YZ450F Oil System?

The procedure to check the oil is to run the engine for a couple of minutes then shut down the engine and check the oil with the dipstick. This makes sense because it allows the oil to fill the oil tank that the dip stick is located in. After shutting down the engine, the oil all apparently drains down into the engine case. If this is the case then it seems that the amount of time between shutting the engine down and taking the dipstick reading would have an effect on the measurement. (The more time that elapses, the more oil will be in the case and not showing up on the dipstick). I haven't had any trouble getting an accurate oil level reading and maybe I'm being a little annal, but after thinking about it, I am wondering just how this oil system works. It seems that if oil is constantly draining down into the case after shutdown, it would effect what you see on the stick. For example, after sitting in the garage overnight the dipstick shows absolutely nothing on the dipstick. Anyone care to shed some light on this one?

What you have is a dry sump oiling system. In a wet sump, the oil is carried in the crankcases, or below them in an oil pan. There is one oil pump which picks up the oil and feeds it under pressure to the engine bearings and valve gear (camshaft, etc) in the head. Then it just drains back and starts over.

In a dry sump, oil is carried in a tank seperate from the engine, and the feed pump draws it from there by way of external plumbing. Atfer it gets where it's supposed to go, and drains back to the crankcase, it is picked up by a second pump, the return, or scavenge pump, and sent back to the tank. The scavenge pump normally pumps a higher volume than the feed side, so that it can outpace the feed pump and keep the crankcase cleared out and oil back in the tank for the feed pump to use.

The oil tank is generally higher than the oil pump (although not necessarily), usually because one reason for dry sump systems is to increase ground clearance by eliminating oil storage lower than the crank. What keeps the oil up in the tank varies somewhat. Very sophisticated systems have a low pressure check valve that is easily pulled open by the demands of the feed pump, but that is strong enough to hold against gravity. These can sometimes leak while standing if foreign materials get caught in the seat. Simpler systems rely on the close fit of the gears in the feed pump to hold back the oil. In truth, it doesn't matter much in a system like the YZ's use, because if the whole 1.7 quarts that the pre-03's use emptied into the crankcase (it often does if left alone for a wek), it isn't enough to cause a problem for the 50-60 seconds it will take the scavenge pump to clear it out.

But it does mean that for the first 5-15 seconds after a dead cold start, there might be no oil in the tank, and thus none for the pressure lubricating system. Under this circumstance, the engine is getting is lube only from splash and left over film. The only oil that could get to thetop end of a YZ would be that dragged up by the cam chain. So take it easy on the throttle for a couple of heart beats, at least.

Some convert to wet sump for a weight advantage (the conversion carries less oil, and I presume they remove the scange pump. There is also the fact that the oil is carried lower in the bike.)

Dry sumps offer a power advantage in that the oil supply will never drag on rotating parts. Oil is moved away from the heat of the engine, so it may cool better. The oil supply can be of any size, without regard for the space available in the engine itself, and dry sumps can be used to increase ground clearnce, or lower the engine.

Technically, I prefer dry sump systems. I do wish that Yamaha had separated the engine from the transmission, as with the CRF450/250, and the British dry sump designs.

That enough light?

That should be enough to keep me thinking for a while! Thanks for the education!

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