more positive pressure displaced out the breathers on downstroke.
It's not clear to me just how much of a change the smaller hole made.
But again, it was by design to OEM specs internally.

I'd suspect it could contribute to an 'out of balance' between positive and negative pressures inside.
(edit: to rephrase, I suspect it was meant to manipulate that balance)
But it'd be more of a concern if you had higher CC pressure / vacuum than was healthy.
(which would probably show itself if it was a big problem to the engine)

60Gunner 9th July 2019 23:44

I never notice any oil puking even if leave I leave the hoses off.

Hippysmack 9th July 2019 23:54

I really believe it has to do with the balance of pressures inside the case.
Good ring seal means less positive pressure inside.
As the rings wear, more positive pressure is induced by blowby.
You may end up good now but not later.
Or it may never be a problem.
But if it does surface later, you might try to go back to the smaller holed bolts to see what changes... and then let us know. :geek

60Gunner 10th July 2019 01:03

I'm not sure if/where smaller holed bolts can be found.

bustert 10th July 2019 01:20

well mr. hippy, i think you are on to something. i knew the holes got smaller but since i never had issue with vent, out of sight, out of mind.
they also changed coalescer design.
put on the tom terriffic hat and the correlation will start to show, so what do you think?? i have a theory.
i do not run hd breather bolts, mine are round stock aluminum that i flatted so it can be wrenched and bored to accept the backing plate and internal vent routers. bolts are easy to make.

bustert 10th July 2019 01:25

well, just had an invention pop up in my head.
get rid of vent to cleaner all together.
the concept was used in the auto world
and if i am not mistaken, the suzuki wankle engine had a similar devise
think deep and in close proximity of the present setup. any guesses???

Hippysmack 10th July 2019 02:02

Small radiator with tiny holes in the top? :laugh

I ain't feeling too terrific right now I guess.
You talking about external PCV?

Hippysmack 10th July 2019 02:45


Originally Posted by 60Gunner (Post 5761559)
I'm not sure if/where smaller holed bolts can be found.

You should be able to use a standard bolt of the right length and drill a hole thru it as they are routed.
The thing is with the aftermarket bolts is the amount of meat that's left over.
I've seen a pic of one broken from too much torque applied... snapped it.
Keep in mind these are 1/2" bolts....

There is also a mod I believe using OEM bolts and spark plug boots to route hoses down.


Originally Posted by bustert (Post 5761563)
they also changed coalescer design.

Yes they did.
In theory,

03< the oil is actually separated by the umbrella.
The umbrella doubles as a one way air valve.
04> the oil is separated by the mesh.
And the umbrella is nothing more now than a one way air valve.

Although there is that extra oil chamber between the in and out of the breather assembly.
Maybe it's 2 stage separation.
But oil in the middle chamber has nowhere to go until it's picked up and suspended again.

60Gunner 10th July 2019 03:01

Ya know, routed like the factory does, it's in a low pressure area and wouldn't raise crankcase pressure as much as routed to atmospheric pressure such as is the case of most including myself. Maybe it's a good thing it's bigger? Smaller in this case may just build too much pressure and blow seals and gaskets?

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