Have wondered about this for awhile but never thought to ask until I saw the previous post; With the old caution about not using too big of a carb, or use a formula to estimate what size carb you need, why did some muscle cars come from the factory with multiple carbs totaling 1000 cfm or more? I realize the secondary carbs are usually vacuum operated, but why so much cfm potential on stock or mildly modified street cars? Thanks.

First off they aren't that big when you use the same measurement scale and secondly they are vacuum operated so they only open up as needed.

Andy was exactly right.

I'm going to expand on Andy's answer. Two barrel carbs are rated at 3" depression and four barrel carbs at 1.5" depression. That makes any two barrel carb's rating 1.414 times the rating if it was a four barrel. That's the square root of the ratio of the depressions.

So that 500 Holley 2-barrel is actually 500 / 1.414 = 354 cfm rated as a four barrel.

Back in the day the "how big" question was answered by magazine writers as [(cubic inches X rpm) / 3456] X volumetric efficiency.

That effectively resulted in a carb that wasn't too big, but as we have figured out, it left quite a few horsepower on the table.

There is a lot of power to be had in getting manifold vacuum to be less than the rating point of 1.5". The problem is that as the carb gets bigger the air velocity gets smaller and it becomes harder to get enough signal to meter the fuel properly. The carb becomes too big when it can't meter properly.

Most carbs have some sort of mechanism that responds to engine demand, so in effect the carb is only as big as the engine needs. Vacuum secondaries and spring loaded air doors are some of the ways to do this. On the Mopar six-packs, the outboard carbs were opened by vacuum. On most OEM dual four setups the carbs are set up to open sequentially with a progressive linkage.

R.

For a street-only car, the only serious mistake is making the primaries too big. Factory engines as small as 230" have used QuadraJets.

the key is velocity controlled. too me this throws all the cfm stuff under the bus. over carbing is exposing too much venturi area too soon in the power band which slows the air column down. basically this makes the carb not flow fuel. there is no such thing as a carb with too much cfm. a carb can't make 1 cfm.

Too add to this, a too small of a carb on a hot motor can be real sensitive and a bear to tune sometimes because of the high velocity however I have Dommy's on cars/trucks that the "pros" said would never work for the reasons you mentioned above as in not enuff velocity..........I've had 1250 Dommy's on my dart for street testing with good results and why is that............

i'd say your keeping the rpm up.

The place where air speed matters is inside the primary booster.

That's one of the criteria and I have a fairly loose 5200+ vert, 12.1 comp, 4.11 gears, fairly light etc........Like Mr.PBody said, he had a 1050 on a 300 something cubic in. small block BUT super light, good heads and combo plus high rpms. I've learned a couple hard humbling lessons lately about high winding SB Mopars. They love CFM and Shiloh and Tory will attest, they have HUGH carbs that some big blocks cant utilize and they run stellar #'s for their combinations.............Fun stuff, so the whole "theory" CFM calculators don't seem to take these things into account at least the ones some speak about on here........

Important for sure and that sets up the rest of the tune...........

Poly, revise thinking downward. The Solex 4A-1 on my Mercedes 280 six is a Quadrajet copy and the engine is 2749cc which calcs out to less than 170 cubic inches.

R.

Not only do SB mopars like cfm as my stock CR 340 did (850DP+2/10ths over a 750DP).. we had a well known racer with a Mk1 Ford Cortina running a Rover 3500c (215ci buick V8) stick car, lots of rpm etc. He amongst all the other racers kept on about my 850 carb being too big, so I persuaded him to try my 850 straight on his little 215ci motor,.....result.. 2/10ths faster, he couldn't believe it, throttle response wasn't as good as his 600 carb of course but I proved a point to them all back then in 1984.

Yes sir, see it a lot where a "calculated" computer program spits out cfm recommendations but there are too many variables to nail YOUR specific cfm needs........

JMO... those CFM calculators tell you the smallest carb cfm to possibly run on an engine, not the correct one.

Yep........And monkey see monkey doer's follow not realizing it....

"Smallest carb CFM to possibly run"?
Definitely not, MUCH smaller works perfectly well.

Why does Holley still say to use a power valve half the idle vacuum and recommend CFM be based on that old equation?

Shouldn't both be updated? Or better yet, be eliminated all together. I'm not sure either one was ever accurate. But we sure know better than to use them today.

Bring the on!

they're trying to make their product idiot proof. dumbing people down seems to be the American way anymore.

You can put a 1bbl carb on a 440 and it will run perfectly well...

Won't do much for power, but some have to pick everything apart.

390's ran great on the older Busch series cars. Not as well as if they had a worked 750+ carb on them... but perfectly well.

some have to pick everything apart

So annoying - someone had the correct answer, and it wasn't you.