Inlet Manifold oil vapor recycling inlets?

Road-race engines and ancillaries - general discussion
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Brit01
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Inlet Manifold oil vapor recycling inlets?

Post by Brit01 » Wed Dec 14, 2011 5:58 pm

Hi,

I have a doubt about the small oil vapor pipes on the inlet manifolds on my Alfa flat 4 boxer.

Each inlet manifold has one small pipe on each side which connects to the oil vapor circuit (or did in my case until the original plastic connector shattered into small pieces).

These were designed I assume to reduce emissions. Smaller ones on the inlet manifolds for when the engine is idling and the larger pipes on the air intakes for when the throttle is open.

I will be installing an oil catch tank (vacuum assisted with the larger pipes that exit from the air intakes).

Now I don't see any reason to maintain these smaller pipes from the inlet manifold connected to the oil vapor circuit.

I am planning to block them off.

Does anyone know of any good reasons not to block these off?

Regards

Chris
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Guy Croft
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Re: Inlet Manifold oil vapor recycling inlets?

Post by Guy Croft » Fri Dec 16, 2011 10:12 am

When using a catchtank to collect oil vapor/gas from the crankcase directly all suction ports on manifold and carb must be blanked off otherwise they will draw unmetered air and make the engine burn lean.

G

Brit01
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Re: Inlet Manifold oil vapor recycling inlets?

Post by Brit01 » Fri Dec 16, 2011 11:04 am

When using a catchtank to collect oil vapor/gas from the crankcase directly all suction ports on manifold and carb must be blanked off otherwise they will draw unmetered air and make the engine burn lean.
Thanks Guy. Yes I was thinking along those lines. I will block them off.

Also due to the really bad quality of fuel here I need to keep the mixture as clean as possible and don't want additional oil contaminants entering the combustion chamber.
It varies from fill to fill and also has a % of alcohol in it.
They state the premium petrol is 97 RON. It is imported from Brazil.

Regards,

Chris

WhizzMan
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Re: Inlet Manifold oil vapor recycling inlets?

Post by WhizzMan » Thu Dec 22, 2011 9:16 am

Be aware that 99% of what you suck out of your engine is gaseous. Your engine has been designed ground up with the idea that you would be having an under-pressure in your crank case from the carter ventilation. All the gasses you are no longer sucking out, including all the water vapor in them, will now remain in the crank case, unless over-pressure will push them out. That means that, effectively, you will have your oil contaminated at a much higher rate than before. For a race car, this is of no matter, since you only have the oil in the engine for maybe one race weekend. For a road car, you will be wearing your engine a lot quicker, or you'll be spending a lot more money on monthly oil changes.

I've seen people do this modification on road cars several times. Mostly because the oil they sucked in was to be believed to be bad for performance. On a road car, if you get noticeable performance loss from oil being sucked in from your carter ventilation, you usually have your piston rings or cylinder walls totally worn out. Yes, you will get knocking and your ECU will react on that, but the problem is not the carter ventilation, it's your engine needing a rebuild.
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Brit01
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Re: Inlet Manifold oil vapor recycling inlets?

Post by Brit01 » Thu Dec 22, 2011 11:06 am

Hi whizzman,

Yes I understand that the crank benefits from having a vacuum to pull out these gases and have seen/felt the consequences of lack of under pressure in my crank on 1 occasion.

These smaller inlets provide a small amount of vacuum only when idling.
These are the ones I'm thinking of blocking off.

The main vacuum pipes from the air intakes will remain in place providing ventilation and an under pressure in the crank.

So I thinking along the lines that at idle these smaller vacuum pipes are not needed.

One important note about what you said being designed from the ground up.
The boxer engine did not really change in design over the years.
The original design used on the alfasud had an open ventilation design with no vacuum.
Only when emission tests were introduced then this recycling system of gases was added to the boxer engine.

Regards

Chris

TomLouwrier
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Re: Inlet Manifold oil vapor recycling inlets?

Post by TomLouwrier » Thu Dec 22, 2011 1:18 pm

hi guys

My engine has a similar setup (EU spec 1608 with twin 40IDF's). The vacuum lines connected to the carbs are opened and shut by a disc valve within the carb body, on the butterfly valve shaft. The purpose of this is that critical idle setup is not offset by false air coming in from the crank case. Once the carbs are opening up, the crank case gases are sucked in and burnt off without upsetting the mixture and running.

The larger lines coming from under the air filter actually do not suck vapour out at all, since there should be very little pressure drop over the filter (it would be severely clogged up). This / these lines allow clean filtered air into the crank case.

The system is called 'positive carter ventilation', circulating fresh air in and extracting vapour out. You are best off leaving it intact.

regards
Tom
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Brit01
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Re: Inlet Manifold oil vapor recycling inlets?

Post by Brit01 » Thu Dec 22, 2011 4:46 pm

I think the Alfa set up is quite different.

The larger air pipes attached to the oil filler pipe come from the air intake between the carbs and filter so there is a high vacuum here especially when the carbs are fully open.
A large volume of air flows through the air intakes here sucking out the gases.
You can see this by the oil film on the pipes.

The smaller pipes are below the carb and suck when it's idling (not much as the hole in the brass pipe is a pinhole).


Chris

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