Most modified carburetors are, in my opinion, only offer mild alteration from a box stock carb. So what are the differences?
In my experience, or exposure to other brands of modified carburetors, the differences amount to small changes in calibration of one or more of the following; main jet, idle fuel restrictions, idle and high speed air bleeds, emulsion well diameter, possibly a billet base (worth no performance gain, but nice to have), possibly billet metering blocks (worth no performance gain, but nice to have), perhaps a change in booster specs - and less frequently a different booster style, and generally most carburetor builders / modifiers are pretty conventional with carburetor sizing.
I don't mention milled choke towers as Holley HP and Race Demon center sections are very common now.
These things can improve part throttle performance - which is very important on many types of racing, and less important on others.
So where is the performance gain at W.O.T. as Joe suggests? If you are replacing one set of 660's with a set of modified 650's, and each are calibrated optimally for the application, there isn't likely going to be any real gain.
I'll reiterate that box stock carbs have a generic fuel curve - by necessity. Carburetor modifiers can take a stocker and incorporate a variety of calibration changes based on experience to better match the needs of the engine / application across the rpm range. I'd stress that every engine is different, and that finish tuning always needs to be done in the car - no matter how good the modifier thinks he / she is.
Is a carburetor worth $300 - $400 more than a stocker? I sold many carbs for $1.000.00 and more than a stocker - and my customers were / are happy. Imagine that. - Remember though, the State of California Dept. of Consumer Affairs Buearu of Auto Repair, the City of Los Angeles, head City Attorney Rocky Del-ghetto and John Bohannon, John Nelson and Roderick Baca (of the BAR) want you to know I am a lier and a thief and that they confirmed it by forcing me out of business unlawfully and criminally (that sounds redundant, but it's a matter of degree, something unlawful may not rise to the level of prosecutable, whereas something criminal is clearly prosecutable. Apparently for CA state employees, perjury isn't a crime), so...
The carburetors I build (and I am sure what other good carb modifiers build) simply reflect the things I had done hundreds of times to "box stock" carburetors on as many different engines to enable the engines to idle better, have better throttle response, to make more torque and horsepower. Pretty simple really. After a while, you see that certain modifications have a specific effect and how it relates to various aspects of engine modifications such as headers, cams, various types of intakes and so forth.
As a result of the time I've spent trying to con people out of money, I mean, looking for ways to improve the Holley / Demon carburetor, to build the better mouse trap, I began building my carburetors larger and larger for a given application.
But, what happens when you put a set of my 900 cfm carbs on Joe's 817 HP 434? (Or anyone else's, I'm not picking on Joe). Will you see an increase in air flow thru the engine? Maybe a very small percentage gain - not worth anything in itself. Will it be a bog monster trying to leave the starting line @ 5000 rpm? Not at all. It's likely the owner of the car will need to do some serious work to get the car to hook. Why you say? I can't tell you, but I'll sell you some.
Other areas to consider; as we discussed or are discussing in two other threads, if you increase the size of the carburetor(s), you reduce vacuum in the intake. Less vacuum means more atmospheric pressure in the intake. As Madbill pointed out in one of those threads, the exhaust has less resistance to overcome in order to evacuate the cylinder, making the exhaust system more efficient or effective. It's like capping the end of a straw in a drink, then you pull it out and no fluid can leave the straw. Then if you allow the tiny opening on top, the fluid comes out slowly. Let's say you double the size of the opening, the fluid comes out faster. Same deal.
What typically happens when you bolt a set of 900 cfm carbs on an engine that is really well designed for two 660's? By increasing the pressure in the intake - or reducing the vacuum if you prefer, and making the exhaust system more efficient, you will may see more fuel in the tailpipe (over) scavenged, you will see more power - but how much depends on how badly the cylinder is over-scavenged. If you are relying on O2's, it may tell you the engine is too rich and you may wrongly lean the engine - when in fact, you need to widen the lca. Of course, this happens all the time with normally sized carbs - one of the reasons you can't trust dyno tuning.
Consider that if you increase the pressure in the intake, or reduce the vacuum, the induction charge will begin to flow sooner relative to valve opening than if it is operating in a greater vacuum - effectively increasing the duration of overlap.
Also consider that if you have a restricted induction system, smallish carbs, the inability or degree to which there is resistance to drawing in fresh induction charge, it will to some degree pull exhaust back into the cylinder. Hmm, built in EGR!
So, how does all this make more power? I don't know. There's a bunch of issues. If you bolt on a somewhat conventionally sized modified carb, the potential for increased torque and horsepower is greatly limited as it limits the extent of modification of the carburetor - but then again, not everybody has Weber Power Plates to market. With conventionally sized carburetors, it is really about simply having the right a/f ratio over as much of the rpm range as possible. A relatively stock carb can probably match peak numbers & a/f ratios as a modified one.
Joe's example if two carbs is not necessarily a fair comparison of a modified carb to a stock one. For starters, most people don't run two carbs - so his example isn't relevant. Additionally, two stock carbs on one engine IS more forgiving of the stock carburetors inadequacies than a single carb application would be.
I don't want to get into telling my competitors all about how to build a better carburetor exactly, so I will refer you to something I wrote on another thread. There is a section where I make comparisons to most Holley's and Demon's. It's the last post, but read it all.
http://speedtalk.com/forum/viewtopic.php?p=33336&highlight=#33336
Troy Patterson TMPCarbs.net TMP Carbs
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