Intake Manifolds

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Marc W

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I ran across an interesting article on Sonerai.net. The author described a mod to the GPAS intake manifolds that he claimed improved idle and gained him 25-50 RPM at WOT. What he did was remove the divider inside the manifolds that separates the front and rear intake ports. He has a slide valve carb so he doesn't know how it would work with the Zenith. I have a Zenith but I might give it a try. Mine doesn't idle as smooth as I would like when its cold but it idles decent when it is hot. I wouldn't do it just to improve idle. The additional 25-50 RPM at WOT throttle has my attention though. I would do it for 25-50 RPM!

The article is at: https://cloud-data.nyc3.cdn.digitaloceanspaces.com/sonerai/data/articles/vw_tips_part 1.pdf
 

fly2kads

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The typical AeroVee installation apparently has uneven fuel flow, as well. Sonex owner Patrik Jonsson went to a lot of trouble to rectify this, creating a plenum system that feeds four individual pipes. His blog covers a lot of fabrication and testing details. He evened out his mixtures and picked up somewhere around 80 rpm in the process. Not too shabby!

Video tour of the plenum system:

His blog entries:
Sonex posts – Patrik's projects

Aircraft intake (and exhaust) systems are often designed more from a packaging and operational perspective than performance. The volumetric efficiencies of typical VW installations reflect this, falling in the range of 73-80%. There's clearly a lot of room for improvement. The VW car guys are getting VEs in the range of 90-94%, even at the rpm range we operate in. That's an apples-to-oranges comparison, but lets us know we can do better.
 

Daleandee

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Aircraft intake (and exhaust) systems are often designed more from a packaging and operational perspective than performance. The volumetric efficiencies of typical VW installations reflect this, falling in the range of 73-80%. There's clearly a lot of room for improvement.
Perhaps Pops will get in on this thread as he has gobs of VW experience!

The above video disturbs me a little as I remember the time I flew with a Red Menance slide carb that was held on by a hose clamp & some bailing (safety) wire. Them days are gone. Those carbs are truly a controlled fuel leak and do little to atomize the fuel entering the intake air stream.

Add to that the fact that the Aerovee intake manifolds are a pretty horrible design as they aren't even near the same shape as the ports in the head.

1592715103861.png

The original VW designed manifolds would seem to me to be a much better choice ... along with a carb that actually helped atomize the fuel.
 

TFF

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Usually if something like that works, there is not enough capacity in the manifold so being able to share just a bit more air/fuel makes it work. Easy to try and if it doesn’t work cheap enough to put back.

Just like any mod, it depends on everything else in the system. One thing may or may not work with one’s engine. Doesn’t mean it does not work overall, just not yours. Opposite, might work on yours and not someone else’s.

I would try it.
 

Mike Stewart

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"Add to that the fact that the Aerovee intake manifolds are a pretty horrible design as they aren't even near the same shape as the ports in the head."

Yeah. AeroVee is dune buggy stuff.

Steve Bennett used my plane (when he owned it) as a testbed for induction experimenting. I've got a top mounted Ellison EFS-2 and Steve's intake manifold design. I imagine that became the GPAS standard design, at least for top mounted carbs. I've found fuel distribution to be equal, based on what I found in the cylinders when I re-did the heads.
 

Dana

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The big problem, I think, is the l-o-o-o-n-g intake, so the mixture has plenty of chance to cool and condense on the manifold walls, naturally it'll be worse when the engine is cold. One common solution is to heat the manifold... Pops had a box sandwiched between the carburetor and manifold with hot oil from the oil cooler line circulating through it. Lots of other guys just wrap some aluminum tubing around the manifold close to the carburetor. I was going to try that on the Mosler in my Fisher, even bought the tubing, but I sold the plane before I got around to it.
 

Marc W

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No doubt Pops oil heated intake improves mixture quality and distribution. That comes at the cost of more tubing, fittings, hoses and space in the engine compartment. Pops is also running single port heads so that might make a difference. Reworking the manifold is a far simpler solution if it works.

I am finishing up some modifications on my CX4. One thing I did was to change the exhaust from 4 into 2, to 4 straight pipes. I have an O2 sensor that I had just behind the Y on the left side of the engine. With the straight pipes I installed it in the front pipe on the right side. I ran the engine yesterday for the first time with the O2 sensor in the front pipe. That cylinder ran lean and it would not richen up at idle no matter what I did to the idle mixture. I need more fuel to run it anymore but I am going to run it again to get some better numbers and then modify the manifolds and see what difference it makes to the mixture in that front cylinder.
 

TiPi

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With the straight pipes I installed it in the front pipe on the right side. I ran the engine yesterday for the first time with the O2 sensor in the front pipe. That cylinder ran lean and it would not richen up at idle no matter what I did to the idle mixture. I need more fuel to run it anymore but I am going to run it again to get some better numbers and then modify the manifolds and see what difference it makes to the mixture in that front cylinde
Before you change your settings, add a length of exhaust pipe to the pipe with the O2 sensor. You need at least 300mm, preferably 500mm of pipe AFTER the O2 sensor. The pulsing of a single cylinder in a short pipe will draw air back in, giving you a lean reading.
 

Pops

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I have no doubt the mod to the intake manifold will help in getting equal mixture in each cylinder. Its going to need some good testing of different stages of the mod to get it a close as possible. You have the same problem when using the single port heads that I use. Posting a picture of the single port intake .
The end with the cap is to provide turbulence for a more equal mixture in each cylinder. Its really a trial and error type of thing. I got it close but not perfect.
The front to cylinders run slightly rich. Not very much by the color of the plugs but there is a slight difference. The CHT of the cylinders are very close. Of course, the engine baffling and airflow above the engine will have an effect on the temps. But, I consistency have the slight darker color plugs on the front cylinders. Also the turbulence is going be difference at difference rpm's. So I tried to get even colors on the spark plugs at the 2650-2700 cruise rpm.

The intakes are 1 1/4" dia Stainless Steel bathroom handrails for the aviation section of the Lowes Box store. Its also 1 1/4" from the center of the down tube to the head to the inside of the end cap. The length is critical and so will the mods on the dual port intakes.
Single port heads on a 1835 cc VW engine.
 

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Marc W

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There was a change. It just didn't fatten up as much as I would like. I didn't get to it today. It is first on the agenda tomorrow.

Also the fuel tank is nearly dry. I may have just been running out of gas.
 
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Flyguyeddy

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Pops, why not just run some mandrel bent tubing straight into the port, like youd see on the vw car?

I have two sets of early monnet manifolds (they are a big open cavity like the OP article suggests doing) but with freeze plugs on the ends. Plan on using them on my sonerai, but with welded ends instead of freeze plugs.
 

Mike Stewart

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Pops mentions dark plug colors. Mine are always a light tan. The slight deposits on head and top of piston were also a light tan. I run very lean at cruise, up high. Pulling probably about 50% power at 11 - 14K feet. I have an "E" bottle for oxygen and use it from takeoff to landing since the pulse-dose regulator and Oximizer cannula makes a bottle of O2 last clear across the country and back.

The Ellison seems to lean on its own to a large extent. I've never understood just how that works but once I've leaned manually while climbing to altitude, if I touch the mixture the engine will start coughing. The Ellison has a mind of its own. It's also amazingly trouble free. In 14 years the only adjustment I've ever made was to up the idle a notch early on. Haven't touched it since. Wonderful carb. EFS-2
 

Pops

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Not dark. Just a little darker, more like a little darker brown than the rear cylinders. Just slightly different. Tried to get the colors exactly the same at cruise rpm's.
 

Pops

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Pops, why not just run some mandrel bent tubing straight into the port, like youd see on the vw car?

I have two sets of early monnet manifolds (they are a big open cavity like the OP article suggests doing) but with freeze plugs on the ends. Plan on using them on my sonerai, but with welded ends instead of freeze plugs.
With the horizontal part of the intake tube coming from the rear, the front cylinder will run a lot richer than the rear. In the auto the intake is coming from the center of the engine. So both cylinders tend to get the same charge.
 

Pops

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Or perhaps something like this but single port?
The front cylinders will run richer than the rear cylinders.
As the charge goes around the 90 deg bent it tends to be richer on the outside of the bent and leaner on the inside of the bend. It would work IF you had some turbulence before going into the head to equal the charge in the ID of the tube.
 
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