[fkemmerer]

Originally Posted by mr. oz

From Pro Systems website:

Carburetors and Electronic Fuel injection are two completely different systems. They share no concepts and each has a different theory.

EFI's claim is this: I will supply sprayed droplets of fuel at the proper air to fuel ratio all the time.

Carburetors claim: I will supply a pre-emulsed froth of fuel and air into the engine at a preset ratio.

The results proved the analysis of the concepts to be correct. In this case, the carburetor was supplying the engine in question with the proper air to fuel ratio, so the EFI's advantage was gone. Remember, EFI has a computer to tune the engine. You have you. If you know how to tune you'll have the advantage. Carburetors (at the risk of sounding chauvinistic) are a man's game. Guessing rarely works. You have to know how to actually tune an engine.

Remember a carburetor is an atomization/emulsion machine. An injection system is a proper air to fuel delivery ratio machine. Two different concepts. If a carburetor can be designed to supply the perfect air to fuel ratio all the time it should consistently outperform EFI. Its design lends itself to have an unfair advantage in atomization.

Obviously adiabatic expansion is the next question on the list. So if we take a good look at the carburetor we see its not only a perfect machine for atomizing fuel, it also has another advantage. The Joule-Thomson effect.

Tests performed using quartz plates and infra red sensors located in the plenum area beneath an NHRA Pro-Stock engine revealed an intake manifold temperature drop on a 85 degree day of almost 20 degrees as a result of the the carburetor creating this effect.

So when your neighbor with EFI is ingesting 85 degree air, your power-plant could be ingesting 65 degree air.

That's a nice advantage.

But let's not skip over the atomization advantage. In a high end designed carburetor the fuel is emulsed to lift it. Its a controlled froth. I won't kid you, it's very difficult to control. Its much easier to build a carburetor that operates on a vacuum to ratio concept. But the fogging advantage is gone. So when a customer asks, why is this carburetor more expensive than that builders carburetor as they look basically the same. Most of it is all in the emulsion package and the time spent flowing it and tweaking it to do its job. Remember in a high emulsion design .001 of an inch is a big deal. They're difficult to balance and require sophisticated equipment that many shops have never seen. Also, don't go poking things into the metering block passages to inspect them or look around. You might just lose 10 lbs of torque.

The disadvantage of carburetors used to be restriction. I remember back 20 years ago before booster technology really took off you had to size carburetors to operate on 1-2 inches of vacuum in the plenum at the starting line. The restriction alone was probably costing these engines a 2-3 percent power loss.

Tests we performed at Sonny's racing 5 years ago showed us numbers of about .6 in the plenum and spikes of about 1.1 to 1.3 in the runner at the finish-line. That's a pretty huge decrease and just for dynos sake when we built carburetors large enough to reduce this number by on average 40 percent we saw an increase of only about 3-5 horsepower on an IHRA Pro-Stocker. SO that advantage for EFI is now also gone.

Now that these same engines can operate on as little as .5 hg of vacuum at the starting line and only 1-1.2 at the finish-line, the restriction is nil. Really it all comes down to getting the air to fuel ratio correct. If a carburetor can do that, it should win the race every time. After all, by design, it's a superior emulsion machine.

Here's some additional things to consider on the questions of EFI vs. carbs. First, in a race motor (ex drag racing), the engine spends almost all of its time operating in the sweet spot of the cam/induction/exhaust and at wide open throttlle. Its not really hard to get a carb to work well in such a narrow range of operation.

In a street situation, things get alot more complicated especially as the engine combination gets more radical. Consider this: suppose you are running a a typical "mid-range" CAM shaft with a power band from say 3000 - 6500 RPM. Most of the time that you drive on the street, you are operating below the 3000 RPM level. Lets also say that you are using a big carb (say 900 cfm on a 427 ci motor) to get the package to really haul from 4000-6000 RPM. When this combination idles or you drive it at part throttle below 2000 RPM, the overlap in the cam will cause serious intake reversion problems which will play hell with the carb's metering circuits. You'll probably have to jet/adjust the idle side so rich to compensate for this that you can hardly breath in your garage when the thing is warming up. How does EFI help? First off injectors (stacks or a throttle body) don't rely on a venturi effect to draw the fuel into the motor. This makes the EFI setup is much less prone to reversion problems and the associated need to get the car rich. Secondly, you have a much greater level of control over fuel (and timing) delivery than you can every get with a carb and no computer. I've had engines that will have a flat spot or hesitation at a certain throttle position and engine speed in both the TWM injected 482 ci FE and my 383 ci blown and injected SBC. In the FI case, I change maybe 4 specific cells in the fuel and timing tables to correct these probelms without effect the rest of the the tune at all. If I had to correct these problems with carbs I would have needed to make changes effecting a much larger area of the fuel and timing curves which would have created performance compromises elsewhere. Also, the large throttle blade area of the big carb choosed for high end performance along with the low vacumn of the cam make it very difficult for the carb to metter the fuel properly at low speed. The EFI system can much more accurate determine the amount of air acutally going into the motor and meter the fuel much more accurately. This is why you can get setups like the one shown below to run well on the street.



Lastly, the EFI controlled motor can make much better corrections for conditions than a carb can. Those with carbs ever notice that their cars perform much better at certain temperature/humidy conditions than others? Ever have you performance car run poorly when you took a drive to a higher altitude? Some of this is about air density and oxygen content but it is also about the carbs limited conditions to compensate for air temperature, humidy, and pressure. EFI systems have sensors that detect and will correct for all of these problems and many will used closed loop operation to deal with other issues and wear. Issues like these (as well as emmisions and fuel economy) are the reason that almost every production performance car made today uses EFI including cars like the Viper, GT500 mustangs, and Z06 vettes.

So why isn't everyone using EFI? Well for one thing, it costs more than a carb (althought the gap is getting more reasonable if you use quad throttle body systems). Its also more difficult to tune. All of the control and flexibility means that there are alot more "knobs" that you can turn. I would say that its not that much more difficult to put a near perfect tune on and EFI system than it is to do the same on a carb based motor (although the carb motor may still be subject to the comprimises listed above). The problem is that you can put a so-so tune on a carb much easier than on an EFI system. There is also the work to retrofit a vehicle that is not set up for EFI (especially an existing older car) to use EFI. Biggest areas of work here are 1) upgrading the fuel system to use a high pressure pump and a return line, 2) beefing up the electrical system to reliabily run the electronics, and 3) finding a spot to hide the computer and running the wiring. None of these things are particularly difficult but they do take some work.

I've built or messed with a lot of performance engines in my time (both race and street motors). The last two projects that I've done with EFI are probably two of the most radical that I've run on the street yet they are the two highest performance and most streetable engines I have worked with.

Net is that in practical terms for high performance motor used on the street, its hard to beat EFI. The biggest downsides are the cost, extra work to install, and the technical knowledge required to tune. If you bite the bullet and deal with these issues up front, you get rewarded with a car that usually performs much better/is more streetable and is therefore more fun to drive. BTRW, these days, the tuning part can be handled by many chassis dyno shops at pretty reasonable cost.

All of this said, a carb really is the best choice for many projects. This approach is certainly much simpler and usually more authentic for 60's-80's based muscle cars. The budget factor is also very important for most of us. I would have to say that one could almost always improve the OVERALL performance (not just peak HP/Torque at WOT) with an EFI system on the street over a carb though.

I hope this helps those of you having the EFI vs carb debate.


- Fred