DCOE 40s or 45s???

[visionaut]

DCOE 40s & 45s share the same basic construction. They both take the same jets, brass fittings and gasket set.  The chokes, auxiliary chokes, horns, and throttle butterflies & shafts are unique to each carburetor size and are not interchangeable.  The DCOE 40 has one choke and auxiliary choke design (aux venturi w/spring clip holds choke),  and the 45 has a slightly different design (choke & aux venturi held by bolts). The 40 and 45 refer to the carb's throat barrel size in mm.

 

DCOEs work to pull air and fuel in by vacuum, using different ‘circuits’ across the RPM range. Simplifying it a bit — vacuum in the Idle circuit is generated by the engine sucking against the closed throttle butterfly, while vacuum in the Main circuit is generated by air flow velocity through a venturi (the choke). A venturi is simply a necked-down restriction in the air passage.

The greater the neck-down, the more pronounced the venturi effect, and the stronger the vacuum generated at any given flow rate. A 32mm choke in a 40mm throat will generate a stronger vacuum than a 34mm choke will (larger choke bore = less neck down = weaker vacuum).  Similarly, if a 40 and 45 DCOE have the same chokes (say a 34), the 45 DCOE will flow more under the same vacuum conditions.

 

When you change choke sizes, you change the overall venturi vacuum strength in the carb.

 

The choke on DCOEs swap in/out in a range of sizes (like jets) per each DCOE carb.  The larger the choke (venturi), the more air fuel mixture can flow through it and the less flexible the engine will be at lower RPMs. The smaller the choke the better the low & mid end is and the sooner it will start restricting flow at higher RPMs. The ideal choke size is the minimum size that provides good flow at your maximum usable RPM. Going larger than that costs you low end and mid RPM power without increasing high end power.

 

The largest choke that will work optimally in a 40 DCOE is a 34 choke (the available chokes range from 28 - 36mm). The smallest choke that will work optimally in a 45 DCOE is a 34 choke (available chokes range from 30 - 40mm).  Optimally here means the choke provides a great vacuum signal and thus the carb responds well to jetting changes.  While larger chokes are available for 40 DCOEs and smaller ones are available for 45 DCOEs they typically will not work as well, and may be peskier to tune.

 

The main jetting set-up on a DCOE is only appropriate relative to the venturi vacuum level present in the throat, and choke size has the most overall impact on that venturi vacuum. So to properly vary choke size, one must be prepared to also re-tune the other carb components affecting mixture in concert.

 

Hope this helps,
Tom

[Greg Mierz]

All this is true, but 45DCOE's is what the Tisa had. If the car is more driver than racer definitely go 40's 34mm chokes. I've run 45DCOE's with 36mm chokes for over 30 years, the bottom end loss isn't noticed 'cause of shorter gearing masking it well. Car will get very high 20's mpg on road with 5 speed and 4;11's now with the 3;91 it'll probably get 30. Tuning of any setup is crucial, Byron will probably agree many sidedraft setups get jetted poorly and don't run as cleanly and responsibly as they should. Motor need at the minimum 9:5 1 or better, cam at the 284-300 degree area, I was lucky at the time of changing from the factory Solexes I had to the 45's was in the nice port job done on the BMW intake manifolds, Signal strength is important, so much affects it. If you already have the 40's it is really a no brainer, use them. You don't want to know how little I paid in the early 80's for factory Webers brand new. Hope this helps

 

[Chris_B]

It will probably never happen, but I would love to see all of this tested on a flow bench. There is some pretty complicated physics going on here and the real world flow is dependent on what is happening upstream and downstream of the carb barrels. However, it does seem like on a standalone basis the 45 DCOE should flow more than the 40 DCOE with the same chokes.

 

I am running dual 45 DCOEs on an IE intake manifold (~45MM, AFAIK), with a Motorsport 300 cam and 9.5:1 pistons. During tuning, I tested 32, 34 and 36 chokes and the only difference I noticed was more robust performance on the top end. The motor pulls strongly through 7K RPMs at which point I chicken out because my motor was not built to handle high revs. I really need to install a rev limiter of some sort.

[TobyB]

Quote

So, what kind of airbox did you use?

I ended up with the biggest- assed flat air filter (Porsche 944?  I forget) I could find as far from the edges of the trumpets as I could get it.

I'm not sure it's the absolute best, but it's far better (lower lap times) than most of the ducted ones out there.  Someone on here had a simple, elegant way

to get cooler air to it, and one of these years, I'll 'creatively apply' the idea (just mask off the carbs so they suck air NOT from the rad)

 

Quote

flow bench

At first blush that makes sense.

But, unless you have a flow bench that spits, pops, and makes neat trumpet- like sounds,

a flow bench will tell you a pretty limited amount about an IR carb.  The flow through the carb is highly pulsed (it's only flowing for about 1/4 of every 2 revolutions)

and the effects of reversion are significant, and very rev- dependent.  A significant amount of flow through an IR carb is  backwards.

The weird stuff that happens at different rpms is... well, it makes

sense, but you kinda have to see it to believe it.  For example, a shroud around the trumpets can make the car far

easier to tune than just open trumpets- with nothing, there's a standing wave of fuel IN FRONT OF THE TRUMPETS

that gets blown around, and can migrate from cylinder to cylinder.  It's RPM and cam dependent, and it looks like.. well, a sheet of rain, really...

Yes, you can prove that the car's theoretically pulling 3" of mercury at 6500 with a 34 mm choke, but what that means for how the thing

works is only one part of the picture.

 

The choke in a DCOE HAS to be a pretty defined restriction to get the thing to work.  So a 40 and a 45 with identical chokes

will really flow just about the same, when they're solidly up on the main circuits.  Now, with a mild cam, the 40 will be

easier to tune on the idle and progression circuits, while the 45 will have (linkages being equal) a much quicker

transition through to the main, in regards to throttle angle.  As in, "a little gas"  will go RIGHT NOW.  Just like the 38/38- might not

really make much more power than the smaller carb, but since it all comes on quickly, you get a far bigger grin putzing around town.

 

But hell, just about anything works, most of it can be made to work pretty well, and at this point, we mess with these things

for fun, so at the end of the day,  if 45s are what you want, just do it.

 

t

 

[LimeySteve]

I run twin Dellorto 45 DHLA's on my M10  , with 36 Chokes and it runs brilliantly through all ranges from cruising around town to track time, watch the video below to hear the engine, it never loads up & the plugs are a great color. Tuned with an wideband 02 sensor. I have the carbs and new manifolds etc for sale as I bought new carbs . Carbs are  for sale have new 36 chokes , jets and diaphragms, are cleaned & rebuilt .

 

 

 

 

[M3M3]

lots of good info here so far. basically boils down to:

 

1/ if all you are looking for is bragging rights to say you have "bigger" carbs...45 is a bigger number than 40.

 

2/ if you are looking for best performance for the type of driving you are doing,  choke size is the primary factor.  with that,  getting the carb body that accommodates that choke size is secondary.  the same engine can be tuned to run with a range of choke sizes, but the choice makes a significant difference in the drivability and power band that is usable.

 

there are several good books that have been listed on the FAQ many times that nicely describe all the tradeoffs in choosing and tuning webers.

[jrhone]

Thanks Marshall, good info here.

[vinceg14]

Question. Is there an improvement in drivability or performance when you use a distributor less ignition systems like the XDI from Electromotive with 40 or 45's DCOEs compared to the 123 distributor?

[TobyB]

Yes, there is.

 

Will you notice?

 

Probably not.

The M10 is not very hard to light.

 

t

 

 

 

[ray_]

On 2/24/2018 at 9:11 AM, LimeySteve said:

I run twin Dellorto 45 DHLA's on my M10  , with 36 Chokes and it runs brilliantly through all ranges from cruising around town to track time, watch the video below to hear the engine, it never loads up & the plugs are a great color. Tuned with an wideband 02 sensor. I have the carbs and new manifolds etc for sale as I bought new carbs . Carbs are  for sale have new 36 chokes , jets and diaphragms, are cleaned & rebuilt .

 

 

 

 

 

How much for that car, Steve? ;-)

 

3.90 diff?

 

Cheers,

[LimeySteve]

Hi Ray , diff is 3.64 , I sold the rolling shell and have just started putting my keeper car back together , I have a 3.90 Posi for that , should be interesting , especially in Hawaii .

[williamggruff]

Revival of the fittest...

 

Since I moved from Maryland (basically at sea level) to Salt Lake (higher than Bob Marley doing bong hits in a hot air balloon, at 5k feet) I've felt the need to increase the choke size in my DCOE 40s. Sadly, they were already at 34mm. Happily, I had a set of 36mm chokes lying around, and I put them in this afternoon to see what would happen. Asskicking good fun happened. While there's a barely perceptible decrease in low rev torque, there's an obvious increase in mid-high rev power. God bless America and pass the dutchie pon the left hand side.