carbs, rated flow and power vs vacume

[grumpyvette]

I get this question in so many versions I guess its time for a thread with some basic info.

 

the question usually goes " what size carb do I need," or something similar,

4v carbs are rated at a flow rate determined with a vacume or pressure drop of 1.5" of mercury, your best power will generally be found with a carb that lowers the presure drop or vacume to between 0.5" and 1.0" of vacume, not 1.5" at full throttle,more vacume at full throttle indicates a slight restriction to flow, now on a street car thats not going to be much if and problem, but on a race cars engine its a sign that your potentially giving away some potential power.

lets look at your comon 600cfm carb many of you guys use, a 0.5 inches of vacume it flows only about 350cfm, at 1.0" it flows about 500cfm, at 1.5" it flows about 600 cfm , rated like a two barrel at 3.0" of vacume it flows close to 780cfm, and if you stuck it on a 600 cubic inch big block spinning 6000rpm youll pull about 6" of vacume and it would flow about 1000cfm plus!

now remember youll try to stay in the .5" to 1.5" range at full throttle, to make good power.

now some of you might notice that the flow dropped NOTICABLY once the vacume dropped and dropping the vacume at wide open throttle tends to help power, provided the a/f ratio is kept near 12.7-13.0:1,AND the engine is set up to USE the flow available to it.

VOLUMETRIC EFFICIENCY

in theory a cylinder fills to 100% full, but the limited time the valves are open and the ports restrictive flow will only allow that to happen at a narrow rpm range

your engines torque curve on an rpm scale closely mirrors the engines efficincy at filling the cylinders, on that same scale, once the cam timing and port flow become a restriction power falls off because theres less fuel /air mix burnt per power stroke, the power tends to keep going up for alittle further in the rpm band simply because theres MORE ,thou slightly less effective power strokes per minute.

at 1000rpm theres 500 intake strokes per minute thats 8.3 per second times the intake valve opens and closes, at 6500rpm thats 54 times a second, not much time when you think about what needs to flow thru that port in the limited time....especially if you remember that of that 720 degrees in the cycle only about 240 degrees have any useful flow potential, so you just cut even that time by 2/3rds

 

keep in mind the rear gear ratio has a huge effect on the time youll spend in each gear before being forced to change gears due to the rpm levels increaseing faster,and the distance youll travel between shifts, will be shorter,it also aids in that your appling more rpms or power strokes per foot of distance traveled, thus more tq gets applied,the converter allows you to get into the more effective higher rpm ranges faster and apply the tq quicker, BOTH are necessary to take full advantage of the power curve, look heres two differant power curves, it should be obvious that youll want to both start higher and operate in a higher rpm range with the second combo than the first, to maximize the effective power to the rear wheels

 

 

 

now back to the carbs size

some of you may have figgured out that to get the lower vacume or restriction, youll want a larger carb or perhaps two carbs, remember were trying to get that .5"-1.0" of vacume at full throttle, and that 600cfm carb is not going to flow 600cfm, at that vacume reading but between about 350-500cfm, so if you have an engine that can take full advantage of the flow it may, and usually does require a larger carb to make max power,that 383 might require an 800-850cfm carb or two 600 cfm carbs (since you double the venturie cross sectional area with two carbs the vacume reading is generally cut to about 1/2 what it was and the two 600 cfm carbs now flow about 350cfm each or 700cfm per pair) yet the carb size is just NOT all that critical,to making fairly decent (NOT MAXIMUM POWER) simply because as the vacume signal goes up, so does the carbs flow rate, and as the vacume signal strength goes down so does the flow

RESPONCE!

up till now we are talking only FULL THROTTLE POWER, but you operate under a wide range of rpms and loads, put that larger carb on a small engine and it makes good power at wide open throttle, but it also tends to have a weak vacume signal at off idle rpm ranges and it may run like crap! so a ballance must be accepted. smaller carbs are generally more responcive, but slightly more restrictive with thier smaller venturies.

 

lets look at a 383, in theory each cylinder requires 1/8 th of the displacement, or 47.875 cubic inches of fuel air mix every 720 degrees, or every other time around on the intake stroke,theres 1728 cubic inches in a cubic foot, so at 6500rpm the 383 would require 1244750 cubic inches, or 720cfm (remember the carbs are rated at 1.5" but you want lets assume .75 inches of vacume, yet we don,t want a carb so big that responce and drivability sucks, in theory we want a 800-850cfm carb at full throttle, but only a 280-400cfm carb at low rpms, thats why a 600cfm-750cfm carb is generally sellected,

but Id also point out that TWIN/DUAL 500cfm carbs will work very exceptiably

having BOTH the flow at high rpms and the small responcive venturies at low and mid rpm ranges.

but since it requires extensive tunning skills they are less comonly used/sellected

 

remember that great set of heads that flow 250cfm at .600 lift??

well of the 720 degrees available in a cycle theres probably less than a tenth of a second to a fifty-th of a second of available time that they acctually get to flow air, yet they are rated at a steady open flow, that head that flows 250cfm on a bench flows A WHOLE LOT LESS ON AN ENGINE WITH MOVING VALVES, one reason why maximizing flow potential is critical

 

QUESTIONS/COMMENTS

[veritech-z]

So I'm trying to get my head around all that...What would I need to know about my engine to be able to effectively determine the flow requirements at a given RPM? Also, what features of a carb will determine what it flows at various levels of vacuum (like the 600cfm example that you said would flow 350cfm @ 0.5")? Is that something you just know from doing it a million times, or is there like a formula or something?

[grumpyvette]

BOTH

but this may help you

 

http://users.erols.com/srweiss/calcdchg.htm

 

http://users.erols.com/srweiss/calccarb.htm

 

http://users.erols.com/srweiss/calcafhp.htm

 

http://www.rbracing-rsr.com/runnertorquecalc.html

 

http://www.engr.colostate.edu/~allan/fluids/page4/effarea/effarea.html

 

http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm

 

http://www.newcovenant.com/speedcrafter/toc.htm

 

http://victorylibrary.com/mopar/intake-tech-c.htm

 

http://www.geocities.com/MotorCity/Track/6992/vizard.html

 

http://www.bob2000.com/carb.htm

 

http://www.mortec.com/carbtip1.htm

 

http://www.rbracing-rsr.com/runnertorquecalc.html

 

http://www.centuryperformance.com/vacuum.asp

[MONGO510]

Grumpy, you constantly amaze me with the extent of your knowledge!

[blueovalz]

Thanks again for another enlightening moment from Grumpy

[rytherwr]

You make my head hurt Grump...I think you forgot the gunrack and two broken arrows....

 

Wayne

[grumpyvette]

??? I just had a two large brandy,s and Im relaxing,so this may be all my lack of understanding here.....but WTH are you talking about????

[John Scott]

I always hear the discrepancies in claimed flow of Edlebrock vs Holley vs Holley HP, vs Demon etc. Demon techs claims their 650 will flow more cfm than a Holley 650, more like a Holley 750. Holley 950HP isn't really 950 cfm with 800 baseplate and 750 body and a 830 with larger bores and body flow more than either. In other words can we believe what the manufacturer claims? Which carbs do you trust as far as claimed ratings? The best one I've run to date with extreme flexibility, great idle even with long cam, is the 4150HP 750.

[grumpyvette]

"Which carbs do you trust as far as claimed ratings?"

ID say NONE!

 

they are all ESTIMATES, close but never correct, from what IVE seen

 

personally I like the DEMON carbs first,qualtiys noticably better and tunning easier on most models ,transition tunning is far better than holley on the race demon line, but its also easier to screw things up big time, as theres more things to adjust, so theres more potential to screw up

holleys second , much more flexable than the edelbrocks, but limited compared to the race demons

and edelbrocks THIRD, nearly bullet proof , dirt simple and easy to adjust but not very flexiable , compared to the demons or holleys, but thats really a PLUS , rather than a problem in some applications. like dual quads

 

you can simplify things, a 750 -850, mighty,race, or speed demon will work on nearly all small block applications, call the manufacturer for the correct part/application

 

http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10001&catalogId=10002&catalogIdentifier=Jegs_Direct&categoryId=24389&parentCategoryId=10271

[rytherwr]

Hee, Hee, Hee....has absolutely nothing to do with anything...that's the point...I make funny... The only way it will fit is if you have a couple more larger brandy's...

 

By the way, great info as usual....