Are carb flow calculators any good?

[green69rt]

I have been diddling around with different carb flow calculators on the internet and can't make sense of them.  Simple to use but the I must be interpreting the answers wrong.  

When I put in my proposed engine 470CI mopar and a top RPM (5500) it gives me a carb of less than 650 CFM at .85 VE.  Even at 1.0 VE the calculation gives 750 CFM.  This doesn't sound reasonable to me.  It asks nothing about cam, compression etc.  In the "Lowly 440" thread, carbs were in the 800-850 size.  I'm confused.  

[BSB67]

I think your talking about the carb sizing formula.  And No, the formula is not that good if max accelerations is what you're looking for.

You first need to understand what that formula is.  The formula is simply mathematically matching the theoretical engine flow at max rpm to a carbs measured flow at 1.5" of hg.   What most people don't understand is the what that means.  Simply, not that much if your sizing a carb for best full throttle acceleration.  There is no correlation to an engines performance and a carb's flow at 1.5" hg.  None.

The 1.5" is the pressure drop across the venturis and is the industry's standard for rating 4 bbl carbs flow.  The higher the pressure drop, the harder it is for the motor to suck air/fuel in.  So if you have a larger carb, it will result in the actual pressure drop to be less, and the motor will likely make more power. The down side is the carb needs some dP otherwise it will not flow the appropriate proportions of air and fuel.  So at some point a carb can become too large, the signal too poor to appropriately proportion air and fuel and power will fall off as well.

Other things matter too, like manifold style (single plane/duel plane) runner length, and plenum volume.

Generally speaking, most will likely experience a quicker car with a carb rated at a higher CFM than what the formula states.

[green69rt]

What you say makes sense but I didn't know about the 1.5" HG spec.  So really the calculation only gives you some minimum size to run over the RPM range (sort of) but for us, I guess the way we would size a carb is thru experience and testing.  That's where guys like you and CDR (my consultant) come in.  So a 850 CFM carb for my engine is not as unreasonable as the calculation makes it seem.

[BSB67]

Probably, but don't remember the details of your build.

[c00nhunterjoe]

Dont forget to take your altitude into consideration as well. A guy in the rockies will need a different carb then a guy in florida with the same motor. But as stated by russ, i hate that online carb calculator.  Generally, a mild 470 would have zero problem with an 850. The wilder it gets, the bigger you can go. Fwiw, my stock stroke 440 runs a 950 currently and each step up in carb has yielded et at the track. im going to swap to a 1050 dominator this season and see what happens if i can snag an intake for it. Granted im on the extreme end with this engine but it proves the point about the calculator.

[Challenger340]

At ANY elevation, even closer to sea level..... there is a phenomenon where actual "size" of the carb butterflies can cheat ambient air pressures to pack more A/F into the intake plenum. For lack of a better descriptive think of it as "pressure times Area" ?
Hence,
why we here running between 2200 ft to 4,000 ft and even higher tend to opt for Carbs with the Larger Butterflies/Venturi's than would be indicated as "optimum" for an Engine from strictly an Engine demand calculator perspective..... as a way to "cheat" if you will....  a few extra ponies.

Example:
While a calculator may indicate a "650" cfm for example as all the Engine requires ? the Larger Venturi/Butterflies present on a 750 or 850 cfm Carb provide more "Area" for whatever limited Pressure is available to PUSH the Air into the lower pressure area(Intake).... providing more power.

And as we Dyno every engine to provide correct Jetting/Stochiometric Carburetor mixture function fuel curve across all rpm's.... we always opt for larger than required carbs.
Also keep in mind here....
the stock 440 Magnum 375 hp AVS Carbs were 800 ? or 850? cfm from factory ?

[c00nhunterjoe]

And the 6 packs were 950 when converted to the 4bbl calculation method...

[BSB67]

I used a lot of words earlier and maybe it was not so clear.  I'll try a different approach.

4 bbl carbs flow ratings are determined at a 1.5" Hg pressure drop.  I would argue that the 1.5" hg is somewhat arbitrary.  A 650 rated carb will flow 750 with a high enough pressure differential.  But it will not flow 650 cfm if there is less than 1.5" hg pressure differential.  Every carbs actual flow at any moment is simply a function of the pressure differential at the time.

So if your goal is to achieve the carburetor's rated flow rate at your max rpm the formula is exactly correct, VE corrections aside.

However, if you can achieve the same flow rate, but with a lower pressure drop with everything else being equal, the motor will make more power.  This is what a larger carb will do.

Here is a simplistic, made up example:  Lets say your motor needs 650 cfm according to the formula.  In theory a 650 carb will give you that flow, but at 1.5" hg.  But if you install a 850 cfm carb, it will achieve that flow probably at about 1.0 - 1.1" hg.  If the 850 has the same fuel/air delivery/atomization performance as the 650, the 850 with it's smaller pressure drop will make more power. Part of that will actually show up as a better VE.

I'll stop here for now.

[Kern Dog]

This is confusing.

[c00nhunterjoe]

This is confusing.

Think boost. A 650 cfm carb with a blower making 20 lbs of boost will move more then 650 cfm of air through it. A dramatic picture but same principle.

And think of your intake ports- the general goal for most guys on here is 300 cfm on the intake port. Well if you have 8 ports that all flow 300 cfm, thats 2400 cfm. Granted every intake valve is obviously not open at the same time, but given overlap and rpm, it adds up.

[XH29N0G]

There is one thing I would like to hear from this group who know better than me (which is why I haven't added my thoughts).  At what point would the carburetor be too big?

I have a smaller engine (450 CI) and have tried various carburetors (all 1.75 butterflies and 830 or larger).  I felt like the largest one which was 1.59 venturis (but no choke horn) was a little less responsive at the tip in (while on the highway at 3000 RPM) and thought it could have been a little too much air.  I am near sea level and also with an engine that is not super radical.  The 1.59 is the size of the typical 850 (the one I tried was supposed to be 1050). 

Any wisdom on this that is relevant to the OP's question is appreciated.  I am not trying to sidetrack this discussion.

[BSB67]

Every thing discussed above is regarding WOT, max rpm.  And note the qualifiers regarding atomization, delivery......

Without a good signal, the carb will lack responsiveness.  

At what point is it too big.....trial and error.  And not all carbs are created equal .  The Carb you describe is approaching the edge, IMO (To be clear, I'm a hobbyist, and not an expert in any of this).  The reason is that the venturi is so big (1.59) and it is still only a 1 3/4" throttle bore.  Again the pressure differential is what make the carb function.

I would suspect that if you installed annular boosters the responsiveness would pick back up.  The annualar booster causes a better signal at lower dP than a downleg.  You'll give up a little flow, but over all power might go up.

Also, the 850 actually has a 1.56 venturi. So between the venturi size, likely thinner throttle shafts and plates, contoured body there is more flow than from the typical 4781 - 850 DP.

My small venturi (1.40") 950, which is actually about the same size or a tad smaller than a 4781 from an actual air flow stand-point, is faster at the track.  Of course this kinda fly's in the face of everything I just said.  But it is simply a better carb doing a better job of delivering the fuel/air.  The increase in overall performance at the track comes from more power in the lower rpm ranges at shift recovery.

[Brass]

Generally speaking, most will likely experience a quicker car with a carb rated at a higher CFM than what the formula states.

Exactly - I think that is the takeaway.  I've learned to go bigger than what the calculators indicate.  In the past, I have tended to err on the side of smaller carbs for the sake of throttle response and drivability.  Not anymore.  My 508 picked up 44 HP to_the_wheels switching from a 700 to an 850.  There has been no noticeable change in throttle response either, and based on the curve of the graph, the power was delivered much more smoothly as well.  The 850 has power-plates to help atomization though.  The 700 did not have those.  Of course now I wonder if I should have gone with a 950.   

[green69rt]

I read all the reply's.   Thanks to all for the info, always good to learn.  Now all I have to do is chose one!

[stuubi]

It's not easy
Holly has propably best one but thats also on the low side.
They are just in the ballpark

[stuubi]

470cid and 5500max rpm...i'd say that 750 Hp is ok,but 850Db is max.Both will work.
You might want to change to 50cc primary acc pump and change bit more aggressive cam for 850.
Or one carb worth looking,Edelbrock AVS 2 model.It has new Booster desing and i tried 650cfm model on 340hp 351W engine and was really happy way it performed OTB.And they have 800cfm model 1912.
No Edelbrock can match low end power to Holley DB but for all day riding,it's worth the try in my book.

BSB67 had good points,those online calculators are mainly for low rpm everyday use.For race,add 100-150cfm and you can ran good numbers.

On dyno,even 350sbc with 450hp will make numbers with 1000cfm carb,but it will not run on street with 2200stall and 3,55 gears on 350lbs car with out serious bogging or massive tuning.
Engine dyno loads engine way different than normal driving.
If you have 3800+stall and 4,xx gears,you can get away from big carb,because theres no load on low rpm,and engine can speed up faster.

A  454cid,6100rpm redline made better afr withn 850 than 950.
On street,850 was hairier,same response than smaller 750 but AFR for everyday commuting was superior with 750.I did tune them by AFR.And engine made 592hp so it's no puppy for 454.
On 1/4 mile,850 was worth 0.1-0.15sec roughly on footbrake car with 3000stall.

Good indication for too big carb,for 85% of the engines running there,is that if you have to put more jets than 90-92 Holley size,you are running too big carb.
With 88 size and afr showing lean,you better get smaller carb.
This is for street/Streetrace engines operatin under 6500rpm.
Just my thoughts.

[DanielRobert]

I've often wondered about the low cfm suggestions too so, do I understand  a dyno can't really give an accurate carb size choice? It can show power differences but not real world performance? So testing at the track seems to be the best gauge of what carb to use,I guess. I need someone to babysit me on my first trip to the track!