carb question

[J.C.]

looking to get new carb,and wondering if i should go bigger.i have an 850 holley. the motor is a 468 11:1 with solid cam and big rectangle port heads.just

[BigBlockBaja]

Have you looked into the volumetric effeciency of that motor? Chances are you have all the crab you need.
Just my .02

[poncho-pwr]

Why are you wanting to get a new carb? You should be able to rebuild the 850 you have. Unless you are going to be turning that engine more than 6-6300rpm I would say you have enough carb, but it also probably wouldn't hurt performance if you went a little bigger just don't go crazy.

[J.C.]

the carb is about 6 years old.it is one of the zinc plated holleys.some of the bolt holes are messed up.thinking of going to a 950 or 1000cfm

[poncho-pwr]

J.C. , I don't think you could go wrong with a 950-1000cfm carb, one thing to keep in mind is if you have a enclosed engine compartment you need to get a "marine" rated carb with the j-bent bowl vents to be safe and be Coast Gaurd legit. If you don't have an enclosed compartment I wouldn't worry about buying a marine version. You might check out the Holley HP series 4150 carbs, I have run a bunch of them and like them a lot. Summit sometimes has pretty good prices on them.

[J.C.]

it is a open motor.that is the carb i am thinking of.

[RCB19]

I'm with BBB on this one. If your motor will not swallow the extra CFM's you will go backwards.

[lilrick]

I'm with BBB on this one. If your motor will not swallow the extra CFM's you will go backwards.
......or ateast not as fast as you want...i don't know about backwards!!!:D

[SmokinLowriderSS]

looking to get new carb,and wondering if i should go bigger.i have an 850 holley. the motor is a 468 11:1 with solid cam and big rectangle port heads.just
Big question is how much more gas do you want to burn for very little added horsepower? Unless you are well undercarbed, that's exactly what you will get.

[75MillerJet]

Big question is how much more gas do you want to burn for very little added horsepower? Unless you are well undercarbed, that's exactly what you will get.
smokin off of what do u base what size carb u should have? still learnin :)

[poncho-pwr]

smokin off of what do u base what size carb u should have? still learnin
I'm not smokin, but I'll answer your question. The formula used for figuring out what size carb you need in order to achieve 100% volumetric efficiency is : engine size x maximum rpm engine will turn devided by 3456. That will tell you how many CFM are needed.
Example: 468 x 6000rpm = 2808000 devided by 3456 = 812.50cfm
In the above example you would need at least 812.50cfm in order to achieve 100% VE.

[SmokinLowriderSS]

Boy am I about to crank one up here, LOL. Rarely will your engine ever manage 100% VE :argue: , For a modestly modified engine, 85% is far more likely, maybe closing on 90%. of that 812, 85% of it is only 690CFM. Holley used to have a chart guide, I'm not able to find it this AM :mad:
Also, I have run quite a few carb size changes through engine simulation software such as Desktop Dyno, which is pretty good if it is fed accurate information for it to work off of. Changing a carb size probably has shown there to make the smallest changes when trying to "improve" an engine's production, unless, as I said, the engine is definitely undercarbed to start with.

[HammerDown]

Boy am I about to crank one up here, LOL. Rarely will your engine ever manage 100% VE :argue: , For a modestly modified engine, 85% is far more likely, maybe closing on 90%. of that 812, 85% of it is only 690CFM...
Exactly...so toss on a 750 and call it good to go.
Sometimes bigger isn't always better.

[SmokinLowriderSS]

Here's the chart from Holley I was looking for:
http://www.holley.com/data/TechService/Technical/Selecting%20A%20Carburetor.pdf
And also a pretty indepth article from Carcraft withy Barry Grant of Demon Carb fame. :
http://www.carcraft.com/techarticles/56919/
And Demon's selection guide on this page, click it to enlarge, it is savable as a jpg so you can enlarge it on your own PC. Includes some advice on intakes.
http://www.carcraft.com/techarticles/56919/index1.html