I say potential because there are many factors that play into making power. Intake Selection with MPI: MPI offers only the potential for increased power. It has more components, though most are pretty reliable so it isn’t a big problem.ĭ. Disadvantages: MPI requires a custom or modified intake, which increases cost and complexity. Batch fire MPI has a wider dynamic range, making it more suitable for higher hp/forced induction engines where idle quality is still an important factor.Ĭ. It can also produce a small increase in economy. This can be an important consideration for a novice, but not a deciding factor if you will be hiring someone to calibrate for you. Because it does not have to wet an entire manifold with fuel, typically throttle response will be improved over TBI, or equal with much less calibration effort. Advantages: MPI almost guarantees equal fuel distribution (however, if you manifold does not provide equal air fuel distribution, the benefit is lost). Some injectors will be spraying on an open valve, and some may be spraying on a closed valve.ī. The important thing to remember is that the injectors are not in sync with the engine. The arrangement varies sometimes an entire bank is grouped, sometimes 2 injectors from each side are grouped, and on 4 channel systems the injectors are fired in pairs. Batch fire means that the injectors are fired in groups. MPI is always a high pressure system, with common operating pressures being 45 and 58 PSI. Overview: As mentioned above, MPI (or MPFI) gets its name from the fact that fuel is delivered at multiple points, with usually one but sometimes two injectors per cylinder. Also, high quality, full-circle hose clamps should be used. Good EFI quality rubber line costs $4-5/foot. Circulating fuel systems aerate the fuel and lead to rapid breakdown of el-cheapo fuel line. Yes, the pressure is lower, but this does not mean you can use carburetor fuel line. Despite being a low pressure system, TBI will not save you much on the fuel system, assuming you do it properly. What you will gain is fast, reliable starting, much improved cold drivability (and actually a general improvement in throttle response) and maybe a little bit of economy, depending on how your carb was set up and whether you run closed loop (more on that later). As you know, power is airflow, so unless your carburetor was restricting airflow, a throttle body will most likely not improve flow. However, don’t expect to gain power either. Contrary to popular belief, a properly sized TBI will not cost you any power when compared to a carburetor. For EFI newbies, I would probably subtract 100hp from those numbers if you will be attempting to calibrate yourself for the first time. Roughly 450hp for small blocks and 550hp for big blocks is about as much as one would want to attempt, unless you are very experienced. However, the more extreme your engine, the less likely you will have success with TBI. Other notes: Despite the concerns listed above, TBI is not to be underestimated, as it can provide plenty of power and very good drivability. Forced Induction engines also have a large dynamic range requirement, making TBI an uncommon choice.ĭ. In a nutshell, it is harder to get a high rpm, high hp small block (which uses very little fuel at idle but a lot at WOT) to perform well under all loads. Disadvantages: Because TBI delivers fuel at the same spot as the carburetor it replaces, it is vulnerable to any manifold inefficiencies (puddling, unequal distribution, etc) that the carburetor was - but since it is not relying on a venturi for fuel flow, typically a calibrator can often “calibrate around” these problems. If you want to run a larger intake (from a Performer to a Performer RPM, for example), you do not have to pay to have it modified - it’s just like the engine was carbureted.Ĭ. Upgrading can be very cost effective and simple. Once the air cleaner is installed, it is difficult to tell that the engine is fuel injected. Because TBI has no fuel rails, it offers the most “retro” look. This also makes it the easiest system to install. TBI also has few components (only 2-4 injectors, usually no Intake Air Temp sensor, etc), which also lowers cost and complexity. Most TBI throttle bodies will bolt to any common 4bbl intake, either directly or with a basic adaptor plate. First, it does not require a custom intake manifold. Advantages: TBI is almost always the lowest price system. It is usually a low pressure (less than 15 PSI) system, but there are exceptions.ī. IT is also sometimes referred to as single-point injection (since all fuel is introduced at the plenum, as opposed to multi-point systems which have fuel delivered in individual runners). Overview: TBI is short for Throttle Body Injection.
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