holley carburetor help

[moose0211]

hi all, i recently bought a 1973 gran torino sport with a 302 v8. i would like to put a holley 4 bbl carburetor on it and i was wandering what size would work best. i did the "find the carb" thing on the website and it said a 560 cfm carb would be best. but then i did some research and some say its too small. So what do you guys think? thanks
-moose

[chuckster57]

(Engine size (CID) x Max RPM /3,456) x VE = CFM

That I have used many times. VE is volumetrics or in laymans terms the % the carb will "flow". Most stock engines will flow about 70%.

so: 302 X 4500= 1359000

1359000/3456=393.229

393.229 X .75= 295 CFM

Carburetor - Tech & Terminology - Popular Hot Rodding

Good reading on Carbs sizing and adjusting. If you leave it stock then I wouldn't get more than about 500 CFM.

[Believer45]

Can't speak to the 302 but I had a 327 / 275 horse Chev in a 67 Impala that would pull hard to 5700 RPM once I put a 650 SpreadBore on it. Carb was not oversize (car did not hesitate or stumble at all, I must say) and, as long as I could keep my foot out of it, mileage was better than I could have imagined (23.5 on the interstate in a 4500 lb or so Impala SS convertible). Going by displacement percentages 302 is 7.6% smaller so carb size, all else being equal, should work pretty well at 600 CFM or so.

Hopefully someone who has worked on the 302 will chime in as well.

[drcampbell]

Originally Posted by chuckster57

(Engine size (CID) x Max RPM /3,456) x VE = CFM
That I have used many times. VE is ...

... "volumetric efficiency".



If it's not obvious where that calculation came from ...

Displacement (cubic inches) divided by 1728 = displacement (cubic feet)
302 cubic inches = 0.175 cubic feet
(haven't calculated anything yet, just converting to a "consistent" set of units. 1728=12 cubed)

Complete cycles per minute = revolutions per minute divided by 2.
(for 4-stroke engines; 2 revolutions per complete engine cycle)
5,000 rev/minute = 2,500 cycles/minute

Each of those cycles (two revolutions) represents the entire engine displacement volume being pumped through the engine once.

0.175 cubic feet of air per cycle times 2,500 cycles/minute = 437 cubic feet per minute.

But wait. A rating of "437 cubic feet per minute" doesn't mean that only that much air, and no more, will flow through a 437 cu.ft/min carburettor. It means that much air will flow through it when the pressure drop across it is what's specified in the testing protocol. If you run the engine faster, or mount the carb on a bigger engine, more air will flow and the pressure drop will be greater.

In keeping with The Everlasting Principle of Maximum Confusion, there's more than one rating point. The SAE Recommended Practice calls for 1.5 inches of mercury (5 kPa) when testing 4-barrel carburettors and 3 inches of mercury (10 kPa) when testing 2-barrel carburettors.

To further confound things, everything has to be normalized to standard temperature & pressure, or the air density will be off, which will throw the test results off. That's why we use mass flowrate, not volume.

[chuckster57]

Originally Posted by drcampbell

... "volumetric efficiency".



If it's not obvious where that calculation came from ...

Displacement (cubic inches) divided by 1728 = displacement (cubic feet)
302 cubic inches = 0.175 cubic feet
(haven't calculated anything yet, just converting to a "consistent" set of units. 1728=12 cubed)

Complete cycles per minute = revolutions per minute divided by 2.
(for 4-stroke engines; 2 revolutions per complete engine cycle)
5,000 rev/minute = 2,500 cycles/minute

Each of those cycles (two revolutions) represents the entire engine displacement volume being pumped through the engine once.

0.175 cubic feet of air per cycle times 2,500 cycles/minute = 437 cubic feet per minute.

But wait. A rating of "437 cubic feet per minute" doesn't mean that only that much air, and no more, will flow through a 437 cu.ft/min carburettor. It means that much air will flow through it when the pressure drop across it is what's specified in the testing protocol. If you run the engine faster, or mount the carb on a bigger engine, more air will flow and the pressure drop will be greater.

In keeping with The Everlasting Principle of Maximum Confusion, there's more than one rating point. The SAE Recommended Practice calls for 1.5 inches of mercury (5 kPa) when testing 4-barrel carburettors and 3 inches of mercury (10 kPa) when testing 4-barrel carburettors.

To further confound things, everything has to be normalized to standard temperature & pressure, or the air density will be off, which will throw the test results off. That's why we use mass flowrate, not volume.

WHAT???? Maybe a typo

If you read the article that I linked to in my post it mentions 1.5 inches of mercury for 4 barrel carbs. There are lots of things to consider when you try to figure out the best carb. I was trying to give him a starting point...The everyday builder doesn't have to know all the technical points of cams, lift duration, single vs. dual plane intakes and such..

[RatherBNtheWoods]

Originally Posted by moose0211

hi all, i recently bought a 1973 gran torino sport with a 302 v8. i would like to put a holley 4 bbl carburetor on it and i was wandering what size would work best. i did the "find the carb" thing on the website and it said a 560 cfm carb would be best. but then i did some research and some say its too small. So what do you guys think? thanks
-moose

It would have to be a pretty hot built 302 to need more than a 600. I know we all want to have 750 dbl pumpers for bragging rights and to feel "bad", but too much carb will kill the bottom end of the powerband. Back in a day gone by, I had a 400 CID FoMoCo with a tall(> 0.600"), long dur cam, and compression that was not 8:1. With a 780 on it I had to use the lightest spring made for the sec, and lowend sucked. Even with a dbl pumper it only ran well at WOT. Cool if you're 16, or 16 at heart, but horrible as a dd.

I would consider a FI system. More power! argh argh argh.

[montanafan]

Having built and raced several cars and engines, I have to agree with MOST of the answers you received. Campbell is correct, but all that being said, I would heartily recommend a vacuum operated spread bore of about 650-700 fpm. It will not stall you out at low RPMs, but will provide all the air you need, and no more. If you find one at a junk yard, the jets will possibly be too small for max performance. The old Carter "Thermo-quad" carbs were excellent if you could find one that wasn't warped. Even warped ones could be repaired, and once you set one up to your liking, you would probably never have to touch it again.
For a street carb, double pumpers never worked as good for me as just plain vacuum operated secondaries. If it needs an acceleration pump for the secondaries, it is giving too much air too quickly.
If you have a low compression, a "big" cam is a torque killer. It would work with a supercharger, or even a turbo.

[444-4D]

Originally Posted by moose0211

So what do you guys think?

I think it is big enough.

[montanafan]

If he is going to leave it stock, it certainly is large enough.

[drcampbell]

First, you have to decide what you want: A quarter-mile bracket racer, an oval-track stock car or something for tootling around the neighborhood with the grandkids?

First decide what you're building, then select components. One size doesn't fit all.

[moose0211]

thanks guys, i think i understand.lol i am thinking of putting 351c heads on it from bush performance. i know that i will need a new intake manifold, but what else will i need? thanks
-moose

[montanafan]

Never having built up a small block Ford, I can give you a couple of pointers anyway. I would assume the 351 heads would have larger valves, which would increase the potential airflow, which could possibly utilize a larger carb. However, those heads may also have larger combustion chambers, which would lower the compression ratio, a bad thing. With the larger, more free flowing heads, you could enjoy the results of a more radical camshaft, more RPMs and a larger carb, with a free flowing exhaust system. The better heads may not be as good for a fairly stock street machine, and may lower the overall engine efficiency without other mods. Your local bookstore probably has great books about modifying a small block Ford.

[94_turbo]

moose--BE CAREFUL. my understanding is--yes, you can put hose heads on. but the biggest hold, is the intake. are they even made anymore??--that was a one off intake, to use those heads on that block--serious. b&a used to make em. and then after that--you have to approach the issues of cam, timing of the cam,redo the dist curve, diff manifolds,or headers. your going to incur a lot of expense--for what???

[fxdwg]

Look for an earlier set of 302 heads, get a 600 cfm carb, use a mild bump stick, a decent torque converter and a set of 3.70ish gears. She'll be a decent driver with enough fun pedal to make you happy. With that big a chassis, unless you're ready to go 429/460, no reason to run that 302 more than that. Used to run a 351W in a 67 Cougar with a simple 2 bbl carb. Once tweaked, it did just fine for what it was. Cheap fun daily driver that always got me to the track and back home again. Your Torino have the FMX or a C4 tranny?

[Got Diesel?]

Originally Posted by fxdwg

Look for an earlier set of 302 heads, get a 600 cfm carb, use a mild bump stick, a decent torque converter and a set of 3.70ish gears. She'll be a decent driver with enough fun pedal to make you happy. With that big a chassis, unless you're ready to go 429/460, no reason to run that 302 more than that. Used to run a 351W in a 67 Cougar with a simple 2 bbl carb. Once tweaked, it did just fine for what it was. Cheap fun daily driver that always got me to the track and back home again. Your Torino have the FMX or a C4 tranny?

I'll second this.

Look on the Corral.net or any Mustang site and you will find more take-off parts than you know what to do with.