carbs and power

[grumpyvette]

I get this question in so many versions I guess its time for a thread with some basic info.
it helps a great deal to know what your working with , each carb has good and bad points

EDELBROCK

http://www.edelbrock.com/automotive_new ... manual.pdf

viewtopic.php?f=55&t=1820&p=4717&hilit=edelbrock#p4717

HOLLEY

QUADRAJET
reading the sub linked data on this site is critical to getting the full info youll need
viewtopic.php?f=55&t=1639&p=3949#p3949
the question usually goes " what size carb do I need," or something similar,
4v carbs are rated at a flow rate determined with a vacuum or pressure drop of 1.5" of mercury, your best power will generally be found with a carb that lowers the pressure drop or vacuum to between 0.5" and 1.0" of vacuum, not 1.5" at full throttle,more vacuum at full throttle indicates a slight restriction to flow, now on a street car that's 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 common 600cfm carb many of you guys use, a 0.5 inches of vacuum 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 vacuum it flows close to 780cfm, and if you stuck it on a 600 cubic inch big block spinning 6000rpm you'll pull about 6" of vacuum and it would flow about 1000cfm plus!
now remember you'll 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 NOTICEABLY once the vacuum dropped and dropping the vacuum 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 you'll spend in each gear before being forced to change gears due to the rpm levels increasing
http://www.youtube.com/watch?v=k-Nx5HEzvlY

viewtopic.php?f=50&t=1652&p=3987&hilit=volumetric#p3987

viewtopic.php?f=38&t=1099&p=2152&hilit=volumetric#p2152

faster,and the distance you'll travel between shifts, will be shorter,it also aids in that your applying 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 different power curves, it should be obvious that you'll 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 figured out that to get the lower vacuum or restriction, you'll want a larger carb or perhaps two carbs, remember were trying to get that .5"-1.0" of vacuum at full throttle, and that 600cfm carb is not going to flow 600cfm, at that vacuum 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 vacuum 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 vacuum signal goes up, so does the carbs flow rate, and as the vacuum signal strength goes down so does the flow
RESPONSE!
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 vacuum signal at off idle rpm ranges and it may run like crap! so a balance must be accepted. smaller carbs are generally more responsive, but slightly more restrictive with their 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,there's 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 vacuum, yet we don,t want a carb so big that response and drivability sucks, in theory we want a 800-850cfm carb at full throttle, but only a 280-400cfm carb at low rpms, that's why a 600cfm-750cfm carb is generally selected,
this is not a guessing game, having a good understanding of what your looking at and a few basic tools will provide more info,

http://www.harborfreight.com/fuel-pump- ... 93547.html
throw a vacuum gauge on the plenum under the carb and run the engine at w.o.t. under load and get a reading
if its higher than about 2 inches of vacuum you might benefit from a slightly larger carb.

viewtopic.php?f=44&t=777

viewtopic.php?f=55&t=2994
but Id also point out that TWIN/DUAL 500cfm carbs will work very exceptionally
having BOTH the flow at high rpms and the small responsive venturies at low and mid rpm ranges.
but since it requires extensive tuning skills they are less commonly used/selected

remember that great set of heads that flow 250cfm at .600 lift??
well of the 720 degrees available in a cycle there's probably less than a tenth of a second to a fifty-th of a second of available time that they actually 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

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.barrygrant.com/fromBarryGran ... ual-08.pdf

http://www.innovatemotorsports.com/products/lm1.php

http://www.classictrucks.com/tech/0806c ... index.html

http://www.stockcarracing.com/techartic ... index.html

http://www.camaros.net/techref/articles/holley_2.htm

http://www.chevyhiperformance.com/howto ... index.html

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

viewtopic.php?f=55&t=1442

viewtopic.php?f=50&t=1853&p=4848#p4848

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

viewtopic.php?f=44&t=745

viewtopic.php?f=44&t=585

http://www.centuryperformance.com/tunin ... g-148.html

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

http://www.geocities.com/MotorCity/T...92/vizard.html

viewtopic.php?f=44&t=1142

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

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

viewtopic.php?f=44&t=1116

http://www.centuryperformance.com/fueli ... g-140.html

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

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

BTW HERE IS THREE MORE CLOSELY RELATED THREADS WITH GREAT INFO
viewtopic.php?f=50&t=1652&p=3987&hilit=volumetric#p3987

viewtopic.php?f=38&t=1099&p=2152&hilit=volumetric#p2152

viewtopic.php?f=55&t=109&p=136#p136

 

[grumpyvette]

It just seems odd to see a fuel return on something that was carburated

NOT REALLY, if your fuel systems set up correctly you NEED a return style fuel pressure regulator in the loop.

http://www.centuryperformance.com/f...tanding-fuel-pressure-and-volume-spg-140.html

Keep in mind, that installing a return style fuel pressure regulator allows the FUEL FLOW thru the fuel pump too absorb and transfer heat away from the fuel pump, generally increasing its life,expectancy and usually supplying a far more consistent pressure and flow to the carb. under high G-forces.

viewtopic.php?f=44&t=773

viewtopic.php?f=50&t=1853&p=4848&hilit=ratio+meter#p4848

 

[grumpyvette]

KEEP IN MIND IF youve got a carb equiped chevy engine combo that produces under 450hp at the flywheel, on that set-up a mechanical pump and return style regulator is fully adequate, and probably less potential problems than an electrical fuel pump,

http://store.summitracing.com/partdetai ... toview=sku

viewtopic.php?f=55&t=1115

http://store.summitracing.com/partdetai ... toview=sku

"ALSO for the 110gph version it said no regulator is required so what is the gain for adding one?"

thats a good question that deserves some info posted,chevy
mechanical fuel pumps throw a supply of fuel in pulsed volume, if youve ever started your engine with a loose fuel line connection to the carb, or tested fuel flow rates, by allowing the pump to throw fuel in a container,youve seen the fuel squirt out in pulses, a fuel pressure regulator tends to smooth out thos pressure spikes the carbs needle/seat sees and provide a much steadier pressure and supply.
NOW theres a pressure overload circuit in the pump that keeps the pressure lower than a set maximum, but its not nearly as effective as running a return style fuel pressure regulator in conjunction with the pump. while its true the pump and carb will functuion without the regulator youll generally find more consistant fuel control, and fuel levels in the carb if you use the regulator even with an electric fuel pump.

http://www.holley.com/12-454-25.asp

always drop back to basics

have you verified TDC on the damper and timing tab?
did you degree in the cam or just DOT-to-dot install it?
whats your fuel pressure?
have you verified the carbs float levels?
does fuel run out the site holes at idle?
are the needle& seat valves working correctly?
whats your ignition timing?
whats your plenum vacuum?
have you verified the engine got no vacuum leaks?
whats the oil pressure?
have you adjusted the valves at idle?
have you done a compression check?
does the distributor timing advance with rpm increases?
have you tried a different carb?
what are the plugs gaped at?
does this happen without an air filter?
have you verified you've have functioning power valves?
what jets are you using?
is there visible fuel flow from the boosters?
whats your battery voltage?
whats your exhaust back pressure?
are you totally sure the fuel free of water and fresh?

 

[grumpyvette]

all the demons Ive used over the years were very good carbs, they are not a HOLLEY CLONE, they are a very similar looking carb, that looks similar to a holley in some cases but there have been certain changes made to the design, and in many cases those tweaks, in the design, resulted in improvements in my opinion.

http://www.moparmusclemagazine.com/tips ... index.html

http://www.barrygrant.com/bgfuel/default.aspx?page=71

http://www.gnetworks.com/v4files/barryg ... ncy%20.pdf

http://www.circletrack.com/techarticles ... index.html

http://www.barrygrant.com/bgfuel/default.aspx?page=64

http://www.gnetworks.com/v4files/barryg ... stions.pdf

http://www.gnetworks.com/v4files/barryg ... 0Track.pdf

http://www.gnetworks.com/v4files/barryg ... ages2%20(3).pdf

 

[grumpyvette]

while we are discussing intake selection, Id point out that the tunnel ram intakes are one of the most cost effective designs , if maximum possiable peak power is a major concern, PROVIDED they are matched to an engine and drive train that allows them to operate in thier designed rpm band, theres far too many guys that assume you can throw a tunnel ram intake on about any combo and get it to work, but the truth is they are almost all intended for the upper mid and high rpm flow rates, and although its possiable to tune one to function, it won,t really be an asset to the combo unless youve designed the engine to take full advantage of the higher rpm power band. so head flow , cam timing compression , displacement and exhaust scavaging need to be carefully thought thru when planing the engine if you want to take full advantage of the intakes potential.

http://www.carcraft.com/techarticles/03 ... index.html

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

http://www.superchevy.com/technical/eng ... index.html

http://www.edelbrock.com/automotive_new ... 0/7110.pdf

http://www.edelbrock.com/automotive_new ... 0/7115.pdf

 

[grumpyvette]

http://www.chevyhiperformance.com/techa ... index.html

" It Does (QUOTE)

Most carburetors employ what is generally called a power valve circuit. This circuit enriches the air-fuel mixture when the carburetor goes to wide-open throttle (WOT). At WOT, intake manifold vacuum drops to almost zero. When this occurs, the power valve opens and directs more fuel into the main power circuit, in addition to fuel delivered by the main jets. The Holley power valve employs a small rubber diaphragm that is opened by a small coil spring. The valve is held closed whenever sufficient engine vacuum is present. At WOT, engine vacuum disappears and the power valve spring opens the valve, directing fuel through a small, precise orifice in the metering block called the power valve channel restriction. This restriction determines the amount of additional fuel delivered to the engine.

Power valves are used most frequently on the primary side of a Holley carburetor. They allow the carburetor to operate with much leaner main circuit jetting for part-throttle fuel economy. Then, when the throttle is slammed open, the power valve adds additional fuel, creating the rich air-fuel ratio needed for WOT operation. Most Holley power valve circuits are designed to add the equivalent of 8 to 10 jet sizes of additional fuel. Holley does offer a power valve block-off part that closes the power valve circuit, but this means the jet size must be increased in order to compensate for the lost power valve circuit fuel. Imagine how bad your fuel mileage would be if you had to add 10 jet sizes to the primary side of your carburetor!"



http://www.youtube.com/watch?v=6vKlAnKdcB4&fmt=18

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

http://www.visn2.com/UsingVacumeGauge.html

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

http://www.valvoline.com/carcare/articl ... 4&scccid=1

http://www.holley.com/TechService/Library.asp

High Flow Power Valves are intended for Alcohol applications while Standard are intended for Gas applications and a Gas application will not benefit from the use of High Flow Power Valves and in fact would run worse not better. So for gas applications stay with the Standard Flow Power Valve.

http://www.nationaltbucketalliance.com/ ... /carbs.asp

http://www.summitracing.com/parts/PHP-15001/

http://www.summitracing.com/search/Bran ... toview=SKU

http://www.percyshp.com/adjustajet.html

heres a neat little option for the HOLLEY carbs, its not really In expensive but it does allow you to effectively change the fuel flow rates far faster than you could changing JETS, its basically a needle & seat valve that adjusts like the float levels do on a Holley carb, it comes with detailed instructions , and if you use a fuel ratio meter or an IR temp gun and read spark plugs it will allow you to adjust the fuel air ratio far faster and easier than jet changes will



ignition timing thats a bit to retarded will tend to make the plugs run cold and foul, setting your float levels and making 100% sure the linkage functions correctly is mandatory, before you start chasing problems

http://www.barrygrant.com/bgfuel/default.aspx?page=83

http://www.holley.com/data/TechService/ ... uretor.pdf

http://www.stockcarracing.com/techartic ... index.html

unscrew the site plugs and adjust the floats until the fuel level just prevents fuel flow from the site holes at idle, youll need a flat blade screw driver and a 5/8" wrench


QUESTION How do I adjust the fuel level on my carburetor?
ANSWER Setting the fuel level should be the first thing you do before attempting to make any further adjustments.The float level should put the fuel level just below the bottom of sight plug hole. You will make the adjustment with the vehicle on a level surface and the engine idling. You will first remove the sight plug, then to make your adjustment you will need to loosen the lock screw on the needle and seat. This will allow you to turn the adjusting nut to raise or lower the float level. Each hex flat on the nut will change the float level approximately 1/32". When you have the fuel level just below sight plug hole you will then tighten the lock screw and reinstall the sight hole plug. Make sure you have a shop towel handy in case you have any fuel leaks from the Sight plug or needle and seat adjusting nut.