bnr517

I know this subject has been talked about a lot. Right now I have a 75 with a ZZ4 and running a Street Demon 650 carb. I'm getting around 10mpg and wondering if going to EFI would help much on the fuel mileage? Not my daily but if I want to take the miss on a day cruise it gets a little ridiculous. TIA

Kacyc3

If your carb is tuned correctly it will take you very long to see any savings if any of swapping to efi.

Goody

You would gain better results from an overdrive transmission swap. What rear gears you running?

stingr69

10 mpg sux.

Would it pay to switch?.... You probably would not drive it enough to make up the cost. You might be better off buying a wide band O2 meter and tuning for better mpg.

bnr517

.373

fishslayer143

get an OVERDRIVE TRANSMISSION ... I get 19MPG hiway with 525 HP and 3.73 gears .. my orig motor/trans and 2.87 gears didn't get close to that

REELAV8R

Even with you 373 and 3 speed auto you could do better than 10mpg if it's tuned well.

I get 19 mpg with a th350 355 gears and 440 hp.

The qjet can get really good gas mileage, it's what I run. But I imagine your current carb could do better as well.

jim2527

What are you all getting in the city?

REELAV8R

Don’t know specifically, mixed driving I get around 16 to 17 mpg. That would be about 70% hwy 30% city. I hardly ever baby it so that includes brief periods of WOT.

7T1vette

You have a vintage Corvette and have put a ton of money into the engine compartment....and you're fretting over fuel mileage????

terry82

a good tune up ,and rejeting on the carb you may only gain 2 -3 mpg.but a good od tranny you would gain 5-8 mpg.

scottjamison

The cost of EFI just for fuel isn't worth it. You have to make sure it is tuned for mileage whether it is a carb or EFI. I have a Sniper and really like it over my carb but it made very little difference in mileage.

Kacyc3

seems like 12 but, was never tuned correctly

jim2527

As others have said there's no ROI going to fuel injection or over drive if your looking for an MPG boost. Both FI and OD have many advantages.... MPG ROI isn't one.

REELAV8R

It may be more than just carb tune affecting your mileage. Timing if retarded or not using a vac advance can ,can be a real fuel waster.
Drawing under hood air vs cold air can also waste fuel,

MelWff

how did you end up with a 650? The engine comes from GM with a 770 Holley vacuum secondary.
https://www.hotrod.com/articles/hrdp...hevy-bolt-ons/

pltmgr

I agree. FI and OD may help on resale.

ConnecticutJunkman

I think they set many of the "ready to run" carbs like the street Demon really rich to make sure the customer has no problems with stumbling or pinging when they bolt them on.

I bet you could find a few MPG with a wideband and vacuum gauge.

I don't like aftermarket gauges so on my cars I have an O2 bung with a plug. When I am tuning I remove the plug and install the wideband O2 sensor that goes to an AEM gauge that plugs in to the cig lighter and just sits on my pass seat. When I am done I remove the whole mess and reinstall the plug in the O2 bung.

What do your plugs look like? Pull a few after a long drive and check them out. All black or clean and tan?

NewbVetteGuy

I agree on other things impacting mileage including timing, but hot underhood air doesn't HURT fuel economy, it actually helps it.
A "hot air intake" reduces the actual amount of air in the engine and then with a properly calibrated carb or efi to those hot temps == less fuel. Hot air = less power, but increased MPG.


Adam

NewbVetteGuy

Very recent and relevant video from Finnegan's garage:
Took his giant 89 GMC Sierra 454 BBC truck towing rig and fixed the retarded ignition timing, installed new spark plugs, and swapped the TH400 for a 4L80e and went from 5.5 mpg to high 8mpg - low 9 mpg.


OD transmission with lockup TC, get some good ignition timing, run the cam a couple of degrees advanced, and long-tube headers for improving fuel economy. IMHO, the benefit of EFI is keeping it tuned, and if you go with ignition control and a knock sensor, being able to run the timing and cruise AFR ("lean cruise") on the ragged edge of detonation to get as much torque at your cruise RPM as you can. -You just can't be safely as aggressive with the ignition timing and running the AFRs lean with a carb or EFI without a knock sensor.


Adam