Mileage, EFI or Carb

redmt · #1 (**permalink**) 04-01-2011, 01:40 PM

With all things being equal, in other words the same engine. Would a relatively stock 428 FE get the same fuel mileage with an 8 stack or with a carb and manifold?

jdean · #2 (**permalink**) 04-01-2011, 03:08 PM

To really get any of the potential gas mileage benefit of EFI, I think it takes an EFI-friendly cam with like a 114 lsa that pulls decent idle vacuum. Running a lumpy cam and then tuning the EFI to compensate for and perform well results in no better gas mileage than a carb. Thats what my own experience is anyway. If the EFI controls the timing you can gain a mile or two in MPG improvement there with some tuning.

bobcowan · #3 (**permalink**) 04-01-2011, 08:08 PM

Generally speaking, the EFI will be more efficient because it is more accurate. A carb has three fuel metering circuits, and the mixture can be adjusted to be pretty darned close in most situations. EFI has dozens of fuel metering points for all kinds of situations. Assuming the same amount of care is given to both tunes (carb and EFI), the more accurate system will be the most efficient.

Efficiency not only save gas, it makes more power. The peak numbers will probably be the same - or real close to it. But the torque curve will be flatter, and the area under the curve will be greater.

Anything with individual runner intakes - carb or EFI - does better with a cam that has a wide LSA, like 114-115*. The narrow LSA tends to cause some reversion as the intake valve closes - there's no common plenum. And then fuel get's pushed back up into the stacks - the famous weber fuel cloud.

Carbs of any kind require a good vacuum signal. And if the intake charge velocity slows down enough, the fuel will fall out of suspension, and become a liquid at the bottom of the intake port. Especially with the individual runners of a weber carb system.

The nice thing about EFI is that's it's not dependant on a solid vacuum signal to feed the cylinders. The fuel is forced under presssure into the chamber regardless of air flow.

I run a huge roller cam, just about the biggest off the shelf cam I could find: 304/314*, .608/.608", 114* LSA. The car is just a joy to drive. It idles very nicely at 850 rpms, with just enough of a rumble to make you smile, but still allow you to keep your kidneys and your fillings. It will cruise all day long at 2,000 rpm's, and get 18-20mpg. The exhaust note is very pleasant around town and while cruising.

But mash on the gas.........

vector1 · #4 (**permalink**) 04-02-2011, 01:04 AM

you could read that as "all things being equal" then both would be the same. too vague or too many variables really. whichever induction flowed less cfm would probably get better mpg.

Rwillia4 · #5 (**permalink**) 04-02-2011, 02:18 AM

Difficult question.
If you tune a carb to an particular RPM used for 'crusing'. I would say a carb would yield better numbers in MPG and power. However, EFI will perform better over the full RPM range and changing driving conditions. The burn of fuel would also be better. I think this would be true if you compare like engines (and cams).

Talk to engine builders, they can usually tweak more peak HP out of a carb'd car, but the EFI power curve is much flatter.

redmt · #6 (**permalink**) 04-02-2011, 07:20 AM

Where this is headed is that I have an 8 stack that needs converting to efi. I have a 406 and a 428 that both will need to be rebuilt. One of them will be built in a reasonable stock configuration. I'm shooting for 400-450 HP. I want it to be drivable on pump gas and even poor pump gas when necessary. I've been running a 9.5 comp 428 in a pick up for years and 87 octane without any issues. I've knocked the timing back and flattened the advance curve some.

I'd like to run the 8 stack because of the "cool factor". I don't want to kill the mileage so bad that it becomes un-affordable to drive.

DonC · #7 (**permalink**) 04-02-2011, 07:54 AM

What they said.
Don't think you have to worry overly much about "killing" your fuel mileage going to and EFI as long as the tune is reasonably close. I went with a mass flo system and, once dialed it, got a slight increase in fuel mileage with no detriment to driveability with a slight increase in both HP and torque. The dyno curves however were significantly smoother after the change.
Probably the single biggest change is the consistency in starting. Doesn't matter if it's cold or hot, just hit the starter and it goes. Over the course of a season I end up running everywhere from sea level to 10,000 feet or more and the EFI does a great job of adjusting to the changes. With the carb it took some serious attention at the higher altitudes.
DonC

eschaider · #8 (**permalink**) 04-02-2011, 09:08 AM

BobCowan and DonC pretty much hit the nail on the head. The EFI will outperform the carbs in all aspects including mileage. Get the system properly tuned the first time and then begin to explore.

For the first time in your life, as a tuner, you will have complete control of all aspects of the tuning process and can quickly "see" what a particular tune variation actually brings to the table. You will be happy beyond your wildest expectation.

The old Weber setups evolved to eventually incorporate plumbing connecting all the intake ports so as to simulate a more conventional manifold's vacuum signaling. You might want to explore something similar.

Ed