How Do I Pick What Cfm Carb To Use?

[toad490]

Hi,This is the formula I have for determining what CFM carb to use.
Engine Displacement (511) x Max Rpm (7000) divided by 3456= 1035.
That seems a little small to me, eh? Isn't a 6 pack 1350 cfm?

[Tarrbabe]

But you must remember that the outboard carbs on the 6 pak were vacume operated. Therefore because of what the engine demanded it got, but not more. It couldn't get more than it could use.

[dave571]

The calculation is correct, but keep in mind that the calculation is at 100 % volumetric efficiency, which your motor can not have.

Therefore, you will not even need all of the number you came up with.

BUT the issue then comes up that the manufacturer's flow ratings on the carbs are bunk.

An 850 demon is too big for ANY stock motor, yet an 850 TQ is great for a 318. Fairly obvious which rating is a load of BS.


My 440 does fine with a 750 mighty demon at 6000 rpm. Good enough to push my big heavy pig into the 11's.

My question on your 511 is, why are you runnning it all the way up to 7K? May I ask what the rest of the combo is?

[Tarrbabe]

It was long ago stated by the (experts) that the old log rams actually went above 100 % on efficency. Yes it sounds odd but with the ram effect it was close to a mild turbo. BUT with todays intakes that fit, it is unlikely.
I would think IF everythig is matched PROPERLY it could use a 1050 cfm very well. If less than perfect match I would go with a 850 for street or a 950 for performance.


Just thinking out loud.

[toad490]

Sure Dave, here it is.

4.25 Keith Black billet crank std/std.
Indy aluminum main caps
Main cap girdle 3/8''
R&R 7.135" ally rods
Bill Miller pistons and pins
Comp Cam solid roller
Intake- .761 lift 328 duration
Exhaust- .729 lift 332 duration
Indy 440-1 heads, 1.6 rockers and intake
Partial block fill
TTI 2 1/8" to 2 14" stepped headers

I'm thinking of getting a set of 1.5 rockers, that'll back the cam down to .714 and .684.

[pishta]

Originally Posted by toad490

Hi,This is the formula I have for determining what CFM carb to use.
Engine Displacement (511) x Max Rpm (7000) divided by 3456= 1035.
That seems a little small to me, eh? Isn't a 6 pack 1350 cfm?

A 6 pack is closer to 700 CFM due to the inability of a 440 to pull 1.5 inches of mercury across all 6 barrels at any RPM. Its all relative to the 1.5 inch factor that holley came up with.
CFM=CID X RPM X CF/ (2 X 1728) where CF is the correction factor considering the VE of the motor 1.00 = 100 Volumetric efficiancy (use 92 for a stock V8 using a 250 duration cam @ .050) Use this handy dandy chart from Vizards book to determine your VE, or get close. Use 6 for stock heads, 5 for pocket ported heads, 4 for street ported heads, 3 for race heads, 2 for Nascar quality heads, and 1 for exotics like 4 valve heads.

[rumblefish360]

An 850 demon is too big for ANY stock motor, yet an 850 TQ is great for a 318. Fairly obvious which rating is a load of BS.

The 850 T-Q isn't very good for the 318 due to the large primarys. Even so, the smaller T-Q is a rated 800. The large secondarys are the flow reasons for the rated CFM. Weather or not the ratings are bunk or dead on, the T-Q can tune the secondary door to fit a wide varity of needs. Thats why you can find them on 318's - 440's.
How they rated carbs back in the day is beyond me, but would absoultey love someone to flow the T-Q's. Even better if it could be done with a liquid to simulate fuel.

Dave, let me ask, BG "Wet flows" there carbs right?
I don't think they did that back in the day did they?
(Just wondering outloud there. )

Also a note on the six pak carbs. There 2bbls rated as so right? Different vacuum readings on there ratings right........

7K!!!!!

[toad490]

Thanks Pishta, that CF is what I was looking for, BTW the last calculation is divide. It works out 1128 cfm with line 4 and 1169 cfm with line 3 @ 290 duration. At 6000 rpm, line 4 is 966 cfm to 1000 cfm on line 3.

As far as turning it 7000 rpm, my heads are flowing 327 cfm so probably not, but I'm thinking of sending them to Indy for CNC porting. If I don't, is being 100-150 cfm too big that critical?

[dave571]

Originally Posted by toad490

Sure Dave, here it is.

4.25 Keith Black billet crank std/std.
Indy aluminum main caps
Main cap girdle 3/8''
R&R 7.135" ally rods
Bill Miller pistons and pins
Comp Cam solid roller
Intake- .761 lift 328 duration
Exhaust- .729 lift 332 duration
Indy 440-1 heads, 1.6 rockers and intake
Partial block fill
TTI 2 1/8" to 2 14" stepped headers

I'm thinking of getting a set of 1.5 rockers, that'll back the cam down to .714 and .684.

Thats one heck of a combo

You going to run the 'big' series of intake, so you can use a king demon, or the holley equivalent?

[dave571]

Originally Posted by rumblefish360

Dave, let me ask, BG "Wet flows" there carbs right?
I don't think they did that back in the day did they?

Also a note on the six pak carbs. There 2bbls rated as so right? Different vacuum readings on there ratings right........

That's my whole point. There is no standard, for CFM ratings that truley relates to the real requirements of the motor.

A tq's rating isn't at the same vacuum signal as a holley, so they aren't a fair comparison to each other. The Holleys and eddy flow ratings don't include the fuel. The demons do, so all of them are like comparing apples and oranges.

I see posted on mpc a lot that a 360 needs more that a 650 holley because the TQ was 850.

It's just BS.
A 650 holley is quite a nice size for a hopped up 360. The 360 I ran in my van worked very well with a 600 eddy. I ran it once with a TQ also, and that worked too. For that matter my 400 in my newport works great with an old 2 bbl on it. Much better than it did with the TQ on it, before I swapped it into the car.

Comparing carb CFM ratings is like comparing factory hp rating s on vehicles, and thinking that will give you a real idea of how it will perform.

[pishta]

Originally Posted by toad490

Thanks Pishta, that CF is what I was looking for, BTW the last calculation is divide. It works out 1128 cfm with line 4 and 1169 cfm with line 3 @ 290 duration. At 6000 rpm, line 4 is 966 cfm to 1000 cfm on line 3.

I agree, you have to devide the whole thing by 2456, not devide by 2 and then multiply by xxx. I got a large number also, but remember 290 at .050 is a sick drag racing nitro/top fuel solid/roller only cam! you sure your running that large?

[rumblefish360]

dave571, Thanks dude. I agree.

[DAHEMIKOTA]

www.mortec.com/carbtip1.htm I know it is for Holley carbs, but it has some formulas.

[bigiron]

Why waste that combo on carbaration... get a 12-71 and a bird catcher and be done... now THAT would be cool...