Dual Weber 32/36 set up

[Blue Milton Works]

Hi new guy here so please forgive my ignorance. I'm a recent proud owner of a 76' 2002. The car needs a complete overhaul so have been talking with my mechanic about different possibilities. Question #1 is has anyone done a dual downdraft set up with Weber 32/36 carbs? I have not been able to find any intake manifolds available or talked about for such a set up but the car came with a couple extra carbs so I figured let's give it a shot. I run a custom metal fab shop so was planning to make some intake manifolds from scratch. My only hesitation is not finding any previous examples. Any input is greatly appreciated.

[John76]

Hi and welcome!

I have thought many times about the same...similar to a Bavaria, but feeding only 4 cylinders vs 6.

If you use the Weber 32/36, it acts like a single barrel carb (32mm) until the 36mm secondary opens.

I have wondered if you need to have 1 carb feed cylinders 1 and 4, and the 2nd carb feeding cylinders 2 and 3,

with a manifold that has the same length runners for each pair. This would help the exhaust scavenging that occurs when two pistons are moving together (one on the power stroke and the other on the intake stroke).

If you use two Weber 38/38s, you would have a dedicated carb barrel for each cylinder...same as a dual DCOE set-up, only vertical.

In that case, heating the bottom of the intake manifold would greatly help the atomized fuel stay in suspension as it makes the 90 degree turn from vertical to horizontal.

Looking forward to hearing if this has been done too.

[TobyB]

Haven't heard of it on a 2002,

 

but common on straight sixes.  Cannon even made an adaptor

to the SU HS-6 that were installed in pairs on Datsun 240s and

TR6's.

 

I've wanted to try a pair of HS6's on an M10- one feeding 1&4, the other on 2&3.

 

Likewise, splitting a 38/38 the same way makes sense.

 

The Lynx manifold does just that for one DCOE

 

Twin 32/36's would be a lot of carb if they weren't split-

and I don't know how they do with the pulsing that splitting them would create.

 

DCOEs are designed for it.

 

t

twin scroll or go home

 

 

[2002iii]

Don't think it would be worth all your time and trouble to make a manifold and then a throttle linkage and then have to play with the jetting to get it running right. 

 

If I was going to do a custom manifold I would set it up for a 4 barrel holley carburetor. You could get a 390 cfm holley carb meant for a dual setup on a v8. You could start with part of an early 320i intake that mates the runners to the heads then you just need to make a plenum that bolts onto that with a 4 barrel carb flange. I bet if you placed it right you could even adapt the stock throttle linkage to work.

[AustrianVespaGuy]

I'm pretty confident in saying the closest setup to what you're describing would be a single DCOE on a Lynx manifold, which is just a great setup IMO, only caveat being air cleaner-to-brake booster clearance:

https://www.racetep.com/manufacturer/carbs-and-injection/weber/conversion-kits/bmw-2002-320i-m10-engine-single-sidedraft-dcoe-conversion-kits.html

[avoirdupois]

12 hours ago, TobyB said:

Haven't heard of it on a 2002,

 

but common on straight sixes.  Cannon even made an adaptor

to the SU HS-6 that were installed in pairs on Datsun 240s and

TR6's.

 

I've wanted to try a pair of HS6's on an M10- one feeding 1&4, the other on 2&3.

 

Likewise, splitting a 38/38 the same way makes sense.

 

The Lynx manifold does just that for one DCOE

 

Twin 32/36's would be a lot of carb if they weren't split-

and I don't know how they do with the pulsing that splitting them would create.

 

DCOEs are designed for it.

 

t

 

 

 

Doable, but the energy/labor expenditure might not justify the effort.  Depending upon manifold design, space could be an issue.  No readily available off-the-shelf throttle connections, etc..

 

 

Edited January 18, 2021 by avoirdupois

[Blue Milton Works]

23 hours ago, John76 said:

Hi and welcome!

I have thought many times about the same...similar to a Bavaria, but feeding only 4 cylinders vs 6.

If you use the Weber 32/36, it acts like a single barrel carb (32mm) until the 36mm secondary opens.

I have wondered if you need to have 1 carb feed cylinders 1 and 4, and the 2nd carb feeding cylinders 2 and 3,

with a manifold that has the same length runners for each pair. This would help the exhaust scavenging that occurs when two pistons are moving together (one on the power stroke and the other on the intake stroke).

If you use two Weber 38/38s, you would have a dedicated carb barrel for each cylinder...same as a dual DCOE set-up, only vertical.

In that case, heating the bottom of the intake manifold would greatly help the atomized fuel stay in suspension as it makes the 90 degree turn from vertical to horizontal.

Looking forward to hearing if this has been done too.

Thanks John 

That makes a lot of sense. The complicated "plumbing" could very likely why it's not an off the shelf option for the M10 

[Blue Milton Works]

Thanks Guys for all the input. I think it looks like I should temper my exuberance for dual carb set up for now. There is quite a long list of other areas that need attention without making more problems to solve. I may still fiddle with some pie cut plumbing since I went ahead and laser cut flanges and made a mock up jig. Thanks again all , Cheers

[Dudeland]

My TBI EFI has dual 50mm barrels and it is way too much for my 75, and it has most of the bolt on goodies that you can get.   Remember that the carb is progressive, so most of the time you drive around on a single barrel.   People that have put in the bit that makes them syconus and most do not like the result.  I couldn't imagine need 2 of them. 

   

[John76]

4 hours ago, Dudeland said:

I couldn't imagine need 2 of them.

 

Sorry, but I disagree.

Puttering around on a 32mm single barrel (32/36 primary) feeding 4 x 500cc cylinders is nothing.

I have a 1000cc 2-cylinder BMW motorcycle and each cylinder is fed with a 40mm carb. 

I would love to duplicate the 65hp twin with my 2000cc inline 4 (should be about 130hp).

Both machines are big bore, short stroke with big valves and almost identical cams...but cycle has 9.5:1 compression vs. 8.3:1 on the 2002.

2 x 32/36 carbs should be a nice compromise....just short of 2 x 40mm DCOE's. The only roadblock I see is the design of the manifold to have equal length runners to cylinders 1 & 4 and 2&3....with heating.

 

[2002iii]

1 hour ago, John76 said:

 

Sorry, but I disagree.

Puttering around on a 32mm single barrel (32/36 primary) feeding 4 x 500cc cylinders is nothing.

I have a 1000cc 2-cylinder BMW motorcycle and each cylinder is fed with a 40mm carb. 

I would love to duplicate the 65hp twin with my 2000cc inline 4 (should be about 130hp).

Both machines are big bore, short stroke with big valves and almost identical cams...but cycle has 9.5:1 compression vs. 8.3:1 on the 2002.

2 x 32/36 carbs should be a nice compromise....just short of 2 x 40mm DCOE's. The only roadblock I see is the design of the manifold to have equal length runners to cylinders 1 & 4 and 2&3....with heating.

 

I would have to agree with this since I have dual weber 40 Dcoe Carbs and stahl header on my stock Factory BMW rebuilt 2.0l e12 head engine, It runs great.

 

There is a reason no one has done dual 32/36's before, it's not worth the time, effort and money it would take to engineer and build a manifold and linkage for them and spend the time figuring correct jetting. Its easier to just buy a single or dual Weber's from redline weber.

[zinz]

Here ya go.. dual downdraft Weber set up for a Triumph TR6...linkage and all on fleaBay, but I bet Redline sells the kit.  You will have to fabricate your own intake...

 

Edited January 21, 2021 by zinz

[Lorin]

10 hours ago, John76 said:

 

Sorry, but I disagree.

Puttering around on a 32mm single barrel (32/36 primary) feeding 4 x 500cc cylinders is nothing.

I have a 1000cc 2-cylinder BMW motorcycle and each cylinder is fed with a 40mm carb. 

I would love to duplicate the 65hp twin with my 2000cc inline 4 (should be about 130hp).

Both machines are big bore, short stroke with big valves and almost identical cams...but cycle has 9.5:1 compression vs. 8.3:1 on the 2002.

2 x 32/36 carbs should be a nice compromise....just short of 2 x 40mm DCOE's. The only roadblock I see is the design of the manifold to have equal length runners to cylinders 1 & 4 and 2&3....with heating.

 

You are comparing apples and oranges. Only 1 cylinder pulls at a time. 40mm per cyl is great, like dcoe webers or your  bmw cycle. 
 

When you add a plenum, each cyl draws from the entire carb volume at once. A 32/36 is Way bigger than one 40mm barrel. 
 

Fun project for the sake of being different, you could always downsize to a smaller 2 barrel weber if the signal in carb was too weak with only 2 cyl pulling on it. 
 

 

[Dudeland]

14 hours ago, John76 said:

 

Sorry, but I disagree.

Puttering around on a 32mm single barrel (32/36 primary) feeding 4 x 500cc cylinders is nothing.

I have a 1000cc 2-cylinder BMW motorcycle and each cylinder is fed with a 40mm carb. 

I would love to duplicate the 65hp twin with my 2000cc inline 4 (should be about 130hp).

Both machines are big bore, short stroke with big valves and almost identical cams...but cycle has 9.5:1 compression vs. 8.3:1 on the 2002.

2 x 32/36 carbs should be a nice compromise....just short of 2 x 40mm DCOE's. The only roadblock I see is the design of the manifold to have equal length runners to cylinders 1 & 4 and 2&3....with heating.

 

I got thinking about this after my reply and probably why my 2x50mm seems like too much is because I can easily run too much fuel.   I also find with the TBI not being progressive, it is a little jumpy off the line.  I would much prefer a progressive setup. 

 

With dual 32/36 you will have to have a common plenum for each carb,  and of course you are going to have to figure out how to balance them. 

 

 

[John76]

7 hours ago, zinz said:

dual downdraft Weber set up

Yeah, that's the ticket!  Hope Blue Milton Works can build a nice manifold for a 4-banger.