Cut in between manifold cylinders

[docrav46]

Have a very strange question.
Why is there a cut between two manifold ?
Picture has been attached
Thank u

[bnam]

Docrav,
Here's my reasoning:

First some background data on intake cam timing and duration. A quick search yielded the following info. I don't know if it is exact, but is directionally sufficient.

intake opens 7.5deg Before TDC and closes 37deg After BDC. It remains open a bit after BDC as the momentum of the AF mixture has built up at the bottom part of the suction stroke and the AF mixture keeps filling up the cylinder even though the piston has started moving upwards.

Now let's look at where the cylinders are in the cycle when Cyl 1 is at TDC at the start of the suction stroke.
Cyl 1 - TDC at start of suction stroke
Cyl 4 - BDC at start of exhaust stroke
Cyl 3 - TDC at start of combustion stroke
Cyl 2 - BDC at end of suction stroke/start of compression stroke

Let's look at just Cyl 1 and 2. When Cyl 1 is near TDC before the suction stroke, the intake is about to open (at 7.5 deg BTDC). At this point Cyl 2 is still completing its suction stroke and the intake valve there is still open for another 44.5 (7.5+37) degrees. The AF mixture momentum is keeping the AF going towards Cyl 2 even as Cyl 2 intake closes further.

Since Cyl 1 intake has started opening, the small channel between 1 and 2 allows the AF flow to start diverting from the #2 manifold into the #1 cylinder and gets the AF flowing into #1 faster.

Similarly #3 benefits from #4.

#2/#4 does not get the same benefit from #1/#3. When #2 is at TDC and intake valve has started opening, #1 was in the exhaust cycle and there was no AF mixture movement in that branch of the manifold that can aid #2. Same for the other pair.

Don't know if this is the correct explanation. Just my reasoning based on me trying to explain why it would be there.

(Sometimes the best way to get the correct answer is to post the wrong one)

Byas
_________________
1971 1302LS Convertible (RHD) owned since '74
Click to view image
1965 Karmann Ghia Coupe - under restoration
1966 Fiat 1500 Cabrio (with 1600 Twin cam)
1952 Citroen TA 11BL

[docrav46]

[VW_Jimbo]

Balancing the intake charge between the two ports. That's my guess. Kind of like running the balancing tube between the two sides on dual carbs.
_________________
Jimbo

There is never enough time to do it right the first time, but all the time necessary the second time!

[viiking]

I agree with VW_Jimbo although I would call it more like a "scavenging" device to remove unusable fuel from the manifold side that is NOT needing the fuel.

My simplistic explanation is that the fuel from the single carburettor is available for both sides of the manifold but will preferentially go into the manifold where the inlet valve is open due to the vacuum. However some must still be able to enter the static manifold which may remain albeit very temporarily in an unburnt "slug". When its intake valve does open, you may get a temporarily rich fuel mixture in that cylinder until it flushes through the new fuel.

The split between the two manifolds allows this residual slug to be removed from the static manifold i.e. a balancing/scavenging tube.
_________________
1968 1500 RHD Lotus White Beetle since birth. In the hospital for major surgery
1966 Lancia Flavia Pininfarina Coupe - in the waiting room
Discharged: 1983 Vanagon, 1974 1800 Microbus,1968 Low Light,1968 Type 3

[Multi69s]

The bottom line is that it was an attempt to even out the idle, due to the fact that the VW engine have exceptionally long intake runners on the intake manifold.
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73 Squareback - 2L, T4, Automatic W/ AC
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[bnam]

[viiking]

[bnam]

I get the first part. But not the second part - “slug” causing lumpy idle - for 2 reasons.

First, Even if there is a slug there - the new incoming share for cylinder 2 will face the same effect and only 95pct will come down the cyl 2 branch with the remainder going to cyl 1. So it effectively gets a full charge. Even if the AF slug were present early, ignition only happens much later - post compression. So it should not matter.
Second, the same effect happens on all cylinders so should be an even effect.
_________________
1971 1302LS Convertible (RHD) owned since '74
Click to view image
1965 Karmann Ghia Coupe - under restoration
1966 Fiat 1500 Cabrio (with 1600 Twin cam)
1952 Citroen TA 11BL