351-4v best carb cfm?

Bubba_Clide wrote:

Hello everyone, Nick here. Im looking at getting a new carb for my car but dont know what cfm I should get. I currently run a 650cfm brawler carb. Im thinking of going to a 700 or 750cfm but dont know which one I should go to or if I should change my carb at all. If you use the formula it says 680cfm is what the engine needs so its either a 650 or a 700cfm. What do yall run on your 4v engines?

Thanks

I was always under the impression that generally speaking you want 2 CFM for every cubic inch...

Jim A
73 Gran Torino Sport

what intake manifold, transmission and rear gear?

72 GT Ute

You have to remember this key statement:

Your vehicle is a system.

That system is comprised of the engine which is a proverbial giant air pump.

Now, if we are talking specifically factory 4V 4 barrel headed Clevelands, stock carb on the Q code (at least in the case of mine) was a 4300D, number tag indicated it was a 780CFM. I had seen other stuff pointing to other Q codes being 735CFM depending on 72,73 or 74 models.

Let's not forget the 74 model year 4V's getting neutered with smaller valves.

When we get into engines...the build of the engine is going to really come into play.

Intakes, Cam Shaft, stock or stroker engine, exhaust (manifolds or headers) Exhaust system tube size, mufflers. Fuel pump (mechanical or electric) and how much fuel that is moving.

Then we sprinkle in the choice of carbs.

Add a side of ignition. (Points, Duraspark or Mallory/Crane/MSD/Davis Unified Ignition/Pertronix)

The...*gasp* the Cleveland cylinder head. Stock 2V, then stock 4V, open or closed chambered?

Aftermarket heads of the 2V, 3V (port size is somewhere between 2 and 4 barrel heads) and then the 4V stuff.

Add the transmission : (Torque converters also come into play on slush boxes..Q codes has a special 10" converter, nothing like the normal H code 2V 2 barrel Cleveland cars had. 4V's needed that converter. (Forget the specifics if it was a higher stall etc)

3 speed auto

4 speed auto

OR...(My fav here) steeping to modern era 6 speed 6R80 with adapter (best option these days if you like slush boxes)

4 speed Toploader

5 Speed Tremec TKO/TKX

6 speed Tremec T-56 and variants

Rear end gearing:

2.75 highway

3.00

3.10 (older 9" cars, uncommon but I have this gear set for Toploader applications)

3.25 Q code C6 cars)

3.50 Q code 4 speed Toploader cars

3.70

3.89

4:10

4:11

4.30

4:56

Tire size:

Stock tires

225/75R14

235/75R15 was common and 28.8" tall

Common choices for Rear tires:

255/60R15

275/60R15

295/50R15

In my case, I've run

295/50R15 26.6"

245/60R16 25.5"

275/50R17 28"

295/45R18 28.5"

The general feeling over the years (I've been beating my head for 25+ years trying to select carbs.)

when it comes to Clevelands is the basic fact: our engines have a drinking problem. Toss some more rear gearing over stock and the right tire size, and awaaaay you go.

But you need the cam to also take advantage of that extra fuel we all want to dump into a Cleveland.

Myself?

My rebuild saga with my Q code 4V, I am running a milder 540 lift roller cam, stock crank, forged flat top pistons to give me premium fuel pump gas life. I have it backed with a Tremec TKX 5 speed, 3.70 gearing, 28.5" rear tires and I went with an Edelbrock AVS2 1913 800CFM carb.

RobbMc 550 mechanical fuel pump with regulator.

I need to spend time tuning it better...but this package combo...it pulls and won't stop pulling. Amazing for driving. I've said over and over my GTS is going to get me into a LOT of trouble if I am not careful.

It's insane how fast I can hit 120+ MPH in a 3800 pound car. And my combo isn't even that insane.

Now...aside from my long winded posting here...(I know, too late) you need to look at all your factors and ultimately the usage.

If the car is a street cruiser, the carb size, lean a bit more conservative, but the rest of the drivetrain will play into the car being "happy" to drive.

If you beat on it, really wind it out and open it up for stretches, then look to a 750-ish CFM carb.

In the Cleveland groups I am in, there are a lot of diehard Holley fan boys.

I am not a Holley fanboy. I prefer Edelbrock stuff.

I run my GTS, and I make no bones about running it wide open. Plead the 5th, call my lawyer kind of wide open (in a safe manor relatively speaking...)

Main thing is post up what you have for the rest of your drivetrain. And also, now is the time to look are replacing all fuel line. SAE30R9 spec EFI hose is pricy, but it's made to handle ethanol based fuels. Do not use regular carb fuel hose. That stuff does not hold up.

Other item...fire extinguisher. Speak from Experience. Better to have one, and not need it instead of needing it and not having one!

Andrew:GTS.ORG admin, '72 Q code 5 speed Restomod
Pondering: #99Problems

Eliteman76 wrote:

now is the time to look are replacing all fuel line. SAE30R9 spec EFI hose is pricy, but it's made to handle ethanol based fuels. Do not use regular carb fuel hose. That stuff does not hold up.

you can get polytetrafluoroethylene (PTFE) lined hose that is not EFI rated, should hit the pocket a little less

72 GT Ute