carb cfm's

George Holbrook · Mar 13, 2017

does any i know what the cfm's are on the 520,620,720 carbs are ?

FBH44 · Mar 13, 2017

Could a person figure it out? Displacement times RPM's?

Mike(NEOhio) · Mar 13, 2017

Displacement times 1/2 RPM would be the air volume but the venturi opening would have to figure in somehow.

George Holbrook · Mar 13, 2017

got that....the question is how many cfm 's does the carb flow..?

RandyB(MI) · Mar 13, 2017

That would have to be done on flow bench with a standard of 14.7 psi at intake end for standardized test data. A two bbl auto carb with that size blades/bore size would run around 250-300 cfm. Those carbs easily fed up to 250 hp V-8 engine with 327 cid at 4500-5500 rpm. Do the math on a 300-400ci JD at rated rpm or even 25% over and will find that carb is not the limiting factor for Hp on JD 2cyl. Remember also when figuring it...they are at best no more than 75% effeciency so that must be deducted off total CFM and engine displaces.

George Holbrook · Mar 14, 2017

sounds reasonable...I guess fuel injection is only thing better...I have a B with 390 cubic inch and by doing the math it needs 160 cfm at 1400 rpm"s

buickanddeere · Mar 14, 2017

Throttle and choke plates are smaller in the 50/520/530 carb vs the 60/620/630/70/720/730 however the cast body is the same size before machining.
Airflow is not just the carb. Attach the air cleaner intake manifold and cylinder head at the same 28" of water column pressure . Now try the airflow with the valve at half lift where it spends most of it's time instead of full lift.
Have a cam ground, clean the rough edges and mis matches from the manifolds and ports. Then have a three angle valve job done on the ports and valves.
Check if the choke and throttle go wide open, you will be surprised how many don't.

RandyB(MI) · Mar 14, 2017

Like "Buick" said above, the carb isn't the "weak point". Kinda like putting an 800 cfm Holley 4bbl on a stock 289 Ford. Your cfm figure is right but that's at 100% volumetric effeciency and assuming you will be at 1400 rpm the whole pull. Put a 71 carb on it with gas manifold and grind out manifold taper it back 3-4". All the carb it will ever need. Otherwise I'd just get a 50 or 520 as their intake tract , cam , ignition is all designed with two throat carb in mind. I know a pretty "stock" 50 here in Mi that sends a few of the "Bubba better than you,s" home quite regularly swearing "can't be a 50 " !

George Holbrook · Mar 14, 2017

All true ..My B has 520 block and head..Using a 520 carb with lp manifold..Bullet cam..Cam am fuel ..

RandyB(MI) · Mar 14, 2017

sounds like you are about the best you can expect a "B" to ever be...without "pressure" or oxidizing fuel. Must be a fun ride.

carb cfm's

George Holbrook

New User

FBH44

Well-known Member

Mike(NEOhio)

Well-known Member

George Holbrook

New User

RandyB(MI)

Well-known Member

George Holbrook

New User

buickanddeere

Well-known Member

RandyB(MI)

Well-known Member

George Holbrook

New User

RandyB(MI)

Well-known Member

Similar threads