Intake Manifolds

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Flyguyeddy

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Mar 28, 2020
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Carburetors don't really belong on VW's, nor do they on the diminutive AO84's, because of those long intake tubes. Keep the tubes, but replace the carb with a throttle body and modify the heads to accept injectors, then get an SDS (or Megasquirt, or an Arduino, or...) fuel injection system. That is the only way to get the most equal charges in all 4 cylinders with the least amount of fuss.
That doesnt work so well when you have a single port head, unless one injector will feed two cylinders.
 

Flyguyeddy

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That doesnt work so well when you have a single port head, unless one injector will feed two cylinders.
Also, the corvair uses a standard aviation carburetor and id say they have a longer intake tract than the vw
 

TFF

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One injector can feed both. That’s no big deal. That is a whole different system so it does not solve making a carb work. And a carb can work just fine.
 

Pops

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Pops, what carburetor are you running, posa? Aerovee? Why is extra turbulence necessary in the intake tract?
Zenith
Turbulence necessary to get equal charge into each cylinder. Using 1 1/4" dia intakes tubes for the single port heads and for a higher intake velocity for more torque at lower RPM's for a straight drive engine. This higher velocity tends to makes the charge to go to the outside wall of the ID of the tube in a bend. Also makes the Zenith a little more prone for carb ice and for the charge to condense into droplets before getting to the head. The hot oil box around the intake at the base of the carb solved the carb ice and condensing of the charge plus cools the oil 20 degrees. Made the engine smoother with more power.
I am very close to having equal charge in each cylinder, but not perfect. I want to take the spark plugs out and see NO difference in color of each plug.
Dan
 
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Marc W

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I wonder if a balance tube between the two manifolds would help? Has anybody tried that?

I did a Google search and the Rotax 912 has a balance tube. Some people have put larger tubes in and got smoother idle. The Rotax is a little different since it runs two carbs. I am going to look at this more.
 
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Daleandee

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Also, the corvair uses a standard aviation carburetor and id say they have a longer intake tract than the vw
Corvairs converted for flight have quite a long intake from each side down to a single carb. I get quite an even burn on all my plugs and the CHTs & EGTs are very close from side to side (not perfect though). It should be pointed out that the WW conversion utilizes the stock GM intake manifold which is offset differently on each side:

Stock location: https://qph.fs.quoracdn.net/main-qimg-f2c46676ebc07cb7da5fa6e4760b4538-c

Flight Engine: https://flycorvair.files.wordpress.com/2019/02/img_6833.jpg?w=630
 

Flyguyeddy

Active Member
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Mar 28, 2020
Messages
29
I wonder if a balance tube between the two manifolds would help? Has anybody tried that?

I did a Google search and the Rotax 912 has a balance tube. Some people have put larger tubes in and got smoother idle. The Rotax is a little different since it runs two carbs. I am going to look at this more.
That was discussed long ago in the sonerai newsletters (my main source of older vw info) but nobody does this anymore that i have seen
 

Mike Stewart

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Oct 11, 2010
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I just posted an Ellison EFS-2 for sale for $500 in the For Sale forum in case anyone's interested. Belongs to a hangar neighbor.
 

Marc W

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Mar 31, 2017
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Colorado
I have now flown the airplane twice with the intakes modified as shown here.
1594593929680.png

I gained over 100 RPM in the climb at 80 MPH. Unfortunately, I made several other changes so I don't know how much of the improvement I can credit to the change in the intake manifold.

The airplane was down because of an engine problem. It had a small leak in an exhaust valve, one rocker arm was riding against a head stud so valve adjustments on that cylinder are suspect and the magneto was failing. I also replaced the spark plugs, installed a spinner and changed the exhaust system. Way to many variables!

The engine does idle smoother and I do think removing the web in the exhaust manifold made a difference. I just don't know how much. I still need to time the climb rate to see if it improved to any significant degree. I didn't do any WOT level flight speed runs either but I expect some improvement there.
 
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