carb cfm?

[jaspermaggie]

how do places come up with carb cfm? I have a holley 750 put on a proform body with no chock tower trimed the screws and blades. Do I have more cfm?

[BracketNova]

I was always told shaving the choke horn was worth 30 cfm. But, if it already doesn/t have a horn I bet 750 is 750. my .02 cents

[drag chevette]

OK, how is CFM determined?

Way back in the day, late '50 early '60s or maybe sooner holley came up with a few different carbs that had physical differences....
a 650 had a certain size throttle blades with a certain size venturi....a 750 was a bit different...and a 850 was different yet.

As years went by people discovered that if they modified these origonal carbs that they could get more cfm from them, (like cutting the choke horn is worth 30cfm and the thinned throttle shaft might be worth about 2-3cfm)

As time passed and engine technology grew so did the need for more air flow....the modifications started happening.
Thinned throttle shafts, no choke horns, and even high flow boosters.

but yet these carbs were still being classed by the origonal baseplate/venturi flow....a 650 was still considered a 650 even though it would flow more air.....the part numbers stayed the same.

Recent times the venturi size part of the classification went away and the base plate is still the standard on which carburetors are judged.

a classic example would be the BG Demon....a 750 demon has a 1 11/16" base plate ...the same as a holley 750 base plate
the 750 holley venturi measures a 1.39" while the Demon is a 1.42"....but yet they are both boxed as 750s?...

Now here is the kicker, while the origonal 750 from way back in the day will probibly only flow 750 cfm because of the choke and huge throttle shaft and other things....the 750 Demon because of the rediused entry and the throttle shafts will flow closer to 930 cfm from the box....yes, i said 930cfm!

as you can see this would obviously make more horse power than the origonal carb.....and the carb builder can say my 750 will make more power than your 750.

OK so why not open it all the way up and make the 750 flow 1100cfm? ...it can be done, but the throttle response and drivability would be greatly deminished and wide open throttle would be the only way to get this carb to run.

a 750 Holley will have a slightly improved throttle response and torque over the 750 Demon, while on a dyno the Demon would make more HP.

its all about marketing and who can make the biggest and/or the badest!

hope i helped, and if and when you want an E85 carb, remember me!...

[wikd69]

OK, how is CFM determined?

Way back in the day, late '50 early '60s or maybe sooner holley came up with a few different carbs that had physical differences....
a 650 had a certain size throttle blades with a certain size venturi....a 750 was a bit different...and a 850 was different yet.

As years went by people discovered that if they modified these origonal carbs that they could get more cfm from them, (like cutting the choke horn is worth 30cfm and the thinned throttle shaft might be worth about 2-3cfm)

As time passed and engine technology grew so did the need for more air flow....the modifications started happening.
Thinned throttle shafts, no choke horns, and even high flow boosters.

but yet these carbs were still being classed by the origonal baseplate/venturi flow....a 650 was still considered a 650 even though it would flow more air.....the part numbers stayed the same.

Recent times the venturi size part of the classification went away and the base plate is still the standard on which carburetors are judged.

a classic example would be the BG Demon....a 750 demon has a 1 11/16" base plate ...the same as a holley 750 base plate
the 750 holley venturi measures a 1.39" while the Demon is a 1.42"....but yet they are both boxed as 750s?...

Now here is the kicker, while the origonal 750 from way back in the day will probibly only flow 750 cfm because of the choke and huge throttle shaft and other things....the 750 Demon because of the rediused entry and the throttle shafts will flow closer to 930 cfm from the box....yes, i said 930cfm!

as you can see this would obviously make more horse power than the origonal carb.....and the carb builder can say my 750 will make more power than your 750.

OK so why not open it all the way up and make the 750 flow 1100cfm? ...it can be done, but the throttle response and drivability would be greatly deminished and wide open throttle would be the only way to get this carb to run.

a 750 Holley will have a slightly improved throttle response and torque over the 750 Demon, while on a dyno the Demon would make more HP.

its all about marketing and who can make the biggest and/or the badest!

hope i helped, and if and when you want an E85 carb, remember me!...

Thats really great information. Here's another question for you:

I'm running a pair of Holley (0-80576S) 4150 HP 750 cfm carbs on top of my 671 blower. How much CFM am I really seeing out of these two carbs ?

[drag chevette]

I'm running a pair of Holley (0-80576S) 4150 HP 750 cfm carbs on top of my 671 blower. How much CFM am I really seeing out of these two carbs ?

I would venture to say that your pulling around 1600-1700cfm at max flow.

with two carbs the signal isnt as strong and the flow will deminish somewhat....the velocity is reduced.

something else that will determine flow is actual cubic inches of the motor.....if you have a 650 carb on a 350ci chevy it will pull pretty good, but if you put that same 650 over on a 700ci motor it will want to suck the boosters through the exhaust as its trying to feed all 700ci through a small port.....and threrfore pull more cfm through increased velocity not volume.

[Hitchcock]

I'm running two 950's, and they wet flowed 830's each on a flow bench. But that is on a N/A motor. Someday I will grow up and get boosted

[wikd69]

I'm running two 950's, and they wet flowed 830's each on a flow bench. But that is on a N/A motor. Someday I will grow up and get boosted

Well, you know what they say...

Babys like bottles, real men get blown. <hehe>

[Hitchcock]

I would love to do a centrifugal blower through a tunnel ram set up. But Have never seen one. If that is not an effective way to get boosted I will eventually go roots. I have spent plenty of time on Littlefields website.

[wikd69]

I would love to do a centrifugal blower through a tunnel ram set up. But Have never seen one. If that is not an effective way to get boosted I will eventually go roots. I have spent plenty of time on Littlefields website.

I'm running a roots blower of course, a street 671. I make prolly 8 psi boost at full tilt boogie and with these two 750's it makes healthy power.

However, if I had it to do over again, I'd go with a ProCharger with either a blow-thru carb setup or a full EFI setup with an EMS control system. These big roots blowers look cool and are really great for nostolgia builds (which is why I have it) but the ProCharger is a lot more efficient (in my estimation). You also have the option of using an intercooler and when coupled with a good aftermarket EFI system, gives ya lots of options for performance tuning you just don't have with this big mechanical roots system. You of course can use EFI with roots, but its a lot more rigid and less tunable than a ProCharger build. Again, my 2 cents...

And on using a tunnel ram, I don't know why you could'nt do that. With a blow-thru setup, you either put the carb in a full box or just cap the plenum on a purpose built blow-thru carb. That could as easily be on a tunnel ram as not. However, I'm not sure what advantage a tunnel ram would provide in that sort of combination.

[Hitchcock]

I would love to do a centrifugal blower through a tunnel ram set up. But Have never seen one. If that is not an effective way to get boosted I will eventually go roots. I have spent plenty of time on Littlefields website.

I'm not sure what advantage a tunnel ram would provide in that sort of combination.

It would look absolutely rad!!
The t-ram, with the caps, not boxed and two big shiny tubes turning and dumping into the motor, again, rad.

[BracketNova]

Photoshop it!

[wikd69]

I would love to do a centrifugal blower through a tunnel ram set up. But Have never seen one. If that is not an effective way to get boosted I will eventually go roots. I have spent plenty of time on Littlefields website.

I'm not sure what advantage a tunnel ram would provide in that sort of combination.

It would look absolutely rad!!
The t-ram, with the caps, not boxed and two big shiny tubes turning and dumping into the motor, again, rad.

10-4

[vegaracer]

I would love to do a centrifugal blower through a tunnel ram set up. But Have never seen one. If that is not an effective way to get boosted I will eventually go roots. I have spent plenty of time on Littlefields website.

I'm not sure what advantage a tunnel ram would provide in that sort of combination.

It would look absolutely rad!!
The t-ram, with the caps, not boxed and two big shiny tubes turning and dumping into the motor, again, rad.

Super Chevy had an orange Nova that was done like that, but I can't find the pictures. It was from about two or threee years ago. Otherwise, do something like this.

[John_Heard]

Wow, bundle of snakes there... Whose car is that?

[sc racing]

Theres a guy at our track with a 67 Camaro with the same type of setup as that pic Vegaracer posted its called " Medusa". I never saw it make a good pass though.