How can you have some extra power with PCV valve

Hi, I am very happy with the PCV valve but I can believe you can make some exta BPH only with this valve!
To create compression or vacum you need energy no?
Cheers yves

You loose less power . Any power consumed by compressing the mixture is gained back when the piston reverses direction. Power is lost when the piston is attemting to pump air in and out through a restriction. If you look at a Jake Brake on a truck , it slows the vehicle by holding one exhaust valve open a little bit so the air moves in and out through the valve and uses a lot of energy. The same thing happens in a crankcase but to a less extent because the compression ratio in the crankcase is much lower. Jim

It's an old hotrodding method that's been proven to work, though it probably wouldn't be noticable on a streetbike. It's not a matter of compression and expansion, though that uses up energy as heat. It uses much more energy to push air from one place to another inside the crankcase because of the drag; the stuff is thick as molasses, at speed. Worst case scenario would be a 180deg twin where there's no compression or expansion at all. A guy that raced Commandos said that he dyno'ed about 5bhp out of using one. Everyone from NASCAR to Formula One couldn't run like they do with air in their crankcases. The reason it wouldn't be be a big deal on a streetbike is that the drag is a function of RPM and it goes up exponentially; we live in the horizontal, not the vertical part of the curve.

Although it's very true that a Commando will have more HP using a proper PCV valve, I consider that secondary. Getting rid of the oil leaks, especially the crankshaft oil seal leak, is the primary benefit for me.

In the past I've seen a number of owners convinced that their crankshaft seal is faulty. They replace it only to soon have the same problem reoccur. Still convinced it's the seal, they then look for the "Wonderseal". I did exactly the same thing. Replaced the standard seal numerous times, eventually ending up with some $30 Teflon seal. Still leaked engine oil into the primary. I then tried the Motormite vacuum check valve. This stopped the oil leaks for a short time, a few hundred miles at most. Finally, I installed the XS 650 PCV valve. Viola! No more engine oil in the primary. Since I run a belt this is very important. The fact I gained HP is a plus. I would never run a Commando without a proper PCV valve again. I think once the PCV valve is installed, the function of the crank seal changes to preventing contaminates from entering the crankcase, as opposed to trying to stop pressurized oil mist from escaping the crankcase. I will say if the crankcase is full of oil from wet sumping no PCV valve will prevent oil from escaping somewhere. Wet sumping is a separate issue. It, too, needs to be addressed if you want an oil (engine oil) free primary.

Do I just go to Yamaha and ask for a xs650 pcv valve?
Thanks,
Marshal

The XS 650 PCV valve is not a Yamaha item. The only source I know of is: http://www.mikesxs.net/products-40.html#products.

It's my understanding the valve may have been designed for a small engine, lawn mower type. I don't know where it came from, only that it works. The fact it is sold on a website that advertises XS 650 parts caused me to assume it was a Yamaha part.

The http://www.coloradonortonworks.com/catalog/engine.asp#1713 PCV valve has had extensive development work dedicated to the Norton engine. The development work was done by Jim Comstock, a regular here. No doubt the valve sold by CNW is superior to the XS 650 valve. I can see it definitely is affixed better. It's been stated the closer the valve is to the crankcase, the more efficient it is. If I were to build a new engine (Commando) I'd pop for the CNW valve.

JimC said:

I think once the PCV valve is installed, the function of the crank seal changes to preventing contaminates from entering the crankcase, as opposed to trying to stop pressurized oil mist from escaping the crankcase.

Click to expand...

Alan Goldwater at INOA put a vacuum gauge on the intake of his Commando and recorded a vacuum up to around 4000rpm with a Motormite. So you're right, the crank seal is working at negative pressure with a PVC valve, up to a point. It also shows that Motormites aren't designed to handle pressure cycles much above 4000/sec.

It seemed to me the Motormite valve worked fine, for a very short time. After a couple hundred miles it gave up the ghost. I know others have posted excellent results with the Motormite, but I can't say the same. My first choice would be the CNW/Jim Comstock valve. If circumstances ruled that out I would go for the XS valve.

To use really high rpm it really helps to have a vacuum to minimise drag. I remember when working in a lab that one day the vacuum pump on the centrifuge failed. The rotor, which was simply a smooth titanium block (once capped), would not spin above about 15000 rpm; it would normally go to 50000. It cooked my samples due to the heat.

Now imagine a non-aerodynamic crank - much resistance, much heat. Racing engines, especially F1 and Moto GP, now run up to 18000 rpm or so. 1960's Honda's used to go to 25000 rpm! Running in a partial vacuum would really help.

Best regards, Phil.

It makes sense that venting the crank case will lower the resistance of the cranks ability to turn freely. Wet sumping (oil in the path of the flywheel) or a dense oil mist under pressure in the crank cases will offer resistance to the crank at high rpms. Racing engines have polished cranks to reduce air turbulence. The more you do to reduce resistance in the crank case the faster you achieve max rpms.

H aving read the responses of the members, i hope not to piss off anybody with the following. you guyes seem to be doing a little "overengineering" of the effects of the PCV,
In the simplest terms,, the pistons move the same volume of air both above & below. the parallel arrangement of the engines means that: when the pistons " fall " air displacement ( equal to swept volume of the pistons ) creates pressure on the cases, just like a syringe. this parasitic loss is relieved by creating a "vacume" beneath the piston (s),, sucking the air from beneath them.
if not for the " oil bag ", a wet sump engine with this arrangement would blow oil everywhere!, hence the necessity for a remote oil tank. not to take away from the responses here , slightly less dense air in the cases will let the crank spin more freely, but i don't think those are the primary concerns. aerodynamic drag is the nature of the beast, when it comes to a spinning crank. more important to evacuate the air from beneath the pistons eh? that is how the bhp is increased. or do you think it necessary to externaly polish the crank to reduce hydronamic loss too?

Several years ago when looking for some extra power on the racebike I thought a bigger breather would be the thing to do. The bike was on the dyno in my shop. I removed the fitting from the back of the mk3 engine case with the 3/8 inside diameter and drilled and tapped the the hole for a 1/4 inch pipe with a 1/2 inch inside diameter. And guess what- I lost about 1/2 horse. I went back to the original size to double check and got it back. Then I started going smaller on the fitting restriction and I gained power all the way up to the point of blowing oil out of the mainseal because of crankcase pressure. This proved to me that there is power lost due to pumping air through the restriction. The less air that was allowed to move the less power was lost. Installing a check valve got everything right . I kept the engine from leaking and got rid of the pumping losses and reduced the air drag in the cases. I saw about 2.5 horse gain over the open breather . The next season I installed a crank driven vacuum pump and got the power increase up to around 4.5 horse. Jim

Jim,
You hit the nail right on the noggin. You were moving the air in and out, breathing through the breather. Power losses. As with the mechanical vacuum you used, the dragbike guys run at least one and some use two electric vacuum pumps. NASCAR limits the volume of the scavenge pumps because guys use them to pump air, not oil out of the engine. Neither have any breather, as such. The only air going into the crankcase is blowby.

Recklessridir,

The point of evacuating the crankcase isn't so much aerodynamic drag on the crank and rods as much as it's pushing air around in the case. They do design and polish cranks and rods for drag but they also contour the inside of the crankcase for minimum drag on Formula One engines. In fact, at one time they designed engines like a stack of sealed V-Twins for just these reasons. The question Yves asked is about possible performance gains with a PCV valve. Did we overengineer it for him? You betcha!

But the best part of the reed conversion is you get your moneys worth at the Best Western. This is after the first days ride to Lake o the Pines. 550 miles at 80 to 90 mph and pulled it in hot. Put the dirty clothes bag under it just to be safe but the next morning there wasn't a drop. Jim

comnoz said:

But the best part of the reed conversion is you get your moneys worth at the Best Western. This is after the first days ride to Lake o the Pines. 550 miles at 80 to 90 mph and pulled it in hot. Put the dirty clothes bag under it just to be safe but the next morning there wasn't a drop. Jim

Click to expand...

:lol: you should change you sig to "no drips"

Jean

Did you back it into the room?

There's a point where your bike goes from being finely tuned to just plain pampered, I think this is it.

Actually I drove it into the room and then turned it around. Pampered it is. But on the way home I rode 800 of the 1000 miles in the rain and right now you can't even tell what color it is. Going to have to pamper it some more today.

Then I'd say it well deserved that night in a warm motel!

Yeah especially when there was a group of those bad boy Harleys setting outside. Might have ended up with something like this.