[goat guy]

Have a 455 30 over. Roller cam and lifters. Roller 1.65 rockers. Ported and polished cast iron heads. It has an Offenhouser 360 manifold. 2 1404 edelbrock 500cfm carbs. Still seems like a lot of CFM. Runs good. Wondering if anybody knows what jets, rods, and springs should be in the carbs? Ordered kits to change if necessary. Any help would be appreciated. Even a contact person that knows that set up. Thanks

[61-63]

.085 primary jets, .070 secondary jets, .051x.059 (or close) metering rods. Actually you can use any size primary jets or metering rods just use piRsquare to calculate the same net flow area the .085(.051x.059) combination produces. And atually it would be better to go with larger, say .095, primary jets, because it will give you a larger selection of metering rods to fine tune with should you put an air/fuel meter on the engine and the initial calibration isn't close enough to dead on to suit you.

[misterp266]

CFM does not double on a dual quad setup. It’s still 500 which is on the small side for a 462” motor.

[tom s]

Not near enough!

[61-63]

CFM is not additive on multiple carb setups so total cfm on your setup is not 1000 but it is more than 500 when both carbs are opened up all the way. I do not have the formula (if there is one) to calculate what it is. misterp266 and Tom s are right you are giving up power putting 500cfm carbs on an engine that big. 625s would be better and people do two 750cfm carbs on 462 cubic inch engines.

[tom s]

I run 750s on 428s.Tom

[61-63]

Tom you may be running them but according to the ages old cfm formula you are over carbureted. The formula is (rpmXcu.in.Xvolumetric efficiency)/3456. Thus a 428 spinning 6k operating at 100% volumetric efficiency (which no or very few engines do) requires only 743 cfm. Two 750s fully open would be more than that probably?. The old bathtub LeMans sd cars ran two 750 cfm carbs on 421s and they too were probably slightly over carbureted. Of course I know your 428 is in a street car with progressive linkage and you are thus running off of the front two barrels of your rear carb almost all of the time so no harm no foul.

[25stevem]

The problem with carb size is never too much maximum air volume being available so much as it’s the point where the air velocity thru the carbs bores does not allow accurate fuel metering.

[Half-Inch Stud]

When i over-carb my engines, i do it well and the engine rewards me with good strret/strip results.

Dual Q-JETs on the Offyl manifold is indeed a performance result to bother with.

[Tom Vaught]

Originally Posted by 61-63

Tom you may be running them but according to the ages old cfm formula you are over carbureted. The formula is (rpmXcu.in.Xvolumetric efficiency)/3456. Thus a 428 spinning 6k operating at 100% volumetric efficiency (which no or very few engines do) requires only 743 cfm. Two 750s fully open would be more than that probably?. The old bathtub LeMans sd cars ran two 750 cfm carbs on 421s and they too were probably slightly over carbureted. Of course I know your 428 is in a street car with progressive linkage and you are thus running off of the front two barrels of your rear carb almost all of the time so no harm no foul.

I have to smile on the formulas used on dual carb set-ups.
It DEPENDS on the Intake Manifold used.

So many things that can affect that basic formula.

Tom V.

[61-63]

Tom that's "the" so-called formula, not dual quad, tripower, single four, etc., etc. And you are correct there is lots left out there plus it's old school. Back when someone came up with that formula the McKeller #10 was THE big Pontiac cam. But still when you overcarb an engine you give up low end torque and blow unburnt fuel out the exhaust pipe and allow it to drip past the rings into the oil pan.

[blueghoast]

Originally Posted by Half-Inch Stud

When i over-carb my engines, i do it well and the engine rewards me with good strret/strip results.

Dual Q-JETs on the Offyl manifold is indeed a performance result to bother with.

LOL I like this guy, Two Q-Jets.
Thats gota be bad ass, Way to go 1/2 inch

GT

[carbking]

Originally Posted by Half-Inch Stud

When i over-carb my engines, i do it well and the engine rewards me with good strret/strip results.

Dual Q-JETs on the Offyl manifold is indeed a performance result to bother with.

If I were sufficiently talented to do the necessary sheet metal work fabricating an air cleaner to attach to the "shaker", I would have the intake with a pair of 800 CFM thermoquads on my 350.

Now it runs a single 850 TQ.

For those that are unaware, the "aforementioned formula" is simply a mathematical equation in a perfect world devised more than a century ago by a carburetor enqineer for internal combustion multiple cylinder 4-stroke engine WITH A MINIMUM OF 4 CYLINDERS.

The equation CFM = (RPM times CID) / 3456

May be more understood if written CFM = (RPM times CID) / (2 times 12 times 12 times 12).

The 12 times 12 times 12 because there are cubic feet on the left side of the equation and cubic inches on the right side; the units need to be the same.

The 2 is necessary because of the relationship of the camshaft and crankshaft.

The equation does not hold if: one, two, or three cylinder engine, or an IR intake (which effectively changes the number of cylinders).

The first mention I have seen of the equation was in a textbook printed in 1912.

Jon.

[Tom Vaught]

Originally Posted by carbking

For those that are unaware, the "aforementioned formula" is simply a mathematical equation in a perfect world devised more than a century ago by a carburetor enqineer for internal combustion multiple cylinder 4-stroke engine WITH A MINIMUM OF 4 CYLINDERS.

The equation CFM = (RPM times CID) / 3456

May be more understood if written CFM = (RPM times CID) / (2 times 12 times 12 times 12).

The 12 times 12 times 12 because there are cubic feet on the left side of the equation and cubic inches on the right side; the units need to be the same.

The 2 is necessary because of the relationship of the camshaft and crankshaft.

The equation does not hold if: one, two, or three cylinder engine, or an IR intake (which effectively changes the number of cylinders).

The first mention I have seen of the equation was in a textbook printed in 1912.

Jon.

Another KEY statement.
When I worked for Holley Carburetor at their Research Center, in Warren, Michigan, Zora Duntov was involved with us on doing several Carburetor Studies.

Ford was the primary $$$ source for the 1969 first DOMINATOR carbs for NASCAR. The first carbs were a true 1050 cfm per carb.

Later 1150 cfm carbs were built as well as dual quad Dominator set-ups.

So that brings me to the point about "THE CARB FORMULA".

Holley was interested in "Individual Runner" induction systems. Weber carbs were used on IR manifolds for years.
Jaguar ran engines with 3 WEBER carbs on some straight 6 cylinder engines.

So Holley Engineering designed a "Dominator" carb with 2.75" throttle blades vs the typical 2" throttle blades.
Each BORE flowed 600 cfm. 2400 cfm carb and 4800 cfm on a dual quad IR set-up. The engine was a 10,000 rpm SB Chevy.
Engine was less than 400 cid. Around 377 cid comes to mind.

So "THE FORMULA" for carb size does depend on a lot of variables, as I mentioned:
Intake manifold, carb size, engine rpm, fuel type, etc.

So 61-63, the formula posted was ONE Formula for one carb on one specific intake manifold design for the average guy playing with carburetors. So it is not "THE FORMULA".

Nothing more.

Tom V.

[carbking]

Tom - you mentioned the Jaguar 6 and Webers.

The Datsun Fairlady (yes, definitely not a Pontiac), ran (2) 46mm Mikuni-Solex (Japanese Weber rip-off?) with an IR intake on a 121 CID four cylinder.

But back to the dual quads, as I mentioned, I do not have the skills to fabricate the air cleaner adapter to run dual quads with my "shaker", but I do have a pair of 625 CFM genuine Carter AFB's on my 390 CID shop truck. I run solid linkage, not progressive, and it runs like a scalded dog.

For the engine posted by the OP, I would suggest (2) 500's are way too small for best performance, although low RPM throttle response might be great because of the higher than normal venturii air velocity. But giving away a ton of performance as the RPM's are increased.

Oh, and the IFF (International fudge factor) for the aforementioned 100 year old equation for an engine less than 4 cylinders is to multiply the CFM suggested by the equation by 4/n where n is the number of cylinders.

Mathematics and physics are your friends!

Tom - I am sure it won't happen, but would be great to see pictures of some of those experimental Holleys, and IR intakes. While I have quite a few experimental Holleys (less than I did, as I donated a few to Holley), all of mine predate 1935, which probably have little to no interest to most members here.

Jon

[61-63]

Tom I must disagree with you it IS "the" formula. Since this discussion about it started in this thread I started clicking around on the internet to see if I could find the source of it. I can't but in trying to so I googled it and fine that I get 190,000 reasults popping up and it is on all manner of websites as "the" formula, Summit racing, inchcalculator, gottsfishracing, greedhead, calculators bag, roadkillcustoms, speedway motors, I could go on and on. I'm not saying it is perfect or that it takes into account every induction/camming/etc. etc factor, since it does not, but it has long been accepted as the go to formula if one wants to know what size carb to put on their engine. And it has value. If a person has a 462 Pontiac and all they are going to do with it is cruise around at 2500 rpm they do not need two 750 cfm carbs running in tandem and the fomula will tell them that. They can still put them on there and the car will still work for them; they will just have to feather the gas pedal all the time. Carbs are like cams; a lot of people over do it because of the sound or the look or the bragging rights.

[Skippy597]

Not to hijack the thread but hopefully add to it.

What carbs do you guys recommend looking for while assembling up a dual quad setup for engines 400cid+? I am putting together a 400 with offy 5499 and was planning 500 cfm carters, but now thinking I need bigger?

Thanks

[Tom Vaught]

Originally Posted by 61-63

I'm not saying it is perfect or that it takes into account every induction/camming/etc. etc factor, since it does not, but it has long been accepted as the go to formula if one wants to know what size carb to put on their engine. And it has value.

Have to totally agree that it is the most used formula for Carb selection for the average application.

Key word is application.

Course a lot of NA carbed engines would be a lot less powerful with a single carb (dominator) in a drag strip application vs the typical dual quad usage.

If a enthusiast is not a carb guy, then the generic formula works great for the Summit people, the people wanting to try a different combination engine size, etc.

Tom V.

[61-63]

Originally Posted by Skippy597

Not to hijack the thread but hopefully add to it.

What carbs do you guys recommend looking for while assembling up a dual quad setup for engines 400cid+? I am putting together a 400 with offy 5499 and was planning 500 cfm carters, but now thinking I need bigger?

Thanks

Get two early 60s Buick AFBs like the 3088 or late 50s Ford/Lincoln AFBs. They are 580-625cfm and would fit that engine fine. Watch ebay and two clean ones of the same number. You may have to buy three cores to make up two good carbs those things are 60 years old now and sometimes have been taken apart and put back together with incorrect components.

[carbking]

Originally Posted by 61-63

Get two early 60s Buick AFBs like the 3088 or late 50s Ford/Lincoln AFBs. They are 580-625cfm and would fit that engine fine. Watch ebay and two clean ones of the same number. You may have to buy three cores to make up two good carbs those things are 60 years old now and sometimes have been taken apart and put back together with incorrect components.

Yes.

OR

(2) matching Pontiac AFB's from the early 1960's. Two common numbers are 3300s and 3326s; which may be mixed or matched. And the same caviat about buying extras mentioned by John applies.

Tom was kind to the dual quads when he stated that a single Dominator would out-perform the dual quads. The issue being manifold design is better for the Dominator manifolds; and there has been essentially no design upgrades on dual quad intakes since about 1969. Most are basically early 1960's design; and manifold design is at least as important as carburetor design. (But I still love my dual quads )

Jon.