The Weber carburettor, size does matter and here is how and why

[@brunodepaula2875]

Awesome explanation.... greetings from Brazil. Have 2 DCOE 40 Made in Bologna for my build, they are in good shape, but here in Brazil is quite rare to find these in good condition and spare parts.

[@D3Sshooter]

Thanks

[@AndrewJay-qz1zg]

I have got an Australian Valiant Charger(1977) with a Hemi straight 6(265 ci or 4.3 ltr)using triple 45dcoe weber carbs with 40mm chokes. This is a factory option from earlier models(VH-1972 E49).Chrysler Aust. took this engine to weber in Italy to develop the correct carbs and internals. My car, and in fact all 6-Packs,drive like they have modern fuel injection(without the fuel economy). Idle at 800, no choke required on cold starts, super easy in heavy stop start traffic, very strong low to mid range torque. Mine has a modest solid lifter cam 242 intake and exhaust at 50 thou lift making 167kw and 400nm at wheels at about 5500rpm.IMO Best carbs in the world for this style of set up. There is no compromise, they work in traffic and on the track and every where in between and with good solid linkage setup dont need resyncing barely ever. These carbs are almost bullet proof. Ps Love your channel and have learnt so much about how these carbs work Thanks🙂

[@D3Sshooter]

Thank you so much for the write up... That seems like a nice car

[@jaliennation9169]

You're the best! Exactly the answers and explanations I needed for my build.

[@D3Sshooter]

Glad I could help!

[@v2gbob]

Great video! Almost fifty years ago I put a single 45 DCOE on my 1973 Datsun pickup. A stock carburetor replacement (I needed one) cost a lot more than buying a DCOE kit. Turned out installing the "kit" was a big educational experience on carburetor tuning. Besides having to do a lot of work matching the manifold to the cylinder head (porting) I also had to recurve the mechanical advance on the distributor because the Weber didn't lend itself to vacuum advance. I also installed a 'power' cam to gain more low end torque. I wish I had known the formula you provided as the 45 DCOE was too large for the 1600cc engine, even with the cam. The chokes available for the 45 at the time didn't go below 30mm, if I recall correctly. Maybe they did? I remember having to make chokes on my lathe that were the next size smaller than whatever was available from Weber to get the engine to run correctly. Once I got everything dialed in, I had a nice increase in power and was able to drive up hills--at high elevations--without having to down shift nearly as much. Love seeing your videos about Webers. Brings back a lot of memories from my younger years!

[@D3Sshooter]

Thanks for the great write up

[@alfaradbean]

You are a real Pro, thanks. I have two Fajs DCOE 40 on my Datsun A14 in my Austin Sprite, I shall calculate

[@graceenginedevelopments]

Great explanation Steve

[@bryanmeyers5476]

Thanks for the video. Great information. Interesting relationship between fuel jet and venturi size. You make complicated subjects seem more accessible.

[@D3Sshooter]

Thnaks

[@stooartbabay]

Excellent video! Thankyou so much :) Would it be possible to do a short update to this but related to SU carbs and issues around the Siamese port? And maybe talking about sizing to correct exhaust please? :)

[@D3Sshooter]

I will have a go at it later if that is ok

[@stooartbabay]

@ thanks so much!!! I have an issue with my MGB type engine where the power falls over quite dramatically at 6400rpm… it is not caused by inlet or head ports… so I think the exhaust is too small but I have no idea how to size it correctly… i.e. are the exhaust gases the same, more or less volume as the inlet gases? No idea!! :)

[@IlPinnacolo]

The reason this method undersizes carbs is because the flow ratings for carbs are typically at 3”hg or 1.5”hg Two barrel carbs are typically flowed at 3”hg. You wouldn’t want this much vacuum at wot peak power as this would be too restrictive. You can scale the flow according to the ideal wot manifold pressure vacuum. This will give you the actual flow of the carb which will be less than the 3”hg rating.

[@mikakolari3201]

You have another great video. But there's one thing I didn't mention. The flow rate of a carburetor is not only affected by the size of the venturi, but also by the size of the carburetor body. In other words, a 40 DCOE 36 mm venturi is not the same as a 45 DCOE 36 mm venturi.

[@D3Sshooter]

Thanks, that is true and has a lot to see with in and out channels and matching to the intake manifold, the venturi's for the 40 and 45 are the same in the sense of their venturi opening, but not in the sense of their outer diameter ( as you stated).

[@WAGONJON]

You stated at the end of the video if you were to increase the primary venturi size you would need to reduce the fuel amount? I was always under the impression if you were to increase the air flow/volume you need to increase the fuel amount also. Please explain. Thank you.

[@D3Sshooter]

Thanks for the comments, and yes I stated that the wrong way around ( I just check the video). An increased Venturi increases the airflow and reduces the airspeed and so the venturi effect. The opposite is also true, a smaller venturi is less airflow but more venturi effect. In order to keep the proper AFR a larger venturi requires a larger main fuel jet, and a smaller venturi requires a smaller fuel jet. Else the AFR might be to rich. Thus yes , YOU ARE RIGHT...... I twisted that around while talking.. sorry

[@FunWithCars716]

Happy New Year from Buffalo Niagara Falls NY USA 🇺🇲👋

[@D3Sshooter]

Thank you and the same to you and yours

[@bjnopoli]

Yes! More weber videos. Thanks.

[@D3Sshooter]

I will

[@clydecotton]

Great video. However with kent crossflow engines, everyone is using 32-34mm venturis with 2x40DCOEs. However using your formula this seems way too large. Why would this be the case?

[@D3Sshooter]

Yes and no, I know the kent engines they have a good flow especially if they are modified. 32-34 are for race engines that are tuned and operate at the 6500-75000 RPM most of the race and at that rate you really need the engine to breath well... There are many other factors that affect the power, cam-timing , valve lift , overlap , compression ratio, AFR's etc.... Typical the kent has the DGAV 32-35 and is a staged carb where the second barrel only opens op only when pushing the pedal at 2/3 (standard). Race cars always have a bit larger venturies and the 32 seems to be OK to me. The example I gave was a very basic engine

[@SlashmanG]

I was about to buy some used Mikunis for my 4age, and I decided against it earlier today. Now seeing this is my sign to buy some new DCOE40s and some 28mm Venturis haha

[@Videoswithsoarin]

webers are better because you can still easily get parts

[@percyfernandozevallosrodri7199]

As personal experience best choice it's Mikuni 44 phh

[@D3Sshooter]

Webers are great and very tuneable, but also demanding in terms of setting up. The must sensitive element are vibrations

[@RobLaing-i2r]

great info thank you

[@D3Sshooter]

Glad it was helpful!

[@stephenlaw1202]

excellent

[@vintage76vipergreenBeetle]

Nice information.

[@D3Sshooter]

Thanks

[@lewiswestfall2687]

Thanks D3S

[@D3Sshooter]

Thanks

[@bobqzzi]

IIRC we made best power with 50MM SK Carbs on my Cosworth Vega. 278HP at 8700 I believe

[@D3Sshooter]

Thanks for the comments

[@johnspelic9861]

Simplifying it further, Weber made a chart showing individual cylinder sizes in CC and choke diameter, while the other axis shows desired peak power rpm. From that chart you can extract that say a 500cc individual cylinder needs a 40mm choke to make peak power at a desired 6700rpm or so. This is all pointless though if your intake valve and port cannot flow the air the carburetor is capable of. With naturally aspirated engines, all the components must match or the engine will bottleneck somewhere.

[@D3Sshooter]

Indeed , there are many other factors that are at play.

[@tanagobg]

Which is the book that you are looking at? Does it have information for DRT Webers?

[@D3Sshooter]

Its a Weber original document - and yes it covers DRT's . I will have a look what ISBN code it has

[@peterolson8350]

No, you can't divide 260 cfm by four when you have one barrel per cylinder, that's way too small. 65 cfm per port means about 65 HP on that engine, and such an engine in the example will probably produce at least 160-170HP

[@D3Sshooter]

Peter, I disagree with you and here is why: Thanks for the comments , however I do not agree entirely. Let me clarify why : To estimate the horsepower generated by a 90 CFM (Cubic Feet per Minute) air/fuel mixture in a single 250cc cylinder ( note that I used a 2ltr engine with 4 cylinders) , the general approach is based on air consumption, engine efficiency, and power factors. Here’s the breakdown: Lets make some assumptions first: Air/Fuel Ratio: Typically 14.7:1 for gasoline (stoichiometric) Volumetric Efficiency: Usually between 85% to 95% for naturally aspirated engines Engine Speed: RPM significantly impacts CFM and horsepower output Thermal Efficiency and Losses: horsepower-per-CFM rules Theoretical Maximal Power: General rule 1 CFM supports approximately 0.5 to 0.6 horsepower in a well-tuned gasoline engine. So, with 90 CFM that is 0.5x90 =45HP and 0.6x90=54HP For the entire engine that is about 180Hp Of course that is theoretical, and in practice there are many losses and max’s out at around 30-40 horsepower ( for a naturally aspirated configuration). Thus in my example in the Video that engine would make about 160 HP.

[@richardtibbetts574]

You both are wrong. God, I swear if I have to butt into a debate in the comment section, one more time, I’m gonna freak out.

[@odl21]

Reality is that the competition engine always makes more power on the dyno with a larger carb than this formula makes. For example my 2.0 pinto revving to 7500 made more with a pair of 45 dcoe’s than 40s. My 4.2 xk revving to 6800 is on triple 48s. It’s not great at low rpm of course but makes a lot more peak power than 45s. I don’t recall the exact choke size is each but you get the idea.

[@D3Sshooter]

yep, but its the choke that matters the most in your 45 and 40's , besides many other settings...

[@erichlausch9886]

🏎🏎🏎🏎🏎

[@jaliennation9169]

so, say the intake port diameter from the manifold to head is 30mm and ur using a 36mm choke--it wouldnt matter and one would be better off using a 30mm choke?

[@D3Sshooter]

OK, here we have a few points that need to be addressed. First the physical barrel opening of the carb must be a perfect match to the intake manifold port diameter. If you have a DCOE 40, then the inlet manifold port must be 40mm . The venturi is fitted inside the carb and needs to be sized for its specific size needed for that engine. In your example, with a 30mm port on the intake manifold should never be used with a carb barrel that is not 30mm. A larger carb ( barrel) will cause a real bad flow and turbulance as the airflow will hit the edge/rim of the intake manifold.

[@danny4310]

Please remove the AI translation! 😢

[@D3Sshooter]

There is no AI translation from my part, can you clarify what it is that you see ?