(theoretically) Huge 4bbl carb on A20A?

[FyreDaug]

Maybe I dont understand the volumetric efficiency of carbs and engines as well as I should but what would happen if you put a huge carb on a small engine?

With alot of tuning and getting it to idle right wouldnt it be fine? What would happen at WOT? Acceleration?

I'm trying to decide on a carb for my rebuild, I dont need anything too wild, but I hope someone can help me understand better. Ive read a couple articles about carbs and motors. So I do understand a bit, but it didnt give wild scenerios. So any help is appreciated.

[A20A1]

Maybe I dont understand the volumetric efficiency of carbs and engines as well as I should but what would happen if you put a huge carb on a small engine?

With alot of tuning and getting it to idle right wouldnt it be fine? What would happen at WOT? Acceleration?

I'm trying to decide on a carb for my rebuild, I dont need anything too wild, but I hope someone can help me understand better. Ive read a couple articles about carbs and motors. So I do understand a bit, but it didnt give wild scenerios. So any help is appreciated.

Acceleration would go down because the carb would be unresponsive. At WOT it would be okay, but still may see negative returns if it's not a vacuum secondary.

There needs to be restrictions in order to create vacuum to properly meter the fuel. If there isn't enough restriction then air just flows through the barrels without pulling out the right about of fuel to go with it.

A good 4bbl for our car would be one with small primaries. Then have extra large secondaries operated on venturi or some other load sensitive vacuum diaphragm.

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I always wondered what would would happen if you had a regular 2bbl venturi/booster venturi carb with one slide throttle type bike carb with out the booster. Mount the bike carb on the stock manifold possibly at the back, Then have the Bike carb throttle activated like a vacuum operated 3rd barrel using the 2bbl venturi vacuum signal.

[A20A1]

These may help you run a larger carb since their design is different then the booster design. It may pull ou fuel more effectively / efficiently.

https://www.3geez.com/forum/showthread.php?t=45378

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[POS carb]

basically the problem is the big carbs are designed to flow big air so when your tiny motor is idling it's pulling less air than it's designed for. The same is true when you dump the pedal and your engine is revving at 7k. Sure, it won't restrict but again the carb's power valve and accelerator pump would be designed to compensate for a larger engine. You can work these issues by playing with the jet sizes and venturri boosters but in the end you might as well buy a cheaper carb, like a holley 2 bbl around 350 cfm, that should be overkill enough without being extreme, plus less manifold modification.

[FyreDaug]

I want a 4bbl carb to go with a 4bbl NX wet kit. Unless it can be adapted I wouldnt have a problem, whats our stock carb flow? I would want something up to 20% more I think

[A20A1]

I think our stock carb flows around 269 to 325 CFM

The correct CFM rating can be obtained by multiplying the horsepower peak X 1.3 . This gives you CFM required.

150 hp needs about 200 CFM carb according to the C&S site.


As I've found, you can get more pull to your carb if you divide the plenum.

Calculated CFM based on displacement

1995 cc displacement
211 to 250 cfm Single Plane Manifold @ 6,800 RPM
230 - 288 cfm Divided Plane Manifold @ 6,800 RPM


2366 cc displacement
257 to 304 cfm Single Plane Manifold @ 6,800 RPM
281 to 351 cfm Divided Plane Manifold @ 6,800 RPM

I've read the average increase is about 52 CFM over single plane, but for smaller displacement motors it could be less.

[FyreDaug]

What would be the beneifit of splitting the manifold though? I know you said you can get more pull to it, but why? You have cyl 1 and 2 fed from 1 barrel and 3 and 4 from the other barrel? Would it make a difference with my stock 2bbl keien(or whatever) carb?

I dont plan on keeping it, infact Im looking into aftermarket carbs for my build, but I dont know where to start