carb sizing

[ragdol]

What is the formula for sizing a carb to an engine. I know it's been dicussed before, but I can't find it. Thanks, Larry

[wayne petty]

about carb sizing...


before you get into carb sizing... as most charts might be for Wide open throttle max performance applications...

if you have been following along the last few days on random posts..

if you are drag racing...  you want max airflow...

if you are running 4:11 gears and a 2500 stall converter, you probably want something with enough airflow to properly operate the engine..


if this application is going to be used for long distance driving..  never seeing the high side of 3000 RPMs... except when passing ... you might want a fairly small carb...

you don't want it operating on the idle transition circuit...  you want it up on the primaries... air flow through the boosters to pull fuel over and through the main jets ....

there is also a LOT of fine tuning done on the idle transition passages... this is best done with heated oxygen sensors and a 30 buck air fuel ratio gauge...   the idle transition passage feeds fuel up through 2500 rpms in most carbs.. sometimes higher..

 one can spend a LOT of money on wide band air fuel ratio meters also... but start cheep...   with the price of gas still around 3 bucks a gallon.. nobody will fault you ... for having oxygen sensor bungs in the collectors..  after tuning..  and verifying.. one can remove the sensors and the gauge...

i know this does not answer your exact question...

but it might help...

[PeterR]

What is the formula for sizing a carb to an engine. I know it's been dicussed before, but I can't find it. Thanks, Larry

The formula is
Cu-ft/min = 0.000289 x (engine size in cu-in) x RPM x (Fill efficiency)

On normally aspirated engines the fill efficiency rarely exceeds 85%

So it comes down to 0.000246 x (engine size in cu-in) x RPM

Easier to remember if you say ¼ x (engine size) x (RPM in thousands)

So for a 400 inch motor not exceeding 6000RPM
¼ x 400 x 6 = 600cuf/min carb

Or a 350 inch motor not exceeding 8000RPM
¼ x 350 x 8 = 700cuft/min carb

[chimp koose]

Its hard to beat a properly tuned quadrajet for economy and performance on almost any street driven V8.

[ragdol]

I'm trying to figure out the carb for a 241 Dodge hemi that I am building for a '37 Plymouth coupe resto rod that will seldom get over 3k. According to the formula I only need a 180CFM carb. I want a 4 bbl.,so I guess the smallest carb I can find will have to do. Thanks for the replies, Larry

[wayne petty]

do you need any extra heads for the hemi...

i have 3 dodge hemi heads for a mid 1950's motor in the back of my VW for decades...



count the springs and valves on the left one in the upper picture... there is a third head hiding behind the porcupine forrest...



i guess i probably should take them apart... clean them up ... grind the valves .. take them to the swap meet...  i just figured i would stumble across somebody who really needed them....  i even offered them to PAW.. they said they have pallets full of them... no thanks..

that small a motor...

http://www.holley.com/0-8007.asp

i even have one of these 8007s kicking around...  somebody milled the air cleaner housing off trying to improve airflow... on a 390 cfm carb..


edit... oh wait... holley did it also...

http://www.holley.com/0-80507-1.asp

[rumrumm]

An old speed shop guy told me many years ago that a good rule of thumb for carburetor sizing is to double your cubic inches, and that should put you in the ballpark. It is not scientific but it works.

[GPster]

And while we're almost on the subject, what engine has got the smallest Quadrajet 4 barrel on it? I've got that 4.3L V6 that I want to run non-TBI. It's got that pseudo cast iron hi-rise manifold on it to let the carb miss the HEI so I'd like to find a carb that would bolt on with-out an inch thick adapter. Even though this engine is fresh,it's cheap and I 'd rather find a '70s or early '80s donor in a junkyard that's close than to do internet shopping. I'm trying to keep my car dreams cheap. GPster

[GPster]

do you need any extra heads for the hemi...
i have 3 dodge hemi heads for a mid 1950's motor in the back of my VW for decades...

Remembering pictures of heads for the bigger "Hemi" those heads with the additional water jacket flanges on the ends were referred to as the heads off of Texas irrigation pump industrial motors. Are they Industrial heads off a boat motor? They don't appear rusty enough to be off a "Salvage". GPster Somewhere there is a Canadian welding instructor that used to be into those motors.

[wayne petty]

these were heads i got from the guy with the big boat that sunk... in payment for some job... back about 1981...

they were spares... he had a 55 or 56 COE dodge truck.. 1 ton.. 2 ton... something big.. that he had ramps and a bed built on to carry his ring type trencher... he dug foundations for housing tracts all over so cal....  and went through to many stop signs as the brakes were just not enough to stop that much weight..  he replaced it with a 63 freightliner that he had the frame extended and built a big steel bed on...


the dodge would overheat going up long long hills...  would blow the head gaskets...  so to save time.. he kept several sets of heads in stock ready to swap on at night..

i have found that every time i throw something out.. i end up needing it within a week or two...


oh the rug they are sitting on is oil soaked... so i never worried about them rusting in the back of the van...


i just googled the casting numbers .. found this confusing link...

http://www.thehemi.com/forums/viewtopic.php?p=806&sid=03f2afcb03eebbc2eeb732f67c48e49c

[GPster]

i have found that every time i throw something out.. i end up needing it within a week or two

You might check around on some Mexican or Cuban sites. They try to make those things run forever and they don't have much luck keeping valve seats in them. GPster

[Learpilot]

I never had a Q-jet to run very good.
The BEST carb I ever owned was a Street Demon 625 cfm on my 350 SBC. It is made in the USA in north Georgia, NOT IN CHINA !!!
Just my 2 cents !!
Rick