I see just about everyone on here, getting 20+mpg, with anything from stock to race engines. Here I am, with a fresh (and very mild!) build, and the best I've squeezed out so far is 17. And that may have been a fluke, as I wasn't doing any type of long haul/granny driving.

the build: 8.9ish compression, OCG 819 (.436, 264* adv, 219*@.50, 109 separation, adv 4*) lash set at factory specs, Hurricane long runner w/ stock holley 350, dual dutras. 447 head,w/ bowl hog and valve guide shaping. 904, stock stall, bands tightened to spec, line pressure bumped up. Manual drums on all 4, 2.76 gears in an open 7 1/4

Like I said, best I've ever got was 17, on a trip down to Springfield, MO. 1 1/2 hour drive, in hilly Ozark mountains, and 6 hours of busy city traffic. On average I get around 14.5-15.5, don't matter if its all in town at 25mph, or on the highway 55-65mph. On tanks with high (75mph+) speed freeway travel, expect 11-12.5.

My normal highway speed is around 60 (24-2500rpm) usually with easy acceleration. WOT is vain, jets are too small, so I typically ease into throttle. I don't have plenum heat, and I just have a 9 inch open element aircleaner. I also have an advance curve that is way too fast (don't know numbers off the top of my head, yellow & blue springs)

Could the lack of plenum heat be the source of this? I park in a garage, and even on 50* mornings it drives fine, once you get a good pump shot in it it runs great.
On a stock setup, once the intake has warmed up the exhaust heat is shut off, therefor using the same theory, once mine is warm this shouldn't be a problem?
Can too fast of an advance curve affect fuel economy?
I believe I've also heard somewhere that too lean can cause poor fuel economy?
Is this just a snowball effect of all the small things?

I'm just really dissapointed that this isn't getting the economy I was hoping for. Takes a little bit of fun out of driving it, with 360 fuel economy and nowhere near the power. All opinions/input gladly welcomed.

A bit hard to diagnose via the internet.
Sorting the advance curve and jetting should be done and should help.
Is the vacuum advance working?
That can make a BIG difference.

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Richard (aka Fingers)

agreed that the ported vacuum advance can make a difference on crusing mileage,
another item is getting the right vacuum operated power valve in the 2300,,

That is often because of a fast-rolling odometer or wildly optimistic arithmetic. Go read road tests of any of these cars we drive, with the possible exception of the Feather Duster/Dart Lite, and you will find that 20+ mpg is considerably higher than these cars actually get.

That being said: 4bbl with giant unheated intake? You're very fortunate to be getting 15ish mpg.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

That's a big manifold for as mild a build as you have. I'd much rather see that engine with a heated Offy manifold or even a Super Six manifold. Holley carburetors can require a lot of fine tuning, and often aftermarket parts, to get good fuel economy. When I ran my 390 Holley on an Offy manifold I wouldn't quite get 20 MPG on the highway. Stop and go driving was around 14 MPG.

If you're really dropping to 12 MPG at 75 MPH I'd suspect a lean misfire and/or excessive timing. Investing in a wideband O2 meter and an accurate plot of the distributor's mechanical advance seems like a wise investment at this point.

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Joshua

Well, that really just got me thinking. I purchased a NOS pod, don't remember the exact number on the arm (its been a while, and I don't remember stuff for crap) but it was somewhere in the 10-11s range (20-23*) I don't think I ever checked it though, just dumped it out of the box and slapped it into the distributor.
Gotcha. That makes me feel a little bit better. I usually use a GPS w/ trip odometer to check mine. My odometer is within 3% of the GPS usually.
I'm finding out that Holleys require much tuning, which equates to buying lots of parts. From them. Wideband AFR meter is in the works. Also, I got the manifold quite cheap (but still more than an Offy) and it was sort of a rash decision. Looking back, I definitely would have done it differently. But as they say, hindsight is always 20/20

Hmmm, I had 3 Darts in the 70's and all got 21-22 mpg or thereabouts. 2 were 3 speed manuals and one was an auto. Driving 65-70 mph in the Appalachians on I-81 and the WV Turnpike. And I usually got 28-29 mpg from my '76 Dart Lite. I was pretty conscientious about keeping track of mileage.

But I would not expect the OP to get that with his set-up.

I agree completely with what Dan and Josh said. Most slants in the real world get 15-17 MPG on average. I don't doubt that some dedicated tuners and modifiers have achieved higher, but most slant owners are not getting 20+ mpg.

I think you have overcarbureted and "over-manifolded" your engine and should step down to a Super Six intake and replace the rear Dutra exhaust manifold with a modified factory exhaust manifold for plenum heat.

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Casually looking for a Clifford hyperpak intake for cheap.

A 350 Holley is a 2 barrel?

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Richard (aka Fingers)

Yes. A big two barrel carb. The small Holley four barrels flow 390 CFM.(though it is calculated differently than the two barrel flow).

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Casually looking for a Clifford hyperpak intake for cheap.

...uphill both ways at 75 mph with the A/C on, right. Sorry, no. That didn't happen.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yes big unheated intake manifold will kill fuel economy, park a big two barrel on it and you start to have problems keeping good air fuel mixture at all throttle openings.

Manifold heat helps to keep fuel droplets in suspension, cold manifold cause fuel to drop out of suspension condensing on walls and floor of runners causing engine to see lean mix. A heavy accelerator pump shot will mask the problem a bit and kill fuel economy particularly at low rpm. Also you may get some cylinder wall washing with raw fuel finding its way into the bores; not good for engine longevity.

Often a four barrel vacuum secondary carb will return better fuel economy in relation to a big two barrel because its primary circuit is smaller than a big two barrel, and it will only open the secondaries on engine demand. In other words most of the time engine runs on smaller two barrel with more desirable throttle plate openings that promote better fuel economy.

But unless your engine can suck enough air, think of an engine as an air pump, a large carburetor just won't see optimal vacuum signal to be able to support consistent A/F mixtures.

Advancing timing past 50 degrees at cruse with a lean mixture will cause problems, having mechanical advance much over 30 degrees is undesirable as well.

I second tuning a Holley can be a challenge, folks here know the struggle I had with my car, but an o2 sensor & A/F ratio gage will help get to an acceptable tune much easer along with a few Holley tuning books for guidance. Titles of tuning books escape me at the moment as I'm not near my book pile; here, and here. I know what you are thinking those are for four barrels, true, but your carb is exactly one half of these 4160 & 4150 carbs. One other spot that is helpful is Holley TV.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

You forgot the part about "in a snowstorm" LOL. And yeah, if you drive 75-80 mph, ix-nay on the 20+ pg-may. (But I never drove like that....)

If I had an A-body car in good repair with a 2.76 rear and I was not getting 20 on the highway I wouldn't be happy. Sorry if some of you don't think it's possible. You may not want to tell Lou and many others that. (And I know how Lou drives)

I drove my truck to the Elk Creek race fully loaded with supplies and checked my mileage by GPS not the speedometer and got 18.7 running 65 MPH for the 300 mile trip. 85 Dodge, super six and 3.21 rear with auto. With my current truck and OD it should easily surpass 20 MPG.

One more opinion in the pile!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Some good responses here, having done this kind of build, I will post some key thoughts...

The Hurricane intake is between the hyperpak and clifford shorty in plenum size, it also tends to come in at a higher rpm for optimum power than the Hyperpak intake, so expect to have the sweet spot be closer to 3500-4000 rpm....If the intake plenum could be split with a plate that will change the sweet spot to be a little lower (closer to the midrange area for daily driving).

When using a larger intake on the street like this the key is to tune for max. vacuum reading and torque...I was ultimately able to get a high of 26"Hg at idle and closer to 21" under load on a flat highway with the comp cam 252 and the hyperpak intake, with lots of adjustments to the timing and the carb (lots of back and forth with one and the other).

The intake will require heat to keep the fuel in suspension as noted, I ended up going with the exhaust box being fed from each side of my duals with a 3/4" pipe to limit the scavenging from the exhaust and how much heat could be transfered to the box...This helped keep the engine happier when weather dipped under 65 degrees, and allowed for winter driving (you will have to change the automatic choke adjustments for summer and winter driving, but even with the hotbox, you can lean the carb down a jet or two since the fuel is staying in the air stream). Water heat is great, but you can wash down the cylinders waiting for the coolant to start up since the engine will need to be excessively rich to start and the choke closed for the next 3-5 minutes. I didn't get to explore the heated air intake option to speed up engine warm up, but was going to incorporate that in my EFI street build later.

I would use a vacc. pump to check whether the vacuum advance pod is functional and really holding when vacuum is applied to it....If you used the Vc-209 pod, this is not the greatest choice for this type of installation it's a lot of timing almost all the time, I would try the new VC-239 can instead, sadly these are only 17 degrees of advance instead of 22, but you have a heavy car with an automatic and sluggy highway gears, so this will help limit being too far advanced on the highway...do you know what governor you have in your distributor? If you had a stockish 1976 dizzy with an 11 governor and fast springs and the VC-209 pod at full advance you could have 22 degrees of mech advance and 22 degrees of vacc. advance and if you just happened to "dial" your static in at 12BTDC...you have 56 degrees which is outside the goal posts of 47 degrees to 52 degrees for a modified street slant....The goal to get the most vacuum is to tailor the curve to allow 10-14 degrees of initial, 30 degrees of mech adv. + initial, and following up with either a "safe" 17 degrees of vacc. advance up to 22 depending on "when" it comes in and if it's beneficial...

On the Holley Carb (except certain 80's OEM models, and certain Fomoco replacement models), the dizzy vacc. adv. hose should be connected to the nipple on the metering block, your vacuum gauge should be attached to the nipple under the fuel bowl...(this arrangement will give you ported vacuum for advance, and a view of the carb's sensing and throttle plate position from below the throttle plates.

I note that for some odd reason, you have a 819 cam, but have lashed it at "stock" lash .010/.020...but the cam should have stated your lash is .010 and .012 (this is also on OCG's lobe table...I would have started at .012 and .014 and looked for vacc. gauge improvements and noted the lash then used that setting from there on out.

The 350 cfm Holley 2 barrel isn't really that big of a carb, it's much like the front half of the Holley 390. I've gotten good success with this carb on a stock late slant six with better response and adjustability (and easier to get parts over the counter in my location) than the BBD or 2280....The Holley 500 is a really big 2 barrel and is known to be finicky on smaller engines without special mods.

That being said, throttle plate position is important in matching the carb to the build, if the build is unhappy with the carb sizing then the plates may need to be open more to get the right air/fuel mix....This can be important because if the plates are 50% open or more on a 2 barrel, and the vacuum reading is low the power valve will open and give it more gas...this most often than not on a holley will tank your mileage big time...driving around with a vacc. gauge will tell the tale, if it has a 7.5 PV installed and when you go up a slight incline and the vacc. dips to 7...you are now getting lots of fuel until the throttle plates are a little more closed or the load goes down on the engine and it shuts...if this happens a lot, you will washdown your cylinder walls and the engine will have a short road life. So this is something to check out...Holley TV should have a good video on Power Valve selection, be aware that it is tailored mostly for the V-8 crowd and there are other options only available if you read the Holley books...for street I run a dual stage power valve so I get partial enrichment on inclines and moderate pedal passing in traffic, if needing the whole thing, once it passes 5 on the vacc gauge I get the full enrichment...this works great on fat late model cars, some trucks, when you don't need a fire hose of gas while passing a semi....it would be good to pull a plug during a moderate drive and check the color until you get an O2 sensor and gauge.




I looked over some of the info you posted and noted that if you had the 819 cam ground at 109 LSA, with about an 8.9 SCR...the dynamic compression ratio is close to 7.3, and if you centerlined the cam at 102 instead of 105, the DCR would be 7.57 so you still have lots of room to play with this engine and get the most out of it (having a known SCR of 9.2 would help this combination of parts a little bit more than 8.9).

I also noted that this cam's flat wide "sweet spot" on the torque band is between 3000 and 3500 rpms...so the engine isn't getting the best motivational efficiency at 2370 rpm (assumed 2.76 gears and stockish 195/75/14 tires). A switch to a set of 3.23's would get you closer to that prime rpm band...but...this may also be outside your comfort zone (muffler drone/sound, higher rpm sound under the hood, etc...).

Recap:

Change only one thing at a time and note if it has a beneficial or negative effect on your car. If you are not a good memorizer, keep a small log book of your settings and store in the glove box (like the little spiral bound note pads for keeping mileage and fill up info...

Revisit your valve lash on this cam and tune it with a vacc gauge plugged into the nipple under the fuel bowl.

Check your timing and map the mech advance/Vacc advance using a dial back timing light and a tach.
Make a note while checking timing to see if you get a better vacuum reading at higher rpms or if you dial in more or less advance. You may need to recurve and rethink the vacc. advance.

Check and note all your current carb settings (jetting, PV, acc. pump cam, shooter size), that way you have a base line, before you change the PV or jetting, etc...

If all of that is maxed out...you may be looking at some upgrades:

Weld in a heat box with a cross over from each side of your duals.
Split the plenum
Take another scrape off the head to get to 9.2:1.
Redegree your cam with a little more advance (102). (Gets a little more torque at a lower rpm.)
Change your rear ratio/ might look at a better convertor when you do this (this may be the key with a cam that likes 3000 rpm and an intake that comes in around that range).

Once you do anything in this category, you will be back to retuning the carb and your timing curve again.

Good luck

-D.Idiot