Demon carb question. Booster type? Annular or down-leg?
02-09-2005, 12:23 PM
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Demon carb question. Booster type? Annular or down-leg?
I'm going to buy a Mighty Demon 850 for my new 496. I was looking on Summit's site at their selection and noticed two booster types. Can anyone tell me the advantages of each? Thanks!
Last edited by The_Dude; 02-09-2005 at 12:36 PM.
02-09-2005, 12:32 PM
#2
Le Mans Master
Carbs: How Big Is Too Big?
Barry Grant Serves Up Advice On Picking The Right Carburetor For Your Engine
By Marlan Davis
Selecting the right carb is not an exact science. No…it’s more akin to nature’s complex tapestry of life, an intricate mosaic of complex variables combining in myriad ways to produce often unexpected results. There is logic and order behind the mystery, but the patterns are not the cut-and-dried solutions of engineering textbooks; instead, they’re derived from the school of real-world experience as learned through trial and experimentation by a leading carburetor expert like Barry Grant, owner and founder of Barry Grant Inc., parent company of Demon Carburetion.
“Formulas Are Useless!”
Grant doesn’t believe in formulas—and for good reason. Textbooks tell us you can accurately select the proper carburetor size based on a relatively straightforward formula (see the formula at the right) that takes into account engine displacement, max rpm, and volumetric efficiency (VE).
Plugging some actual numbers into the equation, what does it recommend for a 350 engine turning 6,000 rpm at 100 percent VE (see the second formula)?
607.6 cfm?! Virtually no one—whether the original equipment manufacturer, aftermarket tuner, or racer—actually installs such a small carb on a high-performance 350. In the real world, everyone knows these engines make more power with larger carbs. Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs. The equation breaks down as a realistic carburetor size selection tool because carburetor flow ratings (in cfm, or cubic feet/minute) are taken at an arbitrary vacuum drop—3.0-inches Hg for two-barrels; 1.5-inches Hg for four-barrels—and there’s no guarantee that, at max-rpm wide-open throttle (WOT), any given engine actually sees the theoretical vacuum drop for which the carburetor is rated!
Pulling Vacuum Sucks
On the other hand, if a carb really does pull 1.5-inch-Hg (or more) vacuum at WOT, it has become a restriction. The carb is actually too small to let the engine realize its maximum power potential. That’s because the greater the pressure drop (the higher the vacuum reading) across or through the carburetor, the lower the air density is inside the intake manifold and combustion chamber. Racers like to see no greater than 0.5 to 0.75-inch-Hg vacuum on the top end. But race cars have high-stall converters and steep rearend gears, and most of them are lighter than the average street car. Racers don’t mind recalibrating the carb on the spot for changing track conditions. They are unconcerned about low-end driveability. That’s why all-out racers, when not restricted by the rules, run huge carburetors—the bigger the carburetor, the lower the pressure drop across it at any given airflow.
Driven to Compromise
It’s a different story for street cars and dual-purpose street/strip pack- ages. For these combos, the limiting factor in carburetor size selection is on the low-end side of the fuel curve. Will the carb meter correctly at lower airflows? Will it have good part-throttle driveability in the engine’s normal operating range? How will it drive in the winter, desert, and mountains?
As in life, so it is with carbs: Compromises are called for—a fine balance. Not too big because you’ll lose driveability. Not too small or the carb becomes a major bottleneck. For most hot, dual-purpose cars, pulling about 1.0-inch-Hg manifold vacuum at WOT, max rpm on the dyno isn’t far off. But there are no guarantees. It’s impossible to know for sure how a carb will perform in a vehicle based on how it did on an engine dyno. Every combo behaves differently; what works in a featherweight car with a manual trans and 5-series rear-gears ain’t gonna cut it in a 3,700-pound ride with a mild converter and 3.23:1 rear gears. As Grant puts it, “You’ve got to do lots of empirical work and go through lots of trial and error. It really takes thousands of carbs before you get an intuitive feel for what’ll work on a given application.”
Grant Us the Knowledge
Fortunately, you don’t have to wait until you’re old and gray to find the answer—you can take advantage of Grant’s gray hair instead. His guidelines are based on years of experience. While they apply specifically to Demon carbs, the general principles are applicable to other carburetor makes as well. The basic parameters revolve around engine load versus engine size as expressed in cubic inches. Compression ratio, vehicle weight, distributor mechanical advance curve, cam duration as it factors into actual running (as opposed to static) compression, trans and converter, and final drive ratio all factor into carb selection.
Barry Grant Serves Up Advice On Picking The Right Carburetor For Your Engine
By Marlan Davis
Selecting the right carb is not an exact science. No…it’s more akin to nature’s complex tapestry of life, an intricate mosaic of complex variables combining in myriad ways to produce often unexpected results. There is logic and order behind the mystery, but the patterns are not the cut-and-dried solutions of engineering textbooks; instead, they’re derived from the school of real-world experience as learned through trial and experimentation by a leading carburetor expert like Barry Grant, owner and founder of Barry Grant Inc., parent company of Demon Carburetion.
“Formulas Are Useless!”
Grant doesn’t believe in formulas—and for good reason. Textbooks tell us you can accurately select the proper carburetor size based on a relatively straightforward formula (see the formula at the right) that takes into account engine displacement, max rpm, and volumetric efficiency (VE).
Plugging some actual numbers into the equation, what does it recommend for a 350 engine turning 6,000 rpm at 100 percent VE (see the second formula)?
607.6 cfm?! Virtually no one—whether the original equipment manufacturer, aftermarket tuner, or racer—actually installs such a small carb on a high-performance 350. In the real world, everyone knows these engines make more power with larger carbs. Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs. The equation breaks down as a realistic carburetor size selection tool because carburetor flow ratings (in cfm, or cubic feet/minute) are taken at an arbitrary vacuum drop—3.0-inches Hg for two-barrels; 1.5-inches Hg for four-barrels—and there’s no guarantee that, at max-rpm wide-open throttle (WOT), any given engine actually sees the theoretical vacuum drop for which the carburetor is rated!
Pulling Vacuum Sucks
On the other hand, if a carb really does pull 1.5-inch-Hg (or more) vacuum at WOT, it has become a restriction. The carb is actually too small to let the engine realize its maximum power potential. That’s because the greater the pressure drop (the higher the vacuum reading) across or through the carburetor, the lower the air density is inside the intake manifold and combustion chamber. Racers like to see no greater than 0.5 to 0.75-inch-Hg vacuum on the top end. But race cars have high-stall converters and steep rearend gears, and most of them are lighter than the average street car. Racers don’t mind recalibrating the carb on the spot for changing track conditions. They are unconcerned about low-end driveability. That’s why all-out racers, when not restricted by the rules, run huge carburetors—the bigger the carburetor, the lower the pressure drop across it at any given airflow.
Driven to Compromise
It’s a different story for street cars and dual-purpose street/strip pack- ages. For these combos, the limiting factor in carburetor size selection is on the low-end side of the fuel curve. Will the carb meter correctly at lower airflows? Will it have good part-throttle driveability in the engine’s normal operating range? How will it drive in the winter, desert, and mountains?
As in life, so it is with carbs: Compromises are called for—a fine balance. Not too big because you’ll lose driveability. Not too small or the carb becomes a major bottleneck. For most hot, dual-purpose cars, pulling about 1.0-inch-Hg manifold vacuum at WOT, max rpm on the dyno isn’t far off. But there are no guarantees. It’s impossible to know for sure how a carb will perform in a vehicle based on how it did on an engine dyno. Every combo behaves differently; what works in a featherweight car with a manual trans and 5-series rear-gears ain’t gonna cut it in a 3,700-pound ride with a mild converter and 3.23:1 rear gears. As Grant puts it, “You’ve got to do lots of empirical work and go through lots of trial and error. It really takes thousands of carbs before you get an intuitive feel for what’ll work on a given application.”
Grant Us the Knowledge
Fortunately, you don’t have to wait until you’re old and gray to find the answer—you can take advantage of Grant’s gray hair instead. His guidelines are based on years of experience. While they apply specifically to Demon carbs, the general principles are applicable to other carburetor makes as well. The basic parameters revolve around engine load versus engine size as expressed in cubic inches. Compression ratio, vehicle weight, distributor mechanical advance curve, cam duration as it factors into actual running (as opposed to static) compression, trans and converter, and final drive ratio all factor into carb selection.
Last edited by SmokedTires; 02-09-2005 at 12:37 PM.
02-09-2005, 03:55 PM
#3
Team Owner
Downleg boosters have a slight bend in the mounting arm, so they sit a little lower in the carb (closer to the throttle blades) to get a stronger vacuum signal.
Annular boosters have discharge holes arrainged in a circle, instead of just one discharge hole. Either are considered to give a stronger vacuum signal than the regular straight-leg boosters.
IMO - I've tried both in my 750 speed Demon and 825 Race Demon. The down leg is better for cruise and partial throttle. but when you open up the throttle you can just see gas spewing out and the atomization is poor. That's where the annular really shine. The fuel is more of a mist at large throttle openings.
The best thing to do to both of them is remove them before you install the carb. Micro polish them inside and out so they have the smoothest air flow possible. Atomization is the key
Annular boosters have discharge holes arrainged in a circle, instead of just one discharge hole. Either are considered to give a stronger vacuum signal than the regular straight-leg boosters.
IMO - I've tried both in my 750 speed Demon and 825 Race Demon. The down leg is better for cruise and partial throttle. but when you open up the throttle you can just see gas spewing out and the atomization is poor. That's where the annular really shine. The fuel is more of a mist at large throttle openings.
The best thing to do to both of them is remove them before you install the carb. Micro polish them inside and out so they have the smoothest air flow possible. Atomization is the key
