Since this has been kind of a hot topic, I have a program in my computer @ work for calculating everything you can imaging engine related, and I made a chart of Carb CFM required for supercharged engines. To keep it simple, I used basic engine size configurations with the RPM range rated at 5000, and needed CFM recorded for 6,7,8 and 9 pounds of boost... depending on what you have. Again, this is a calculation based on the facts above and for use as a general guideline. If anyone wants to give me some specifics about your engine size, rpm and maximum boost level, I will be glad to run them in the program and give you results tailored to your engine. Hope this helps some of you get a little closer to what you need to avoid driveability problems, and the bull**** that drives us all nuts!

Heres some more terms we have been seeing lately....

*Supercharged engines generally require carburetors with 40% to 50% more C.F.M. than normally aspirated engines.*

Volumetric Efficiency
Before you can determine the correct carburetor size for your engine, you must know its volumetric efficiency. Volumetric efficiency is an indicator of how well an engine can breathe. The better an engine's "breathing ability" the higher its volumetric efficiency. It is expressed as the ratio of the actual mass (weight) of air taken into the engine compared to the mass which the engine displacement would theoretically take in if there were no losses. The ratio is expressed as a percentage. It is quite low at idle and low speeds and varies with engine speed. Volumetric efficiency should be computed at the expected operating RPM or your engine application.


Use the following examples as a guide to estimate the volumetric efficiency of your engine.
(A) An ordinary low-performance engine has a volumetric efficiency of about 80% at maximum torque.
(B) A high performance engine has a volumetric efficiency of about 85% at maximum torque.
(C) An all-out racing engine has a volumetric efficiency of about 95% at maximum torque.


A highly tuned intake and exhaust system with efficient cylinder head porting and a camshaft ground to take full advantage of the engine's other equipment can provide such complete cylinder filling that a volumetric efficiency of 100%, or slightly higher, is obtained at the speed for which a system is tuned.


The graph below can be used to find your airflow requirement. It's based on 100% volumetric efficiency so any indicated airflow must be multiplied by the volumetric efficiency of your particular engine. Use a carburetor with an airflow rating equal to or slightly smaller than the air requirement of your engine.

This is the calculation to determine CFM based on 100% Volumetric Efficiency, which is very hard to achieve, but using this figure makes the calculation a bit more simple, and will get you close enough to fine tune things by doing a few
plug reads...

Engine Size (CID) X Maximum RPM / 3456 = CFM @100% Volumetric Efficiency
EXAMPLE: 350 CID X 6000 RPM = 2,100,000 / 3456 = 608 CFM

<<all calculations are based on @ 5000 RPM to make the results relative to the same conditions>>
Engine size....Max RPM 5000....Max Boost....Recommended CFM
454.............................................6 psi...............924.9959 CFM
.................................................. 7 psi................969.6905 CFM
.................................................. 8 psi...............1014.3850 CFM
.................................................. 9 psi...............1059.0796 CFM

468.............................................6 psi...............953.5200 CFM
.................................................. 7 psi................999.5928 CFM
.................................................. 8 psi...............1045.6656 CFM
.................................................. 9 psi...............1091.7384 CFM

496.............................................6 psi..............1010.5682 CFM
.................................................. 7 psi...............1059.3975 CFM
.................................................. 8 psi...............1108.2268 CFM
.................................................. .9 psi..............1157.0561 CFM

502.............................................6 psi..............1022.7929 CFM
.................................................. 7 psi...............1072.2128 CFM
.................................................. 8 psi...............1121.6328 CFM
.................................................. 9 psi...............1171.0527 CFM

540.............................................6 psi..............1100.2154 CFM
.................................................. 7 psi...............1153.3763 CFM
.................................................. 8 psi...............1206.5372 CFM
.................................................. 9 psi...............1259.6981 CFM

I also have many other programs I will list below (listing the most basic) and more, but it would be too time consuming to list them all. If anyone wants a calculation on something, ask me, and if I have it, I will run your numbers for you. These are very costly programs, and I have them so I don't meltdown a motor doing a Dyno run for a customer who has miss-matched components, or doesn't know what he is doing. I don't need that liability (even though they sign a release assuming all risk) it is still bad word of mouth to melt a guys new motor when it isn't my fault.....

A few of the programs that might apply for Offshore tuning etc. I have are...

Max compression for a given Blower pressure
Blower pressure needed from effictive compression ratio wanted
Blower pressure needed for HP gain
Normal HP from blower pressure and HP
Effictive Compression ratio from adding Blower pressure
Blower drive ratio and RPM
HP gain by adding Blower pressure

Cam duration, centerline, lobe separation angle from cam events
Cam timing events from duration and centerline
Valve lift from changing rocker ratio
Cam duration needed from compression ratio
Piston travel and crank rod angle vs crank rotation
several different compression ratio figuring programs based on chamber size, deck height, dome volume, cyl cc, etc.
Horsepower change from a change temp, humidity, Barometer and Altitude
Calculate vapor pressure/dew point/Dyno correction factor/Air Density
Estimate fuel flow for a given engine size
Calculate Injector size in lbs/hr / fuel pump flow from HP
Max HP from Injector size in lbs/hr fuel pump flow
Calculate change in Injector flow rating from fuel pressure change
Calculate needed fuel pressure change for desired Injector flow rate
Estimate intake and exhaust airflow from HP
Calculate minimum fuel pressure
Calculate intake runner length
Estimate intake valve size and port area in sq. inches
Calculate header tube length
Calculate optimum pipe diameter
Calculate affected RPM from tube diameter and tube lenght
Calculate minimum collector length

Bla, Bla, Bla... there are about 75 more but I am tired of typing, and late for the Seafood festival. If you need
anything engine related calculated, just ask, as I probably have it and will be more than happy to help if I can. Time
to go where my wife can't "FindMe"...........

A while back there was a thread about how much HP is being lost due to summer head & humidity along with boat speed. Since you have the programs how about taking a 650 hp blown 454 in early spring with 50-60 degree day compared to the same motor middle of summer with 95-100 degree day with 75% humidity. I do not think that your program can take the water temp difference going to the intercooler so I am guessing that the number you come up with will be on the high side.

I am running a Whipplecharger at 6 psi and run a 825 double pumper. This was recommended by Whipple. I was also told that a draw through supercharger can actually pull more cfm thru than what the carb is rated for because of the vacuum effect. Aren't carbs cfm rated based on naturally aspirated data? My setup has run flawlessly for 2 seasons now. Another note though--this carb is a Demon and is designed to ONLY be used on a supercharged application. It has very different metering blocks from a N/A carb.
Try running my numbers and see what you get. Oh yeah it is intercooled also and my boost readings are right at the cyl head opening after the intercooler.

Dave

P.S. Motor is a stock454 Magnum with IMCO exhaust.

FindMe:
Are you taking Type of Cylinder heads, Cam profile, static compression ratio, rod length, cam profile, vehilce weight, type of intercooler, or anything else into consideration with your program ? Also your "CFM" number are you saying this is what the engine will flow, or a carburetor size ?


Turbojack:
Using standar dyno HP correction numbers as follows:
Your winter run 50 degrees, 0% humidity, and 30.15 barometric pressure. gives you a correction factor of .9828 or an adjusted altitude of -951'. Now go to 100 degrees, 75% humidity, and drop the barometer to 29.6 you get an adjusted altitude of 4580'. This would be the same as taking your 650HP engine down to approx. 600HP.

Bigwavz:
Carburetors work based on air traveling through the venturi creating a low pressure region and pulling fuel through the boosters. As far as the CFM ratings all manufactures rate them differently, some dry, some wet, and some at different amounts of vacuum. Put a carburetor on a flow bench, pull both air, and fuel through it, but just change how rich or lean it is.... This will change the amount of cfm that is flowing through the carburetor. Most manufactures pick a cfm number for there carburetors based on what the should flow at WOT if they are on the appropriate application. Take one of the barry grant king demon carburetors. We put a 1090 carburetor on a 632" motor we all knew that it was too small, but at 7,000 RPM it was pulling over 1500 CFM since there was more vacuum signle below it. I hope this helps, if you have any other questions please give me a shout..

Or you can use this formula, wich assumes 100% VE.

[(Cubic inches * RPM) / 3456] [(boost / 14.7)+ 1 ] = CFM requirements.

Example:
502*5000=2510000/3456=726.27

6lbs/14.7=.40816+1=1.40816

726.26 * 1.40816= 1022.7066

Looks close enough to the computer program.

DAVE

Also consider that a carb's CFM is directly tied to the pressure differential across it, so carburetor ratings are not quite as critical as we might initially think. The math goes something like this:

Frated = Reference airflow
Vrated = Reference vacuum

Fact = Actual Airflow
Vact = Actual Vacuum

Fact= sqrt(Vact/Vrated)*Frated

For example, a 500cfm @ 1.5" mercury rated carburetor theoretically will flow 705 cfm at 3" mercury vacuum.

That's not to say there is not a difference between a 500cfm carb and a 700cfm carb when each is flowing 700 cfm. The larger carb will definitely deliver a larger mass of air, although it is only about 5.5% larger, not 40% larger as the number might initially imply. This is because, ignoring temperature differences, the 700cfm carb is delivering about 95% of the outside air density whereas the 500 cfm carb is delivering about 90% of the outside air density.

what is your capacity in the marine engine world? you stated that you do dydo pulls for customers. are you an engine builder too? do you do some assembly? or do you just do dyno runs?

Lets see...

557ci*5600rpm=3119200/3456=902.546

7lbs boost/14.7(a bit lean no?)=.47619+1=1.47619

So, 902.546*1.47619=1332.3293 "CFMages"



Lets redo the A/F, shall we?

7lbs boost/12.8=.546875+1=1.546875

Again, 902.546*1.546875=1396.1258 "CFMages"

Wow i thought having a pair of 750's was on the small side...not after this calculation.
BTW, this would have made my HS Math teacher Miss Shilff proud.

Back4More, thats pretty good, and I agree... you should get a Gold Star... hell, i'm proud of ya, and I wasn't even your math teacher... lol My program breaks it down a bit further, but ended up being 13324486.8941 CFM, so you nailed it. That one was easy, all you other guys, I'm not duckin you, I just got home from a 3 day unexpected road trip and will give you all answers (or as close as possible) as soon as I take a power nap to shake off the jet lag.

Rumrunner... I pretty much tried to lay out the parameters I used for those calcs in my post, maybe not clear enough.... they were general calcs done with the same rpm, 100% volumetric efficiency, and conditions which would indicate as stated, pretty much a tits set of heads, and very closely matched components to compliment each other (thus the 100% VE) that I would kill for on a motor to motor basis, best case senario, as a general guide and for the sake of arguement.

Dean, I will try to outline what I'm into but I will say there isn't enough of my ass left to do it all anymore, but at some point and time... I have got to do a lot of high tech **** with a lot of respectable people including Horward Arneson, and been lucky enough to have learned from the best, and have invaluable resources from doing so over the past 30 years, and usually know who to ask if I don't have an intelligent, or logical answer... some stuff I will just bail on, but I know my limits and will never mis-lead, or inform anyone... meaning theres a lot I don't know, except I DO know, when I'm in over my pointed little head...lol As far as marine connections... I have lived on the Sacramento/San Joaquin river for the past 35 years, had a 21 ft Hydroflyte hydro with a 392 Hemi that ran 121 mph in the 1/4 mile, built several Offshore boats with no failures partly from being very knowledgeable about marine VS automotive issues, internal clearances, and these days I mostly help people solve computer engine management issues due to owners installing incompatable parts and goodies and in over their heads, Reprogram all GM ECM's from 1993 to date, and have the equipment and software to do so, and am currently checking on what it will take for me to pop a mercruiser ECM's cherry... (very intense)... but that is taking some time, and lots of ass kissing... I am a AC Delco, and Ford Motor Company factory trained advanced electronics tech with certification in every phase of advanced electronics possible, a 4 time ASE recertified Master Tech (whoopee), and digging every hour of my day to take more in...

****, I didn't intend on all of that... I will get back to the rest of you asap... I need a warm woman and........ Be back after ~Refueling~

FindMe:

I'll be intereasted to see what else you come up with.



Back4More:

Your math was great, but I disagree with your outcome. Carburetors work based on air velocity, so put a larger CFM carburetor on the same engine (since it is only an air pump moving the same amount of air) will actually run leaner.