by admin | Mar 30, 2014 | Carburetors, Tech How-To |
1.) How do I select the right size of carburetor? Carburetor performance is largely based on the air-speed traveling though its venturis. If they are too big the air speed will be reduced, too small and vice-versa. Probably the best instruction on carburetor choice is found in the Demon Carburetor Selection guide. It is based on camshaft duration at 0.050in of valve lift, the type of intake manifold (dual plane or single plane) and the type of transmission (manual or automatic) and stall speed. Beyond this it is always best to discuss carburetor selection with a qualified technician. Click here for the Demon Carburetor Selection Guide. 2.) What is one of the most commonly overlooked carburetor-tuning issues? One of the greatest obstacles faced by amateur carburetor tuners is failing to understand the essentials of initial ignition timing. Ignition timing is affected by a host of elements including fuel type, mixture strength, combustion chamber shape, compression ratio, temperature and humidity. Bigger camshafts and larger cylinder heads with matched intake manifolds require increased ignition timing to promote better air velocity and maintain efficiency. Thus the carburetor is helpless in its plight if the initial timing is late or the timing advance curve is slow. These two potential timing impediments frequently exhibit poorer starting and drivability troubles. The ignition is always timed to fire the spark plug before the piston reaches Top Dead Center (TDC) in the cylinder. Firing before TDC is necessary because of the time it takes for the flame front to ignite the air-fuel mixture in the cylinder. Demon’s carburetor selection guide consists of helpful recommendations on ignition...
by admin | Sep 12, 2013 | Carburetors, Tech How-To |
By Sam Logan. Photographs by Moore Good Ink: Download text and hi-res images here. The cardinal rule of carburetor tuning is Ignition First. Once the static ignition setting and the ignition advancing mechanism in the distributor is correct, the air-fuel mixture can be tuned for full power and fuel efficiency. High-performance carburetors, intake manifolds, cylinder heads, camshafts, and other tuning components are all dependent upon correct ignition timing; if the spark is not delivered at the proper time to the combustion chamber, the quest for optimum power or economy is impaired. But the distributor has vanished! Tuning contemporary hot rods involves electronics and computer software. Sensors abound. They sense Manifold Absolute Pressure, Mass Air Flow, crankshaft position and so on. They report to an ECU (engine control unit) that constantly ascertains all the variables and tells each spark plug when to fire. Where there was once a distributor, multiple coils now exist, often one on each spark plug. Still, what a joy it is to understand the psychology of the hot rodder who lusts for a carburetor and a distributor. And, ironically, older vehicles can be simpler to tune. They require no fancy equipment or computer knowledge, often just a timing light, a screwdriver and a few wrenches. Brief background on points-and-coil ignition Before sophisticated electronic management systems arrived, we used the points-and-coil ignition system that first appeared on the 1910 Cadillac. A distributor was employed to determine when each spark plug should fire. An engine-driven mechanical cam in the distributor rotating at camshaft speed operated a set of breaker points. The points switched electrical current to the coil which converted it...
by admin | Jul 12, 2013 | Carburetors, Tech How-To |
Text by Sam Logan. Pictures by Moore Good Ink. Download text and hi-res images here. Engines produce vacuum, and over the past 130 years engineers have contrived ingenious ways to harness its power to the engine’s induction system. Aided by barometric pressure, ignition and compression, the carburetor creates the air-fuel mixture that promotes combustion. Through a labyrinth of small-bore drillings in the carburetor, the vacuum draws a potent mixture of air and fuel. So formidable is the mixture, the carburetor has empowered naturally aspirated full-bodied 500cu in drag racing cars to speeds in excess of 213mph in a distance no greater than 1,320 feet! It works by pressure differential; high pressure flows toward an area of low pressure. On starting and at idle, the area of lowest pressure on a carburetor is under the almost closed throttle plates (see second illustration below) and as the engine gains speed, the area of lowest pressure switches to the boost venturii (see first illustration). On 4150-style carburetors, the boost venturii reside within the main venturii and low pressure is created by the constricting shape of their bores, which causes air speed to increase and, as a result, a siphoning effect is introduced at a boost venturi’s tiny air-fuel discharge orifice and through the air-fuel metering circuits back to the fuel bowls. (The faster the air speed passes through the venturii the more fuel is withdrawn from the fuel bowls and thus unleashing optimum engine power.) As the fuel bowls are vented to barometric pressure—an area of greater positive pressure than that of the discharge orifice in the venturii—the air-fuel mixture is compelled...
by admin | Apr 12, 2013 | Carburetors, Tech How-To |
Text by Sam Logan. Photography by Moore Good Ink. Download text and Hi-res images. In the dyno room, Lamar Walden’s first production road-going 409 fitted with twin 625 Street Demon carburetors revved with enthusiasm to 6,293rpm and generated 602.4 peak horsepower and 602lbs-ft torque at 4,600rpm. For some, the emergence of the new Street Demon and the re-emergence of a new 409 have been the biggest hot rodding news of 2012. Earlier in a comparison test, twin Edelbrock AVS carburetors had generated a commendable 553hp @ 5,800rpm and 575lbs-ft torque @ 4,470rpm. Why had the Street Demons triumphed by 49hp and 27lbs-ft of torque? “I imagine,” said Lamar, “its supremacy lies in the size of its secondary throttles.” Unlike any conventional 4-barrel layout, the Street Demon is a three-barrel carburetor inhibited by few restrictions—the secondary throttle bore is one big opening. It’s taken Demon a while to compose a convincing answer to the Edelbrock AVS. But once Larry Tipton, Demon’s distinguished senior designer, focused his creative energy on the new design in 2010, we suspected a beacon of carburettor ingenuity could be in the works. And when it appeared on May 25, 2012, it not only looked the part in a market where appearance is of primary concern but also it proved to be a very strong performer. Based entirely on a brand-new concept this innovative three-barrel 625 Street Demon has noticeably smooth contours especially around the air entries, unlike its Edelbrock counterpart. Though both carburetors have dual mounting points, accommodating square-bore or spread-bore manifold mounting without adapters, Tipton is particularly pleased with the effectiveness of his triple-stack boost...
by admin | Dec 17, 2012 | Carburetors, Tech How-To |
By Sam Logan. Photos by Moore Good Ink. Download text and hi-res images 1 of 2. Download text and hi-res images 2 of 2. Before the new Street Demons were first released to the public on Friday, May 25, 2012, the carburetor company dispatched twenty to thirty pre-production units to discerning carburetor critics for testing and evaluating. For this purpose Street Demons were sent all over the country—even to Australia. Tests were conducted at sea level and in the mountains, in cold and in hot conditions, in stop-go traffic and in engines with unfathomable camshaft timing—profiles contrived for noise rather than power. Some even found their way onto towing vehicles, lugging heavy trailers up hills. All testers were invited to “Have at it,” as they say and were actively encouraged to present their findings—warts and all. One of these pre-production Street Demons appeared at Automotive Service and Performance (ASAP), a hot rod tuning shop in Gainesville, Georgia. Known as the poultry capital of the world, Gainesville nestles in the Appalachian foothills on the shores of Lake Lanier about 50 miles north of Atlanta. It has an elevation of 1250 feet. ASAP’s Bobby Tow had a 1972 Dodge Dart lined-up and awaiting the fitment of the new Street Demon. The Dart, now owned by Rick Ellis, President of the North Georgia Mopar Club, is one of around 250,000 produced in 1972 and was supplied originally with a Carter two-barrel carburetor, which was replaced by an Edelbrock 600 some years ago. Now the Edelbrock is being replaced by one of the first available Street Demons. Here in this sequence of photographs...
by admin | Dec 14, 2012 | Carburetors, Tech How-To |
Part 2 of 2: By Sam Logan. Download hi-res images and text here. If you ever have the chance to discuss engine tuning troubles with the legendary Jon Kaase, you’ll rejoice in his simple logic. “Most engine troubles,” declares Kaase, “are centered on spark or fuel—if it has the former it is either getting too much or too little of the latter.” In the first part of this series of Handy Tuning Tips, we asserted for the benefit of the young enthusiast that the most valuable attribute of the engine is its production of vacuum. When the vacuum is applied to the carburetor it draws a fine mixture of air and fuel through its labyrinth of tiny drillings. When the mixture is supplemented by spark and compression the engine acquires perpetual motion. But as Kaase reminds us, the air-fuel mixture must meet the correct proportions. In Handy Tuning Tips Part I, we attempted to give the potential engine tuner a command of the fundamentals. In systematic fashion we discussed the opportunities and potential problems with the idle circuit, particularly the seemingly unending troubles with poorly adjusted transfer slots. We continued with float levels, accelerator pump shooters (nozzles), and air bleeds. We concluded with an illustration of the carburetor’s base plate and identified its leading features. Here in Part II we compare the vacuum secondary-style carburetor with the mechanical secondary model, we explore the reasons for increasing the initial ignition timing, and we examine the essentials of modular carburetor’s metering block. Debunking the myth of selecting carburetor size by formula Using a formula to select carburetor size is a dubious practice....
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