Carburetor Sizing Chart

[toobroketoretire]

Here's a carburetor sizing chart I have posted on other forums over the years that shows how many cfm your engine will consume at 75%, 80%, 85%, and 100% volumetric efficiency at 6000 rpm. Note the 100% column only reflects off road track racing engines running open headers so it does not apply to street engines running mufflers. Summit Racing recommends using 1-1/2 cfm per cubic inch for street engines and the 85% column almost exactly reflects their formula. I suggest you print this chart for future reference.

You'll notice a 350" engine only consumes a scant 517 cfm at 6000 rpm (at the 85% efficiency) which proves a 600 cfm carburetor is more than enough. In fact a 600 cfm is enough for a 383" and 396". It's not until you reach 427" that you need the larger 750 cfm. I'm running a 750 cfm vacuum secondary Holley on my beefed 454" and under full power @ 5500 rpm the secondaries are only opening about 90%.

 

[toobroketoretire]

When it comes to selecting camshafts and carburetors the MOST common mistake by a novice is selecting TOO BIG for the application. Yes, killer cams may sound really cool at the local drive in but when combined with tall gears and an automatic transmission they are a disaster. Good for the Bonneville salt flats but NOT good for the street.

 

[dap220]

Very interesting. I just converted to an edelbrock performer 600 and have been wondering if I should have gone bigger while I was at it. Sounds like I should be good. Although it does put a damper on my dream of dual quads on top of a high rise manifold!!!

 

[toobroketoretire]

Very interesting. I just converted to an edelbrock performer 600 and have been wondering if I should have gone bigger while I was at it. Sounds like I should be good. Although it does put a damper on my dream of dual quads on top of a high rise manifold!!!

And a 350" engine running at 85% efficiency at 5000 rpm only consumes 430 cfm. An engine running at 85% efficiency is a HOT engine like a LT-1 but an L-48 will only consume 387 cfm at 5000 rpm. It makes you wonder why some forum members are putting 750 cfm double pumpers on their L-48's. Lost their minds?

 

[toobroketoretire]

I am a member of another forum and I have always been astonished over members putting HUGE 750 cfm double pumpers onto their L-48's when those engine can only consume 379 cfm at 5000 rpm. For an L-48 a 500 cfm Edelbrock or 600 cfm Holley would be more than enough and would give the very best fuel mileage. So why are they installing those huge 750 cfm Holley double pumpers? Yep, on the recommendation of well-meaning but VERY ignorant forum members who don't understand anything about carburetors nor how to select them. Like the blind leading the blind.

The really nice thing about the Rochester QuadraJets is their ability to be used on just about any 250" to 500" engine because of their AVS rear butterflies. The Edelbrock AVS carburetors use the same technology but Holley took a slightly different route by using a vacuum diaphragm to open the secondaries. Regardless of the style used they limit the amount of throttle opening to prevent uh "too much too soon" and reducing power. That's why those carburetors will always produce a lot more power than a double pumper at low rpm's. The double pumpers are intended solely for high rpm track racing where engine speeds never drop below 5000 rpm.

 

[Bendeco]

How many cfm does a renegade move. Do you still have your 82? Hi buddy!

 

[toobroketoretire]

How many cfm does a renegade move. Do you still have your 82? Hi buddy!

Yep, I still have it but a seldom drive it now that I got hooked on 454" power. But to answer your question DCS claimed around 750 to 800 cfm from the Renegade. Once I installed it my engine really woke up at 3500 rpm. I have often read the stock CrossFire intake only flowed about 425 cfm because of it's very small cross section runners.

 

[toobroketoretire]

High Venturi Velocity Is Needed To Atomize The Fuel

The 340 hp 327's used a 600 cfm Carter AFB and the 360 hp 396's used a 600 cfm Holley; both using the proven vacuum secondary technology to prevent over-throttle and power killing fuel standoff. It wasn't until power levels reached 375 hp when the 780 cfm Holleys or QuadraJets were used and they too used vacuum or AVS technology. As the 600 and 750 cfm mechanical secondary carburetors (aka double pumpers) meter fuel so poorly at low rpm's under full throttle the factories never used them but yet now days we have thousands and thousands of ignorant people using them because they don't understand how carburetors work. It requires VELOCITY thru the venturi to atomize the fuel and when that velocity drops to one half (such as in double pumpers at low rpm) the engine stumbles as the fuel enters the cylinders in big droplets instead of a fine mist. That's why the double pumpers are never suitable for street use and only suitable for continuous high rpm operation where venturi velocity remains high. But as people love the name "Double Pumper" they will continue buying them for the cool name alone.

Over the years the 750 cfm "double pumpers" have become extremely popular among the 350" C3 owners but those huge carburetors should never be used on street engines and certainly not on little 350's that only consume around 500 cfm at wide open throttle operation. The mechanical secondary carburetors are solely intended for high rpm track racing applications where the engine speed remains at 4000 rpm or higher. For street 350's the Rochester QuadraJet is a perfect carburetor as it'll only allow what the engine requires. Next a 500 cfm Edelbrock or 600 cfm Holley with AVS or vacuum secondaries would also be ideal carburetors.

 

[toobroketoretire]

Very interesting. I just converted to an Edelbrock performer 600 and have been wondering if I should have gone bigger while I was at it. Sounds like I should be good. Although it does put a damper on my dream of dual quads on top of a high rise manifold!!!

In another forum I have seen a trend for L-48 owners to install HUGE 750 cfm double pumpers which is a HUGE mistake for two reasons: The first reason being a 750 cfm carburetor is way too big for a street 350" engine and the second reason being double pumpers don't belong on street engines that operate between 550 and 5500 rpm. They're okay for track racing where the engine speed never drops below about 5000 rpm but as street engines seldom reach 5000 rpm they're just not suitable.

 

[toobroketoretire]

You can use a 750 double pumper on a street 350" but it'll run awful at the lower rpm's because of VERY slow venturi velocity at full throttle. And as a street 350" engine can only consume a scant 429 cfm at 5000 rpm at an 85% efficiency there is no point in using a HUGE carburetor like a 750 cfm. That's why Holley always recommends a vacuum secondary 600 cfm carburetors on little 350's to prevent too much throttle opening and the loss of power it creates. People have been convinced MORE is always better (aka More's Law) but that only applies to certain things.

 

[kpic]

It's Moore's Law and its focus is computers.

"Moore’s Law "Transistor density on integrated circuits doubles about every two years."

Try again.

 

[toobroketoretire]

I was surfing thru the internet earlier today and stumbled on an article about the 427" Ford engines. When reading the specs it said they used 650 cfm carburetors but it didn't say which brand. I thought that was interesting as it almost exactly matches my sizing chart which says 427" engines at 85% efficiency at 6000 rpm will consume 631 cfm.

 

[kpic]

New correct design holley carb for your camaro. Exact for all 1969 big block 427 , ZL1 , L88 high performance models. 850 cfm delivers the needed fuel to give optimum response to your ride. Features original GM part number 3955205 holley list number 4296 - on the front air horn, just like originals. This is a brand new in the box holley carb. Gold iradite plating as factory units. Fresh to date technology with the right old school look. This is the only version Holley carburetor double pumper carb ever to be installed by Chevrolet from the factory.

No, you're wrong as usual about the double pumper.

1969 Carburetor, Holley 4296 - 850 CFM , 427 , L88 , ZL1 - 3955205

New correct design holley carb for your camaro. Exact for all 1969 big block 427 , ZL1 , L88 high performance models. 850 cfm delivers the needed fuel to give optimum response to your ride. Features original GM part number 3955205 holley list number 4296 - on the front air horn, just like originals. This is a brand new in the box holley carb. Gold iradite plating as factory units. Fresh to date technology with the right old school look. This is the only version Holley carburetor double pumper carb ever to be installed by Chevrolet from the factory. Tested and guaranteed, ready to rock!

You really need to learn how to do a search. :L

You do realize the big power 427 Fords had 2x 4bbls?

Point is, the folks here are Chevy people, not Ford.

Anyhow beings you brought it up...

If you'd check out the big HP Ford 427s; you just might learn something.

C3AE-9510-C 63-64 406-427 Ford / Mercury 427-406 carburetor 4V or 2x4V

C3AE-9510-BJ Sec. carb (mounts rear) for 2 x 4V setup No Choke butterfly 600cfm

C3AE-9510-BK 63 1/2 - 65 427 Ford/Mercury 67 428 Shelby GT-500 Primary carb (mounts front) for 2 x 4V setup w/Choke butterfly 600cfm matching carb C3AF-9510-BJ

C5AF-9510-BC 65-67 427 Ford Secondary card from 2 x 4V 710cfm each

Ford and Mustang Holley 4160 carburetors

@X 4V means 2 4 barrel carburetors. Please read before you post.

There are low HP versions of both small and big block Chevy and Ford engines. It seems the high powered ones say you don't know what you are talking about as usual.

I know the Chevy, Dodge, Ford and Plymouth engine engineers didn't know what they were talking about as usual. Or is it you don't know what you are talking about as usual??

 

[toobroketoretire]

The dual quads and 3X2's were a marketing gimmick and certainly not needed for any of those high performance engines as a single 830 cfm Holley or QuadraJet would have been more than enough. When I restored my big block '71 I had considered dual quads but realized a single 750 cfm #3310 was more than enough, less expensive, and a lot less trouble to maintain.

 

[kpic]

As Grumpy Jenkins said:
"A properly set up and correctly tuned tunnel ram intake is designed to maximize an engines power band in the 5000-rpm-7500-rpm range on most v8 muscle car engines, its never going to be ideal cruising at below 5000-rpm but that does not mean it can't be used or that its going to be useless on a street driven muscle car, provided the owner knows how to adjust and tune it."

Do you understand what he said or is he just another fool in your eyes??

He also stated:
"AND I'M ALMOST ALWAYS AMAZED AT THE COMPLETE MIS-MATCH OF PARTS, MOST GUYS ASSEMBLE THAT THEY THEN EXPECT TO RUN CORRECTLY.
and as usual, a bit of careful research into matching engine compression, cylinder head, port cross sectional area,runner length, cam timing, cam LSA,valve lift and duration, head flow potential,collector length, and exhaust header design,in general to inhance scavenging, the cars gearing, carburator ,fuel/air ratio,fuel atomization, ignition advance curve,tire diameter, drive train gearing,etc. to match the intended power band will go a long way towards getting any tuunnel ram intake to perform well."
tuning a tunnel ram intake | Grumpys Performance Garage

Look at the detail and depth a pro goes to; very few match ports. That being said very few realize the affect of a forward facing step in the air passage.

It is all what one is after. Some want that extra between 5K-7.5K and others do not. However, as Grumpy said, it all boils down to the tuner's skill with adjusting and tuning it.

However, I doubt anyone would consider the old 2x4 426 Hemi or the 3x2 389 pokey due to having too much carburetion. It seems the old muscle cars were a lot quicker than you choose to admit.

Properly tuned and setup, an L88 is right quick. Add a tunnel ram and 2x4s properly tuned and setup as a package, it is even quicker. It is a basic fact of life. You can debate it all you wish.

Without doubt, a single 4 barrel is a lot easier to tune than 2X 4s; however, that does not mean anyone else is stupid because they can and you can't.

It seems you are clueless how many hours are and were spent in testing and tuning by the dummies at the factories.

 

[toobroketoretire]

What is the point you're trying to make?

 

[kpic]

What is the point you're trying to make?

If you haven't gathered that by now; I and everyone else who has answered you are truly astonished.

 

[toobroketoretire]

You guys are just too smart for me to deal with.

 

[toobroketoretire]

The ideal carburetor is one that is sized barely big enough to feed your engine during normal usage. And wide open throttle at 6000 rpm is not "normal" usage for most C3 street engines.

 

[Antz81]

The ideal carburetor is one that is sized barely big enough to feed your engine during normal usage. And wide open throttle at 6000 rpm is not "normal" usage for most C3 street engines.

So you're saying the correct size carb is one that starves the engine for fuel and air at wide open throttle?