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.

Why Electronically Fuel Injected?

by: Justin Rhoads

Why is it that everyone in the world today wants electronic fuel injection integrated into their induction system? It seems like a simple question on the surface but becomes a very tough, technical dispute between the EFI supporters, and the traditional carburetion crowd.  Throughout this article I will discuss the nuances of both and why you should arrive at the conclusion that EFI, commonly known as Electronic Fuel Injection, is the system for you.

Let’s start with a little background on both systems and how they work. Carburetion is a very simple, tried and true, method of delivering fuel to an engine. If you are over the age of 25, you most certainly remember the days of the Holley 4 barrel raining supreme atop all the potent engines.

Carburetors work on a very basic principle; as air travels through the unit at a given velocity, the atmosphere around it that is not at velocity, will be at a higher pressure. This pressure differential forces fuel through the jets and into the intake tract. On a carburetor, the size of the jets (small orifices machined into each jet give it its respective “size”) determines the amount of fuel passed into the free stream regardless of any atmospheric conditions.  However during sudden accelerations, this affect is far too slow, producing stumbles and even killing the engine. This problem lead to the introduction of the accelerator pumps that are now utilized on almost every carburetor in existence. These pumps are fitted to the primary, and often times secondary throttle shafts in an attempt to solve this problem. These pumps allow the carburetor to function smoothly by inducing a large “hit” of fuel when the throttle shafts open rapidly. Again, the amount of fuel introduced is determined solely by the parts, not actual conditions. You should be noticing a trend here; a carburetor meters fuel as it was assembled/adjusted to, not how it may need to in order to meet actual engine demand.

Electronic Fuel Injection on the other hand does not meter fuel as it was assembled; using a vast array of sensors and the processing power of a computer, the EFI system meters fuel as needed by the engine given the CURRENT CONDITIONS. Utilizing the O2 sensor, the engine management computer constantly adjusts the amount of fuel induced into the intake tract; contrast this with how a carburetor does the same and it’s a no brainer. The system will also use sensors such as the intake air temp and the manifold absolute pressure sensor to gauge the density of the air entering the intake tract. This again, heightens the system’s ability to accurately inject the proper amount of fuel.

Another key component of the system of the throttle position sensor accompanied with the ability to adjust timing on the fly. The system not only uses the TPS to determine driving needs and loads on the engine, but couples that with the ability to optimize the timing for every condition the engine may find itself in. This is just a quick rundown of a few of the sensors that these state of the art systems  employ hundreds of times a second to ensure that your engine has exactly what it needs.

Now, what does all this mean for you? It means that your engine, regardless of size/output, will have better brake specific fuel consumption! Brake specific fuel consumption is the measure of how much fuel your engine is using to produce its output when run on an engine dyno. It is the primary way to measure the efficiency of the unit.

EFI will also allow the induction system to function properly when conditions go beyond what a carburetor can handle. An example of this is a very large camshaft in a high horsepower street car. You want the engine to turn 7800 RPM, but you want it to idle around town as well. A carburetor will struggle greatly here since a camshaft with enough duration @ 50 to run at that RPM will lead to an engine that is pushing the air/fuel charge back up through the carburetor. Knowing that a carburetor has to have air passing through it to properly meter fuel, what do you think happens to the quality and accuracy of fuel delivery when the charge travels back up through the carburetor? I along with many others can speak from experience that bad things happen, resulting in an engine that refuses to idle on its own, be smooth; essentially be anything like a streetable engine. For the longest time it was blamed on the camshaft being too large to be streetable… Once the EFI systems began to be employed, that myth was put to bed and people began to see that with the heightened abilities of the new systems, even monstrous camshafts can be tamed on the street! So why EFI? If you want the best fuel system for any engine on the planet; EFI is the only way to go (ask an F-22 pilot how the fuel in his aircraft is metered)!