The Black-art Of Carb Sizing
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".:confused:
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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Re: The Black-art Of Carb Sizing
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.
