{"id":84357,"date":"2022-03-14T10:54:48","date_gmt":"2022-03-14T17:54:48","guid":{"rendered":"https:\/\/www.racingjunk.com\/news\/?p=84357"},"modified":"2022-03-14T10:54:48","modified_gmt":"2022-03-14T17:54:48","slug":"high-speed-bypass-greater-control-in-mechanical-fuel-injection-racing","status":"publish","type":"post","link":"https:\/\/www.racingjunk.com\/news\/high-speed-bypass-greater-control-in-mechanical-fuel-injection-racing\/","title":{"rendered":"High Speed Bypass: Greater Control in Mechanical Fuel Injection Racing"},"content":{"rendered":"
While mechanical fuel injection is frequently used by many motorsports racers, several are missing out on the fine control provided by this type of engine.\u00a0 One such control utilizes a high-speed bypass to divert fuel to the engine and maintain control over the engine\u2019s power.<\/p>\n
Both normally (naturally) aspirated and forced induction engines with mechanical fuel injection (MFI) typically use fuel pumps that are bigger than necessary for their purposes.\u00a0 They can then benefit by controlling the amount of fuel to the engine using one or more bypass jets to return excess fuel to fuel supply.<\/p>\nThis Rons Fuel Injection system mounted on a drag car is one of the most cost-effective fuel systems available today. A simple throttle body in the top center of the photo connects to a standard intake manifold. The amount of fuel delivery is determined by the fuel split between the nozzles and the bypass jetting \u2013 including a main bypass jet and a high-speed bypass jet if so equipped. These bypasses provide greater control of fuel to the engine to maintain correct air\/fuel ratio (AFR) throughout the run.<\/figcaption><\/figure>\n
MAIN BYPASS JET: Racing mechanical fuel injection jetting is made up of nozzles to the engine and one or more bypass pathways back to the fuel supply.\u00a0 While individual nozzles control fuel flow to each cylinder, the main bypass jet determines overall fuel flow to the engine.<\/p>\n
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A bigger main bypass is used to reduce engine enrichment<\/li>\n
A smaller main bypass is used to increase engine enrichment.<\/li>\n<\/ul>\n
The current enrichment is needed for best performance.<\/p>\n
A positive displacement fuel pump with a standard jetting layout makes a linear \u2018fuel curve\u2019 corresponding with engine RPM.\u00a0 That is, the fuel to the engine-per-revolution is essentially constant.\u00a0 As RPM goes up, fuel goes up.<\/p>\n
HIGH-SPEED BYPASS: A high-speed bypass is used to shape the fuel curve. That shape depends on whether the engine is normally (naturally) aspirated or uses forced induction.<\/p>\n
Normally aspirated: Torque Peak vs Horsepower Peak<\/span><\/b><\/p>\n
For a normally (naturally) aspirated engine, the fuel to the engine-per-revolution varies between the torque peak and the horsepower peak.\u00a0 The torque peak provides the highest volumetric efficiency meaning more fuel-per-revolution is needed.\u00a0 It usually occurs at a moderate RPM.\u00a0 The horsepower peak occurs at higher RPM and has less volumetric efficiency causing less torque.\u00a0 More fuel-per-second is needed for the higher horsepower, but less fuel-per-revolution is needed because of the reduced volumetric efficiency.<\/p>\n
The horsepower peak does more work than the torque peak even though the torque at the horsepower peak is lower.\u00a0 That is because there are more pulses of power per second.\u00a0 Hence, more work is done up to the point where torque at the high RPM is so low that the greater frequency of power pulses cannot overcome the torque reduction.<\/p>\n
A HIGH-SPEED BYPASS: In order to maintain consistency between these two fuel consumption scenarios, a high-speed bypass can be used to tailor different fuel amounts for both of these engine speeds.<\/p>\n
A common & best practice for both conditions:<\/p>\n
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torque peak with high-speed bypass closed<\/li>\n
horsepower peak with high-speed bypass open.<\/li>\n<\/ul>\n
A valve is used in the high-speed bypass circuit to control the opening point.\u00a0 It is usually one of the following:<\/p>\n
\n
pressure-controlled device that automatically tips in with greater RPM<\/li>\n
solenoid device that is off-or-on<\/li>\n
manual valve that can be off-or-on, tipped in, or partially opened by the driver<\/li>\n
variable flow slide valve controlled by an automated system.<\/li>\n<\/ol>\n
A pressure-controlled device is most common to automatically control the opening point of the high-speed bypass.\u00a0 A poppet or diaphragm valve can be used.\u00a0 The pressure-controlled valve is typically set to open at about 600 RPM over the torque peak.<\/p>\n
One example of utilizing a pressure-controlled valve is called flat lining<\/em>.\u00a0 The torque peak of a typical drag or sprint car 2-valve racing V-8 engine is often around 5,200 RPM.\u00a0 The high-speed pressure valve would be set to tip open around 5,800 RPM.\u00a0 As engine speed goes up beyond that, the pressure valve continues to tip-in further.\u00a0 That reduces the fuel to the engine.\u00a0 That reduces the fuel-per-engine-revolution to match the reduction in volumetric efficiency as the engine RPM increases.<\/p>\n
Some fuel injection combinations with the high-speed function are set up using only a high-speed bypass valve without a jet.\u00a0 The valve alone provides some fuel restriction for high RPM lean-out.\u00a0 The amount of fuel bypass flow restriction is provided from the open valve by itself. \u00a0In a typical methanol drag or sprint car V-8 engine, it is approximately equivalent to a 0.070 to 0.075-inch diameter jet size by itself.\u00a0 If that bypass flow from only the open valve by itself is too much, it causes a curve that is too flat.\u00a0 The top end would be too lean.\u00a0 To compensate, a restrictor jet is added to further reduce the high-speed bypass flow when the valve opens.<\/p>\n
The combination of valve flow restriction and restriction from a high-speed jet determines the fuel curve reduction as engine speed climbs to the horsepower peak.\u00a0 Again, for drag or sprint engine setups, 7,600 RPM is a typical horsepower peak engine speed.\u00a0 The high-speed bypass flow is from a combination of the valve and jet restriction.\u00a0 A high-speed jet size around 0.070 inches diameter is common.\u00a0 \u00a0\u00a0Ref. High Horsepower Tuning for Mechanical Fuel Injection for Normally Aspirated Racing Engines<\/a>.<\/p>\n
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