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Weber 40 DCOE mods for more flow

Five Reviver

Silver Level Sponsor

Sorry DW I haven’t been able to but for consolation here’s the latest 40/45 DCOE comparison video from Penguin Motors
 

spmdr

Diamond Level Sponsor
So, the above video confirms, when using 40s, 32 to 33 mm chokes are about as good/big as you can

effectively use.

This tells me the Z World chart above has some validity and my efforts (moding 40s) are on the right trail.

In this test, optimized 40s made 144 HP and 140 ftLBs

Optimised 45s made 160 HP (+16) 152 lbs (+12) (with no loss vs 40s, above 2500 rpm).

So, Five Reviver, In light of the above, I think you should put in for a Name change.

More like Five REVer...?

BTW, Reviver/'REVer, did you match (increase) the intake size to the New 45s?

DW
 

Thor 1211

Silver Level Sponsor
So, the above video confirms, when using 40s, 32 to 33 mm chokes are about as good/big as you can

effectively use.

This tells me the Z World chart above has some validity and my efforts (moding 40s) are on the right trail.

In this test, optimized 40s made 144 HP and 140 ftLBs

Optimised 45s made 160 HP (+16) 152 lbs (+12) (with no loss vs 40s, above 2500 rpm).

So, Five Reviver, In light of the above, I think you should put in for a Name change.

More like Five REVer...?

BTW, Reviver/'REVer, did you match (increase) the intake size to the New 45s?

DW
Doesn't all this depend on whether or not the tightest restriction in intake tract is the venturi? So, if the manifold runners or intake port on the head are only 38mm, for example, then isn't a 45 venturi too large for that? I wonder if the Ford X head in the video has at least 45mm I/D all the way through? So many questions . . . .
 

spmdr

Diamond Level Sponsor
Thor, I'm not an expert on areo or even Physics, but my guess

is the intake path is a variety of restrictions.

Improving/reducing Any of the restrictions improves overall flow.

And under the heading of flow is the component of velocity.

Carbs function with velocity changes.

In the performance world, the trick is to get the least velocity

change in the carb and still have the carb function.

In the Alpine case, the stock (S5) intake valve is about 39mm.

There is a general relationship with intake valve/port size and

max usable Carb venturi size.

The point with this thread is that the typical 40 DCOE with STD

30mm chokes is too small/a significant restriction for good High

RPM Alpine engine performance.

BUT, going to the required bigger choke size (36+mm) is beyond the functional

range of the 40 DCOE.

....And how much can the 40 DCOE flow be improved?

So Thor, 45mm chokes wouldn't be much of a restriction.

45mm Webers come standard with 36mm chokes.

DW
 

Five Reviver

Silver Level Sponsor
DW I took out heaps of metal from the intake port.
As to REVer, this Alpine keeps teaching me that it takes a lot of reviving to enable the revving, which in turn leads to more reviving. A nasty little circle!
 

Jay Laifman

Donation Time
Adding a comparison of two different chokes I have for the weber. The one with the squared intake and thicker midsection is factory original. The one with the curved intake, and sharper midsection is from an unknown source that I'm trying to find.
 

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spmdr

Diamond Level Sponsor
Jay, what sizes of chokes are looking for/playing with?

I'm looking for 36mm chokes (for 40s) with the wide leading edge.

DW
 

Jay Laifman

Donation Time
These are all 32s for 40s. I found a set of 33s that I'll probably buy as well. This is for the 1300 cc Alfa racing motor.

One thing I don't know, the "better" set with the nicely curved intake side and sharper mid section, all are great except one has a bunch of small pock holes in it. I don't know if those would cause any harm. If anything they might further disturb the air flow and keep the fuel atomized. But, I'd like to find a smoother one. Or maybe those holes can be filled with epoxy or something. Or just ignored.
 

spmdr

Diamond Level Sponsor
Jay, I would suggest you NOT use the "better"

rounded front edge chokes.

Because they do not match the size of

the Aux venturi.

I have the same issue with the 34s I bought.

I think the better (laminar) flow is with

a choke with the same size leading edge inside diameter

as the Aux ventury exit edge inside diameter.

However, there is a case to be made that SOME

turbulence is desirable in strategic places

in a Carb, to enhance fuel vaporizing??

...at the expense of flow???

I'm having a hard time seeing how turbulence at

the transition of the Aux venturi to the choke is a good thing.

BUT, maybe it is??

DW
 

Jay Laifman

Donation Time
Interesting. That makes sense. Great to know. I guess the open question then is if the faster opening to the trailing end adds other benefits.
 

jdoclogan

Platinum Level Sponsor
As I am immersing myself into the physics of carburetors. This thread became interesting as I have a stepping of the cliff moment. I've been reading John Passini's 1992 book Weber Carburettors Tuning tips and Techniques. I may quote " " Passini from time to time.



I will first start by quoting the Pegasus (auto racing) website regarding the Weber carburetor choking venturi and air flow (https://www.pegasusautoracing.com/p...hA1pdpwHIBKpZBke5fK9xR_CCnu5gwOxoC_uMQAvD_BwE):

What size do I need?
The venturi (choking) size is the first step (after choosing the carburetor) in tuning a Weber carburetor to your application. It is tempting to use the largest size for the most airflow, but this can hurt performance. Too much airflow impairs low-end drive ability. In fact, the car may not start or idle at all. And remember that the engine has a limit to how much air it can use. Feeding it more will not produce more power.


Well, that is the generic approach. As Dan and I are out-of-the-box thinkers that may be true --- or not.



As to fuel /air mixture:

If you can imagine in achieving an atomized mixture of air and fuel. There would be this tiny liquid droplet being mixed with air (for the oxygen content of 21%) "enclosed in a sort of spherical halo of vapor." "The petrol-air mixture is formed when the emulsified fuel is broken up by the air stream through the venturi as it emerges from the nozzle." It is very difficult to determine when complete vaporization has occurred. "Vaporization takes place much faster when pressures are low." At low RPMs vaporization has normally completed before combustion. At high RPMs a certain amount will most likely not vaporize until combustion. "This makes it very important that the mixture should be as finely atomized and homogeneous as possible, otherwise the rate of pressure rise during combustion will differ from the desirable one and adversely affect the power output."

Regardless of the outcome for any application the amount of optimum air flow and vaporization is nearly impossible to determine. It is known that, " The Stoichiometric mixture, the chemical one, for current petrol (1992) is about 15 parts of air to one of petrol, This in practice is too weak under the majority of conditions, 14:1 is normally considered the weak limit and 11:1 about the richest. With petrol the only mixture worth having is the right one for the conditions."

Well, that leaves us with little to nothing. Other than seat-of-the-pants or dyno testing.

For me, I have three pairs of NOS Italian 38 DOCE Webers (two sets for the two Harrington Le Mans and one set for the #41 Sebring Alpine). I can put in different sized Venturi chokes (probably standard 30mm for 40 DOCEs) and achieve the same performance as DOCE 40s. Now for selecting from the many combinations of tuning options with further reading.

Note: Welcome for any comments about my 38mm mating orifice from the 38 DOCE to the 40mm mating orifice of the intake manifold. I'm thinking no problem because there isn't any restriction only expansion. With a 30mm choke venturi it would be like an elongated venturi effect.
 
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spmdr

Diamond Level Sponsor
Jerry, Pegasus may be fine for general parts info,

BUT their info about carb choke sizes reveals their

lack of knowledge about carbs.

To suggest ANYTHING about a choke would effect starting

OR idle, clearly relieves them of any form of credibility

on the subject of carb knowledge.


I have read John Passini's early writings:

Weber Carb 1.) Theory c. 1968

( a very odd book, 6 chapters, NONE with a title/subject...)

...And his later Tuning tips and Techniques.

Also, we forget how much MORE work it was divining what a

carb was doing back when your primary tuning aid was YOUR

ability to "READ" spark plugs!


Your 38 DCOEs will likely NEED all the help they can get.

DW
 

alpine_64

Donation Time
A while back on the forum.... I'm thinking 3 iterations ago... Around 2003 ish....we got an insight into the value of info from Pegasus..

I forget the young guys name who had a red SV he was building and modifying (and ironically I think Jerry you may own this car now via another member ) but he was taken for a lot of cash setting up his Weber motor with parts and advice that failed the basic logic test...

and any stories of Pegasus came out.... The irony they seem to keep finishing new customers....
 

jdoclogan

Platinum Level Sponsor
To quote myself, "I have a stepping of the cliff moment", going on right now. I'm descending into the pit of unknown and absorbing as much understanding as the 'ol brain will allow.

Good to know the Pegasus folly gents.

DW, that is why, regarding the Pegasus quote, I stated, "Well, that is the generic approach. As Dan and I are out-of-the-box thinkers that may be true --- or not."

I was engaged in learning applied mathematics in the '70s, well before Passini's book. My brilliant instructor and friend, Dr Wollkind, taught me mathematical applications to fluid dynamics. A whole new view of the World began to take shape and influences my thinking to this day. My shop work now engages me in the more kinetic/tactile approach to molecular integration that merges my senses with the theoretical. The videos in this thread are useful information along with your flow chart DW. Keep moving forward with your vision.

I do recognize the many variables (cam selection, manifolds, headers/exhaust systems, bore-pistons-rods-balancing and so on) we all face when addressing the best configurations for our chosen driving application. This is where I, and I would think others, get the difficulty in choosing the best carburetor's composition. Sheesh, where is the Sunbeam Genie when you need them the most?

I had a good pair of 45s that i picked up years ago while attending the Bakersfield vintage drag races. I now regret helping a friend out by selling those to him.

From what I can tell. The 40 DCOE body is the same as the 38 DCOE except for the mating opening to the intake manifold. The 38 opening can be machined to 40 or even 45. So, is the body of a 45 DCOE the same as the 40 - 42 - 38 DCOEs ?
 

spmdr

Diamond Level Sponsor
Jerry, as occurred to me, when talking with you,

I suspect (a web search did NOT fill the gaps of info, yet),

the 38 DCOE is the older/first version of the 40 DCOE

They both use the same parts, Aux venturi, chokes.

And it is looking like the Aux venturis are really best

sized for the 38s.

The 40s are just a half baked Stretch of the 38s.

IF the 40 DCOE was a clean sheet of paper design,

it Would have at least a 40mm ID AUX venturi.

SO, moding the Aux venturi out to 40mm ID is just putting

it at the size it SHOULD have been from the beginning.

The 45 DCOE IS a clean sheet of paper design, size wise.

DW
 

spmdr

Diamond Level Sponsor
Jerry reports the 38 DCOEs DO have 40mm

velocity stacks, the same as std 40 DCOEs.

That supports the theory that a 40 DCOE is really

just an under developed stretch of the 38 DCOE.

...that's my story...and I'm sticking to it, ...for now...

DW
 

Tom13

Donation Time
Hi, a friend of mine is selling a tuning set with Lynx manifold, headers, Weber 45 DCOE, the matching levers, valve cover with CNC milled inscription, the required gaskets and a freely programmable distributor from 123 Ignition. The ignition timing can therefore be set via the PC.

The set was set up on the dyno on a 1725 cc engine and measured at 101 hp - Flywheel - with the original cylinder head.

Anyone interested

regards

Tom
 

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