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DOHC motor needs plenum mod 

 

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 Post subject: DOHC motor needs plenum mod
Posted: Mon Dec 20, 2010 6:56 am 
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Hello i have a 2.3 ltr DOHC 4 cylinder motor.

Technical question.
My carby is 2 bbl. My head is 16v straight 4 cylinder.
The design of the intake manifold allows for the primary venturi to feed 1 intake valve in each head chamber, the secondary venturi feeds 1 intake valve per head chamber aswell.
There is no plenum chamber to equate the 2 feeds.
My problem is this, it seems stupid because the valves on the secondary dont get hardly any fuel during driving.Also to make matters worse they have a small breather feed from the crankcase just under the secondary butterfly.

I took off the carby today looking for problems and put a LED light down the 2 seperate feeds from the throttle body.
Primary feed is in good condition.Secondary feed has carbon and crusted oil deposit as far as i could see.

I have a leaky valve already and i am thinking if cutting out the spacer plate under the crby to allow for a plenum chamber to unify the charged mixture.
Anyone had any exsperience with this design , it seems silly to me .
most of my car work was 20 years ago with 5ltr V8 and a plenum chamber was actually a good design .Whats going on with this design. I wanna hack it and do the plenum mod?
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 Post subject: Re: DOHC motor needs plenum mod
Posted: Mon Dec 20, 2010 5:00 pm 
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I understand that this design is used for better low revs by utilizing only 1 valve and 1 manifold runner per cylinder , alot of engines carry this design.there is no internal butterfly to unify the charge at high revs. My main concern is distributing the crank case gases that have clogged out the walls of the the manifolds secondary system.
Its really quite bad and i can only pressume that the valves at the end of the secondary runners are coated aswell.
PLus my van is a constant LPG/Autogas runner so i fear that my secondary valves in each cylinder are not getting the cooling/washing effect of the air/fuel mixture aswell as the crank fume coating.
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 Post subject: Re: DOHC motor needs plenum mod
Posted: Mon Dec 20, 2010 6:09 pm 
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Joined: 19th Aug 2010

Location: North
Oustide Australia

Yes i do see the merit in such a well thought out design.
This main problem with the breather being ported directly under the secondary throttle body seems inadequate for the LPG/Autogas system.maybe if fuel was used it would be washed down and self cleaned.
Still need as many thoughts on how i can resolve this.
Maybe i can plumb another breather feed into the active primary throttle for the crank vacum.

I should show that there are 2 crank breathers.
1 is a small 6mm hose that is feed directly under the secondary throttle.
2 is the usual 10/12mm hose that goes on top of the air cleaner and is directed into the intake before the carby venturi.The 6mm hose is my problem.
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