get a demon!!

assuming maximum efficency the formula is

(cubic inches) X (Max RPMs)
----------------------------------- = Max CFM Flow
3456


If you get more than your max cfm flow you *may* DECREASE PERFORMANCE, and require smaller jets.



-Jeff

 

For your aplication a 550 cfm to a 600cfm. I would stick to a q-jet Best street carb in the world.

 

Be better running with a duel plane and a Holly #4160, 650 vacuum secondary carb.

 

First off, Holley doesn't make a 650 CFM vacuum secondary. Second, that carb formula is the biggest crock of **** ever. I agree with the dual plane manifold. Get an Edelbrock RPM Air Gap. The best carb for the first setup is going to be a Holley 600 double pumper for more performance, or a Holley 600 vaccum secondary for a little more economy with performance. If you go on Ebay the 1850 CFM Holleys, (the vacuum secondary carbs) are a dime a dozen, $20 if that. The 4776 Holley 600 (the double pumper) is usually in the $75 to $100 range. I would get the double pumper.

I would build a SBC 400 if I were you but if you go with the 327 get a Holley 700 DP.

 

Don't sell those little 283's short. A friend of mine ran a 283 in a big old 65 Impala as a P/Stock 4 speed in NHRA for years. Ran high 12's. Iron heads and intake. Lot's of work but man did that thing sound sweet!

 

http://www.holley.com/HiOctn/ProdLine/Products/FMS/FMSC/0-80783C.html

 

http://www.holley.com/HiOctn/ProdLine/Products/FMS/FMSC/0-80783C.html

I stand corrected. Now they do have a 650 vacuum sec. But, it is a 4150. It is a common mistake for people to call the 1850 a 650 CFM carb.

In that case I would use the 650 vacuum or the 600 DP.

 

I have a little problem with a few of those statements, First of all that formula is usually pretty close to the mark second I don't think I would ever run a mechanical secondary carb with an auto definitely not on the street and 3rd a 650 holley would almost certainly be to much carb. I agree with auto-part the q-jet is the answer.

 

I have a little problem with a few of those statements, First of all that formula is usually pretty close to the mark second I don't think I would ever run a mechanical secondary carb with an auto definitely not on the street and 3rd a 650 holley would almost certainly be to much carb. I agree with auto-part the q-jet is the answer.

That formula doesn't take into account any of the important info on selecting a carb. You will never make a decent power using that formula. With a big cam, nice heads etc. you will be severely under carbureted. How could you use that formula for forced induction or tunnel rams? You mean to tell me 600 CFMs is going to cover every 350 Chevy. Yeah right.

Double pumpers and automatics are fine together as long as you pick the right combo. You can see I picked a smaller DP then the vac. secondary. Stall and gear ratio have a lot to do with carb size. So do compression, cam, and head size.

A 650 vacuum secondary isn't necessarily that. It uses engine airflow to open the secondaries. If you have the carb adjusted right then it will only open as much as needed. Maybe 500 CFMs at part seconadry throttle opening.

A Q-jet with it's massive secondaries as compared to it's primaries, is usually going to bog when you floor the gas. The square bore is much more forgiving and easier to tune.

I guess you just have to learn all of this with experience. You can't believe everything you read. That formula is a perfect example. I still stand by those carb picks.

 

According to that formula, my 331 SBF stroker shouldn't even have a 600 cfm carb on it. In fact the guy that runs the dyno I use said the Edelbrock 750 was too big! Until the first pull on the dyno that is. That changed his mind quick.

I think that formula is best suited for picking a carb size on a strictly stock engine. As mentioned above, too many other factors must be considered. David Vizard believes that a high performance engine should have two cfm per horsepower. And another wise man once said......Give it what it wants not what it needs.

I'm waiting for my Proform 750 carb to show up on Tuesday. Summit shipped it yesterday.

I'm running AFR 185 heads with 2.02/1.60 valves, 58cc chamber (cr 10.5:1) and a Buddy Rawls custom cam. Although my tranny is a C-4 (tweaked that is!), I've got a 2600 stall converter and 3.70 gears so I don't expect any problems running a mech secondary carb.

I also don't expect great mileage! :mwink:

 

I'm glad I'm not the one trying to make a decision based on the suggestions given. Everybody has an opinion and an armpit.. *see below
Contact the tech support staff of HOLLEY, BARRY GRANT/DEMON and EDLEBROCK and give them your specs. See what they suggest (especially true with a DEMON). It's a free professional service, albeit bias since they each want to sell you there product. Then look at costs, ease of tuning/maintenance if that's a consideration and then decide if you want to buy used, EBAY or start fresh with a new unmolested carburetor...somthing that an "expert" hasn't modified.
There are professional carburetor builders that may be able to provide you with a custom built carburetor made just for your specs.
If the car is primarily a street car, then conservative will give you better low end to mid range performance, exactly the conditions you'll be driving under. Fuel economy will also be improved.
If the car is primarily a race car, then the sky is the limit.
If the car is a combination, then be sure that that is taken into consideration.
Carburetors are one of those things that too much is not always a good thing.
*Like a deordorant, everyone's armpit is different and what'll work on one won't work as well on another.

 

According to that formula, my 331 SBF stroker shouldn't even have a 600 cfm carb on it. In fact the guy that runs the dyno I use said the Edelbrock 750 was too big! Until the first pull on the dyno that is. That changed his mind quick.

I think that formula is best suited for picking a carb size on a strictly stock engine. As mentioned above, too many other factors must be considered. David Vizard believes that a high performance engine should have two cfm per horsepower. And another wise man once said......Give it what it wants not what it needs.

I'm waiting for my Proform 750 carb to show up on Tuesday. Summit shipped it yesterday.

I'm running AFR 185 heads with 2.02/1.60 valves, 58cc chamber (cr 10.5:1) and a Buddy Rawls custom cam. Although my tranny is a C-4 (tweaked that is!), I've got a 2600 stall converter and 3.70 gears so I don't expect any problems running a mech secondary carb.

I also don't expect great mileage! :mwink:

I agree, Im running a 800cfm DP Holley and there may be a better carb for my set-up but I havent found it, and its just a little 355, it seems the formulas dont actualy take into account for things like stall, or CR, just CID V/S RPM...or whatever...there a thousands of variables you could take into account, but for the street, if you cant figure out what you need just buy an edelbrock...there must be thousands and thousands of muscle cars running around with these carbs and illbeit none or few actualy know what CFM edelbrock they have....

 

Your forgetting the most basic thing,
It has vacuum secondaries so it will only open as much as the vacuum signal require's like lluciano77 , 2wld4u & mstngjoe is saying.
If the motor only needs 575 CFM that's ALL it will get <THIS is a example only, it just may go up to 650.
Now this #4150 - 650 CFM is a very good carb for what he need's and is a recommendation from a few of us.
Just my .0202.5 worth.

2DoorCaprice, Please let use know what kind of carb you end up buying, then the results.

Thank's, LOL
George :thumbup:

 

Thanks for all the replies!

Glad that I'm not the only one a bit in doubt, (I'm a v8 newbie as well...)

Anyway when the 327 is finished is would like to pull some very streetable 350 HP from it...

Engine is in my '59 Corvette, so It'll definetely be 90%+ street, with the occational "eat this -punk!" at the lights, and almost certainly <10 1/4mile pulls during the year (sport is hardly known here in Denmark.. sad....really sad)
Converter is stock (1200rpm stall), but I would consider going up to a ~1800 stall version.

Cyl heads will most likely be my #462 camlehumps!

I did ask the tech depts and they all got the same engine info mentionend including the 327 350hp expect the stall:

Holley tech: The 670 Avenger ( I suggested the 570)
BG tech: 575 Speed Demon ( I suggested the 650)
a race engine builder: 1st=725 Road demon, 2nd=625 Road demon ( I suggested the 575 Speed demon)

I currently have a #4777 650DP, and I have no trouble bogging the engine. Getting it to pull good requires some practicing.
Since the gf will be driving also, (and beating some punks as well) I think that we will get the Vacuum carb.

Doing the calculation is ok when you take into account the Volumetric efficiency of the engine. This mentionend formulae only goes for 100% VE.
Very few street engines have that normally, you'll not have any more that 80-95%.
A 327@6500rpm with 100%VE = 615cfm,
A 327@6500rpm with 80%VE = 492cfm,
A 327@6500rpm with 95%VE = 585cfm,

On the wild side:
A 332@7000rpm with 100%VE = 672cfm,
A 332@7000rpm with 90%VE = 605cfm

Now this will give credit to BGtech for the 575, and some to Holley Tech.
BG claims that their Road Demon 625 equals the Holley Street Avenger 670.

I'm afraid that by selecting the RD725 I'll have to mess a lot around with the jetting in order for it to run good with my current 283, and still have some work to get it to run with the 327.
Fuel economy is a factor, but maybe I'm just too unrealistic...

Thanks!

Bjarne