Fuel Injection or Carb any actual numbers to compare?

[R Thomas]

Like many of you I imagine the engine choices available have a lot of options. I am sure that for many it comes down what can I run in my state based on the registration process. But I read something the other day that gave me pause to think. It said that low end torque and performance was much better in a fuel injection engine than one with a carb. Since I live where I can go either way I was hoping some of the engine builders here would comment on that claim. Both setups have advantages and disadvantages I would like this thread to focus on performance numbers across the rpm ranges.

[wallace18]

I built a 33 with an Edelbrock carb and a type 65 with Proinjection III. IMO the Fuel injection is superior in every way except initial cost. It runs better, holds idle better A/C on or off. And best of all the ethonal in fuel does not screw it up like it does to carbs. JMO. Check out my build threads for more info.

[skullandbones]

I can't compare my engine with carb vs EFI because it started out as EFI. I have done multiple carbs on almost every hot rod I've ever had. So this was to be my first EFI hot rod. Also, it's my first Ford engine and first roller cam. I'm impressed with the SBF. As far as EFI, I can tell you that all EFI systems do not perform better at low and mid range torque levels compared to a carb. I started with a Holley Systemax and 75 mm TB. It was strong at 4 to 6k (very sudden surge in power). Then I changed to a short runner and small plenum with the same injectors and TB. It has changed to where the power comes in at 3 to 5k. So I think the air/fuel induction setup whether EFI or carb depends on the other variables around the system (runners, plenum, single or dual plane, etc). Of course, the cam is a game changer as well. I also had the same E cam in these two setups. I notice that I may have lost 1 or 2 mpg with the change as well (about 20 mpg). Overall, I would say with the power and mpg as it is now, it would be hard to duplicate with a carb. Another aspect of the EFI vs carb question is the tuning of each. I plan to switch to a MegaSquirt PiMP to replace the ECC-IV engine management system. It will allow for complete tunability where the stock system is somewhat limited in it's adaptability. If you have a wide band A/F sensor, it will make tuning either easier and better.

Each system has it's own idiosyncrasies as you know. I like the idea of the engine starting every time with no flooding. I know those do not represent stats as you are used to seeing but actually they are zero failure statistics which are very compelling. Even when mine is not running perfectly, it still starts!

Good luck,

WEK.

[RonnieType65]

For my MKIV, I used a Holley 750 DP on an Edelbrock Air Gap and tuned it on a dyno. One pump of the pedal and turn the key and it fires to life every time. The car made 320rwhp on 91 octane and it is no slouch in the acceleration dept. Whatever low end torque I may be giving up to EFI, the cars weight more than makes up for. I like the idea of having a more "period" type setup, this is why I went carb.

[edwardb]

In very general terms, no the statement isn't true. The right high quality carb properly tuned will have dyno numbers across the board on par with EFI. No advantage either way. So I wouldn't use that as a selection criteria. But there's the rub. Guys throw various carbs on their builds -- sometimes the wrong brand, the wrong type, the wrong size, etc. and their version of tuning is to try a few different primary jet sizes -- and then are disappointed with how they start and run. Conversely, an EFI system is installed, and assuming all goes well and it works as designed (not always a small task) it immediately adjusts the starting, idle, mixture, etc. for a good clean running setup. Add into this altitude, or especially changing altitude where an EFI system can adapt, and EFI comes in even stronger. So, yes an EFI system has significant advantages, and there's clearly a reason there are no longer carbs on OEM cars. But more powerful? Not necessarily. For our purposes, a carb can be made to run very well. But it has to be the right carb and tuned properly.

[Corsair]

so what everyone is trying to say is that it doesn't really matter how the fuel get's in there, so long as it does... If i'm reading this right, EFI is a bit more costly and fiddly initially, but more day to day user friendly, while carbs are cheaper and easier to get going, but have more "personality" and more fiddly to get optimized?

[Bob Cowan]

Generally speaking, the carb will make a little more peak power; WOT at the upper rpm range. But EFI tends to make more power under the curve. And that's because EFI provides more accurate fuel delivery in those ranges.

[skullandbones]

The NHRA Pro Stock class is going to EFI next season so I would say there will be a fast track learning curve as those guys extract every ounce of power they can. They will have tricks you have never even imagined. It's probably to make the class more reflective of the word "stock" since, I guess, all OEM autos have EFI now. So what hasn't been discovered about EFI will be discovered sooner than later. As far as the carb setup, you just have to want it no matter what the downside is. A professionally setup carb is still a very good option, though.

WEK.

[2RR2NV]

as others have said, it's more about what is important to you... initial price? easy starts? tuning? setup difficulty? knowledge of carb tuning (my major downfall), etc...

Me, If and When I build an FFR, I'm going EFI for the specific reason of excellent day to day manners and long distance capability. Power..? yeah it's nice, but I'm not building a race car. It's gonna be my cruiser that is unique. Mind you, I'm gonna go with TBI so I can keep it "looking" more period correct whereas MPI is easily noticed from any angle.

GL with whatever ya decide. Enjoy!

Scot "Z"

[Bob Cowan]

Power..? yeah it's nice, but I'm not building a race car.

For most race cars, power under the curve is more important than peak power. You need max torque to pull out of a corner - as that's the place you'll shorten your lap times the most. And, as conditions change - temp, air density, etc - the EFI will compensate for that and keep the engine at it's best.

I live at 6,200', and race at anywhere from 4,900' to 5,500'. A couple of years ago we raced in Tulsa, OK, (700'). While tha carb guys were retuning and messing around, we drove off the trailer and onto the track - computer did all the tuning for us.

About the only situation where a Carb might be consistantly equal to (or maybe better than) EFI is drag racing; almost entirely WOT.

[UpstateCobraGuy]

Personally, I went carb because it looks right in these cars (Roadster), I fully understand fuel injection has its advantages. IMHO the alien head EFI intake just looks wrong. I also wanted a car "close" to the originals as I could afford, again carb won out. I know now there are a ton of carb look alike systems out there and I'm sure they have there benefits, but I subscribe the "KISS" principle, so carb won out again. Lastly, even at 300 HP these cars are overpowered, and my carbed fed Roadster can scare the crap out of me.

Mission accomplished!

Pat

[tirod]

What a lot of guys miss in the comparison of EFI vs Carb is that the cam is different to get the maximum potential out of the system. There are plenty of stories here and TOS about builders putting together the motor and topping it off with a fuel system that they could never get to run right. And AFAIK, it usually traces back to the cam.

Carb cams tolerate a lot of overlap, and carb tuning allows compensating for low vacuum simply because a carb has a initial enrichment device - the accelerator pump. Some carbs have two, and some "race" carbs pumps are high capacity. The purpose it to overcome the low vacuum at initial tip in, when it goes even lower and there is no siphoning from the main jets until the engine gets up into the revs.

What carbs do is use a mechanical injector to cover up something the normal analog circuits can't because physics work against it. This is also why some carb set ups seem to have almost no power off idle and may even buck under 2,000 rpm - which could be cruising rpms in final drive. It's a symptom of too much "race" tuning in search of high rpm peak horsepower, and if there is any one issue with hot rods and kits, it's bragging rights over a dyno number that can be backed up with an independently paid slip.

BUT - like Bob and the other experienced racers note, you can tune for more hp in a useful range rather than on peaky high rpm. You tune for more bulk from 2k thru 6k, where you can use it coming out of corners and shifting gears. Rather than tune the engine to run like a diesel or two stroke with a narrow rpm band and a whole bunch of gears in the transmission, you get it to roll off the line with no sputtering or extended clutch slipping and then just keep building more and more torque.

One caveat, as mentioned. Do you really need a 500 hp motor in a car that weighs less than 2,400 pounds? It's already pushing single digit pounds per hp, vs daily drivers with double digit numbers. As in 6:1 ratios vs 15:1. You can use a near stock 350hp motor and the majority of the performance gain is weighing up to 1,800 less than the next car at a stop light. The performance isn't from massive horsepower - it's from eliminating a lot of mass.

EFI tends to run well because of two factors, long runners in the factory manifolds, and much higher vacuum at idle. EFI needs high vacuum to keep velocity up so that the mass air sensor will respond to the changes in airflow - which would be difficult if it went from little vacuum to none like a carb does. This is why a lumpy carb cam does NOT work with EFI and is the source of a lot of issues. The classic post is one where EFI could never get tuned quite right, the owner rips it all off and installs a Holley four barrel, and responsiveness and tuning suddenly dial in as if it was made just for that motor.

Well, no, the motor was made just for that kind of fuel system, largely because of the cam. EFI has no separate mechanical injector that is foot operated to fix the fuel air ratio off idle until the circuits start pulling enough vacuum to get the jets working. It has to read air movement if mass air to even "see" the change.

With the older speed density systems that change is "seen" by throttle position, and, again, by a sensor in the intake reading air pressure. The system also has one reading ambient barometric pressure, too - which is how a EFI car rolls off the trailer and on start up - within milliseconds - adjusts the fuel/air injection rate to match ambient. Where a carb guy has to manually turn needles or even change jets and rods to accomplish it. That usually involves a tear down of the carb on the spot, which is a messy and delicate operation which also tends to require new gaskets more often than the owner wants. Much less race carbs with vertically split fuel bowls which soak the gasket 100% of the time.

The difference in EFI and carb is less the potential maximum hp you get, and a lot more where you get it and how easy it is to keep getting it. Short run race car parts are meant to be almost disposable - the cars certainly are after a few seasons when they start getting outclassed ( and the Daytona Coupes are a classic example, literally stripped, then sold off in 1967-68.) EFI is longer term and delivers a broader range of performance in more weather and environments where it doesn't need constant maintenance. It's much more like a Quadrajet vs Holley comparison. Qjets could go 10 years with no maintenance - Holleys are usually pressing it to go 20 months on the street.

Choose which you wish, understand the pros and cons, but don't make the mistake of thinking it's something you top the motor off with. It's exactly the opposite - it's something you build the motor up to, from the bottom up, which brings out the best performance.