Positive crankcase ventilation
05-27-2007, 11:22 AM
#26
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RE: Positive crankcase ventilation
Four-stroke oil routing to the cam gearing:
I've had my TT91FS apart a few times and noticed a small hole inside the engine. All one has to do to see it is remove the back plate and look just above the rear bearing. (It has been a while so I'm pretty sure it can be seen without removing the cylinder.) This tiny hole is drilled a short distance through the case and opens in the cam gear housing. The crankcase vent is located at the bottom of the crank spline(?) so that the blow-by oil travels through the drilled hole, around the cam gears, down past the crank spine, and out the bottom of the case.
That is how the front end of this engine gets lubricated.
By creating a slight negative pressure on the vent line I would think it would help facilitate the lubrication of the front end.
I get very little blow-by residue coming from the crank vent and considered opening up the little hole going from crank to cam a bit. I don't know if I made this mod or not... I don't think so.
Even after about a gallon and a half there is little residue. Of course the compression is still greater than some of my two strokes indicating the rings and valves are seated very well.
High speed air passing over a tube perpendicular to the flow will create a vaccum in the tube. The pressure in the tube is greater than that of the air stream. IE paint sprayer. PLacing the crank vent tube anywhere in the exhaust stream but NOT in a chamber where pressure builds will create a vaccum in the line. I would think it would be the same for the intake manifold.
There is more exiting the crank vent than entering (on the up-stroke) so I see no adverse effects in making this mod. I for one do not want the dirty residue from my TT91's crank vent going back into the engine though.
I've had my TT91FS apart a few times and noticed a small hole inside the engine. All one has to do to see it is remove the back plate and look just above the rear bearing. (It has been a while so I'm pretty sure it can be seen without removing the cylinder.) This tiny hole is drilled a short distance through the case and opens in the cam gear housing. The crankcase vent is located at the bottom of the crank spline(?) so that the blow-by oil travels through the drilled hole, around the cam gears, down past the crank spine, and out the bottom of the case.
That is how the front end of this engine gets lubricated.
By creating a slight negative pressure on the vent line I would think it would help facilitate the lubrication of the front end.
I get very little blow-by residue coming from the crank vent and considered opening up the little hole going from crank to cam a bit. I don't know if I made this mod or not... I don't think so.
Even after about a gallon and a half there is little residue. Of course the compression is still greater than some of my two strokes indicating the rings and valves are seated very well.
High speed air passing over a tube perpendicular to the flow will create a vaccum in the tube. The pressure in the tube is greater than that of the air stream. IE paint sprayer. PLacing the crank vent tube anywhere in the exhaust stream but NOT in a chamber where pressure builds will create a vaccum in the line. I would think it would be the same for the intake manifold.
There is more exiting the crank vent than entering (on the up-stroke) so I see no adverse effects in making this mod. I for one do not want the dirty residue from my TT91's crank vent going back into the engine though.
05-27-2007, 11:47 AM
#27
RE: Positive crankcase ventilation
Good point...I don't like the idea of sucking the residual oil (and what might be floating around in it ) back through my engine(s ).
An Augmentor tube is a tube within a tube...the augmentor tube is approx. twice the dia. of the exh. pipe, and is bell-mouthed at the inlet...the end of the exh. pipe just enters the bell mouth, and the exh. gasses flowing through the augmentor tube pulls a vacuum...
They are frequently used to augment cooling in full scale aircraft by helping to pull air through the cowling.
The crankcase vent tube could be inserted into the augmentor tube, at the proper angle so that a nice vacuum could be created and all the oil would be exited in the same place...
I did a google search, looking for a picture but found zip? Anyone else got a pic or find anything?
As far as a crank scraper...that's a good idea, but I'm thinking that the crank to case clearance is already quite tight, and that the actual case itself acts as sort of a scraper?
An Augmentor tube is a tube within a tube...the augmentor tube is approx. twice the dia. of the exh. pipe, and is bell-mouthed at the inlet...the end of the exh. pipe just enters the bell mouth, and the exh. gasses flowing through the augmentor tube pulls a vacuum...
They are frequently used to augment cooling in full scale aircraft by helping to pull air through the cowling.
The crankcase vent tube could be inserted into the augmentor tube, at the proper angle so that a nice vacuum could be created and all the oil would be exited in the same place...
I did a google search, looking for a picture but found zip? Anyone else got a pic or find anything?
As far as a crank scraper...that's a good idea, but I'm thinking that the crank to case clearance is already quite tight, and that the actual case itself acts as sort of a scraper?
05-27-2007, 11:54 AM
#28
RE: Positive crankcase ventilation
Yes, the TT91 FS uses that little hole above the bearing. I was surprised when I saw this as it is much harder to produce than the hole in the crank like OS uses. A passage that small and long is likely to clog if the engine is not run often. I understand Saito also made a few engines with sealed main bearings without provision for lubricating the cam box, read it here somewhere. I have little experience with Saito engines however.
All said the idea in the original post is acceptable. I've seen several posts on RCU about people retrofitting PCV to the intake manifold, so that should be just fine on engines that weren't supplied with it. It's strange, if you look at the first FS-40 (circa 1981) the crankcase casting has a boss for a fitting below the cam box. The engine however used a backplate breather fitting.
All said the idea in the original post is acceptable. I've seen several posts on RCU about people retrofitting PCV to the intake manifold, so that should be just fine on engines that weren't supplied with it. It's strange, if you look at the first FS-40 (circa 1981) the crankcase casting has a boss for a fitting below the cam box. The engine however used a backplate breather fitting.
