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Old 19th July 2014, 07:09 PM   #1 (permalink)
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How does the venturi exhaust effect work?

So I've got a venturi port in my exhaust that looks like this: -



I've checked the connection to the engine and it does indeed create a vacuum effect. This is connected to a valve on the side of the engine which I believe goes down to the crank case.

The valve has the following properties: -
  • When gas is blown from inside of the engine the valve shuts
  • When gas is blown from outside of the engine, the valve opens

I have no idea how this is supposed to work as I would imagine that as soon as the engine builds up any kind of pressure, the valve will just shut. Given that the venturi port is also creating the vacuum, wouldn't this just allow the engine to build up and up?
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Old 20th July 2014, 08:49 PM   #2 (permalink)
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Any ideas anyone?

I just don't see how this can release positive crankcase pressure if the valve shuts, especially if the suction from the venturi effect assists.
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Old 20th July 2014, 09:08 PM   #3 (permalink)
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If you look at flow around an obstruction, high pressure upstream and low pressure downstream ie suction.
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Old 20th July 2014, 09:39 PM   #4 (permalink)
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I think this is what you're after...

Automotive Crankcase Ventilation Systems Diagram PCV
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Old 20th July 2014, 09:56 PM   #5 (permalink)
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Hey matt, thanks for your reply however I kind of understand the functionality of a PCV system but this is not what I am describing.

From my exhaust system, I have the venturi port which connects to a valve that leads to the crank case. There is no feedback into the intake on my system.
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Old 20th July 2014, 09:58 PM   #6 (permalink)
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Read the article, the valve will be a simple NRV.
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Old 20th July 2014, 10:22 PM   #7 (permalink)
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Hey matt,

The link you posted makes sense however there's no specific example that looks like mine.

I'm just confused at the operation of this valve that I have been told is the PCV valve. It shakes when rattles and thus seems operationally fine however I don't see how it can be in the open state if there is suction from the venturi port. The valve operates as I posted in my original post.

Basically it looks like this: -

[PCV valve] -> [ports next to the rocker cover] -> [another valve - maybe this is the NRV?] -> [venturi port] -> [exhaust]

So if the PCV valve is shut when suction is occurring or there is positive pressure behind the valve, how is the pressure in the crank case being released?

Basically, I'm getting oil blown out of the dip stick however I've checked the compression, all good with all cylinders very good so I'm concerned that my ventilation system is a problem.
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Old 21st July 2014, 11:21 AM   #8 (permalink)
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If it rattles when shaken, that'll most likely be an NRV / check valve.
It's probably a ball bearing type.
PCV would be self regulating if there's no contol and it wouldn't 'rattle'.
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Old 15th August 2014, 11:46 PM   #9 (permalink)
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I hope it helps

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wanted: R33 GT-R 400R sideskirts & overfender
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Old 16th August 2014, 07:41 AM   #10 (permalink)
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I'd have thought the nozzle was the wrong way round in the flow, surely you want the gases to be drawn from the low pressure side, not high. A pipeline quill is usually the other way around.



Actually, scratch that, just seen the draw hole.
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Old 16th August 2014, 08:31 AM   #11 (permalink)
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Hehe, thanks for the replies.

So the reason I'm worried about the PCV system is that oil is getting sprayed out of the dipstick tube on high boost.

I've checked compression and it's really good, as I expected. Now, at the end of the dip stick, there is a breather that is supposedly designed so that the engine can breath in through this and then exhale through the PCV system.

The Venturi effect port in the exhaust creates the vacuum that lets the pressure out.

I don't quite understand how the gases could ever escape the PCV system if the PCV on the valve cover shuts when sucking hard on it from the exhaust side.
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Old 16th August 2014, 08:32 AM   #12 (permalink)
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Regarding PCV valves rattling, the typical check to see if it is working is to rattle it and see if it makes a noise. If it does, it is usually working. Quite a few posts for this online.
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Old 16th August 2014, 08:47 AM   #13 (permalink)
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Quote:
Originally Posted by matt j View Post
If it rattles when shaken, that'll most likely be an NRV / check valve.
It's probably a ball bearing type.
PCV would be self regulating if there's no contol and it wouldn't 'rattle'.

Quote:
Originally Posted by matt j View Post
I think this is what you're after...

Automotive Crankcase Ventilation Systems Diagram PCV
Check diagrams, it says "PCV" check valve.

Quote:
Originally Posted by edizio View Post
Regarding PCV valves rattling, the typical check to see if it is working is to rattle it and see if it makes a noise. If it does, it is usually working. Quite a few posts for this online.
Terminology: PCV to me is a Pressure Control Valve, NRV is a Non Return Valve (check valve). a PCV is usually a self regulating diaphragm valve where as a check valve is a ball and spring and will rattle if free.

What you are referring to is simply the check valve, they use PCV as "Positive Crank-case Ventilation" so that should clear it up.
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Old 16th August 2014, 09:19 AM   #14 (permalink)
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Sorry if my explanation is poor, see the following: -



That's basically my setup.

I still can't see how if the venturi is causing the suction which closes the PCV (as tested by sucking on the valve after removing it), is releasing crankcase pressure.

The catch can doesn't need to be on the oil dip stick but apparently for this setup, the breather does. I was thinking of putting a one way valve on there that allows it to breath in but not out.
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Old 16th August 2014, 09:24 AM   #15 (permalink)
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Surely PCV in this context is positive crankcase ventilation? Essentially a check valve to keep blowby gases flowing out of the crankcase to prevent excess pressurisation.

Sounds like your check valve is the wrong way round - you want positive pressure coming out of the block, and charge pressure not going in!
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Old 16th August 2014, 09:37 AM   #16 (permalink)
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Yeah, that's what I thought; obviously the one way valve you see in the image stops the exhaust gases actually getting in through the valve cover but speaking to the guy who built the car for me, it's set up like that for the following reason: -

Quote:
As for the PCV valve by the fuel rail, it is working correctly. The valve is suppose to close when boost is made. This is to stop the boost pressure from entering the crank case via the valve. So if you suck air from the pipe connected to the valve cover, it should close, but if you blow through it, it should be open. The black valve on the exhaust should have an arrow painted on it that shows the direction of flow. It should flow from the valve cover to the exhaust.
I don't really want to keep bothering and I'd rather understand it fully myself.
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Old 16th August 2014, 10:12 AM   #17 (permalink)
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Is the PCV plumbed from the valve cover to the exhaust? If so then the only breathing from the crankcase is via the dipstick so the breather there is for gas to get out, not in - without decent ventilation then it's no surprise the oil's spraying out of the dipstick tube!
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Old 16th August 2014, 10:14 AM   #18 (permalink)
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Yes, that's correct, as the diagram shows. I thought the point of the PCV was to ventilate crankcase pressure? Surely it going to the exhaust and being sucked out from the venturi effect is allowing it to breath? (forgive the ignorance)

http://i.imgur.com/LUKKoj8.png

Otherwise it would just be going straight back into the intake; surely this is allowing it to breath more?
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Old 16th August 2014, 10:46 AM   #19 (permalink)
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Your setup clearly isn't scavenging well enough if there's oil coming out the dipstick. It could be down to the vacuum generated by the venturi nozzle - if the vacuum isn't strong enough then you won't be drawing enough pressure from the crankcase. What size is the line which leads to the exhaust?
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Old 16th August 2014, 10:51 AM   #20 (permalink)
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Can you not post a picture of your actual set up.

For reference, here's mine, there is communication via the crank case and head via what would normally be your oil cap.

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