2240310 1972 BMW 3.0cs

[Ohmess]

Pretty good price on those!

Still trying to identify a cost effective way to fabricate my version. Xometry wants about $400 each for aluminum printed parts.

Most of the sand cast parts are going to be about like the Karsundparts and dbilas parts. Looks like the dbilas parts are SLIGHTLY nicer.
I think any sand cast part, and possibly a metal printed part will need to have the flange surfaces milled. I'll probably start there with the Karsundpartz units, just to get the car operational. That will be next year sometime.

I'll need to add a boss for the vacuum fitting... Has anyone explored if having one carb with the vacuum fitting causes any issues? Seems like as the brake booster cycles it would momentarily change the AFR for that one cylinder. If so, would it be better to spread that effect out across all six intakes?

In the meantime I ordered a set of three of the above units in ABS, to use as a fit-check. ABS was $53 for three!

Korman spread the manifold vacuum take off across two cylinders by putting fittings on two different intake runners.

As to the top to bottom differential on the manifolds, there may be reasons for that. The stresses would be higher on the top mounts, so they might be fatter to accommodate more stress. Alternatively, they might be attempting to get a particular angle on the carbs when mounted. I recall @Stevehose had some performance issue in right hand turns with his VGS setup because the manifolds were canted up slightly to allow for more clearance over the brake booster. When he turned right (or was it left?) the fuel in the bowls drained more quickly than the needle/seat allowed them to refill.

Just spitballing here, but if I were having those surfaces machined, I would think you want that surface at whatever angle creates a level surface on the fuel in the bowls.

 

[ScottAndrews]

According to @Stevehose the optimal angle for the carbs is tilted up 5 degrees. The Karsundpartz manifolds are angled down at 18 degrees. I originally thought the cylinder head would be 30 degrees from vertical because the engine is canted over 30 degrees, but apparently the intake side of the head is not 90 degrees from the plane of the top of the block, so a potion of the 30 degree tilt is taken up by the head, which means the manifolds do not need to be 25 degrees to get a 5 degree tilt on the carbs. I'll probably wait until my engine is in the car and then measure the angle of the intake side of the head.

It is also possible that the 5 degree tilt was to prevent any fuel from leaking out the front of the carbs, but that that tilt created a slight difference in fuel delivery in right vs left turns. Seems like Webers have been used a lot in racing, so I would be surprised if this starvation was a common issue.

 

[ScottAndrews]

I have spent quite a bit f time over the last year or so pondering the best configuration for the triple DCOE setup. I started out buying a set of Karsundpartz manifolds from Pierce. Right out of the box I was disappointed. The finish quality is poor, and from @boonies experience it sounds like the issue isn't just finish, but dimensions.

There is a lot that can go wrong with these parts.

First the flange faces on both sides (head and carb must be co-planar). If they are not, then the manifold will tilt to one side on the head, causing a leak, or the carb will tilt on the manifold, also causing a leak. The lack of co-planarness can show up as an angle between the two flange faces, or as a change in height, or both.

Second the flange mating surface must be flat and smooth. If it is bowed or rough, then again the potential for a leak is high.

Here is a sketch of how things can go wrong. This is also a good argument for keeping the manifolds in pairs versus making one big manifold. A big six hole manifold will have two sets of six flanges, and all six of each set must be flat and coplanar..

I think the right solution is to do what Lance was doing, but maybe with more precision than sanding on a flat table. Ideally you need to put the part on a mill and flycut the faces of one set of flanges so they are co-planar and smooth, then turn the manifold over and set it at the proper angle, and flycut the flanges on the other side. I would do this with sand cast parts as well as 3D printed parts

I also think that if you are going to all the trouble to have independent throttle bodies (which is what triple Webers actually are, then the vacuum for the brake booster shoudl be distributed across all of the carbs. That will spread out the vacuum effect on mixture and will also assure than any variation is not isolated to a single cylinder. I learned this the hard way on a Ford V8 I built. In that car I had connected the brake booster host to a port on the intake manifold. The car always had a little misfire shudder in the background. One day a friend pointed out that usually the booster vacuum comes off the base of the carb. I changed that and voila! no more misfire...So it seems you want to get the vacuum from all of the intake runners, not just one. On the CSI cars they draw it from the manifold plenum. The other way might be to use a small vacuum pump instead of the intake, but that's getting pretty far afield of original...

 

[boonies]

According to @Stevehose the optimal angle for the carbs is tilted up 5 degrees. The Karsundpartz manifolds are angled down at 18 degrees. I originally thought the cylinder head would be 30 degrees from vertical because the engine is canted over 30 degrees, but apparently the intake side of the head is not 90 degrees from the plane of the top of the block, so a potion of the 30 degree tilt is taken up by the head, which means the manifolds do not need to be 25 degrees to get a 5 degree tilt on the carbs. I'll probably wait until my engine is in the car and then measure the angle of the intake side of the head.

It is also possible that the 5 degree tilt was to prevent any fuel from leaking out the front of the carbs, but that that tilt created a slight difference in fuel delivery in right vs left turns. Seems like Webers have been used a lot in racing, so I would be surprised if this starvation was a common issue.

The angle of the engine is 24.7 degrees (pic below) - 18 degrees (K&P manifold) = carb. angle of ~6.7 degrees
-18 degrees

 

[ScottAndrews]

The angle of the engine is 24.7 degrees (pic below) - 18 degrees (K&P manifold) = carb. angle of ~6.7 degrees
-18 degrees

Yay! I was hoping you could make that measurement!

So the optimal angle for the carbs at ZERO tilt would be 24.7, and for 5 degrees would be 19.3.

I'll research the 5 degree recommendation. Not sure if that's official or a wive's tale...

Is that a 3 liter engine? Looks like the dip at the top is about the same as the K&P manifold, which is WAY too big for the M90. The M90 throats are significantly larger as well. I'll probably end up with several models, one for the M90, and one for the 3.0 liter M30

 

[boonies]

It is the original engine, but a 1977 manufacture head, I found a picture I grabbed while dis-assembling for the engine compartment refresh and engine rebuild.

 

[ScottAndrews]

It is the original engine, but a 1977 manufacture head, I found a picture I grabbed while dis-assembling for the engine compartment refresh and engine rebuild.

Interesting to see the thermostat housing offset in the wild..

I noticed that you are using a stock distributor. IIRC @Stevehose was using a 123 Dizzy. I know you have been fighting vacuum leaks, but I also wonder if some of you ills maybe timing related. I know Steve spent a lot of time tuning the advance curves..

 

[boonies]

That picture is quite old, I have been using the 123 for several years.