Thread: The Black-art Of Carb Sizing

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One of the most important and sometimes confusing finishing touches to a hi-perf street engine is carb size and type. Holley is one of the most common so most of what I write here will pertain to them. One of the most common misconceptions is that you must run a small carb in order to have good low-end performance and drivability. What this usually does is cut top-end HP considerably. A lot of hot rodders never reach max potential from their engines because of carbs. Some down-size the carb in order to prevent the dreaded "over carbed" syndrome while others really do "over carb".

Choosing a carb is really very simple if you just follow a few guide lines. Holley has a formula for choosing CFM but it is very conservative and should be so. Many hot rodders think they're pushing 420hp and must have a 750 dp or even an 850dp, when in all reality they are pushing about 300hp and are killing their engine with ring wash from raw gas. A double pumper carb has no place on a street engine...What is really need is a variable size carb. One that will save low end torque and still have top end horse power. That is exactly what a vacuum secondary carb does. It lets the engine decide how much carb cfm it needs at any given time.

Some points to take into account is as follows:

1. Never use a carb on a small block street with venturi size larger than 1.4. Almost all 750s and some 850s fit here.

2. Thinning the butterfly shafts adds considerable air flow and booster signal.

3. Streamlining the venturis with fine sand paper to remove casting flash help turbulance and enhances booster signal.

4. The addiction of a K&N stubstack greatly boosts air flow and booster signal.

A completly stock 350 chevy can easily handle a 750 vacuum secondary carb and gain considerbly and give up nothing. A big mistake often made is to think that a vacuum type carb is great for economy and drivability but compromises performance. It is true that is great for economy and drivability but compromising performance couldn't be further from the truth. If any thing it enhances performance. A little bit of tuning on the vacuum springs can reap big rewards.

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I would hardly call choosing a carb.(a black art).Choosing the right carb is the simple formula of CID X MAX. RPMS DIVIDED BY 3456.After that you would have to figure out your volumetric efficiency of your engine.Most street engines are around 80-85 % efficient.Go to www.Holley .com and then click on Tech. library.When you look at this chart and read what they say you will see that a stock 350 does not need 750 cfm.After all an engine only needs so much air.

do the math, be honest about rpm range and you'll find a mild 350 for street use needs only 400 cfm. at 6k it only needs 600. over carbing kills more street motors than anything else. if your going to go to the track every weekend fine. but if it's a driver lose the 750.

You hear and read so much on this subject that I am almost at the point of beleiving and saying that it is simply a 'variable'. What I mean is, doesn't it really depend on the 'nature' and application of the motor in question? Example, a low hp/rpm motor will need less and a high hp/rpm motor will need more,BUT,only if they are driven 'that' way, low rpm only,or high rpm only. Sometimes, 'application' is the determining reason. But a carb that varies and exactly fits the application of the moment,whatever it may be,,,,does it exist?

I personally err on the side of less carb for a street engine. And how many people really know what their efficency percentage is? Add to that 2 tenths off your ET wont really be noticable on the street anyhow.

The reason I labeled it a black art is because of the controversy when it comes down to it, everyone thinks they have the simple answer. L@@k at this feed back so far. Many different answers. Some of which are fine points and others that a fine opinions.


Everybody including myself, wants a street engine to perform like a street engine with street engine manners, drivability, economy, etc etc............. But what is the harm in running a more power ful carb WITHOUT GIVING UP ANYTHING ON TORQUE. If you were to go back and reread my article, you will see that I am not shooting for highest hp numbers only.


The most common racer mistake is to assume that bigger CFM equats bigger HP. The next most common mistake is to assume that a carb can have to many CFM for an engine.... Confused by the contradiction there?? The problem isn't with the carb having too much CFM but, not enough booster signal to go with the available CFM. Given enough booster signal for atomization, you can give an engine all the CFM it needs at top and still deliver down low.

CHECK OUT THIS COMBO:

Chevy 355
9.8 CR
Comp 268H
Roller tip rockers
Chevy 186 heads
Edelbrock Performer RPM
Holley 750 reworked to flow 820cfm
1-5/8 headers Dynomax 2-1/2 mufflers


RESULTS:

TORQUE: HORSE-POWER:

@2K-353 152
@25-356 178
@3K-375 205
@35-398 265
@38-406* 288
@4K-404 314
@45-398 345
@5K-395 373
@55-368 377
@58-347 381*
@6K-335 380
@62-310 374

THAT IS A NICE, DIRT CHEAP COMBO THAT IS VERY MUCH AT HOME ON THE STREET. I RAN THIS ENGINE FOR ABOUT 3-1/2 YEARS WITHOUT A SINGLE PROBLEM. IT HAD A SMOOTH IDLE AT ABOUT 725, PULLED MORE VACUUM THAN I KNEW HOW TO USE, AND HAD A STOCK CONVERTER WITH A 3.42 REAR GEAR. I HAD IT IN A '76 NOVA AND RUN 12-90s @ 107. SHIFTED AT 6500 WITHOUT EVER A GLITCH UNTIL ONE-DAY AT A STOP LIGHT I JUMPED OFF WITH A STANG AND LOST OIL PRESSURE AT THE TOP. PICK-UP FELL OUT THE PUMP AND IT WAS BRAZED, JUST NOT GOOD ENOUGH. NOW I BRACKET THEM AND BRAZE.
THIS WAS MY EVERDAY DRIVER CAR FOR WORK AND ALL.

Originally posted by shevy not heme
You hear and read so much on this subject that I am almost at the point of beleiving and saying that it is simply a 'variable'. What I mean is, doesn't it really depend on the 'nature' and application of the motor in question? Example, a low hp/rpm motor will need less and a high hp/rpm motor will need more,BUT,only if they are driven 'that' way, low rpm only,or high rpm only. Sometimes, 'application' is the determining reason. But a carb that varies and exactly fits the application of the moment,whatever it may be,,,,does it exist?

A LOW RPM NEEDS LESS CARB AND A HIGH RPM NEEDS MORE CARB...........ALAS THE VARIBLE CFM VACUUM SECONDARY THAT LETS THE ENGINE DECIDE WHAT IT WANTS. THAT IS A VARIABLE CARB FOR A ENGINE THAT IS RUN AT VARIABLE RPM/POWER

F

Yes Camaro_ I know, ( I said some dum,not plum dum ) but read again, I said that fits the application 'exactly', meaning, that there is no carb that will exactly or anywhere near 'properly' fit the bill on a hypo SBC, unless it delivers, say under 300 hp and you never punch it.Put any carburated car on a dyno and do a gas analysis and you will see. But maybe I'm a cheap butt that thinks that plugs and mufflers should last forever,and that my grocery getter should do wheel stands anytime there is a pretty girl on the sidewalk.

In my last reply I should have stated 'exhaust gas' analysis cause Denny already mentioned the 'incoming' variables and imperfections. Now if some 'top tuner' whiz would just tell us all how to build the sbc combo that would give a 100% utilization,,,100% of the time ,,,,then this carb question would be solved. Or hey,,,if shevy would perfect the street/strip combustion engine then we could get out of the hotrod dark ages. Well I've gotta go now my car is in the driveway warming up and I see smoke and water coming out of the tail pipe,,,geesh wonder why?

Originally posted by shevy not heme
In my last reply I should have stated 'exhaust gas' analysis cause Denny already mentioned the 'incoming' variables and imperfections. Now if some 'top tuner' whiz would just tell us all how to build the sbc combo that would give a 100% utilization,,,100% of the time ,,,,then this carb question would be solved. Or hey,,,if shevy would perfect the street/strip combustion engine then we could get out of the hotrod dark ages. Well I've gotta go now my car is in the driveway warming up and I see smoke and water coming out of the tail pipe,,,geesh wonder why?

Well SHEMI, that could be a new tyme steam motor you got there or it could just be running very efficient..............You will never get any engine perfect, But with an on board O2 analyzer you can sure tune a carb well enough to make EFI fanatics impressed. The carb has many many tunible circuits starting at idle then going to transit, then going to main jets and then tuning power valve circuits. One major problem with carbs is that Holley tunes their carbs to be Universal and that don't fit most engines. It gives you the impression that a carb was intentionally built to deliver the complete wrong mixture at any given rpm level. Carb tuning is complex and you got to have some type of O2 sensor to get it close at all driving speeds. At idle you should only be pulling fuel from the idle discharge under the butterflys, part throttle is going to use mostly transition circuits, cruise is going to use main jets, And WOT is going to use power valve circuits. So you must tune everone of the circuits by them selves one at a time starting at idle and tune on up. Its not easy and it is time consuming but it makes more horses with less fuel.

Yes sir,and you can tune Harley's, oil furnaces and banjos to impress people also, .

Originally posted by shevy not heme
Yes sir,and you can tune Harley's, oil furnaces and banjos to impress people also, .

SO WHAT RAN OVER YOU???? A SHEMEY OR A HARLEY!!