For the parameters of this discussion, spread bore or square bore is dictated by manifold design. Bolting one design carb onto the other design intake with a simple carb adapter results in fuel distribution issues that will have a big negative effect on part throttle fuel economy and performance. Its perfectly possible to modify a square bore intake to work well with a spread bore carb, but that sort of work is well outside the scope of this thread and the HAMB.
George, I think you're a bit overwrought.... You say the formula is 'meaningless, and has no place in the real world' yet even Vizard quotes it and goes on to say that 'Chances are, that doing nothing more than this, the results will be good'. That doesn't sound 'meaningless' to me. Now, that's not to say I'm disagreeing with any of your quoted material, I'm not (and I spent quite a bit of time digesting it). Will using this formula guarantee the absolute best choice in a carb? Nope... But I'm not claiming that any number it generates is 'absolute' either; this is simply a 'guide' that if used correctly, will give a reasonably decent ballpark figure. The factory CFM numbers, whether they're accurate for a given application or not, at least give you a basis for comparison between carbs. Hot rodders suffer from the 'if some is good, more is better' disease on a regular basis and we both know that this results in some pretty doggy combos more often than not. The Torquer intake I ran on my Comet I got for FREE as a trade for a OEM 2-barrel intake and carb after a kid installed it and a 660 double-pumper on a stone-stock 289/C4 Mustang (and threw away the original parts) that was virtually undrivable with that carb/intake. It really wasn't the best manifold choice on my Comet either, but the price was right, the 4-speed made it livable, and I made money after selling the 660... So for the average schmo who doesn't have a flow bench, dyno time, or the bucks/expertise to buy/build a custom carb, that 'meaningless' formula (if used realistically and/or conservatively) will spit out a number that will be 'good'. I've tried both sides of the 'big/small' carb debate and personally prefer 'small' for the generally better daily drivability, but that's me. If there's a better way to calculate carb size without all that shop equipment, we're all ears.... And I will say that I'm very intrigued with those Summit Autolite copies. They always were a pretty decent carb, but got changed out for the 'better' Holleys (more is better disease at work) back in the day and I hate to think about how many of those I threw away...
Its no closer than a guess, which you don't need a formula for. Overwrought? No not really, not when the very same formula is thrown around here on the daily basis as an absolute, and as I have clearly demonstrated, the formula is based on a false, incorrect assumption, namely that a flow number generated at 1.5" hg pressure drop has some mathematical relationship to how much air your engine is consuming at an entirely different pressure drop. It is not "scientific" its wrong. Plain and simple. As I said in my last paragraph, if you want to use the formula, knock yourself out, I don't mind. Some will "get it" some wont. But there is no direct relationship between what a carb flows at 1.5" hg on a flow bench, and how much air it is going to flow on your engine at WOT at an entirely different pressure drop. I am not going to argue the point further, theres really nothing to argue. Some will get it, some wont. Sacred cows tend to die slow and lingering deaths, I get that. As far as a better way to use the formula, go back and read again, its actually in there, between the lines. You haven't read thoroughly enough, and you haven't grasped it.
Oh, and the 660 being undriveable? Don't doubt it for a minute. Unfortunately you have jumped to the wrong conclusion. It has nothing to do with the 660 being "too big", 660's were designed and calibrated to be used on tunnel-ram equipped small-block MP cars, which do not have much manifold vacuum. The reason it would barely run is because the stock idle feed restriction (IFR) in a 660 is WAAAAY too big for a typical single four bbl street motor with 10-15" of manifold vacuum at idle, it is calibrated to provide enough fuel at idle for an engine with 4-5" of vacuum at idle, and its a bit large even for that. Even on tunnel ram applications, it was commonplace among serious carb tuners to reduce the IFR by around .015, I know, because I did it. Should have put a 650 double pumper on it, would have run a treat.
@falcongeorge Tell me what was wrong in my old setup Back in the late 70's I had (still have it) an LS5 454 out of a '70 Vette in my '57 Chevy PU...I pulled the Stock intake and Q-jet off, and put a Torker intake and Holley 750 dual feed, vac 2ndry carb on it....it was a dog until about 3000-3500 rpms, worse than the factory set up...could a different carb or tuning of that carb, make it come alive down low? After a summer of jets and PV changes, I swapped in a Holly Dominator? (have to look to be sure) dual plane intake, ran the same carb, and had lowend power like I'd never seen before. Now I know the dual plane is better suited for a cruiser, weekend warrior setup, but could that carb have been tuned to work with the Torker intake? Other particulars, truck has 3.08's and TH400, tall L60-15's in the rear... Was it just the intake or?
back around 1973-4 i had a '301' 283 vette motor with a hot cam in a '56 vette. i tried (and kept) a holley 2 bbl on an adaptor on the 4 bbl aluminum manifold (brand ?) because it ran so well on the street. sold it to a dealer from salt lake that way. as for the edlebrock carbs... i have 3 of them on different cars. how do i identify which model carb do i have?
I am sure some of the carb and intake stuff is like a fingerprint no two the same unless one spends the time to fine tune it.
MP racer Bob Thompson Feb. 76 Hot Rod, talking about how he sets up 660's for his BB chevy MP motor. Too bad I wasn't there when you were trying to make that carb work on your Comet Steve, I would have taken a couple pieces of wire about .020 in diameter, cut them to about 3/16" long, bent them in an "L", and dropped the short leg into the IFR, trapped the long leg between the metering block and the gasket, and viola, it would have started responding to the idle screw adjustment, and the carb would have worked fine. Did it literally DOZENS of times. Of course, actually re-sizing the IFR itself, as Thompson says here, is a better way to go. I do this as a routine part of tuning, I remove the stock bushing from the metering block, tap the hole with a 10/32 bottoming tap and use short brass set screws I drill with a wire drill set. 80% of "the carb was too big" stories out there are based on the IFR's being too large for the application. How in the hell anyone who has even the remotest idea of how a carburetor functions can see that the idle is too rich and outside the range of adjustment through idle screw tweaks, and take this information and arrive at the conclusion that the carb is "too big" is honestly beyond my comprehension. The two are completely unrelated. Carburators are devices that meter fuel based on the pressure drop in the venturis. In fact, if a carb is really "too big" it will be lean on transition, and the way to crutch an excessively large carb for the application is to ADD more fuel. The idle circuit has nothing to do with the carb being too big or too small, it usually has to do with taking a carb way outside its design envelope, and is a calibration issue. I should note here, its not spelled out in the article in so many words, but Bob is also effectively moving the IFR from the top of the well to the bottom. Some Holleys have the actual IFR at the top of the well, some have the IFR at the bottom. Moving the IFR to the bottom of the well is common when adapting a "race" carb like a 660 or 4500 dominator to a street application, not that Bobs MP car was a street car by any stretch of the imagination.
Bingo. And ANY application will benefit from fine tuning ALL the circuits in the carb, and it is SO MUCH EASIER in this day and age, with the use of wide-range lamda sensors.
Open plenum on the torker. the larger plenum area lowered the signal to the boosters. You needed a bigger primary side accelerator pump shooter, more pump shot, to cover up the lean spot until the pressure drop got higher and it started to pull fuel from the booster venturis. Honestly, the 3.08's were probably a little tall for a torker and I would expect the dual plane to be better in that application, but increased pump shot would have made up about 95% of the difference. I'll add this too, if the carb is 'right" in the first place, and you add a 1" open spacer to the combo, you will need more pump shooter to bring it back to optimum, same reason, the increase in plenum area has dampened the signal to the boosters.
Thanks George. The joys of youth and not knowing squat, nor where to find the info, even the speed shop that sold me the set up (was a machine/race shop as ) wouldn't give any advice except, "change jets, try this PV"..lol
George, I never even tried that 660 on the Comet. The kid had been at it already (along with his 'buddy' that talked him into this in the first place), I had a fresh 600, and knew another guy that was hot to buy the 660 so I sold it ($$). This motor was never a 'scienced' out combo, it was assembled out of 'parts at hand' for the lowest $ amount possible after my X blew up the 289 that was in it. I didn't have a lot of time to invest in it either (family issues at the time) and as it was my go-to-work vehicle, get 'er running was the word. It ran good if the Rs were up with the 600, but was really doggy in traffic and wouldn't do better than 15 mpg (which was a budget issue for the miles I drove daily). The 450 I put on more-or-less fixed most of the issues, but I did take an expected hit on top end power. I was about ready to dig back into it again when my back blew out, but after the diagnosis/surgery/recovery during which the car sat outside for about a year and a half because I couldn't drive the 4-speed, it had deteriorated to the point beyond what I could fix then and I sold it. Still miss that one sometimes.... it was fun to drive.
Again, it probably wanted more shooter with the open plenum of the torker, the 600 probably needed a little more shooter, or better yet, an F4B instead of the torker. I like single plane intakes in general, but they are not suited to combos with excessively tall rear-end gearing. The 15 mpg is a little more mysterious, just making a total WAG years after, should have maybe tried a slightly different power valve, it was maybe opening very close opening at your cruise vacuum, every time you stepped on the gas a little in traffic it may have been opening??Without being there, just a WAG.
In for a penny, in for a pound, so for the sake of clarity: 1) the flow bench isn't a carb tuning aid (unless you start swapping booster venturis, which is well beyond the scope of this thread) the reason I brought the flow bench up is because experience using a flow bench will make it abundantly clear why "the formula" is incorrect. When you are flow testing a carb, or an intake port, or anything else for that matter, it immediately becomes clear that flow numbers are not absolute, how hard you suck (laymans terms) on the orifice you are testing has a DIRECT correlation on the cfm number you see on the manometer. And I think we can all agree that it is not a given that your engine is going to be pulling 1.5" hg at peak power WOT. That's actually what the formula as handed around REALLY calculates, how to select a carb that will make your engine pull in the region of 1.5" hg at wot. If you are targeting 1.5" hg @ WOT peak power, you aren't gaining driveability over a properly tuned, properly sized carb or anything else, you are just leaving between 15 and 30hp (and some torque, believe it or not, and I can support this with dyno test info if necessary) on the table for no good reason. 2)there is no direct relationship between "general street driveability" and carb size expressed as cfm. Go back and really read post #42 on booster venturi design. Really read it and digest it, there is a TON of really good information there. Read the stuff I posted from Pro Shop carbs too. 3) one thing that is more or less true, an undersized carb will help hide sloppy carb tuning SOMEWHAT better than a bigger carb, ASSUMING the same booster venturi design, which is rarely the case, even within the Holley line.
I'm going to add this diagram here from the old HP Books Holley book, its a real good one. Funny thing is, if you read close, the diagram is right, but theres an error in the text, or depending on their intent, its at the very least misleading, and it could really mess with your head if you are trying to understand this... also important to note, the idle and transition systems are not depicted in this diagram.
i wish they would bring back the 4011 as well.............but spreadbores just don't sell in big numbers. And never have....
I honestly never blew one even without the power valve protection. Its 95% internet bs, with about 5% basis in fact.
I've blown Holley power valves but using a 4-hole spacer (even if it's just a thin sheetmetal one) under them helps a lot.
I have been in this game all my adult life, ran a speed shop for a year and a half, and yes Holley carbs doblow power valves. It is just a general problem with them.
I will re-iterate just so its clear, since 1991, holleys have had a check ball in the passage to the power valve, and it is a 5 minute job to retro-fit to pre-1991 carbs. So rupturing power valves because of a back-fire is a non-issue, and on carbs made since 1991, its impossible. Heres photos showing how to retro-fit it to older carbs. The carb in the example is a 500 2bbl for a circle track application, but the process is the same. The last time I did one was around 25 yrs ago, I didn't know I was gonna need photos for the HAMB at that time, so I didn't take any. So you drill part-way through this passage, using a drill bit 1/16 larger than the check ball you are going to use, I use a 5/16, so I can use a 1/8 spring that is common in ball-point pens. I wrap a skinny strip of masking tape around the bit to use as a depth stop. Then I cut the spring so that the top of the 1/8" check ball will sit even with the top of the base plate, and drop the spring in the hole, and drop the ball on top of it. The extra 1/8" around the check ball allows vacuum to operate the power valve, but the balance of the passage inside the main body that leads to the power valve is still the original diameter, and smaller than the check ball. In normal operation, the check ball is floating in the passage on top of the spring. When there is a violent backfire, the pressure wave forces the check ball up against the smaller diameter hole in the main body, and blocks it off, the pressure wave never reaches the power valve. Remember, Holley has been doing a similar mod straight from the factory since 1991.
Seems nobody here cares for the new Demon "Street Demon"? 625 cfm, little tiny primaries, large, wide open secondaries. I put one on my chevelle and loved it. Got one on the bench for the el camino.
Not the new ones. They come from the factory with a fail-safe check ball. It is no longer a "general problem" with them, and you are way behind the times.
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