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Hello, I am building a motor for my 76 Alfetta I don't know if I should use the Weber 40s I have, or buy the 45s. The motor is going to have the centerline performance head with either the 11:1 performance cams or the 11:1 and euro cam combo. The pistons are the 10.4:1. I don't understand if the intake is 40mm how I will gain with the 45mm webers. Thanks
 

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You will discover that for any form of street use, either 40's or 45's will be fine. Currently, 45's are more available on E-Bay. The limitation is the maximum choke size that will fit a 40. On the other hand higher air flow velocity through the smaller 40 mm butterfly can be an advantage, depending on the head and manifold porting, valve seat size and cams used. If you do contact AlfaR7 for cams or head work, head his advice on which Weber bore would best suit your particular application. If you HAVE good 40's there is little cost vs performance gain with 45's. If you HAVE good 45's, they can be set up to match about any application. Just my opinion form 45 years of Weber use.
 
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Why restrict yourself to 11mm cams if the head is going to be modified? There is no reason not to go for more lift. I have 12.3 intake and 11.7 exhaust cams from RJ and they are great for road use. I have 40DCOE's with 34mm venturis. It makes good torque from 3000 rpm and power beyond 6000 rpm. I have a self imposed rev limit of 7000. The motor is so sweet that my wife drives it when she goes shopping. It gets 24 mpg, if that is of interest.
 

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I agree with alfaparticle. I also have RJ cams and they are excellent. Not the same cams as Ed but with VVT on the intake cam. I had CB12 cams and RJ's are the better choice. I started with 40/34 and now I have 45's with 36 mm venturis.
The car is FUN to drive on small roads and still no problem in the city thanks to the VVT.

It makes good torque from 3000 rpm and power beyond 6000 rpm. I have a self imposed rev limit of 7000.
I would say for my engine 3000 to 6500 but the power really kicks in at around 4000 and it revs good to beyond 7000. I try to stop at 6500.

I suspect that if you start with the 11,1 cams you will change them later anyway? :D
You save your money if you buy highlift cams from the start.

I talk about a 2 litre engine.
 

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It depends upon how you time the cams. I had mine at 100 intake and 102 exhaust and there was good power at 7000 rpm, but I did not want to run the motor at such high rpm. If I had stayed with that timing, then switching to 36mm venturis would have made sense. Now I am running at 102/104. The power is a bit lower down and there is less to be gained and more to be lost with bigger venturis.

To answer the original question. If you are interested in a flexible road engine that is not going to be revved beyond 6500 then 40DCOE's with 34mm venturis should work fine. If you want more high end horse power at the expense of low-mid range torque, then 36mm venturis in 45 DCOE's may be better. But your choice of cams will have more influence on overall performance. 11 m cams are poipular because that is the most lift that you can use without modifying the head. I had CB47's in two engines and they were a good inprovement on Euro cams but nowhere near as good as the ones that I have now. More lift = more torque. More duration and overlap = more high rpm power at the expense of low-mid torque.
 

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The original question; The engine was very flexible and nice with 40's.
If you are going to fit 10,4:1 pistons you need to remove the head anyway, why not do the simple cutout for bigger cams at once?
(For my old cams I meant CB48 with 12mm lift)
 

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Gordon's absolutely right; the real question is the choke (primary venturi) size.

The choke size will determine the maximum power and torque characteristics--roughly speaking, the larger the choke diameter, the more top-end power and the higher the maximum revs; the smaller diameter, the better throttle response at mid-range speeds and the greater the mid-range torque. (Take a look at the diagram on the second page of the attached PDF.)

View attachment Weber DCOE setup.pdf

If you decide you need chokes larger than 36mm, you'll need to go to 45s; otherwise, Weber recommends barrel diameters of 1.1 - 1.4 times the choke diameter, which essentially means that either 40s or 45s will work fine. So, unless you need to run 38mm chokes or larger, stick with the 40s you already have.

But it's the choke size that matters most--not the barrel size.
 

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I've had 36 mm venturi's in my 40 DCOE's for years. LOTS of power (11:1 CR, cams, head, etc); still climbing at RPM'S I don't want to go to on the street! Only short-comming is a bit fast idle, probably from the cams, at 1000 - 1200 rpm.

If you put 45's in, you'll probably end up with 36 mm Venturi's anyway for street use.

Robert
 

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Richard Jemison
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Carbs & Choke tubes

To have decent drivability there has to be some restriction to speed up the air/fuel mix as it passes through the venturi and expands out. Too little expansion, no atomization. In all but a Track car the standard fitment 34mm choke tubes fitted to 45 DCOEs are probably best. Particularly if fitted with the correct airbox and ring type auxillary Venturi for air flow the 40 can`t achieve.
Although the OE fittments for the 40 DCOE 32 calls for 32mm and the 40 DCOE 2 calls for 33mm Chokes I would not suggest any larger than that to maintain drivability.
The "Venturi" shape of the chokes is poor as cast, and can be improved, and the shape can be shortened which helps the effect.
What invaribly happens is the owner wants big venturies (choke tubes) and loses low end response (big flat spots). Actually the smaller correctly shaped venturi will flow well enough that the motor will produce better over the entire RPM range.

11mm cams are barely an improvement over stock, but for some, they serve the purpose as "1st cams" to replace stock cams, that after a minimum of 10 to 15 years or much more are well worn..
 

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The above experience mirrors my own. I have run 36 and 38 mm chokes in a 1600 GTA engine on the street, but it is an ..... unpleasant drive. The 36's & 38's were both modified chokes as Richard mentions. 35's made up in shop with a proper internal airfoil shape work well with THIS PARTICULAR engine. I would not recommend anything that large for useful low end torque and power. The flow must be matched to the engines volumetric efficiency at specific running speeds with specific cam timing and lift. More simply, it's all a compromise.
If you are only looking for absolute top end Hp near redline, big chokes offer that. In the real world, the engine must accelerate up to mechanical and design limited redline, with SOME LOW END TORQUE to get it there. It's all a compromise.
 

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In volume 1 of Weber Carburettors by John Passini, he gives a formula from Weber for calculating BORE size. It is:
B = 0.8 to 0.9 x SQRT(V x rpm) where V is cylinder volume in liters.
For a 4 cylinder, 2 liter motor making power at 5000 rpm, the recommended carburettor is 40 to 45 mm. If this motor is making power at higher rpm then the recommended carb is 45 or larger.
Alfa and other manufacturers that supplied DCOE's as original equipment appear to have generally followed this guidline. Several books list OE Weber setups and 32mm is the largest venturi that is listed for 40DCOE's. This emphasizes Richard's point about having a decent difference between bore size and venturi size for good operation.
 

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Richard Jemison
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What??

B = 0.8 to 0.9 x SQRT(V x rpm) where V is cylinder volume in liters.
I was just going to say that!:rolleyes: Yeah, right:p

Ed is very knowledgeable on tuning the later emission type Webers.

Below is some early data on OE setups on Alfa Weber Carbed units, and pages of Weber OE setups and for their suggested and over the counter setup.
 

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Richards notes on the above pages show that beyond the "factory" set up, some further "adjustment" is often required.
Now, lets see... Engine CC capacity divided by # of cylinders, X RPM @ anticipated peak Hp= "(number)". Square root of "(number)" is then theoretical ideal main vent size.

Example: Alfa @ 1648cc's divided by 4 = 412 cc/ cyl. Max power @ 7500 (cam design).
Then 412 X 7500 = 3,090,000. Square root is 1,757.8395, divided by 50 = 35.15 as maximum USEFUL vent size.

That works!
(Formula from Gronway Parry (408) 267-5933. {Thanks Gron!})
 

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RJ,
Thank you for posting the scans!
Unfortunately, the scans are of non-Alfa published documents. I have the benefit of access to many tens of thousands of pages of Alfa published documents so I can see first hand the inconsistencies of what Alfa says about carb application and calibrated parts values fitted to their own vehicles versus what someone else says Alfa fitted to their vehicles.
A few examples from the first scan; the 127 main jet listed for the 40 DCOE 4 (which is also missing the /3 designation meaning the jet has a ball in it) was changed to a value of 1225 when it was discovered that the ball caused drivability problems; the Giulia Super used no less than three different 40 DCOEs (24, 27 & 33, I think) and the main jets for the 40 DCOE 32 fitted to the Series 1 and Series 2 1750s are different.
As a result of these (and other) inconsistencies, I've put together a spreadsheet that includes application and (most) calibrated parts data as well as the type and publication number of the Alfa document(s) from which the data was obtained. If you, or anyone else, would like this spreadsheet, I'd be happy to send it to you.
 

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That's why I said 'no less than three'. I thought that was at least one more but heck, I can't remember every application. :rolleyes:
 

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When I fitted Dellortos to my Fiat I used a similar formula and calculated that the choke size would be best at 32mm for a four cylinder 1608cc engine. But Alfa calculate that a 30mm choke is correct for a four cylinder (barely any smaller) 1570cc four cylinder.

As it happened I had two sets of chokes so I tried both. On the 30mm ones it was terrible, didn't want to go past 5000 rpm. Put 32s in and it was fine, pulls nicely from 1000 all the way to 7000. The maximum power figure was 99 bhp (9 more than stock) at 5460 rpm (it still has standard cams) which is about the same speed as a standard 1600 Alfa engine produces its maximum of 109 bhp.

So the question is, how did Alfa get away with 30mm chokes on the 1600 when these same chokes didn't work at all on the similar Fiat? I've driven plenty of 1600 Alfas of all descriptions and while the engine occasionally feels a bit small for the car (1600 Alfetta sedan anyone?) it never feels breathless like the Fiat did.
 
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