Carburetion vs. Electronic Fuel Injection: a horsepower perspective

by: Justin Rhoads

It’s long been known that efi offers the ultimate in fuel delivery and control; but why is it that many feel that the most power will be made with a carb? It doesn’t seem natural that having anything but a correct air to fuel ratio would result in the most output. This is a question that I have been pondering for quite some time; why would factories, professional race teams, and superbikes develop all use efi systems as opposed to a carburetor when they can use any type of induction they want?

The answer that I have arrived at is simple; they use it because they get more output with the efi set up. Now I can already feel the emails getting sent out, directing me to the numerous threads around the internet where a well intentioned racer, engine builder, or tuner has taken a carburetor off and installed multiport efi in its place and lost power. What follows is an explanation to that phenomenon.

When you look at any induction system, one must consider the application for which it was designed for. In the above case, a Formula 1 intake manifold (a really amazing piece of technology) was designed knowing exactly where, how, and under what circumstances the fuel was to be introduced into the intake tract. This is crucial because fuel changes the density of the fluid; which in turn changes the required optimum geometry. This affects EVERYTHING that the intake is designed to handle, pulse wave tuning and velocity control are the largest concerns.

I highlight this point because an intake manifold that is designed with having a carburetor on top is designed to function with a given density from top to intake valve. When you convert a single plane intake to a multiport configuration the injector will be located near the end of the intake manifold runner (usually located near the nitrous boss). Now while this trend started at the factory there is a very specific reason they put it there and have since moved on to bigger and better things; unfortunately the aftermarket and various shops performing the modification have not.

In the EPA lab and tuned port injected corvette indeed needed to have the best possible fuel economy, the lowest possible emissions, and to do that multiport efi was born. They located the injector very low in the lower manifold (with a very long runner above it) for one very specific reason. At the very low operating RPM of this application, injecting the fuel very near the intake valve ensures that is drawn into the cylinder very quickly. This is important because the longer the fuel have to travel to get to the combustion chamber, the more likely the chance it will fall out of suspension; hurting everything that multiport injection was designed to provide.

Now, that’s a great idea but anyone that’s driven an L98 powered Corvette notices that they do not make good power at elevated RPM. There are many well documented reasons for this, but I will mention the one that is applicable for this discussion. The very low placement of the injector insures that fuel does not have time to fall out of suspension during lower RPM operation. It also, insures that during very high RPM operation, the fuel does not have time to properly distribute amongst the air.

These are the inherent problems with the conversion/comparison. In moving the injector down into the runner you have changed part of what the design was relying upon; you now have most of the intake manifold flowing only air. You also have created a situation that is not optimal for a high performance engine with the placement of the injector itself. These are just a few of the issues and to most these would not appear to be a large problem; but to an engineer that deals with fluid dynamics, it’s a real game changer. The geometry will have to be altered to compensate for this change; in addition, the equations used to compute pulse wave timing will become more complex. A good system will employ a shower nozzle injector for high RPM usage, eliminating the problems associated with a single, low positioned injector.

This is often over looked and simply written off as “carburetors are for hp, efi is for emissions”. I would go way out on a limb and choose a well thought out multiport efi system any day of the week for anything I race.