What a difference the right size carb can make

[LeeMS]

Ok here goes,being a carburetor rebuilder and set up tech for 23 years at the same shop I have to throw in my two cents. Cfm will change all sorts of power bands. I Built a 66 muststang fastback approx 2800 lbs. 289 cid,some very nice ported and poished gt40 valved heads, tri-y headers, top loader 4 speed trans, ran 9in rear end with 411 and 370 gear ratios. Eng dynoed 330 ph at 7000 rpm with a holley 750 mech sec anular discharge carb. Working at a carb shop afforded me lots of opions, ran a 450 cfm vac sec.600 and 650 mech sec, 715 vac sec,the 750 and webers. the smaller the cfm the quicker the responce, the webers made lots of noise nice top end, the dyno carb was the worst on the steet. Ran the 650 the most , the best of both worlds, good throttle responce and great mid range torque. I liked the controll you have with mech sec. So depending on your rpm limits 600 cfm mech sec to 6000 rpm, 650 cfm mech sec to 7000 rpm. To sum up, don't over carb or over cam for the steet. So much for carbs I'm well into the 1990s with my first lt1 FI now!! Lee

[Guest ZFury]

Carb tune is also very important. Most people just throw on a carb and expect it to work. There are a bunch of things to tweek on a Holley to make it run right. Front and rear jets' date=' power valve, accelerator pump squirters and cams, and vacuum secondary springs to name the most common.

 

That 600 may just run better because it just happens to have a tune on it that complements your engine combo. [/quote']

I think that is the best advise on this thread. I have never seen an accurate mathmatical equation that will give you your max carb CFM allowed. I have seen one for minimum. But there are so many variables, figureing the max would be quite difficult for any mathamtician. Even the minimum equation is somewhat of a guesstimate. 600CC & 1L motorcycles have a carb set-up that allows more CFM than most cars. And they give great fuel ratios at low rpms and all. On the 2.4L datsun motor they use Triple carbs that flow like 1000 cfm or more on stock cams and exhaust. Tuning is the key. Just because when you bolt it on, it doesnt run the way you want, doesnt mean the carb is too large.

[Twoeightnine]

LeeMS...ZFury...both excellent points!

[grumpyvette]

"Originally Posted by Pyro

Carb tune is also very important. Most people just throw on a carb and expect it to work. There are a bunch of things to tweek on a Holley to make it run right. Front and rear jets, power valve, accelerator pump squirters and cams, and vacuum secondary springs to name the most common.

 

That 600 may just run better because it just happens to have a tune on it that complements your engine combo. "

 

 

Ive got to agree..I can,t even tell you all the times when I had guys tell me a carb was too large or small, when simple tuning tricks made a HUGE improvement in both the responce and power curve almost reguardless of the CFM rateings, its usually NOT the CFM rateing , its how well the carbs set up, jetted,and adjusted, in fact Ive had many cases of exceptionally good results useing DUAL QUADS, with two 600 cfm carbs on most 400-500 cubic inch displacement engines, correctly set up. its not at all uncomon to get an extra 10-15 hp off a good dual quad set-up over a single carb and they sound better and run smoother when correctly adjusted. yes, I usually sellect a single holley or demon 750cfm carb on a 350-383-396 sbc.

but on big blocks in the 454 plus range duals are a very valid option, 8 small venturies can be fine tuned to run a more precise fuel air curve than 4 large venturies in most cases, but IT does take some experiance/knowledge/skill

keep in mind the intake used, the port size, runner length, cross section and the plenum volume, the headers used, the transmission and rear gear ratio, plus factors like displacement compression and cam timing ALL effect your results and required tune.

its USUALLY simple things like the wrong float levels, wrong fuel pressure, metering rods or jets, perhaps a bad power valve, or badly adjusted accellerator pump.

 

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

 

http://www.goldrush.com/~rhuish/temp_data/carbtune.html

 

http://www.jason.fletcher.net/tech/carbtuning/carbtuning.htm

 

http://edelbrock.com/automotive/eps_intro.html

 

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

 

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

[Nealio240z]

It only cuts out right before it shifts (4000-5500 RPM ). My timing is set to about 18 degrees BTDC. Im running a Holly fuel pump @ 7-8 psi and 97gph. I have not even touched the carb adjustment... Any ideas why it is cutting out? could it be the coil?

[Guest ZFury]

I really like those dual quad set ups. But honestly, I like the design of the two barrel over the quad, and have been itchin' to get one of these.

[Guest ZFury]

Here are some pics of some extreme manifolds for some big CFM intake. Both of these would probably would need head work done to get them to work properly. Especially the second one. (the second one was made by Edlebrock)

P.S. these intakes are for a Hemi. Though they still show what it takes to over carb a motor.

[deja]

That 600 may just run better because it just happens to have a tune on it that complements your engine combo. "

 

I agree totally. If I had the equipment and time to tune the 725 I would. But since I'm going LT1 soon I'm going to pass on that. I know it took me months of playing around with the Holley on my AMX to get it as good as it was and I'm sure with the right evaluation equipment it could have been better.

[Twoeightnine]

There is a difference bettween the right carb tuned properly.

And a large carb detuned to match your gear. Yes?

[Guest ZFury]

What do you mean by "right"? I mean there is a difference between 300hp and 600hp. The more CFM the better, period. If you have to detune a large carb, it doesnt mean it is not done properly. If large carbs came out of the factory at really lean settings they would be just bolt on upgrades for some. I will stop the maddness there.

[grumpyvette]

a carbs function is to efficiently blend atomised fuel droplets into the air entering the engine in a totally controlled and adjustable manor, while the size of the venuries and the pressure drop across the bores effects the results ,the DESIGN of the carb more than the bore size or flow rate will effect the power curve. yes its true that a fixed displacement engine can only use a set flow at a given rpm, and a 350 is well served with a 600-650cfm carb, but a slightly larger or smaller 450cfm-750 cfm of the correct design will produce very similar power and throttle responce once correctly tuned.

its carb design and tune levels more than just rated flow rates

you don,t DETUNE a large carb, you adjust it so it gets the correct mix and ratio of fuel at the correct time, changes like swapping booster styles, jets and power valves ,accellerator pumps, jets and linkage cams, ETC. can be used to match the engines requirements

[Guest ZFury]

and a 350 is well served with a 600-650cfm carb' date=' but a slightly larger or smaller 450cfm-750 cfm of the correct design will produce very similar power and throttle responce once correctly tuned.

[/quote']

No, if you get the same/similar results with a 750cfm than with a 600cfm carb. Your 750cfm carb is not tuned properly. Why do you all type this stuff?

[grumpyvette]

OK lets discuss it!

I build enought engines to know what works both on the street and the track, where am I wrong??

 

most chevy V8 engines make max power with a fuel air ratio between 12.6:1 and 12.9:1 fuel air ratios

most chevy V8 engine can be designed to make thier best power in an rpm range where the piston speeds are between 4000fpm and 4500fpm at aproximately 12:1-12.5:1 cpr with 110 octane fuel as long as the heads and cam and exhaust scavaging can fill the cylinders at that rpm range with little restriction., and that ,with little intake flow restriction, is important,add a RESTRICTOR plate or a smaller carb,and increasing the plenum volume and static compression, or a carb spacer and changing the cam timing, can compensate, to some extent

dynamic cpr needs to be between about 8:1-9:1 if youll use current street fuel,and vacume readings in the plenum at max throttle should be less than 1.5 inchs of vacume .

dual plane intakes generally can effectively use larger carb flow rates than single plane intakes, but again the combo used has some flexability, in what will work

 

intake design and port cross section , exhaust scavaging,cpr,head flow , displacement,effect your results, but theres a great deal of wiggle room in the carb size that CAN be used

[Guest ZFury]

Yeah there is alot of wiggle room due to a cars air/fuel mix density in the cylinder can widely vary. You can acheive a 12.9:1 fuel mix with a 600cfm carb and a 1000cfm carb on any typical V8. BUT!, the amout of 12.9:1 air/fuel mixture in the cylinders of the car being carbed with a 1000cfm carb will be greater (thus meaning it is more dense, more fuel and air in the cylinder). Here is a pic to help me explain something.

the scale may be a little off, but you can see what I am talking about.

 

You should never be over carbed as long as the area of the opening of the carb ports is less than or equal to the total area of the valve port openings in the cumbustion chamber or intake ports on the head, you will use which ever is smaller to calculate. Some, as you can see in the pics above of those intake manifolds I posted, go even larger. But what I am saying is that you shouldnt have a problem IF. Not all pistons fire at once of course, but there should always be enough vacuum if you dont exceed that area. This is sort of a guesstimate and a rule of thumb. I mean I could go on about how the area of the cylinder exceeds the area of the intake valve so duration of cam, and stroke can come into play. But that is way more than I want to get into. But those variables I just said are why the valve opening, or port opening areas can be exceeded by the carb ports. But to what extent? Or when will you lose a decently variable throttle response? I dunno.

[Twoeightnine]

Ok Ok heres the deal.

The information as listed below is a compilation from my carb tuning manuals for CARTER, HOLLY, EDELBROCK. Their business is engineering carburetor systems. Period. Indisputable. Period.

 

There is no magic formula for determining the correct size carburetor for each aplication.

Different carburetor sizes can produce similar horse power figures at maximum rpm but, can vary greatly at low to mid range aplications.

 

The KEY is airflow velocity through the carburetor, which produces produces a vacuum signal to draw fuel through the main boosters.

 

Too LARGE a carburetor will not produce optimum vacuum to start the main circuit untill reaching high RPM levels. In street aplication large carburetor installations consume the accelerator pump shot prior to proper curcuit flow resulting in "bog".

 

Next....

 

A direct quote from a professional race tuner with a slightly different formula that brings the same results.

 

 

cubic displacement divided by 2 (ex: 351=175.5)

maximum rpm divided by 1728 (ex: 6000 rpm=3.47)

multiply those two figuires (ex: 175.5 x 3.47=608.98 cfm)

Hold on we are not done. You now have to factor in volumetric efficiency. A Nascar race engine runs at 85%-90% volumetric efficiency. A normal street car wil be 75-80%, depending on what you have done with your heads and exhaust.

Ex: 608.98 x 75%=456.74cfm

With carburetors, bigger is not better. You will not get very good results with an 850 double pumper on your small block Chevy. If you are after good idle quality, fair fuel mileage, and good performance, stick with this equation. The boys at GM and Ford used this equation for many years and it works! You can not change the laws of physics.

 

And now taken from the Muscle Car World Wide Registry...

 

A larger carburetor or throttle body will enable the engine to draw in more air, but one that is too big is almost as bad as one that is too small. On a fuel-injected car, a throttle body that is too big will put too much gas in the combustion chamber that just flows the exhaust unburnt. It is much more of a problem on a carbureted car. There, an oversized carb will make the engine actually perform worst at part throttle. A simple formula for calculating the correct carburetor setup is:

 

CFM (amount of air the engine needs) = Displacement (in cubic inches) X Maximum RPM / 3,456

 

 

 

The result is usually rounded up to the next largest off the shelf carburetor sized. Therefore, a Chevy 350 built to redline at 6,000 rpm would need 608 cfm of air flow (350x6000/3456). A Holley 4150 carb rated at 650 cfm would be a good choice. For multi-carb setups, don't forget to add up the CFMs for each carb!

 

Once you find the right carburetor, keeping it tuned correctly is key. Also, carb spacers and carb re-jetting usually help, but it requires trial and error to find the best combination.

 

And if one needs some extensive class time on carburetors and how they work in general please see the link. Its a dandy.

 

http://www.musclecarclub.com/library/tech/carburetor.shtml

 

If one is going to the drags, one can get away with some hefty cfm. Provided you have the gear and the RPMs to utilize it.

Big is cool, but not always better.

[A. G. Olphart]

Here are some pics of some extreme manifolds for some big CFM intake. Both of these would probably would need head work done to get them to work properly. Especially the second one. (the second one was made by Edlebrock)

P.S. these intakes are for a Hemi. Though they still show what it takes to over carb a motor.

 

 

Zfury, I'm afraid that I must take exception to this post.

 

Four Webers is one venturi per cylinder: Standard procedure for those carbs, and they are famous for doing what they do, very well.

 

 

The old log manifold was built for racing. If you put it on a stock 331 in your great grandmother's 53 Chrysler, yes, it would be a bear to drive.

 

Six Stromberg 97s (155cfm each- don't know if that is at 1 1/2 or 3 inches, found it here: http://www.vintagespeed.com/newprod.htm) on the logs would provide 930cfm. That isn't out of line for an all out engine of about 400 cubic inches. Think smoothed rocker arms with mouse trap springs, rev kits, and a main bearing girdle on a 392 hemi and you will begin to get the idea.

 

I know Grumpyvette's credentials... I guess that I can accept you as "BAD".

 

 

 

Nealio- With 18 degrees initial, what is your total mechanical advance? With loud mufflers at high speed, you might not hear some very expensive warning signals (around 34-36 degrees total is generally good for an early SBC).

 

The quadrajets are great carbs when set up right, good throttle response, mileage and power. A book I've seen recommended http://www.amazon.com/gp/reader/0895863014/ref=sib_dp_pt/102-4342387-3648144#reader-link

 

Is it possible that someone put a cheapo rebuild kit in your carb??? If the float/needle valve is small (happens in Holleys with two of them) that would starve the engine at high RPM.

 

 

 

BTW: I have no credentials:twisted: .

Edit note: I do actually know the difference between four and six.

 

<>

[jt1]

originally posted by Zfury

 

No, if you get the same/similar results with a 750cfm than with a 600cfm carb. Your 750cfm carb is not tuned properly. Why do you all type this stuff?

 

Yeah there is alot of wiggle room due to a cars air/fuel mix density in the cylinder can widely vary. You can acheive a 12.9:1 fuel mix with a 600cfm carb and a 1000cfm carb on any typical V8. BUT!, the amout of 12.9:1 air/fuel mixture in the cylinders of the car being carbed with a 1000cfm carb will be greater (thus meaning it is more dense, more fuel and air in the cylinder). Here is a pic to help me explain something.

 

Zfury, you need to understand physics better, specifically Bernoulli's Law. With two carbs of different cfm ratings on the same motor, with both of sufficent cfm to minimize manifold vacumn, the difference is the velocity thru the carb. That's why you can have two different cfm carbs on the same motor, they can both run well and produce close to the same power.

 

Grumpy is one of the most knowledgeable people on this board. When you start telling him he's wrong, you better get your facts together.

 

Perhaps you should consider changing your sig line from "I'M BAD" to something else.

 

John

[Twoeightnine]

Zfury' date=' you need to understand physics better, specifically Bernoulli's Law.

John[/quote']

 

Yep Yep.

If one intends to go large, one must realize that your usefull RPM range will be very high.

The air velocity must be high to create the proper vacuum for the desired fuel air mixture. As per Bernoulli.

Physics.

[Nealio240z]

No one answered my question about 12 replys ago.... Thats okay

[Pyro]

Neal,

We need a bunch more information about your engine. Maybe you should start your own post.