Hello

Newbie here. I'm the proud owner of a '68 Barracuda by way of Mecum last week. Got a great deal (I think) and love the car. The previous owner had a custom built 360LA dropped in and it's been dyno'ed at 450 HP. I was lucky enough to receive the JMS Racing Engine reports on it as well. It has a 750 Holley on it as well.

It's been over 30 years since I've driven a fuel hungry machine and the sub-10 MPG takes a bit to stomach, mostly because I want this thing as a daily driver. The owner mentioned that his next upgrade was an EFI.

I'm curious if anyone here has done the conversion and if that improved your mileage at all. I know it's not cheap, but TBH, even a 50% improvement would be worth the effort to me.

Thanks in advance.

 

10 MPG is kind of low ... What kind of gears does it have ?? FI is not cheap but can do wonders for mileage, you can tune it with your laptop...

 

IMHO. a gear change would be easier and cheaper. and no tuning needed. that much power would have no trouble with a 3.23 for in town driving. i had a 300 J with a 500 in max wedge engine with a 2.73 gear and it would walk away from any thing. even at low speeds.

 

2014 5.7 only shows 14-15MPG around town......
So your trying to get 15MPG around town per-say????

A gas miser 360 will be a whole new drive train build. "In MY Opinion"
360 LA 0.030 Overbore
264* Cam "Mile-A-More"
2-Barrel EFI w/Fresh air intake.
727 With a gear vendor
2.73
28" tire
Keep your foot out of it, should get around 14-15MPG or better around town.

Just my 2 cents...

 

Save your money, tune the carb and ignition. Got vacuum advance? Does it work? Your total all in advance should be 35 degrees at least for max mpg. Yes a gear swap will help but it'll cost money and tuning is almost free. My 4000# b-body gets mid teens and that's with a built 500" stroker and I beat the bag out of it. The difference is that it's got a razor sharp tune (and that's with an 850DP carb!). For the record I've looked into efi as well, but I have two little girls that keep spending all my money

 

EFI, depending on what is wrong, will not magically fix fuel mileage.

In this day and age, as cheap as a "wide band" O2 setup is, that might be a good investment.

As said.......you need a good advance curve AND vacuum advance.

You need to get the gearing in decent range. You have not told us a thing. What are you running for gears and tires, high stall? converter, and what RPM at "some" mph like 60 or 70

This could be so many things I'd get tired of typing.

What have you TRIED to alleviate this?

 

As I stated, I'm newbie and I don't wrench at all so my engine knowledge is pretty minimal. My engine experience ended back when I put the engine block back into my '62 bug w/ the 4 bolts on the tranny and pieced it back together. Suffice to say my buddy had to help me get it to turn over.

I know my info is sparse and all your questions seem like ones I should be asking as well. I'm trying to contact the previous owner as he seemed open to answering any questions on the car. The car did come w/ all the receipts of the engine and drivetrain work. I'm still digging through all of it to understand what he had built, granted I'd prefer just to ask him.

At this point, I haven't done a thing, just trying to understand if EFI was a valid option only because the previous owner mentioned.

Overall, it seems most suggest carb tuning vs the EFI. I've asked in a few other Mopar forums and the consensus is the same. In fact, I've only had one poster recommend it.

Appreciate everyone's help/input and more, detailed info to follow. This is putting me in the solid direction though.

 

Well, gee, if you want an actual answer, how about filling us in? You still haven't answered a thing about tires, gears, cam, and general engine build.

 

And ignition timing, very important for mpg and drivability.

 

Ok... I think I can make this easy for you.... It's going to help us here at the site...

Call JMS and talk to the builder. Mike Johnson is a good Dude. He has a good head on his shoulder and is no spring chicken to what he builds. He will do you a solid on the info you need for one of his motors...

http://jmsservicecenter.com/contact/

By the way.... JMS did the Twin Turbo build in the Blue Duster that is in this mouths Car Craft...
Car craft guys shoot it down the other night at Irwindale.... You can se photos over at PhotoBucket under the name Bowtie0069... Check it out...
JMS also just got done doing a Street Hemi with 2 AFB's... Super Nice looking Motor...

 

Take it with a grain of salt..
17 mpg from a Hemi ?

 

That was such a cool build...lol

 

NOT just that mileage "claimed" but with "Hilborn" style (EFI) stack injection!!!!

 

Yeah, I'm taking this slow and working on the carb tuning 1st.

Got a hold of previous owner and he admits the prev mech ran it rich for break in as the car only has 900 miles since restoration/rebuild.

Other info that might help posters provide more pertinent feedback:
360LA
Tremec 5-speed with scatter shield (converted from automatic)
Rebuilt 8 3/4 3.92 rear end
Custom built Holley 750 double pumper
Comp cam (waiting for details on that)
Front wheels - 215 70R14
Rear wheels - 255 60R15

I dropped at my local shop today just to have a safety inspection. They work on all my late model cars but I know the owner has some muscle cars and races, so I know they can at least take a look. Admittedly, I only recall seeing older GMs though. Today, there was a covered, LONG car on the floor w/ a tall tail and I immediately knew what it was. Underneath was an orange Superbird owned by the senior mech, who also said he owns a 70 Cuda 440 Six Pack. I think I'm in good hands.

 

You need to put a 3.23 rear end in it !!

 

You gotta take this thing to the track.

 

I see that anybody has recommended a carb change. I suspect that Holley 750 is a double pumper and double pumpers aren't made for gas milleage

What about a change for a street demon, Carter/Edelbrock AFB o AVS or maybe a thermoquad?

 

Oldskool.. Sound like your in good hands there and here! Those 750 dp's have eyes for gas stations! Lol.. I've got a 650 dp on my 383 and I'm always stopping and that's with 3:23 gears!

Before changing things, as mentioned Make sure the timing is set right, vacuum advance, engine set up right, after break in. But give a little time with what you have.. Maybe you don't need to do much at all to increase you mileage.. I've learned when buying a car from someone else even if they sound knowledgeable, they may not have things set up correctly. Check it out, Then go from there. Also, record what you change and what difference it makes if any. That will help you learn as you go. I've tried to change one thing at a time when possible so I can see just that.... My 2 cents..

Good luck!

 

Yeah, I agree w/ you Myraman. Seems the previous owner made it look nice but left a few things out.

My mech cleaned up a bunch of electrical, apparently he had to go back and forth w/ the dash because every time he was about to close it up, something stopped working. At least now all the lights work, w/ the exception of the high beams.

He tuned up my carb. Apparently, it wasn't running rich, it was running thin w/ a really high idle. It idles lower and lost the loud growl but I'll take it. Why? Because it seems mileage is much better since I drove back/forth from work which about 30 miles and the full tank needle didn't move past 'F'. Last week I would've been easily at 1/4. I'm monitoring the mileage, but it's looking good.

 

PLEASE do NOT "figure" mileage in this manner.

1......While I'm on a rant make certain your odometer is accurate. In this day and age, it's easy and cheap to get / borrow / buy a GPS. I buy used ones off Craigslist for about 40 bucks. I use one for a speedometer.

2......And figure mileage over an average of at least 3 tanks by using "fill it up" and your accurate GPS or known accurate odometer.

 

Maybe

Doubtful

Probably not

OP says it has a Tremec. First order of buisiness is to determine RPM at some known speed

And or LIST the rear tire size (27"), rear gear (391) and the OD ratio of the Tremec

The Tremec site shows all the way from .64 to .82. So if this box has a .82 OD that's an effective axle of .......................3.20!!!!!!!!!!!!!!!!!!!!!!! !!!

So if the Tremec is a .82, this should give us close to 2800 RPM at 70 mph. That is not really all that BAD

To put this in perspective, my old V code RR ran almost exactly this RPM and got "just touching" 14 mpg. This was not being particularly careful, as used to just "love" on ramps.

It's been said before, above...............

1............Make certain the timing is optimized. This means initial, distributor curve, and total. IT ALSO MEANS using a vacuum advance!!!!!!!!!!!!!!!!!!!111

2..........AFTER you get the timing optimized, check out the carb jetting. Frankly, in this day and age, buying an aftermrket O2 wideband setup makes SENSE. Innovate, etc, there are others.

3......A 750 Holley IF PROPERLY SET UP may not be hurting you that bad. My old RR ran an 800 double pumper. This was originally a 6 pack car (440-6)

4.......You said this engine had been dynoed. By who? You have documentation, or did the owner just "claim" that? This thing MIGHT live better with less camshaft if you are trying to get mileage. But "do" the distributor and carb retune BEFORE you worry about the cam. Depending on who actually put this together, cam TIMING might not be the best, either

5.......Don't discount "other problems" You SURE you don't have a fuel leak? You SURE the neighborhood ne'er-do-wells are not supplementing their lawn mower money with YOUR gas?

 

No worries. I'm logging everything w/ my iPhone. I've got an Automatic hooked up to my ODB in my FJ and I've been logging my routes. I have all the data to know the mileage on my standard routes so I'm using this along w/ logging my data (gas fill ups, mileage, etc) on the Cuda.

I'm an engineer by trade, so I may not know cars well but I am **** enough track data.

I have original dyno printouts from JMS as well.

 

Great. I think I'd try and figure out what you have for a cam, and "maybe" put a smaller one in. But I would go through the timing / carb first and optimize that