I have a 250 with an offy intake would a summit 600cfm carb be to much? What carb would you fellas recomend besides a holley 390 or edelbrock 500?

I would go 465 Holley 4 barrel with vac secondaries..but overall I would still go with EFI, although it seems to be a bit of a bugger to set-up!

On a Chevy 350 V8 a 600 cfm carb will support 375 HP with 400 ft lbs of torque.Inlines have different cfm requirements,but not that much different.390 Holley is really still too big but works ok.A 350 Holley 2300 two barrel works very well from my actual experience on extensive carb testing with both a modified 261 Chevy and 302 GMC with water heated intakes.Again,the 250 engine is different than the two I mentioned....I never got a 500 cfm Edelbrock to function properly on the 261 or 302.The carb was good at part throttle but always a bog when the secondaries opened no matter what jetting or fussing with the secondary air flap.Some guys think a bog is good, the engine stumbles,then picks up and they think it's accelerating harder,but it's actually slower.
From a fuel distribution point of view,two carbs is better than one ,but the centrally located carb can be made to wrok out ok.

Some guys think a bog is good, the engine stumbles,then picks up and they think it's accelerating harder,but it's actually slower.
X2 - this is very common, and very important.
The human body is not too sensitive to absolute forces (you can't tell how fast a plane is going), but far more sensitive to changes in force. Anyone can feel + or - 10% change, better drivers can tell down below 5% change.
Recovery from a bog feels like "coming up on the cam", but is instead going from very bad to only slightly bad.
Other things that feel like an improvement, but need more exploration and diagnosis:
Late spark curve
Late secondary opening
Too much overlap for the exhaust system

If Tony will permit?
"Supports" in this context means that the carburetor will be within the manufacturer's designed range of vacuum, and the air bleeds and venturis will function predictably without major tweaking.
If the engine is much larger, of course peak power will be down (compared to a larger carb), but also the metering circuits will be operating outside their intended range and may need expert advice.

Yes,that's correct...a carb is designed for a certain engine situation. The Holley 600 ,out of the box,will be jetted just a touch rich ,for safety,at full throttle on typical 350 cube V8.Same for similar sized Carter and Edelbrock.
In theory the Holley 390 is pretty close out of the box for less powerful engines,like 180-250 hp.But not always.
The simple carburator size test;rig a vacuum to the intake,run the engine from let's say 2000 rpm to redline in second gear.When you first go wide open the vacuum gauge needle will zero then rise up a bit as the engine nears redline.If the vacuum rises to about 3-4 inches ,a larger carb may help.
My observations found that GMC 302 inline 6 with a single central 4 barrel carburetor needs a richer fuel mixture at maximum torque peak than the carb will supply without some fussing with power valve restriction channels on the Holley.The Edelbrock or Carter wouldn't respond to metering rod or air flap adjustments.Finally I bumped into some tech info about fuel bleed holes in the secondary venturi clusters on some Carters.I drilled holes on a Carter 400 CFM 4 barrel on the GMC and it cleared up 95 percent of the bog and detonation.Supposedly all engines need a richer mixture at peak torque,then a less rich at it winds up to peak HP.
I believe many guys run too rich a fuel mixture on modified carburated inline engines and thus the complaints about poor fuel mileage,especially with GM 235-261's and GMC.I run street engines on the verge of detonation with lean mixtures ,aggressive spark timing,180-190 degree thermostats heated intakes and relatively high compression with as tight a quench as practical.Downside is any problems with fuel mixture curves show up instantly as heavy throttle detonation,it's a slippery dlope and you have to be careful.But the return is super sharp throttle response and good fuel mileage.And I don't drive with the throttle wide open for more than 10 seconds on the street.

And it only takes 20 years to learn it...

No,about a month of tuning to figure it out The truth is very few people care about this stuff,in fact I might think it's a big deal and it's not.
Most want to bolt on a carb and headers,and that will double the power,giddy yap and go.They might not have the time or desire to fuss around for hours messing with tiny drills and timing lights then making repeated timed runs between two landmarks on a deserted back road to gauge performance modifications.

I'll bet a dollar that a recent BOCES graduate needs a weee bit longer than a month...

Thanks! that enriched the knowledge library.

I used to substitute teach at the local BOCES when construction was real slow.I usually did building trades electrical but did a low term sub for auto shop.Half the kids were ok,the other half were just using trade classes to avoid regular HS.Maybe 10 percent of the good half were real sharp.
The best tradesmen I know aren't always the sharpest but they are very observant.

No one ever wants to hear this... the Weber series of 2 bbls, with the tiny 32mm primary, are almost ideal for smallish inlines, maybe unless it's heavily modded for high flow and high revs.

Easier to tune and setup than the Edelbrock and Carter carbs too. Don't let association with 'import scene' make you over look them. $400 new. You can change idle and primary jets with a screwdriver with the carb on the engine. No gaskets below the waterline. No stupid power valves.

You can have performance and mileage both.

The little primary means good vacuum signal and optimum flow low speed on the street, no bogging! The bigger sec flows like crazy with 250 cu in at 3000+ rpm. Work out the flow rates, the Webers are just right.

Let's not forget that Pontiac used 750 CFM Quadrajets on their Sprint sixes that were as small as 230 ci in 1966. Suffice it to say that it was tuned for the application and not used "out of the box" but then,that should be the case for ANY carb,really.

If tuned correctly (perhaps with the aid of a good Q-jet book such as Doug Roe's) even the Quadrajet's 750 cfm is not out of the question for providing excellent power and drivability for our beloved 6's.

Give that man an "AMEN"(and thats a loud one!) And Doug Roe put a Q-jet on one of his off road 181 four banger Vic? Hickey Mini-boots in the 70's. (also on a turbo'd chevette road racer)

Swap meet prices can be in the $10 range if you're lucky.

Now the primary butterflies (2 of them) is 35MM in Hank Talk, and the secondaries (2 of them) is 57 MM (1-3/8" an' 2-1/4" respectively in American talk) You can see why the secondaries have the adjustable air valve feature.

Not to start a war over the "new" and "old" carburators (because I think they all are good) BUT the Offy and Clifford manifold are made for the small base 4 barrel cars including the Rochester 4GC, Carter WCFB and the Holley 4200 aka teapot. These can still be found and all flow, from what I heard, around 350 to 375 CFM. The Rochesters and Carters are found on early 50's Cads, Olds, Buick, Packard etc all in the 300 in+ range. Kit for these carbs exist at fair prices and are easily adjustable with jets or metering rods. I would not use a holley 4200.....

I am currently replacing the teapot on my 56 Ford Victoria to a caddie Carter. It will still hide under the "stock" air cleaner

Hey just another option.....................

The doug roe rochester book has a good chapter on the 4
gcs,their sizes and CFMs, the 4GCs even have tunable secondaries (with a locking screw-WCFBs are only tunable by adding or grinding the secondary wieghts-like an AFB)

Is the "teapot" carb like the stock Ford 4 bbl carb used on the '56 Ford 312s--looks like its guts have been pulled up and out? Some times the swappers think they have a piece of gold just because of the older carb's age .If I had the teapot carb, I think I would keep it covered with a big air cleaner so's not to scare small children.

The "teapot" 4200 Holley was used as the primay 4 barrel on FOMOCO products from 54 to 56 and on the 2-4 barrel T-Birds of 57. They were all made to work with the "Loadmatic" all-vacuum distributors. Since I'm putting in a 57-62 centrifigal/vacuum distributor I'm changing the carb to work with it. Late in the 56 year Mercury went to the WCFB.

All the fuel circuits on the 4200 Holley were above the main body of the carb. Rebiult and restored ones go for $400 if you really want one..........JD

The 500 does have a propensity to bog on fast throttle opening. I used one on a 302 V8 for a while and finally got it to perform well on accels with the right combination of pump cam, pump squirter, carb heat, inlet air heat, and power valve.

If after fooling with all these variables it still bogs have the metering block surfaces fly cut. As built they are unmachined and sometimes not flat which leads to aeration in the pump squirter circuit.

Look at two I flycut. Note that after a .005 cleanup cut they still do not clean up completely. The pump squirter transfer boss is often the last area to clean up. Also notice the carb on the left and how “spongy” the metal looks. Pray you don’t have one like that.



The 350 and 500 share some components but base plates, metering blocks, booster design and venturii sizes differ. If the bowl designs are the same (cathedral or side-fill) then floats and inlet needles can be interchanged.

I ran two diff holleys on my '67 half ton short wide bed 250 with a cliff intake and hedmans/flowmasters. The first was a 450 cfm holley "economaster", a small butterflied carb like a wcfb/4GC, but with MECH. secondaries. With a certain amount of "copying" from a holley book, I was able to duplicate the mods holley said was necessary to run it on a 238 buick V6 (close enough in size-my carb was a replacement on a 350 chevy). After mods,(one of which was to add a set of idle holes and transfer slots under the SECONDARY butterflies to handle the slight bogs) this was a nice carb.

Later I put on a stock 600 cfm "belly-button" holley with vacuum secondaries (NO DOUBLE PUMPER). With virtually no mods, other than idle screw and pump adjustments, it ran real nice too. It seemed that since the back barrels would not open 'til engine needed them, that the old 3.70 geared truck had more pep (from bigger front bbls than other carb) at bottom end making it the more fun of the two carbs.

Now aint those front bbls on the 600 the same setup and situation as a 500 cfm holley TWO BBLs have?. Therefore why cant the 500 two bbls work well on 250s? Most overcarbed motors are going to exhibit bogs, and stumbles. The 600 cfm was really nice, and so was the smaller 450 cfm mech, no-vac secondary carb(after modin'). Vacuum controlled secondaries is the saving virtue. Q-jets, factory mopar thermo-quads ('cept they got a lotta smog stuff on 'em) an' I guess a pair of Pintos too, and most controlled (mech or vacuum-vacuum the better) secondaries carbs should work fine. There are some good carb books on the market-ya ges gotta find 'em and read'em snowman.

The 500 holley 2 bbl is the front 1/2 of a 750 4 bbl. So small throttle changes make a big air flow change.

Oliver,
Deja Vu, I thought I replied already cuz I looked up my test data but maybe Exploder blew up while responding.

I had Gtech tested a stock 250-Powerglide-3.08 using 4bbl carbs ranging from 390cfm to 625cfm. There was no clear trend with CFM, it mattered a lot more that the carb was "happy" - right jets, good shooters, launch technique - you could "stomp" on the 390cfm but not the bigger ones. But if launched correctly, the 390cfm cut a 2.574 60ft and the 600cfm Carterbrock was close behind at 2.580 sec. That is too close to call by far.

In 0-60mph te 600cfm Carterbrock actually won, 10.13 sec vs 2nd place 10.21 sec for the 450cfm Holley. Again very close.

At 1/8 mile et:
11.238 450cfm Holley R4548
11.266 600cfm Carterbrock 1406
11.284 390cfm Holley R6390
11.324 500cfm Autolite 4100 c3af-t
To show how it depends on carb, some "similar" carbs ran
11.406 600cfm Carterbrock 1405
11.538 450cfm Holley R3492 before tuning
12.130 500cfm Autolite c5af-f before tuning, what a dog
Most of the 1bbl Rochester B carbs ran from 11.50 to 12.50 so those last couple 4bbls were real dogs before tuning.

A lot of this was really testing the primaries. The secondaries on the 390 opened about 1/2 way, on the 450 a little bit, on the 500 a little bit, and on the 600s hardly at all tho the secondary transition circuits did help.

Bottom line, if U have a 4bbl in any of these size ranges, TRY IT and play around, if it runs good then just chill and shop around til U find a good deal on just the right CFM. It makes some diff but not that much.

DeuceCoupe,
in your testing, what intake manifold showed to be the quickest 60'? The Offy or the Clifford?
I found it.
Clifford
2.580 best

Offy
2.574 best


MBHD

So, you can say that all those 4 barrel carbs were too large for the tested engine? Maybe a 300 cfm 2 barrel ?

Yup and I knew that. Partly I was just rebuilding some carbs and this was the easiest engine to work on and test em.

My formula for cfm is roughly "double your horsepower" so about a 300-350cfm 4bbl would be perfect, but there is no such animal unless you go back to the early 1950s. A 500cfm (at 3" Hg) 2bbl is about the same size but the whole point was to go progressive.

But the surprising point was it didnt matter. I really thought the big 600cfm Carterbrocks would be soggy but they weren't.