To be rude as you asked for why ask a question you know the answer to???

Unless the carburetor is significantly restrictive in the RPM range the engine is used the engine will pull nearly identical air thru all the carbs. The difference is in how much horsepower is lost pulling air thru the smaller carbs, pumping losses. Each carb on the same engine will have a different WOT vacuum reading, the smaller the carb, the higher the vacuum. Higher vacuum will help with atomization and vaporization leading to better distribution and homogenization of the air and fuel, but with more pumping losses. The trick is to find the ideal balance for best net HP.

Always a little curious for all the info/data we do a deep dive on for heads and cam, but use one number, generally, to describe a carbs potential.

Why wouldn't flow numbers be available for 0.6 to 1.8" Hg in 0.10" increments with corresponding venturi/booster signal?

I have a 4781 with the airborne milled. It runs very nicely on my car. I also have a small venture 950. Certainly the 4781 out flows the 950. After extensive testing and tuning, and to my surprise, the 4781 could not run with the 950 on my car. Corrected data had the 950 on top by 0.15 sec and 1.5 mph.

The vacuum changes based on the motor below the carb and it's size compression and power has a lot to do with it and I've done enough testing on THUMPER to where it's wore out but the info gathered is priceless. I find that a too small of a carb on a hot engine is way more sensitive to tuning changes and is less forgiving that a bigger carb on the same combo. Lately I've built the same CNC'd cast HP-1050 1.720 bore Dommy's which flow about 1180-1200 according to BLP and they work very well from my who knows what hp Dart to a guy who made 891 hp today at 7400 rpm's on his BB Chevy and others in between............Fun and never ending............

Back in 1985 we tested the std 750DP the car came with on a lesser tune vs a 850DP 4781 w/choke horn milled and a 50cc pump on the front on my stock 340 10:1 with a HS Strip Dom int.,+ open 2" spacer, 590 solid, 1.3/4" race hdrs., 4200stall, 4.30's, 10x28's. Much to the surprise of all the other racers who said it was too big and kept on about it, it ran 1.9 faster and we only ran the motor to 6100. Not really enough motor/rpm to warrant that carb but it worked for us and we proved a point, and I bet not many today would run that carb with such a tame motor.

I'll just stop now, before I annoy anyone else with those irritating laws of physics.

QFT / Pro Form 950 main body
1.45 venturi diameter
1.75 base plate
890 cfm

No real surprises. They flow more as the diameters get larger. Just wanted to satisfy my curiosity as to what the differences would be on the bench. Gonna hit the track this Friday and see what the 1.45 body carb can do.

It certainly seems, even though I’m sure it’s not just this easy, that on hot engines bigger is better. I used to run a downleg 830 on my fairly mild 451 thinking it was “the correct” carb for my engine. I then put an annular 1000 HP on it, and (like most others report) it went faster by about 1-1/2 tenths. Surprisingly to me, it not only went faster, but it seemed to run crisper and more responsive down low as well. Lesson learned: if you want to go fast, don’t be afraid to reach for the real speed parts.

What's your testing method? As in, the configuration of the carb fixture, flow bench type, flow test depression, testing with all 4 venturi open vs only one or two "active", etc.

Also, what is the combination on which you're going to run the 1.45 v carb and current ET & MPH?

As we've discussed before since I mentioned seeing a similar .2 and 2 MPH improvement comparing the same carbs, flow numbers and traditional engine dyno testing don't factor in shift recovery on the track.

That's what you'll get from Braswell.

Yeah, sounds like a Braswell 4825. Now I'm curious if it's a 2-circuit or 4-circuit, since the 4825s are available in both.

Open spacer adapted to flang on flow bench.
SF 1020 bench
20.4 inches of water
Full throttle all 4 barrels open

Duster 3350#, 904 5800ish converter, 4.56, 28x10.5
360 11.75, .528 252°@.050 108+5, heavily worked M1 single plane
Aforementioned 1.375/1.687 carb 2" tapered spacer
My ported 596 heads 2.02 / 1.6 flow 265 / 180
1.75 Hooker Super Comps
Best pass
1.40 60ft
4.32 330
6.64 1/8 @ 101.3
8.74 1000ft
10.52 1/4 @ 126.3

Nice setup.

Do you use any type of entry to help direct the airflow into the carb?

BTW, other than your 60-ft being a tad quicker, your time is virtually identical to my best from my last passes before I pulled the engine.

Are you going to back-to-back your 750 with the 1.45v carb?

Just to throw something in the mix that somewhat contradicts the “bigger is better” theme.....

The biggest was still the best, but just barely.

3 carbs, all tested at the track on the same day, back to back, no other changes.
-built from scratch ProForm HP750, 4779 metering blocks, std Holley baseplate
-built from scratch Holley HP950, Holley HP metering blocks, Braswell stepped boosters, baseplate with thinned shafts
-CFS modified 4781 with annular boosters and thinned shafts

All ran within a few hundredths ET and 1MPH....... running 11.40’s with my 383.

I tried a low profile air bell that somewhat mimics my air cleaner, it picked up a few cfm.
Probably just going to run the 1.45 carb as I probably won't get a lot of hits, and I'm trying to iron out its unstable fuel curve. Modified a set of scrap metering blocks thinking it might actually want more emulsion not less. We shall see LOL.

In my testing on my junk and at LEAST 180+ carbs for customers thus far, most of the time myself or my customers set a new best and run cleaner and faster. Not because of "theory" or Hot Rod magazine said so but actual facts like above in fast's testing. FACT; The more power you make, the more air flow needed to get the best performance out of the combo ESPECIALLY if the size or tune was off on the previously tested carb. What I see more than anything though is not just the carbs being too big or small but the actual calibration within the carbs are off for the cfm and intended application and the over emulsified, too big of high speed bleeds to compensate just kill em and make them APPEAR to be wrong and when tuned properly, the car/truck or whatever runs and drives much better.
I've put more "Too big" Dommy's on more stuff than I can remember and every single one is happier and faster than before even small blocks are getting reported 12+ mpg's with the baddest carb ever, the all mighty DOMINATOR..............One example; a basic but cammed up .030 454 Chevy in a flat bottom boat that had two 3-circuit 1050's running a blubbery as usual 120+ on the water then after my usual 2 circuit conversion, he goes 130+ and the plugs lasted all season and he's beyond happy so it's not just about will this or that cfm be right for your combo it's about a properly calibrated carb or carbs.............

I'm interested in hearing how you've set up the carb, and what you're seeing w/ the fuel curve issues you mentioned.

I think there's that balancing act between raw airflow, booster signal, fuel atomization, and whatever other variable you wanna throw into the mix.

When I listed out all my carbs on a recent SpeedTalk post and asked peoples' thoughts on which one(s) seemed to be the best match for my combination & the most worthwhile putting my effort into getting the tune right, I remember that Mike Laws said he thought a couple of my smaller 4150 carbs (up to the 1.50" venturi) would do better overall than my big 4150s (1.58"+ venturi), but that a small 4500 (assuming he meant 1050-ish) would also be a good choice if I didn't already have so much $$$ tied up in 4150s.

I think that he, like some other carb folks I'm aware of, isn't a fan of a 4150 with a venturi that exceeds some particular ratio of venturi-to-throttle bore size. These guys talk about keeping that ratio in check and gaining flow capability by scaling up to a 4500 architecture, rather than carving a bigger venturi into a traditional 4150 and jacking up the venturi-to-throttle bore ratio.

At some point I suppose you're left having to use annular boosters to get back some of the lost signal, but then you've just blocked up some of that big(ger) venturi w/ the annular booster and lowered your peak flow capacity. Oh, there's that balancing act thing, again.

I'd love to try ALL of the options... but then I'd end up with even more carbs that I'd need to sell than I do already.

I made a few shake down runs at the local 1/8 mile track and the wide band skipped around by 3/4 of a point or so. My 1.375/1.68 carb was very steady,and the 1.375/1.75 carb was pretty behaved too. All these carbs have 3310 primary blocks with 2 .027 e holes and 26 to 28 HSAB. Despite the 1.45 carb's erratic fuel curve, it still managed to tie the best ET and generate the best MPH at the local 1/8 track in worse air. I added a third .027 e hole to a set of scrap 3310 blocks, and will also try bumping up to the supplied .032 HSAB and see what happens at the 1/4 mile track Friday.

If you haven't already, I'd suggest putting some red or green Loctite around the base of the 1.45" v main body's boosters where they're supposed to mount flush against the body. The last couple of QFT/Proform main bodies I bought didn't have the boosters swedged tightly enough to keep me from rotating them with my fingers, so I corrected their alignment w/ the venturi bores and added Loctite to seal 'em. Nothing like a hidden air leak to cause a mystery tuning issue...