Throttle Body Injection
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Multi Port Injection
Comparing the Performance of Carburetors
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TBI and Port Fuel Injection
TBI, or Throttle Body Injection, is a nomenclature that specifies a particular method of metering air and fuel in a marine engine. The throttle body mechanism resembles a carburetor in that it has a throttle plate that opens and closes, admitting more or less air into the engine. But, unlike a carburetor, it has no float bowl, main jets or airbleeds. Instead, there are two fuel injector nozzles that squirt a continuous stream of gasoline vapor directly into the air flow above the throttle plate. A microcomputer senses throttle position and adjusts fuel flow to match.
What comes out at the crankshaft is a flatter torque curve, more horsepower and better fuel economy. In other words, TBI boasts better all-around performance than a carburetor. Moreover, these and all other Electronic Fuel Injected marine engines are easier to handle. Even ice-cold TBI motors start instantaneously and keep running without sputtering or coughing. On the launch ramp that pays big dividends, especially for inexperienced helmsmen.
If the technical nature of a TBI system sounds ridiculously simple, that's because it is. While TBI costs more than carburetion, the price is still considerably less than the more sophisticated forms of EFI. In short, a TBI upgrade provides the efficiency and economy of EFI with little additional expense.
With multi port fuel injection there's still a throttle body. But instead of just two injectors there are eight: One for each cylinder. And instead of being located above the throttle plate, the injectors are mounted on the intake manifold a short distance upstream of the intake valves. In terms of efficiency, port injection is to TBI what TBI is to carburetion. With eight injectors mounted in close proximity to the combustion chamber, instead of injecting a continuous stream of gasoline vapor like TBI, port injection pulses fuel flow. Additionally, a microcomputer more precisely times the delivery and quantity of gasoline. Here again, power and fuel economy are optimized.
Obviously, this claim of technical superiority gives rise to a number of questions, like: Can you feel the difference between carburetor and fuel injection at the helm? The answer is a resounding, yes. Compared to any EFI-powered boat, a carbureted rig feels like it needs a tune-up, even when the engine is actually in a high state of tune.
What about TBI versus port injection? Can a helmsman actually tell the difference between the two? The answer depends on boat size. On a small boat, say a 20-footer, a port-injection engine will walk away from a TBI rig. But on a heavier boat, like a cruiser, even with the 40-hp difference, you probably won't actually feel a real seat-of-the-pants difference.
Naturally, with all the additional plumbing and computer software, port-injected engines cost more than TBI. Both require specially trained mechanics to troubleshoot the fuel systems. On the other hand, both styles of EFI engines are generally more reliable than carbureted marine engines.
Contemporary marine engines, both TBI and multi port fuel injected, feature sophisticated ignition and engine management system. Microprocessor spark timing monitors the engine's vital signs. Should low-grade fuel cause engine knock, the module reacts immediately by retarding ignition timing so the knocking stops. The rev limiter keeps the engine from self-destructing should big water ever pitchpole the bow into a wave and leave the blades freewheeling in thin air. Another benefit of electronic monitoring, if either the cooling or lubrication system should fail, an audible alarm alerts a helmsman before damage occurs.