Weber question

[Cliff Beer]

I know DCOE isn't a weber carb type typically fitted to SBF but I've seen some dual DCOE configurations and it's pretty interesting.

Here's a DCOE question.... My understanding of the various configurations of DCOE (4,8,12,18,151, etc.) is that other than the obvious jetting differences, the primary difference is the placement and sizing of the progression holes drilled in the side of the throat. Is that correct?

The progression holes affect the mixture progression as the butterfly comes off closed/idle to part throttle. Makes no difference at WOT. A DCOE4 will have a different set of sized progression holes than a 151 for example. I've always tried to stick with the OEM reco'd DCOE when fitting these for this reason (alfa, MG, Jag, etc.). Is this correct? Are there other differences in the carb itself across the various numbers?

Thanks

 

[Jac Mac]

So are you thinking of the application where a dual DCOE adapter bolts onto a single 4bbl intake manifold Cliff? If so remember that with the DCOE carbs on this setup all for barrels will open at once which might require a throttle linkage setup that has a progressive opening rate. The Aussies used this setup back around ~1980 on the Commodores and Falcons in sedan touring car racing.

 

[Cliff Beer]

Jac Mac,

Yes, i understand that.....a progressive opening rate facilitated by use of 2-then-4, or 1-then-2 carbs, is a way to manage the issue of keeping flow rates, proper atomization, and AFR managed at low flow (idle/slight throttle) v. high flow. I believe that's the basic idea in a Holley four barrel with 2 barrels doing most of the duty except at wide throttle openings/WOT.

But that gets back to the original question I was raising....the various configurations of progression holes drilled in the side of the throat of a DCOE (and most other Weber carbs) are there to control the AFR as the butterfly comes off idle and the progression circuit kicks in when coming off the idle circuit. Bigger holes drilled further up the throat and closer to the plane of the closed butterfly will add more fuel than smaller/fewer holes drilled farther downstream of the butterfly.

My study and use of Weber carbs over the years leads me to believe the difference between the same basic configuration is just the jetting and progression holes (choke rings aside, that is). In other words, a DCOE40-151 is different than a DCOE40-36 in these two regards. One real world example some years ago was my father's ferrari 308GT4 was running like crap when he bought it and it turned out there were four different DCNF40's configurations on there, each with the same jetting but different progression holes in the throat.....so each set of two cylinders was getting a different AFR off idle. We matched the DCNF's by doing some swapping with DCNFs in inventory and she ran sweet as could be.

 

[Jac Mac]

Yes, your correct in the difference in various models of webers especially those built for a specific engine type & tune.
Ive often wondered if Weber ever entertained the idea of a progression slot like the Holleys rather than altering number of and size of holes. The 'slot' could have been made removable to use longer and /or wider variations for tuning purposes.