How To Choose The Right Size Carburetor! | Booster Signal Is Key! | Its Not The Size Its The Signal!

[@ryanbrunson777]

I think you're right with the idea that for power and drivability, total carb cfm is less important than maintaining good booster signal at all engine speeds. For an engine to make the most power it can, airflow needs to be restricted the absolute least, and the right amount of atomized fuel needs to be supplied for that amount of air. The inherent limit of a carburetor is that it relies on a venturi to create a pressure drop to supply fuel, and a venturi by nature is a restriction. This is why injection, electronic or mechanical, has its advantage over carbs for power. Injection doesn't rely on a restriction to the airflow to supply its fuel. For guys like us that love carburetors and horsepower though, we need to figure out how to make the venturi area the biggest it can be while still being small enough to supply the right amount of atomized fuel. This gets tricky because for every engine speed the optimum venturi area is going to be different. Varying the venturi area for different engine speeds is the only way I know around this. One method to help this situation is to use a spread bore carb. A spread bore carb will have smaller venturi area on the primary side than a square bore carb of the same size in CFM. My carb experiments are with Quadrajets for this reason, but there are also other spread bores, even Holley ones. Another method is keeping air from flowing through the secondary venturis at low speed WOT. You're right that vac secondary Holleys aren't a good enough solution for max power since they rely on manifold vacuum to open the secondaries, but mech secondary Holleys allow air to flow through the secondary side when the engine isn't ready for it, so that isn't great either. Other carbs like the Qjet, Thermoquad, or Edelbrock use spring loaded air doors on the secondary side to stop secondary airflow until engine speed is high enough, which I think is better than vac secondaries, but it's still technically a restriction. I think that another solution other than the two typical Holley methods or an air door would be worth pursuing. One idea I haven't seen yet is hooking the secondary throttle shaft to an actuator that only opens when a full throttle switch is triggered and when the rpm is higher than a set point. I'm curious of your thoughts on all of this.

[@newguysgarage6802]

I think an adjustable variable venturi would be a good idea, and I do have plans for one! Also I believe a lot of the issues come from the old booster design in of itself, I believe there are ways to make the booster more sensitive thusly requiring less depression to function 🤔

[@ryanbrunson777]

​@@newguysgarage6802 I think so too. Booster efficiency as far as the droplet size, and how much signal it takes to get the booster to flow are important. Annular boosters and double ring boosters help with this. The double ring booster not only increases the pressure drop with the second ring, but I think it helps break up the fuel too.

[@newguysgarage6802]

@@ryanbrunson777 shear boosters are sweet too! Making the fuel go over the lip and get sheared off breaks it down to the smallest possible droplets.

[@deanstevenson6527]

28:56 🥝✔️ David Vizard says, on a Flow bench, the difference in flow is about 1 cfm different. Air horn removal is beneficial for compromised air flow into the venturis, where primary to secondary entry can be ghosted. Stub Stacks, Bell mouths, Velocity stacks, the old David Vizard books show just what geometry is best for air entry. Variances dont change the peak cfm flow a great deal, but you can loose 5 % or gain 5% with the wrong or right carb air cleaner, Stub Stack, Bell Mouth or Velocity Stack combination. Air horn casting removal is okay, but it wont gain you as much as a great whole combination that isnt working against other factors. I love the look of choke horn removal...

[@newguysgarage6802]

The carburetor project I am working with @JosephNowak were we cut down the choke tower on the 600 holley carb gained over 80cfm more flow with it removed! Lol

[@deanstevenson6527]

@@newguysgarage6802 I believe you on that 84 CFM increase figure, which is percentage terms is no different to taking a Holley Weber 2 bbl Pinto carb and taking it from 227 cfm to 336 cfm in his 1973 2 liter Pinto. He hacked that bad boy up like a MoFoEr. His issue, as will be yours, is making the carburettor function as a fuel delivery device right through the rev range. But I've gots me my popcorn, 🍿, Yaaaar. 🦀 Signed. Eu-Dean Von Carbz...

[@brokentoolgarage]

Good stuff! I may throw a video together on the subject (vacuum secondary stuff).

[@newguysgarage6802]

Do it! 😃 That is what im hoping people will do with this topic 😁 .......mechanical secondarys for the win though 😎

[@brokentoolgarage]

@@newguysgarage6802 Thanks!

[@eddiepiecart6030]

I like your show. You make stuff understandable for ordinary folk with out too much bullshit.

[@newguysgarage6802]

Thats what I attempt to do anyway 😅 Mostly because I am just an ordinary folk explaining it the way I see it 😁

[@markbogle8062]

I think is how you set the carb up and the jets you put you in it

[@newguysgarage6802]

@@markbogle8062 it goes way beyond simple jet changes and mixture screw adjustments 😅

[@eddiepiecart6030]

Good point mate. That's why they have large accelerator pump nozzles, and three stage intermediate systems in Dominators, which is why you can in some instances use one on a small block (pure performance)

[@newguysgarage6802]

Exactly! And it looks bad ass too! 🤣

[@eddiepiecart6030]

Try using depression instead of vacuum, pumping losses to explain that missing thing in your glove, remembering air is compressible and elastic, which isn't taken into consideration the basic CFM formulae. Atmospheric pressure does all the work in a carburetor.

[@newguysgarage6802]

Depression is the correct terminology for sure, though most people find vacuum to be a more understandable term when explaining things 😅

[@claytonstoolbox]

Love the tunnel ram, I wish I could find one for my 351w.

[@newguysgarage6802]

I had been looking for years before I found this one lol They are rare for the sbf for some reason!

[@johnweaver8470]

Nice explanation of booster signal keep the videos coming

[@newguysgarage6802]

I will! And thank you for watching 😃

[@eddiepiecart6030]

Imagine how much reversion pulse loss that big plastic thing would contain. Might work you know

[@thejunkyardnecromancer]

So what you are telling me is to switch to multi-port EFI. Haha

[@newguysgarage6802]

Nope because then you lose all the chanrge cooling affect and will make less power for more money lol 😆

[@ronaldrussell5481]

This should spark some interesting conversation! I would say Holley and other carb manufactures have some math that is appropriate for this problem.

[@newguysgarage6802]

I am hoping other KZbinr's will make videos and we can have a big debate 😃

[@HeadFlowInc]

Here’s a safe equation that never fails to work. CID x RPM/3456 + 50= Base CFM 302 x 6500= 1,963,000/3456= 567.99 + 50= 617.99 cfm Id recommend between a 600-650 cfm carb Volumetric efficiency plays a big role in the “optimum” carb cfm. The above equation assumes a 100% VE, a street engine might only be 85-90% where a finely tuned race engine can easily exceed 100% VE. Note: A street driver spends less time at WOT so be sparing in carb side don’t push your luck. Note: Only run a DP mechanical secondary carb with a manual transmission or automatic w/ matched torque converter for the camshaft and rear gear. (Dumping too much fuel too soon sucks)

[@newguysgarage6802]

See I just think there is way more on the table than the formulas will tell you lol Its not about cfm as much as its about booster signal which can be optimized 🙂

[@HeadFlowInc]

@@newguysgarage6802 You can have fun and learn more about carburetor function. What you’re learning is “Optimizing” the fuel delivery to a specific combination. The formula I shared will get any engine running in its happy space and is proven to work. I didn’t claim it was an optimized setup to every combination. Have fun traveling down a path well worn by racers and engineers for decades. 👍👍

[@lawrenceriffe3809]

Depilz on the intake 650 x ² high risers turbos

[@lawrenceriffe3809]

Freaked u wat really sayin watch demonology hez fun n gittin it done

[@newguysgarage6802]

@@lawrenceriffe3809 I couldn't understand a single thing you said but im sure it was right on the money 🤣😁👍

[@Redneckairflow]

Pumping loss is the word your looking for

[@newguysgarage6802]

Yep lol 😆 I figured it out eventually 🤣

[@jeffreyurbain9604]

Weird to ask on this video. What’s your opinion on spread bore carbs?

[@newguysgarage6802]

Very good street / strip carburetors! I want to get my hands on a spread bore holley and play with it a bit to see just how good it can be 😃

[@Redneckairflow]

Random ramble and thought that I have no proof of but might spark new test. In my basic understanding you need plenty of plenum volume to feed each intake stroke. So if that is true then you need enough CFM to keep the plenum full. So more plenum volume then the larger the carb you need to keep from the carb from being the bottle neck at wide open throttle. Hence single plane style intake generally have smaller carbs than say a tunnel ram with a significantly larger plenum. Dual plane intakes are the exception due to only using half the carb to feed each cylinder so they need a larger carb compared to a single plane. Just my thoughts

[@newguysgarage6802]

That could defenatly be the case 🤔

[@stuartwall8212]

Well, if I dont know what my engine wants, should I start with a carb that is too big or too small?

[@newguysgarage6802]

Are you more concerned with ultimate performance or street crusing? Were will it live 99% of the time?

[@stuartwall8212]

@@newguysgarage6802 Street car of course....but we all wanna nab as much as we can and still drive to the track.😝

[@newguysgarage6802]

@@stuartwall8212 what size engine? 🤔

[@stuartwall8212]

@@newguysgarage6802302. I have a 600 vac secondary, GT40 heads with mild bowl blend, dual plane intake, headers. I have a stock ho cam....which is probably the weakest link in my set up, but I am gonna milk a 12 second pass out of it before ejecting it. I wont change anything until I get the stupid tires to hook up.

[@stuartwall8212]

@@newguysgarage6802 Ford 302, 600 vac secondary, Performer RPM, GT40s with bowl blend, headers, GT cam. I think the carb is big enough to do low 13s or better....once I solve traction issues, I will know. This is nearly the smallest four barrel .....and I cant imagine how a bigger carb would help at the moment unless I change alot of other stuff. So I guess I fall in the start small camp.

[@deanstevenson6527]

Nice. 🥝✔️ CFM is a dead measure. If its independent runner or port on port, like an 8 barrel, Dual Quad V8, use Eduardo Weber Graph of venturi size in mm verses cm³ of one cylinder. It gives an indicated peak power rpm for any given size. Measure your venturi size in mm, calculate your cm³ per cylinder, and you'll get the peak power rpm figure. Hp then is rpm x total cubic inch displacement x 4400. If your car is 1bbl or 2bbl or 4bbl, then the ideal venturi size is based on the Total Cubic inch displacement divided by square inches of venturi area. 127 cubic inches per square for 1bbl, 81 cubic inches per square for 2 bbl, 66 cubic inches per square for 4 bbl. Peak power rpm and peak hp is then determined by the Eduardo Weber formula, this time using the the average venturi size in mm if one cylinder was doing the drawing. Then, for a 1, 2 or 4bbl, use peak rpm x cubic inch displacement times 7500, 6000, or 5500 respectively. The Eduardo Weber ( E- W) formula is published by the US PMO Carb company, which has a great, more elaborate version of the E-W graph. This system eliminates Volumetric Efficiency , CFM, and gives you target peak power rpm and net horsepower levels. David Vizard uses the Weber Venturi size method for carb sizing ib everything from Minis to Le Mans style V8s.

[@jeremypike9153]

There is really no such thing as too much carburetion. The carburetor is the restriction so the more you can open it up the better. However this doesn't mean that it will run good at low rpm or part throttle operation. The more open the restriction is through the carb the better it is for power production at wide open throttle. This is why Individual throttle bodies or stack injection always makes the most power. You can tune each cylinder to its optimal afr for the most amount of power if a carburetor or throttle body is separated out for each cylinder with separate intake runners. It's really odd to me that this is still a question being asked since it has been answered for decades now. I understand why manufacturers have never run this setup though. It would require so many more parts so the cost is the reason why. How to carburete an engine for maximum power production was really solved before the horsepower wars of the 1960's.

[@jcnewbee8124]

Horsepower x 1.5