Twoeightnine
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Posts posted by Twoeightnine
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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.
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Now I am a member of Hybrid Z. I'd like to learn more about the 280ZX and look forward to hearing from other members.
Katie
Welcome. Thats an awesome ride from an awesome member. Im sure I dont have to tell you how fortunate you are.
Glad your aboard!!
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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.
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There is a difference bettween the right carb tuned properly.
And a large carb detuned to match your gear. Yes?
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OK OK Story time...cant help it.
It was a beautiful summer day in Ft. Worth Texas. At the time I was riding a KZ 900. I was taking the long way home and cutting through a suburban homes section of town. This is an older neighborhood that had brick homes set on about a half acre lots. Well trimed lawns and kids playing in the yards. I was kicked back instead of paranoid as I usually was cause it was calm, sunny and no cars. I twisted the throtle as I went throgh the gears and had reached about 50 MPH when I noticed a car comming my way up ahead.
I came to the usual attention, as cars are the enemy, glanced at the speedo and let off the gas. (going to fast, of course, should be going about 35).
When the car was about 100 feet from me I see the drivers hand stick out the window and wave at someone off to my right. Or was he waving at me? To find out, I look over to the right and see a gentleman waving back at the driver. OK.
I turn my attention forward again and there it is. The driver had turned dead ass in front of me to apparently vist with his friend that he was fixating on. I turned my head for ONE second.
BAMMMMM! Not even time to do anything but suck in what I though would be my last breath. Not a skid mark. Not even time to close my eyes.
I cleard the car as the bike hit and rotated me up and forward. After I skid to a stop 50 feet on the other side of the car on my face, with a FULL FACE HELMET on, the last few feet finally rotated me over on my back with a thud. As my vision slowly came back to me I got to hear thoes famous words. I knew it was comming and was mumbling "no, no dont say it". But here they came. The driver first says " man, are you allright". But thats not the words Im talking about. Ready? Here it comes.........here it comes......."I didn't even see you man".
No kidding?
Awesome scar as a souvenir from that one. The car was a pinto station wagon and the side chrome buckeled into a taco shape when my front tire impacted the car. As I cleared the body of the car, the chrome went into and almost through my left leg. Nice yes? At the time it made sense to quick check what he was waving at. Guess not.
Got more. Any others?
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Shame to see a perfectly good z go to waste.
Sell it to me now while there is still time!!
Hey I road for years...hard core....still got my pony tail !!
Wont tell ya I havent dropped one however. Be safe.
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LeeMS...ZFury...both excellent points!
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Very nice mister.
I call that... a sculpture.
You must have some long winters up there!
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Q jets are completely adjustable' date=' unlike a holley that you have to change springs on for the secondaries to open. The qjet has a screw and a allen set screw on the passenger side top to adjust the spring tension on the secondaries. 3/4 turn from touching for a big block and 1 turn for a small block. Of course that's just factory settings.
My monte went 12's with a qjet on a 406.[/quote']
A q-jet is a sweet thing when they are working properly.
I'll venture a guess that you may have had the rpms up just a tad before launch. hmmm?
I'll say this, the sweetest sounding engine I have ever had the fortune to punch was the afore mentioned 396 with a 780 QuadraJet! It just seemed made for that block!! Good stuff.
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Laymans terms.
Take your cubic inch displacement, times that by your TOP RPM requirements, divide that figure by 3456. Times the sum by the volumetric efficiency. Dont for get your decimal. Example 75% VE =.75.
Or you can just read the first three numbers in your final sum.
Remember to be realistic about your VE. A well tuned built engine with all of the goodies will fall about 95%. Some what stock...75% or less. A tired stock engine...65%.
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Carbs can be a bitch unless one gets lucky.Or does so serious matching.
A good breathing engine can consume a lot more fuel than it needs. this can lead one to believe that the engine needs that much.
Ok, Ok every one loves numbers and formulas, so here you go.
One of the first things to determine is what size carburetor do you need? A number of factors come into play. What cubic inch size is your motor? What do you do with it? Race, street use, towing, street use with occasional trips to the track? What type of intake manifold do you have, split plenum, open plenum, tunnel ram, individual runner? How fast do you spin the motor? What is the volumetric efficiency of the motor? Do you have a manual or auto transmission? What is the rear gear ratio? Do you want to get the best gas mileage possible or do you want to develop the most power possible? Keep in mind that a carburetor is just one part of the engine combination. All of the parts need to work together. Putting a larger carburetor on is NOT going to immediately put 100 more horsepower at your disposal. The carb needs to work with the other parts you have chosen and your intentions concerning how the vehicle is going to be used.
One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency.
The math formula is:
CARB CFM =
cubic inches X rpms
---------------------------- X VE%
3456
So if you had a stock, low performance production motor of 350 cubic inches and you wanted to spin it to 5 000 rpms max and it had a VE% of 80%, the formula would determine a required carb CFM of 405 CFM. If you had a warmed over street performance motor of the same size, but it was capable of 7000 max rpms and it had better heads, camshaft, headers and a performance intake that raised the VE% to 95%, the formula would give you a minimum required carb CFM size of 673 CFM.
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Timing advance?
Q-jets have delayed secondary operation.
If it cuts out when you nail it, timing advance.
If it cuts at the top end of the rpm sweep, she may be drowning.
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Is not CFM a measure of FLOW? Yes.
Heads, intake, and exhaust are the all the members of flow. Yes?
And its not just pure CFM, its the method of operation and the type of carb. Secondary size is part of the total CFM. Ever looked at a big vacume secondary? One would think that the primaries didn't even exist! Like dont drop your screw driver! Other factors include the vacuum requirement placed on the carb for excessories.
One should account for the weakest link in the flow.
It is common for moderately dressed engines to be over carbed.
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Sorry...couldn't resist!
The quadrapuke 780?
I had one on a 396 Impala. Would suck birds out of the sky when the massive secondaries flopped open!
But bought a Z28 with an LT1 350 that sported a 650 CFM.
66 Galaxie with a....thats right.....a 289 HiPo strapping a 480 CFM.
1970 327 Nova sucking through a 600 CFM.
But one of the worst cars I ever shouldn't have owned...74 guttlas Cutlass. 350 GM with a 780 Quadradump.
Flow is the deal, do you still have iron exhaust?
Maybe you can talk your friend out of the 600. What brand?
Enjoy the ponies regardless!!
Thanks for putting up with me!
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Was just about to ask about your new avitar!
Must say...very fitting!
It's a keeper!
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May I say............told ya.
I remember that you said that you didn't want to buy another one to drop your CFM. Well just keep the 600 and duck your friend!
Just kidding. Yep, the small block is often overcarbed. 600 CFM MAX!
Stock small blocks can even be happy with as little as 435.
But like Dr Hunt says...some day you will have some big displacement and carbs dont seem to be getting any cheaper!!
Congrats!!
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Well at least you will only be passing through Amarillo!
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"She" sounds a little wacky!
What if the"old man" gets back and puts the kybash on the sale?
Poundz9oh9...a check out might be in order. if it's a real deal, I can see why folks want to end the auction early.
Im going to go home now and check my spare part inventory!
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Larry must be close enough to cover Amarillo!
Past two days we have been pounded by rain! More to come!
From famine to feast! The place greens up quick with a little rain.
Havent seen any old folks floating down the road yet however.
Keep up the good work Larry!
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God bless Bondo.
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How about these Terry?
Want these very much!
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Yep.
Cut a donor section. Match it to your rig. Weld it. Putty and paint.
Hell paint the whole car!
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SBF = Small Block Ford.
:mrgreen: 
What a difference the right size carb can make
in Gen I & II Chevy V8 Tech Board
Posted
Had two.
Both were adjustable as to the activation point. As for the streetability, mine seemed to be a little picky. But I must confess that my knowledge was limited at the time. Probably could have had them adjusted differently but, basicly I had it begining its swing at just past half point. That way I rev it from the line and still get good mid range torque. There were also available various eccentrics which would alter the exponentiality.
It's a performance carb made to go. And go now!