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Multiport fuel injection (MPFI) systems are not as adaptable to dual fuel fumigation conversions as their throttle body counterparts. The main problem with MPFI is the long intake runners designed to improve engine torque at certain RPMs. While they do a great job of improving torque, they also create a huge volume of fuel mixture in the intake manifold while running on propane. These manifolds are designed to supply only air to the individual cylinders and gasoline is injected directly into the cylinder's intake port. Because fuel metering on fumigation propane systems is done at the converter, this huge volume results in a large time delay (known as fuel transport delay) between the time the converter adjusts the fuel mixture to the time the exhaust oxygen sensor can measure the O2 in the exhaust. Besides mechanical problems in the engine, even just starting an MPFI engine with a wide open throttle can cause a lean fuel mixture backfire, which can destroy the intake manifold. Franz Hofmann extensively discusses this type of failure of GM, Ford, and Dodge engines in his web site under backfire issues. The best solution for this problem with MPFI engines is propane port fuel injection. In this case, propane is introduced at each intake port (near the gasoline fuel injectors) and individual solenoid valves sequentially control propane injection. Propane port fuel injection also eliminates the large restriction in the induction system caused by the mixer. While not being a bolt-on conversion like fumigation, port propane injection eliminates the need for specialized adapters to fit a mixer upstream of the throttle body. Unfortunately, EPA/CARB requirements are model-specific for propane conversions so EPA-certified universal systems are not available for North American vehicles. While it is possible to bolt a fumigation onto an MPFI engine, this is most expensive option in the long run. Besides the high probability of having to replace intake manifold(s) from backfire failures, fumigation systems are less fuel efficient than injection systems. If your car or truck spends very much time on the road, the added fuel efficiency of injection systems quickly makes up for their increased capital cost. If you consider that the lowest cost 8-cylinder injection kit (ie, Technocarb's ESIP parts package) costs $1834.81 (including optional components), the tank costs upwards of $800, the fuel lines and filler valve cost around $400 (depending upon the vehicle), the hardware alone costs over $3000. There great deal of work required to figure out the wiring harness, fabricate brackets, and create the propane fuel map for a universal kit. All of this work can easily take 20 hours or more to do properly. If you assume a shop rate of $90/hr, the cost of the most economical injection conversion starts at around $4800 and goes up with more expensive systems and/or bigger tanks. If you find a conversion shop that is charging significantly less, you need to wonder where the costs are being cut. There is not much the conversion shop can do to cut the cost of the parts. In Canada, the current propane installation code requires Annex G certified tanks which pretty much eliminates the reuse of used tanks. The only place to save is in labor hours and this is not where you want the cuts. A fast job can end up costing you much more in future routine maintenance and repairs. Ask the installer if he is putting the level of care in the conversion that was done with the SVIS Impala conversion.
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