carb cfm argument

[doggystyle]

i have a buddy that i got into an argument about carb cfm, he said you can take any carb and make it bigger, he said i could take my 500cfm and turn it into a 750 or whatever size i wanted by changing the jets and re-drilling the small fuel passages, i dont think he is right, is he? i thought that the bigger the cfm the bigger the venturies and other misc stuff, i mean there is no physical outside difference between a 500 and a 750, so am i on the right track here? i would love to tell him hes full of it! thanks

 

[slimlynn1]

The CFM rating of a carb is the amount of air it will flow at a certain pressure, I think they use 1.5 in Hg. So if he can raise the flow rate by reworking the venturies in some way than yes it can be done. Will it be cheap. No! So more or less he is full of it.

 

[Grim-Reaper]

He's right and wrong. The rating is the amount of ir it will flow. You only indicated that he referenced the amount of fuel. You can bore out a carb but it would require larger butterfly be made.


A car works of a "Venturi effect" If you look down the bore you Will find it necks down a little in the middle or will have the fuel Horn in there to create the same effect to create more suction to pull fuel in. This improves atomization as well. Carbs don't have a great deal of material that you can remove so for the most part you would not get a huge amount of gain in most carbs by trying.

Now there are a couple carbs on the market that do have changeable ventures.


The quad has small primaries to improve throttle response and fuel use. You see the throttle blades are counter productive to fuel atomazation. with a smaller primary you have the blades open more so less obstruction to mess with fuel atomazation but the diameter being small keeps it a resonable amount of air flow. The secondaires take charge of the high flow demands.

 

[Thunder]

There is no way on Gods green earth that you can increase the CFM rating of a carb by drilling out the fuel passages and re jetting. The only thing that will do is destory a good carb. Unless your doing it to run methanol or nitro methane.

The only way to increase CFM is with bigger venturies. Or smaller things like cutting off the choke tower and shaving throttle shafts.

 

[dyeager535]

As a for instance if you will, the 750 and 800CFM q-jets (factory) utilize different castings. The 800's primaries are cast 50CFM "larger" than the 750.

People do up the CFM of these carbs, but typically it's by shaving the throttle shafts, throttle blades, remove casting flash, etc.

 

[Z3PR]

The big question is why would you want too when you can buy a rebuildable carb of what ever cfm you want for fairly cheap.

 

[doggystyle]

well this all started when i asked him if he new of anyone with a good 600-650 cfm carb that i needed a new one because the 500 that i had was too small. he said that i didnt need a new one, i could just re-jet my current one, i told him he was full of crap, otherwise they would just sell re-jet kits instead of different size cfm carbs, then he said that the cfm was all controlled by fuel consumption- the more gas= higher rpms=more airflow. i know he is full of bs i just wanted to sound a little more educated when i tell him his eyes are brown

 

[Fierospeeder]

well this all started when i asked him if he new of anyone with a good 600-650 cfm carb that i needed a new one because the 500 that i had was too small. he said that i didnt need a new one, i could just re-jet my current one, i told him he was full of crap, otherwise they would just sell re-jet kits instead of different size cfm carbs, then he said that the cfm was all controlled by fuel consumption- the more gas= higher rpms=more airflow. i know he is full of bs i just wanted to sound a little more educated when i tell him his eyes are brown

because this is real life and not the internet. Just tell him "oh thats how it works" then go call summit up for a larger carb.

 

[diesel4me]

Why is a 500 CFM "too small"???

Unless you have a built big block or a hot cammed small block,you shouldn't need more than 500 CFM --I've run 450 CFM holley carbs on a mildly built 454 on the street,and it ran stronger and got better mileage than the 600 CFM holley it came with AND the original Q-jet carb!....Bigger isnt always better when it comes to carbs...many times "leaner is meaner"!....

 

[blazer74]

Some quadrajets are detuned to a lower cfm by restricting the amount the secondary air valve (upper butterfly) opens. This can be corrected easily. but otherwise drilling the passages and jetting only increase richness.