Horrendous fuel mileage

Now that my '72 Dart is finally on the road, I have noticed absolutely abysmal fuel consumption. I suppose I shouldn't be surprised because big cams are not known for economy, but still...

It's a 451 with a slightly reground Mini-Express (.652, 272@.050). 10.3 compression. 4780-2 double pumper with a Quick Fuel 12-700 baseplate, converted to 4-corner idle and adjustable orifices. I'm still adjusting and trying different settings but at the moment it has 70/78 jets and an 8.5 PV driving .053 PVCR.

Idles at 1200 rpm, 8" vacuum, 13:1 AFR on the gauge. Highway cruise 14-15:1 at 60 mph (3000 rpm, 3.91 gears, A833) and 14" vacuum. Tends to go over-rich with the whole t-slot flowing before the mains come on (12:1 at low speed moderate throttle). Idle timing is 27 degrees, slow centrifugal advance to about 35 degrees at high rpm, plus a vacuum advance can (to ported vacuum). Goes to around 48 degrees revving in neutral.

It's a reasonable estimate but I used at least 5 gallons of 93 pump alcohol-free, and have 21.9 miles on the odometer (correct speedo gear)
No, it's not leaking anywhere or blowing black smoke...

Now that does includes a lot of idling, throttle winging, and heavy pedal application on the back roads up to 40 mph and 4000 rpm, and a few miles on the highway doing 50-80 blasts.

Many years ago I had an A-body with a basically stock 383 except for .484 purple cam, DP-4B, 750 vac sec, A-833 and 3.23 rear. Averaged around 12 mpg mixed, could get 15 highway if I stayed at 60 mph, and my worst tank was all stop & go 1/4 mile at a time, 8.3 mpg.

So I would like to know what kind of fuel consumption others with big cams (who actually bother to measure) are getting.
Thanks.

Either there IS a leak or the plugs must be SOOT BLACK

Why don’t you start with a real MPG test instead of guessing what you have - you mention idling, winging the motor, etc.

Top the tank, warm the motor up and go 30 or 40 miles sustained speed then top it off again.

That would be the first thought, which is why I included all the data I could think of. The AFR gauge on the dash doesn't agree with you ... nor is there any black smoke even on flooring it, or pig-rich smell to the exhaust at idle. If that much gas were leaking somewhere it'd be easy to find by nose and by eye.

I think it's just operating in an inefficient rpm range for much of my testing (3500 and below), plus working both those accelerator pumps a lot of the time

I'm sure it will do better at highway cruise, and that is in the plan once we get some decent weather and aren't on stay-at-home lockdown... not sure the local authorities would consider Mopar cruising an essential activity On the other hand, I would be plenty of distance from others except at the gas station!

Even if I'm off 50% which is really unlikely, that'd still be only 7 mpg.
What I really was looking for is other people's mpg with strokers and big cams

are you running a mechanical fuel pump ?

That 3:91 gear isn't helping the mileage any. That combo sounds like 8 to10 mpg to me.

Ya I think about 8.... my stock 68 gtx 440, auto, 3.91 got 11 at 60, with light brown plugs, near white pipes, no afr gear back then. It got a little worse in town, way worse on saturday nights.

Try locking out your dizzy at 35 and hook the vacuum advance to manifold vacuum. You'll have to dial back your idle, but it should help considerably. You can always swap it back to what you have if you don't like it. Your low end responsiveness and mileage should improve significantly.

low rpm 8" of vacuum with an 8.5" power valve? i'd start here. low rpm is always the fuel waste'er with a bunch of camshaft overlap. loose torque converters waste energy and low gears don't help matters. holley DP'ers aren't known for fuel efficiency either.

Sounds about right to me My Savoy with a 493 ,620 cam and 3,54 gears, 1050 QF carb. gets about 40 miles on 15 gallons of fuel just normal driving with a couple of full throttle blasts. It really drinks the fuel on 100 MPH blasts.
Gus

Thanks for the comments.
The common "wisdom" of PV 2" below idle vacuum is pure baloney, especially with a big cam. Holley just says that for people who don't have a wideband sensor Others say half the cruise vacuum which is what I'm closer to. The PV does not flow fuel at idle.

My cruise vacuum is 14" and I also made my PVCRs a normal size (.053 on a .073 main jet). That is another area I'll experiment with someday, maybe try a 9.5 or a 6.5 depending on what it likes. It's neat to be cruising at 14:1 AFR, push the throttle to accelerate and watch the gauge swing down to 12.5 just as intended



I agree... and the DP has two accelerator pumps and they do get used. Out of the box, the DP is known to be way rich since it's intended for strip use WOT, not cruising, and the PVCR is ridiculously oversized.
Like I said, I'm trying to maximize the (relatively) low speed manners. No torque converter here, I am strictly a gear-rower

Thanks, it's on the list of possible tweaks already
Since I drive it on the street mostly, do I want to be locked out? Concerned over pinging at low speed As you say, I can try it and see.

What I have now is actually a super-light spring so that it cranks easy, then it hits the loop of a heavy OEM spring as soon as it starts... So to lock it out and still be able to start hot, I'd just put in another super-light spring instead, at least for a test.

40/15 = 2.7 mpg
So I am in the ballpark, at least for the "too much is just right" crowd.

Oh well, I didn't build this toy for economy but I may need to get a bigger fuel cell!

Sounds like WOT mpg on a Hellcat !

if the power valve is fluttering, and I think it is, at idle or low rpm then it will bleed fuel. I don't know where the power valve should be half of cruise vacuum comes from but I wouldn't prescribe to that for a moment. i say half of your lowest vacuum reading.

What do the spark plugs look like?
Does the exhaust smell like fuel? Or bring tears to your eyes?
What's the stock PV supposed to be?

Most similar experiences I can relate:
My 438" Street Hemi RR, pretty big old-school low-vacuum Crower cam, 4.10s & 4-speed could burn through gas like crazy before we tweaked the carbs.
Like a full tank in 100 miles, with a few bursts of acceleration.
It'd wet the plugs and burn your eyeballs standing near its tailpipes.
36* total + vac advance, original AFBs, headers, 10.8:1, MSD6.
Leaned it some with staggered jetting, leaner metering rods & stiffer springs for the rods.
Got significantly better mileage - guesstimate 6-7MPG, including a few "exercises" every time I drove it - and the plugs & aroma cleared up.
Had a cammy 446" RB in a '64 Savoy, 727, 3000 stall & 4.10s, also thirsty, never accurately checked MPG but seemed about 6-7 as well.
Never tried to maximize MPG on either, mostly town & "hey, the road's empty" - so I'd bet on better #s at sustained cruise @ light throttle.

I bet on an actual “all around” mileage test, without excessive wot blasts and throttle winging...... you’ll end up at 7-9mpg....... which wouldn’t surprise me at all........ and is about what I’d “expect”.

I also think you’ll find that any kind of “enthusiastic” driving will very quickly cut that average way down......like to 40-50%.

My 493" RB has 10.2 to 1 compression and cranks 200 psi warm. It is an aluminum headed stick car with high-quench, reverse-dome pistons and heart shaped chambers. We only have 91 octane out here on the left coast, but I never hear any detonation or have hard starting issues with the dizzy locked out at 35*.

I am running the old DC mechanical carbs, and never go full throttle bellow 3000 rpm.

I have iron heads on (for now), sometime later on this year I will put the Dwayne-massaged Speedmasters on. Good quench for both.
The most initial timing I could run with good hot starting was 22 degrees. At 24 it would begin to fight the starter sometimes.
Plugs are light tan BTW.

Time to get a better fuel tank sender, too. The foam float in my current (vertical sliding) one is hard and brittle... and with the tank full to nearly the bottom of the filler opening, the dash gauge only reads 3/4! Yes, I have the proper resistance (240-33 SW gauge). As thirsty as this beast is, I want a more accurate gauge.

Thanks for the comparable data points, everyone. Today I went and got some more gas in cans, and filled it up actually slightly further than before, one 2 gal. can and there is still a gallon left in the 2.5 gal can. So it actually used 3 to 3.5 gallons (not 4.5) for 21.9 miles of blasting around. That's more like it (6 to 7 mpg), I can live with that.

Will report back once I put a few more highway miles on it. Now to clearance the #2 header tube where it's rattling against the shock tower...without removing the header

Yeah my wife always tell me "every time you drive that car it costs you $50 " Norwalk Raceway is about 45 miles from my house and the gauge is below E when I get there
Going to put some 3.23 gears in the Dana and put an 800 Thunder series AVS carb on and see if I can get 50 miles out of a tank.
Gus

big block, big cam, double pumper..... geez what do you expect?

From my original post:
Since I got 8 to 15 mpg from a mild 383 with 3.23 gears, I figured 6 to 10 with this one.
Well, I'd like to get 10 mpg on the highway at a steady 60, since I already know it cruises nicely at 15-15.5 AFR there. We shall see

An overdrive would change your world.

Spinning 451 cubes at 3,000rpm still requires a substantial amount of fuel to obtain 15-15.5 AFR, the shear volume of air you are pumping per minute is substantial.

The amount of fuel you need to run at that rpm is probably what my little Hemi consumes at 100mph in 6th.

Well, sure, but I can buy a lot of pump gas for the $5-6k it would cost me to install one
Especially since I'll probably only drive it a few hundred miles a year...

Don't forget that the throttle is mostly closed (14" vacuum) and the VE is way down. It's not like the cfm is anywhere near what it is at WOT


Could be, it's a much more efficient and modern engine - although I don't know what body it's in (drag coefficient). My '72 Dart is not quite as aerodynamic as a brick but it's close

We've found than an OD is not nearly as much of a fuel saving idea as much as it helps you enjoy the car more than you think you could.

We put a GV in a Lil Red Express, for about $3,000, and my father-in-law couldn't be happier. The fuel savings will never add up on paper, but the pure enjoyment it brings is worth it 10x over. It's much MUCH nicer to just get in an go somewhere, you don't feel the stress though the car as it screams down the road. You can carry on a decent conversation and make it to a reasonable destination without constantly monitoring the fuel gauge.

It depends on your location, but here in Texas we can run 100-150mi round trips at 75-80mph to attend a decent car event, I do not miss the days of running 1:1 904 with 3.91s.

That said, enjoy! I know my Warlock is good for about 250whp and gets about 8mpg, those trucks are barns on wheels...but I do have a a518 OD in the shed for it

Well if the 11 minutes to a tank is right, I figure 1.98 mpg on a demon wide open at 200 mph, hellcat would get better.

Work on the AFR so you can get from 14.3 to 15.5 AFR at light throttle cruising under 2500 RPM and see how much that improves the gas consumption on the highway when cruising
I like to see from 12.8 to 13.3 at WOT(full street exhaust system 3.0 pipes to the rear bumper) on our local pump gas with no ethanol in it

Hmmm, my Volare had a 11:1 360 with a way too big cam and a big carb. When I went to the track, I'd put 5 gallons of race gas in it. I could make 3 warm up passes, make 3-4 round passes. Of course for every 1/4 mile pass there was the 1/4 mile back to the pit stall. Roughly 3-4 miles put on it a day. I'd have a small splash of gas left over. So roughly 5 gallons to go 3 miles at 6000 rpm comes out to about .6 mpg.

Could be worse.