aaronsimms637

I can build a carbed motor for 1 grand less than a efi.motor. So id like put a carb on.my 351w i no.a guy whose done a few and hell help.me.if i.need to but wjats the gas millage difference. I dont underatand how a efi can be better if u have a built moter and u need a a gallon a minute at wide open throtle then ur gonna use tht.much gas no.matter whats supplying it. So can someone help me with how its more efficient if it is

sylver91

EFI is way more efficient then carb. EFI is actually allot easier to manage then a carb setup. Sure its easy to adjust the jets whether you want a leaner or richer mixture which can correct some problems with the engines performance, but the problem is that rich/lean mixture stays the same across the the entire engine RPM range, so you are using the same mixture of fuel on the highway and in town. Thats why carbs had to run idle circuits and choke systems to be able to let the engine idle properly and start right depending on the temperature outside.

Now with an EFI, the fuel is automatically metered for idling, low cursing speeds and highway driving. Thats done by changing the timing of the injectors which spray the fuel inside the intake and mix with the air giving it a much better and more efficient burn. The computer can adjust for hot and cold starts by adjusting the injector pulse for either a richer or leaner burn. Combined with the readings from the O2 sensors, it knows if its burning too much fuel or too little and will compensate, something you can't do with a carb.

Carb and EFI all do the same thing to run an engine, and a properly tuned engine will run the same, just EFI is easier to manage, provides a more efficient engine under changing conditions and when you know how it works and what it does its way easier to fix.

Atomic Monkey

The last post is not entirely correct about carbs. Carbs use an enrichening circuit dependent on manifold vacuum, which corresponds to load conditions and throttle position. Holley uses power valves that operate at a specific vacuum, Rochester uses a spring loaded plunger and needles that move in and out of the mains. Speed density EFI operates on the same principles utilizing the MAP and TPS sensors, then add a few extra sensors for cold start enrichment, and fuel trim.
Here's my opinion on the debacle; if you already have the all the EFI components, spend the money on something like the Moates QH and the supporting software, just make sure they have definitions for your computer. This is what I'm doing to save me from tuning in a carb. Messing with air bleeds and jetting can be a major time consumer depending on if you're ok with "good enough" or if you're a perfectionist. And to do it right, it requires a wideband O2 sensor anyway.
I looked at it this way, do I spend the money on a carb, intake, and distributer, then do all the tuning leaning over a fender??? Or do I spend the money on the programmer and software, and do the tuning while sitting in the truck with the ability to change things on the fly, and do some data logging???
Another upgrade I got from a junkyard truck was a MAF computer and harness. It really didn't cost much, and it offers more flexablity when it comes time for tuning.

aaronsimms637

Alright thx for it posts ill let u know what I decide to do

sylver91

Well I sure wasn't going to type down the entire inner workings of a carb and I'm not one to copy paste.

Beside all that you mentioned is wayyyyy more work and money then I would ever put into a truck like this, tuner and software cost more then its worth, especially on a pre 96 truck lol.

Atomic Monkey

Just dropping some knowlege after the principals of carburation according to lawnmowers was put out there, "rich/lean mixture stays the same over the entire rpm range"... really??? I'm just kidding, all in good humor!
The OP was about building a 351, asking about effiency and mileage, carb vs. EFI. I say about the same can be achieved with either, its all a matter of tuning the setup, which something that can't be done with EFI without a tuner, or getting a chip burned.
Personally I've always gone to carburated because I always found deals on the parts. The most efficient carbs I've built have been Quadrajets, the metering seems to be more precise, especially for enrichment. The other advantage was having the smaller primaries. As long as the secondaries weren't being used, the mileage was good... hard to do though, love the way those things sound cracked wide open!
Its roughly $500 for the tuner and software. How much will a carb, intake, and distributor cost? Then hopefully the truck came with a low pressure pump in the tank, and if it has a computer controlled tranny that's another issue. This is why I'm choosing the tuner route. I have carbs on the shelf, but my truck came with a high pressure fuel pump, and I would have to buy a bypass regulator in addition to the distributor, and the intake. I would need the computer for the tranny, 4 wheel drive, and speedo. I would have to put a TPS on the carb for the tranny, and hack the harness to get the fuel pump and distributer to work. That's as far as I went with it,
I guess the only question I would still have with going to carburated with my setup is would I end up with a limp mode for the tranny without the distributor being connected

sylver91

Lol what I was trying to say was say if you were to change the main jets for a leaner or richer adjustment, it will be the same when running the engine throughout the RPM's. So say I need lean condition for economical driving, but at the same time need a richer mixture at WOT for more power and adjust between themselves, I don't think I can do that with a carb.

Atomic Monkey

Absolutely you can do that with a carb. Automotive carburators are fairly complex. A person can't just change jets and expect a linear change to the mix across the board. That's what makes tuning them correctly such a pain. Jets are not the only thing that dictate the mix. In a Holley for example, the cruise mix is dictated mostly by air bleeds. It is entirely possible to setup a carb that runs too rich at WOT, then too lean at cruise, and vice versa.

aaronsimms637

Well.im looking at a holley 670 carb and it has a e40D but when i.do.the swap.im.putting.a standard out of.another truck.in it and on the e40D the only electronics on it is for reverse light and.overdrive which didnt work on.mine so.i.unpluged it and it still works fine but the 4wd will work.without a computer its.manual not electronic on.mine. And for a performance carb it 450 for a intake its 250 for a distributer its 350 and thats it if i was going to put bigger injectors 450 a intake 600 a ignition system 200 and a tune 400 i wud rather spend 1050 on a performance carb set up and be able.to do.anything.to.the motor then spend 1700 on a efi system tht is les reliable more expensivr and everytime i upgrade need a tune

Atomic Monkey

Just out of curiousity... what year is your truck?