Looking for 2nd opinion on Weber downdraft idea...

sreams

sreams

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I've been running Weber 32/36 DGVs on my M30B35 for quite a while now, and I believe I have them tuned about as well as they can be.

From what I've read and from my understanding of the CFM requirements of a 3.5L M30, I believe the DGVs are not adequate for the motor at WOT. I believe there is untapped power to be had. The Weber 38 DGES seems like the obvious choice, although I think I might still want to have a progressive setup for the better gas mileage it allows (I currently get a bit over 20mpg on the highway). I'm actually almost happy with the response provided by the primary 32mm barrel, so I'm thinking a 38mm primary would be perfect in a progressive setup.

So, I had the idea of getting the Weber 38s, and swapping the linkages from a spare set of DGVs I have to make them progressive. For those of you who have experience with both carbs, does it look like such a modification could be made? Does it sound like a good idea in order to end up with the best of both worlds (economy vs performance) on a 3.5L M30?
 
Actually... looking at pictures of the two carbs, it doesn't look like the progressive setup could be moved to the DGES carb. Oh well.

-S
 
I've got an old 4-bbl manifold I'm thinking about attaching to my B35. I have several Q-jets in my garage that are candidates for the conversion.
 
I used to run 2xWeber 36/36 DCD on my bone stock Euro 3.0 for more than 10 years and that worked very well. The DCD is a progrssive type with mechanical operated secondaries. Revved happy to 6,500 rpm and put out 25 mpg at best (about 12 at worst - german Autobahn blast with +110 mph average)

Cheers
A
 
That's a long stretch for cylinder's 1 & 6 to get mixture vs. 3 & 4 so some cylinders would run lean or rich depending on setting.

Mike Goble said:
I've got an old 4-bbl manifold I'm thinking about attaching to my B35. I have several Q-jets in my garage that are candidates for the conversion.
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'69 2800cs said:
How about trying to run triple 32/26's? I've never seen it done, but I don't see why not.
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Besides the custom intake manifold issue that stevehose raised, there is also a consideration of total venturi area relative to displacement. For sidedraft carburetors, where each carb. barrel is dedicated to one cylinder, there is a rule of thumb for calculating venturi diameter in mm:

Venturi dia = [ single cylinder volume (cc) x max rpm / 2500 ] ^ 1/2

If you think about it, this limits the maximum average airspeed through the venturi at top rpm, as well as the sets a minimum airspeed at idle. Put too big a carb on a car, and you will get a lumpy idle as the airflow becomes to slow too properly atomize the fuel. I don't know if triple 32/26's would result in too much venturi area, but maybe.

No idea if a similar formula exists for progressive carbs. Progressive carbs introduce the complication of just the primary venturis at idle but both primary + secondary at maximum rpm.
 
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I've been reading a bit more and now I'm unsure as to whether or not the DGVs are adequate.

The CFM calculators I've found suggest the B35 will need 300-375cfm total depending on its volumetric efficiency. Information on the DGV that I've found suggests that it can provide anywhere from 225-300cfm, depending on where you look. Since each carb is servicing half the motor (1714cc), and since the pulses for cylinders 1-3 and 4-6 are evenly spaced (153624 firing order), now I'm starting to think the DGVs should actually be more than enough. Is there some reason why this wouldn't be the case?

-Scott
 
Mike Goble said:
I've got an old 4-bbl manifold I'm thinking about attaching to my B35. I have several Q-jets in my garage that are candidates for the conversion.
Click to expand...

Stevehose said:
That's a long stretch for cylinder's 1 & 6 to get mixture vs. 3 & 4 so some cylinders would run lean or rich depending on setting.
Click to expand...

That happens in any intake manifold, I'm not going to overthink it. I'll just be the only guy on my block with a 4-bbl M30. It will definitely be enough carburetor. 35mm primaries and 58mm secondaries, dual accelerator pump circuits, dual boosters in the primaries, all the really nice stuff that goes with a Q-jet.
 
Mike Goble said:
That happens in any intake manifold...
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Not really. With the dual Zenith manifolds, the difference in runner lengths between 1/3 and 2, and 4/6 and 5 are not very much at all compared to the difference between 1/6 and 3/4 on a single-carb M30 manifold. The sidedraft manifolds have no difference between runner lengths.

Looks like the single-carb runner lengths for 1/6 are almost twice as long as for 3/4:

https://www.bmwcca.org/classifieds/images/listings/8457_1_full.jpg

The Zenith manifolds, by comparison, have much more similar lengths:

http://www.lesliewong.us/bmw/images/solenoids1.jpg

It would be interesting to but an AF meter on each exhaust port when using the single-carb manifold to see what the differences are between the inner and outer cylinders.
 
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sreams said:
It would be interesting to but an AF meter on each exhaust port when using the single-carb manifold to see what the differences are between the inner and outer cylinders.
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In thinking about it, why would a longer manifold run result in a leaner mixture? The carb puts out a specific air/fuel ratio - it has no idea how long the runs are downstream. It isn't as if the carb thinks: "Oh, now #1 or #6 are about to fire - I'm going to release less fuel". Or: "Here comes #3 or #4 - I'm going to give them a bigger shot of fuel".

Now I will agree that a longer run will offer more resistance to the flow of the fuel/air mixture. So #1 & 6 would receive less fuel/air than # 3 & 4. At "half throttle", # 3 & 4 might get 50% of the volume of the WOT fuel/air mixture, while #1 & 6 are getting 46% (say). But, an AF meter wouldn't pick that up.
 
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i'm no expert on this Jay ... but perhaps its a lag issue due to distance traveled for the fuel.
 
I believe the longer the manifold the more condensation of fuel mixture along the walls therefore a disparity in charge from short to long tubes and resulting different mixtures.

jmackro said:
In thinking about it, why would a longer manifold run result in a leaner mixture? The carb puts out a specific air/fuel ratio - it has no idea how long the runs are downstream. It isn't as if the carb thinks: "Oh, now #1 or #6 are about to fire - I'm going to release less fuel". Or: "Here comes #3 or #4 - I'm going to give them a bigger shot of fuel".
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Every wet-flow manifold with unequal runners will have distribution discrepancies. In the case of a single carburetor on an inline six the center two center runners tend to run richer than the outer ones. The Mopar Hyper-Pak /6 manifolds had various tricks for improving the distribution, like dams, turtles and port shaping.

fig211.jpg


fig212.jpg
 
Stevehose said:
I believe the longer the manifold the more condensation of fuel mixture along the walls therefore a disparity in charge from short to long tubes and resulting different mixtures.
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I understand that if the flow rate of the fuel/air mixture is low, and the walls of the manifold are cool, then the atomized fuel can condense out. Fair enough. But, where does it go? Unless the fuel somehow violates the conservation of mass principle, once it's in the #1 or #6 runner, it has to end up in the cylinder eventually.

The Marty Roach article from the Senior Six Register newsletter contains the quote:
All six of the intake charges vary slightly from one another. Air/fuel mixture velocity is different in each intake runner because of the differing plenum volumes and the varying signals present at the jets. Cylinders closest to the carburetor will get richer mixtures than those which are further away.
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I'll buy that resistance of the longer runners (e.g., those going to #1 and #6) would result in slightly lower airspeed at the carburetor and that might make the venturi/jet system less efficient by causing the fuel/air ratio to fall as the airspeed falls. But the difference in flow rate between # 3 & 4, and #1 & 6 would be minimal - I'm guessing a few percent. Also, at 5,000 rpm, the intake pulses have a period of 24 milliseconds - can the carb really react that quickly? So I'm skeptical that variation in A/F ratio is enough to be measurable.
 
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In reading these four barrel chronicles they don't talk about doing serious work to the intake manifold to lessen the effect of going from a square bore carb to a spread bore manifold. Adapters are cool, but even the 4-hole ones are brutal as there are still a lot of sharp angles and chunks of aluminum in the way, as shown in the pic. I've done dyno testing of some square carbs on unmodified manifolds and the results were not good. Removing the entire center would be my first step if I was going to put a square carb on it. Fortunately I have a handful of Q-jets to choose from, and they fit just fine with a little metal removal from the baseplate

IMG_0839_zps9dea48c1.jpg


My manifold seems to have a larger plenum and shorter runners than the one in your pic. Putting a couple of dams between the 2&3 and 4&5 runners would make a difference.

IMG_0838_zps53a5d8c3.jpg
 
Mike Goble said:
Removing the entire center would be my first step if I was going to put a square carb on it.
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From your second image, it appears this is a dual-plane intake. Are the left and right banks separate or are they connected? If they are separate, that adapter on top is turning your dual-plane intake into a single-plane intake, and what's left of the "divider" in the center is now a huge obstruction. That manifold would probably work decently well without the adapter and with a carb that was meant to fit it. If you use it with the adapter, I would definitely grind out all of that metal in the middle.

-S
 
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