I see this discussion a lot on this board and pose the following questions for the purpose of helping me understand carb. selection; 1. A 462 CID engine (as described above) has the mechanical potential of flowing 775 CFM @ 5800 RPM. This figure is presuming that ANY conventionally aspirated engine could function @ 100% Volumetric Efficiency. Through a dual plane manifold with mufflers attached 90-95% VE would be a generous estimate making 736 CFM the maximum amount of air this engine would ever require. Wouldn't a larger than necessary carb. create slower air flow, therefore, less efficient atomization in the lower RPM ranges? I use a Holley 650 CFM carb on my stock Buick 455. At the very top of the rev range I lost a bit of performance from the 800 CFM Q-Jet it replaced. Through the remaining 4500 rpm this carb massively outperforms the larger one it replaced. Part of this, maybe most, is the result of product design. Some, I beleive, is the result of better venturi function due to the smaller size. I use my smaller carb as a poor man's rev limiter. Waddell Wilson states in "Racing Engine Preparation" that the stresses on internal engine parts are increased by a mulitple of 10 for each 1,000 rpms. Given the large journals, IE high bearing speed, and heavy rotating and reciprocating mass of the Buick motor, aren't we tempting fate by turning these engines beyond 6000 RPM? My instincts tell me the Bigger is Better argument, regarding Carbeurators, is more akin to male ego issues than hard science. I look forward to being educated on this matter.
Part, but not all, of the issue is HOW ARE CARBS RATED, and WHAT PLUMBING (Manifold) are they used with? Put another way: An engine needs a certain amount of airflow. You've said your engine "theoretically" needs 775 CFM. What carb size actually delivers that amount? (Hint: it's not the one rated to flow 775 cfm!) A Holley four barrel is rated to flow "whatever" CFM when there is 1 1/2 inches of STEADY (That is important for later discussion) vacuum to draw air through it. A Holley two barrel is rated with 3 inches of STEADY vacuum. More vacuum=more airflow. Therefore, a "500 cfm" four barrel, handicapped by having less vacuum to draw air through it, actually flows LOTS MORE air than a "500 cfm" two barrel. And, at full throttle, max power, you want LESS than 1 1/2 inches of vacuum in the intake manifold. This is why you see small blocks with twin 600's or 750's on top. You see where this is going? As the vacuum under the carb at WOT is reduced, you have to have a larger "rated" carb to flow as much air. Now we add the manifold type to the equasion: A fully-independent runner manifold (Like most performance motorcycles) has one carb throat for each cylinder. The carb is "sucked on" VERY STRONGLY every fourth stroke, and in between intake pulses, the carb is "resting". (SUCK!, pause, pause, pause, SUCK! pause, pause, pause.) You therefore need a HUGE carb, enough to satisfy the peak demand, but 3/4 of the capacity over time is "wasted" because there's no vacuum to draw air through it. A 180-degree, dual-plane manifold has one primary and one secondary carb throat for four cylinders. (One half of the carb feeds one half of the cylinders) In addition, there is a fair amount of plenum space that the independent runner manifold above doesn't have. So, having connected four cylinders to one side of the carb, the air flow becomes more even, with less downtime. (Suck, pause, Suck, pause, Suck, pause, Suck, pause.) The carb throats are "resting" only half the time instead of 3/4 the time as in the independant runner design. (This is not quite true, the plenum area softens the intake pulses and "spreads the load", allowing the carb to be "sucked on" almost constantly) An open-plenum, or single plane manifold connects ALL the cylinders to ALL the carb. There is a large plenum area. This almost eliminates the intake pulses, making the vacuum draw very smooth. It allows the WHOLE carb to react to ALL cylinders. The carb experiences NO downtime. (ssssssuuuuuuuucccccccccckkkkkkkkkkk...) So, for a given RPM and displacement, what engine can get by with the smallest CFM carb: The one where the carb is only "active" 1/4 of the time, and has to flow all the air in distinct bursts, or the one where the carb is flowing air 100 % of the time? You got it: the more the carb barrels are shared, the smaller the carb can be rated and still deliver enough air. Even if an engine "theoretically" only needs 775 CFM, it needs that amount of CFM at a lower vacuum level than carbs are rated for, (which means it needs a carb rated for MORE than 775 cfm) and depending on the manifold, you may need a "correction factor" to take into account the way the cylinders can draw on the carb. Edelbrock says a dual-plane manifold needs a carb approximately 1.2 times as large as one mounted on a single plane manifold. As for a 600 making more power than an 800 in the midrange...Yeah, maybe. I wonder if the improvement is due to the CFM difference, or the carb design differences, or just that the 600 happens to supply the right fuel curve at that RPM and the 800 Q-Jet could have been tuned to provide the same or better results.
Got it. Appreciate your take and the caveat you stated in your last paragraph is the same one I stated in my post. I assumed the "below the carb" pressure was a standardized factor used in establishing CFM. If not, then it seems buying a carb. using advertised size would be a crap shoot. Plenum type and distribution issues are the compromises we deal with if we want a vehicle that will idle with a hood we can see over. The assumption that VE would be effected, only, on the intake side, discounts the harsh angle the exhaust must take to exit the engine. That, in my opinion, would be more of a detriment to increased VE than a .2 loss of carb size. Doug Rowe, in his book on Qjets does not mention a multiple factor in establishing carb. requirements. I enjoyed the "suck" usage in your description of the pulse factor. Wonder if chrysler used it when they were playing with cross rams in the 50s.
To a great extent, "everything depends" on everything else. You're right, exhaust efficiency is a factor, as is compression, cam profile, cylinder head flow efficiency and cylinder head burn efficiency. Even booster venturis make a big difference! The Holley "straight" boosters are the least efficient according to Dave Vizard, and that's what Holley uses in their most popular carbs. My understanding is that Demon uses "wet" flow numbers to rate their carbs, so a "750" cfm Demon flows more than a "750" cfm Holley. Go figure. It really is tough to know what you're buying. In the end, what matters is whether or not you're happy with the way your engine runs. Sounds like you are, and that means "you've won".
I paid dearly for that "win". Did a 460 in the mid 80s and had $5200 invested in parts. (all my machine work was free) double 660 center squirters and every piece of hardware you could ge a sticker for. I just hate seeing people throw money at parts that net very little in "real world" perfomance. Most of us are driving street legal vehicles and our driving habits very seldom enter the realm of extreme mechanical theory. My takes are offered as a practical alternative to the notion that every performance solution can be solved by spending more money. Once you get to the 450 HP range the cost of each Pony beyond that point gets very expensive. My guess would be about $50 per. My old beater has a 0 to 60 time in the low 6 second range. I have $775.00 invested. To get it in the 5 second range would probably cost me $5,000.00. I tire of seeing people shamed into spending money on technology they neither need, nor will use, for their particular application. No offense intended to those of great means, but you can have a lot of fun, get a bunch of "thumbs up" and use your imagination without having to spend a ton of money.
Tom and Schurkey, Well said! I also learned the hard way with my 361 Chrysler. Cam, duals, intake and carb swap, and I *still* kept the 2.76's in the rear. My priorities weren't in the right place, and I spent too much $$$ looking for gains that could have been easily found for low $$$ if I had just used my head instead of "wow, that cam sounds cool, but it still doesn't shred the tires." With that said, I too have adopted the low buck + smarts = goofy smiles/lots of fun philosophy. Long live the sleeper!
