jlp66stang

OldSchool!!!! [sm=icon_rock.gif]
$300 bucks and you're rolling.

JamesW

Man...where is Soaring when you need him. He'd be all over this thread...

Aussie66Fastback

[sm=funnypostabove.gif]

Scott H.

.....

gothand

Here is my experience with carb and EFI for the same engine, a Ford Racing GT-40 crate motor with the B-cam:

CARB

Started with a Holley Street Avenger, RPM air gap, MSD ignition/dizzy and had it dynotuned. Dynotuner (old school place with gobs of experience) suggested switching to Edelbrock Thunder AVS carb which I did and they re-dynotuned. It ran noticeably better than the Holley. After driving it a bit it was fine tuned again. Overall, car ran well, pulled real strong and got lousy mileage.

EFI swap

Explorer intake, Accufab TB, Pro-M MAF, A9L computer, stock distributor, 190 lph fuel pump, 24# injectors - After minor garage tuning and replacing a distributor with a bad PIP sensor, it drives like a dream. Turning the key, it fires up instantly and idles great. Throttle response is much better than the carb and the motor feels at least as strong as it did with the carb. Mileage has not been checked yet but early indications are that it is greatly improved.

Overall, I am quite pleased with the swap and would do it again in a heartbeat. What seemed so complicated when I started this project is far less daunting now. Mydriveway tuned EFI setup runs much better than the professionally tuned carb setup. No matter how good the previous setup was tuned, it did not have the ability to adapt instantaneously like the EFI does.

The one 'drawback' to the swap was the cost. If I had a stock motor and had more time and the desire to modify a stock harness, I would have a lot more greenbacks in my wallet. That said, I was able topick up a lot of used performance parts for very good prices on Ebay.

Let the flames begin. [sm=badbadbad.gif]

Jeff

67mustang302

Megasquirt requires a lot of work, but can be done for dirt cheap and works really well. And you can get carbs that perform as well as EFI systems, they're just not cheap. I've seen carbs listed for $1,500 and higher, finely tuned race carbs though. Carburetion has gotten a bad rap over the year, mainly due to poorly designed and poorly tuned carbs. A well tuned and thouroughly modern carburetor could put most EFI systems to shame. In the end like anything about performance, it really comes down to how much money you have.

cmanf

There has been several good pros and cons on both sides of this on here.
I did a ood thing and went blowthru carb and paxton s/c on my 347.
Price was high to have it built but that was what i understand and can handle working on by my self from this point.
Ive seen quite a few laptop tuner guys that tune on the fly and have screwed up things so bad not knowing what they dont know.

I cant even use my home computer without it crashing so I knew a carb fit better for me.
Funny my aeromotive and msd 6 are controlled by boost so when the boost goes up so does the spark and the fuel to feed it.
Kinda tunes for itself by pressure when needed.

Scott H.

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67mustang302

Well, first of all F1 cars run EFI because part of the FIA F1 program is technological spearheading. They run the leatest and greatest of all the technology out there. A modern Formula 1 car is actually quite a bit more technologically advanced than even the Space Shuttle, from new composites to aerodynamic breakthroughs, tire technology to pushing the limit of thermal efficiency with engines. The Space Shuttle was designed over 20 years ago, F1 cars are redesigned every few weeks. F1 cars have without a doubt the best EFI system on Earth, with hundreds of sensors monitoring the car as it's running, adjusting a/f ratio and spark timing on individual cylinders as the car is on it's way down the track. The driver can even use a computer control to adjust the sway bar stiffness and front/rear bias as he's driving. But, the EFI system an F1 car uses costs more money than all the cars combined on your average car sales lot, so you get what you pay for in that instance for sure. But if you compare F1 cars to Nextel Cup and NHRA Pro Stock cars, the Nextel and Pro Stock cars are carb'd, and are producing the same, or even slightly better cylinder pressures as the F1 car is. All of them being in the 225-235psi of mean cylinder pressure range. All naturally aspirated, and all very impressive. Obviously the cars with the carburetors are able to burn fuel just as well as the EFI cars are.

As far as the bikes, many of the newer bikes are making better power not simply by going to EFI, but by using better technology all the way around. A lot of newer bikes have gone to Teflon impregnated aluminum cylinder walls, and extremely light weight internal alloys, all of which comes together to produce more power. Now, given the small size of a sport bike and the way it moves, in a case like that I think EFI is better, since it's quite difficult to build a carb that can maintain consistant fuel flow under the severe angles and direction changes that a bike sees.

As far as carburetors, modern carbs arenearly as good as modern EFI in terms of metering accuracy. Carb's don't simply have just 4 circuits. They also have an emulsion chamber that aerates the fuel BEFORE it discharges from the circuit, and the emulsion chamber itself will have a higher or lower fuel level based on the fuel demand generated by the airflow in the venturis, and air is bleeding into it through low speed and high speed air bleeds. That means that as fuel flow changes while going through the circuits, the amount of air bleeding into the circuit changes as well, and therefore the a/f ratio is altered slightly. Airbleeds are actually a controlled vacuum leak in a carburetor.A well set up carb, with properly sized jets and air bleeds, and well designed venturis and boosters,is capable of being sensitive enough to changing conditions in air pressure that it can varry the a/f ratio precisely enough to remain almost as consistant as EFI, and in some cases as consistant as EFI. So, EFI still has, and in all liklihood, always will have a slight advantage in metering accuracy. But the carburetor has several advantages that EFI doesn't.

The first is that a wet flow manifold setup, like a carb(and TBI as well) has a "wet" air/fuel charge moving down the runners, as opposed to just the "dry" flow ofair only inport injected EFI. This allows the fuel/air charge to absorb heat from the manifold that is used to vaporize the fuel in the charge. Since fuel has to be in vapor form to burn, the more rapidly it's vaporized and the more completely it's vaporized, the more complete the combustion is as a result. As the fuel absorbs heat and vaporizes, it causes a cooling effect to occur on the charge, slightly increasing it's density, this evaporation effect is like a swamp cooler. Also, the expansion of air after it passes through the venturi causes the Joule-Thompson effect, which further helps to cool the inducetion charge, increasing density. The cooling effect though can allow for an increased level of detonation resistance, meaning that a slightly more agressivge tune can be used.

The other main advantage is that EFI simply delivers raw fuel, a carburetor delivers some raw fuel, but since it's an emulsifier it also delivers some fuel in a vaporized state, andsome of it in the emulsified state. The foamy/frothy fuel that's delivered is much closer to a vapor state than raw fuel, which means that most of the fuel delivered by a carburetor is more readily vaporizable(and some is delivered already vaporized), and therefore more readily combustible, than fuel delivered by EFI. This also means that less energy is absorbed by the fuel during the compression cycle to allow it to vaporize with a carburetor than with EFI, meaning that carburetors allow for a slightly higher thermal efficiency as the fuel is mostly already a vapor and the energy absorbed isbrining it closer to a point of combustion, rather than a point of state change(from a liquid to a vapor). In other words, EFI setups have to use more energy during compression to cause a state change in the fuel, which means less energy leftover to heat the fuel to a point where combustion will occur as rapidly. Also, since the fuel travels the length of the manifold, which is under slight vacuum, the vacuum helps to lower the vaporization point of the fuel, further increasing the amount of vaporization that a carburetor allows. That's why too big a carb causes a power loss, not only is metering signal, and therefore acuracy, reduced, but without enough vacuum the fuel won't vaporize as well, and power loss results, especially at lower engine speeds.

Also, since the fuel is delivered farther upstream in carburetion, more time for atomisation is allowed(an even distribution of fuel molecules throughout the charge). Better atomisation means better and more complete combustion. EFI offsets this with injector nozzles that are of a specific design, and higher pressure, but since the fuel is delivered right at the port there is less time for atomisation. Cylinder swirl though with well developed heads is considered to be the biggest factor by some in atomisation. This has lead to a current understanding by many that atomisation is pretty much equal in both systems, but I thought it might be worth mentioning.

What all this means is that though EFI has a slight power producing advantage in terms of metering accuracy, a carburetor has power producing advantages in terms of vaporization and combustability. Carburetion's power producing advantages generally tend to offset EFI's power producing advantages. Enough so that when you compare well developed and tuned EFI setups to well developed and tuned carbureted setups, most of the time the carburetors make more power, even if only slightly. The problem is that the VAST majority of carburetedengines are either improperly setup, and/or improperly tuned. Carburators have certain advantages due to physics that are simply unavilable in an EFI system, and when those advantages are used properly carburetors come into their own.

All this says nothing of emissions or fuel economy, this simply adresses aspects of how a carburetor makes power vs. EFI.

All that being said, when you compare a well setup EFI vs a well setup carb, either one can give the other a run for it's money. Although, the more modern engine management systems are starting to gain the advantage over carbs, primarily due to the ability to control things like valve timing and induction velocity. Though you could argue that induction velocity can always be kept at a maximum with a carburetor, by using a properly sized and tuned vaccuum secondary carb. But it still ain't controlling valve timing.

Starfury

Oneproblem with the way carbsdeliver fuel into the engine is that you have to wait for the vacuum signal to get up to the carb, then wait for the fuel to get down into the chamber. MPFI takes into account throttle position, engine load, engine rpm, etc and simply injectsan exact amount offuelas an extremely fine mistdirectly into the intake runner in the head when required. This tends to result in better throttle response across the board, something that's moreimportant in street than cars that run atWOT all the time. Not that this makes a huge difference assuming the carb is tuned properly (I think my throttle response is just fine once I get into the cam's power band), but it is a difference, and something to counter the improved fuel vaporization of a carbed system.