built up 318 oddly low MPG's? hp rating?

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73 Plymouth Duster

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Hey guys so I have been working on my 73 duster for about 16 months and have been driving it recently for the past 2 months getting the bugs worked out of it, I have just recently gotten the kickdown hooked up right and adjusted and other odds and ends. I also took my GPS with me to check the speedo and odom. the speedometer is about 6% off and the odometer is about 11% off both too optimistic it will say 63mph and traveled 30 miles when in fact it is like 60 mph and 26.8 miles miles. That's a problem in itself that I need solved. I am planning on getting a new speedo gear to match the odometer then just deal with the speedometer being off.

Primary question: I just filled up today and my odometer said I went 130 miles so about 118 miles realistically...and it took 12.5 gallons to fill it. So I am shooting under 10mpg in a 318!?! I'll list the engine specs below and let you guys have a go at what you think I can do to increase mpg.

318 with new run of the mill pistons (put in when I bought car) checked clearances and torqued bolts and called it good. they are about
-8.5:1 compression
-bored .030 over
-Comp Cam 262, so a bit spicy.
-dual plane air gap high rise intake
-holley 650 cfm carb (too big?)
-not sure if they are "x" or "j" heads but they were hardened seats, valve job which included 2.02 intake valves and stock exhaust valves. decked heads .01
-used .040 head gasket (not .028)
-shorty headers
-dual exhaust with balancer pipe in between (h-pipe)

-took to Tom's performance in town, who ended up ripping me off a bit but they re-jetted carb from 64's to 66's I believe. and they "drilled restrictors in accelerator pump" and plugged vacuum advance. (all for more HP) double checked timing to 14 initial and 35 all in like I had it. A bit high I know but that's where it just seems to like it.

So since then I have hooked the vacuum advance back up to a ported spot on my carb. and I run regular gas with ethanol, tried non-ethonal premium and didn't notice much difference. Any guesses on why my milage sucks haha my park brake isn't on but I feel like it's something that obvious, thing sure does sound great though. and with 3.55 posi it likes to get up and go.
 
for mileage...keep the vacuum advance hooked up..

do you know how far down in the bore the pistons are?

need to know what heads you got...x or J ..340/360 head has large chambers that will lower you compression ratio...
 
mileage...

you want higher compression 9:1 or better will get better mileage

weight: less weight gives better mileage

trans: manual trans or automatic with lockup converter = good mileage. regular converter on a 727 = very bad lol

i had a 68 dart with a 340 and 4 speed, i used to get 19mpg. sold that car 25 years ago i regret it now haha. it's gonna be a while before i get this duster running
 
What gear ratio? Do you have a tach and do you have a vacuum gauge? What kind of drive, I live in Portland so if your driving the gorge or over the mountains in the wind and driving rain, your economy will be less.

You need to buy a speedo gear to match your rear gears.
 
How much overlap on the cam? If it's too "spicy" a cam, you could be sending a good amount of unburnt fuel out your exhaust. Does it smell rich?

Also, do you run a fuel return line or just a feed?

That carb sounds big for a mild 318. I had a 600cfm carb on mine (truck cam, 360 intake, single 3" exhaust, pretty mild) and it was way more carb than I needed.

Just my thoughts.

-Mike
 
Also, how good are you at keeping your foot out of it? It sounds like there's a good amount of power-adders here. Are you good at leaving that power on the table, or is your right foot a little heavy? Try driving for a week straight as if there was an egg between your foot and the gas pedal. I did it once when I daily drove my valiant with the 318 mentioned above. Jumped from 11-12 mpg to 16 mpg, changing nothing else.
 
Agreed on keeping the vacuum advanced hooked up. The 340/360 heads are hurting mileage two ways. They lowered you comp ratio and the ports/valves are to large. The velocity is slow through the intake and head to break up the fuel. (Atomize)

Your speedometer gear change may not be a 100% perfect fix. Slight adjustments can be made through tire size diameter changes. Also note that the older cars mechanical speedometers are never dead on perfect.

When the time comes, try a taller tire. There are speedometer cal ' or tire size calc's on line that will show the differences in change tire diameters. If your running a 215/60/15 tire now, a 235/60/15 may work out very well. Check the calc's on line.

A slightly taller tire will help in mileage. Very slightly though.

Add a MSD ignition to enhance fuel mileage. A regular 6A box will do well.
 
Also, how good are you at keeping your foot out of it? It sounds like there's a good amount of power-adders here. Are you good at leaving that power on the table, or is your right foot a little heavy? Try driving for a week straight as if there was an egg between your foot and the gas pedal. I did it once when I daily drove my valiant with the 318 mentioned above. Jumped from 11-12 mpg to 16 mpg, changing nothing else.

I'll second this, as even in my Org 318 Engine and only add on I know of is a Street avenger 4bbl and intake, I can vary from 12 to 16mpg pending on the foot.:burnout:
 
OK whoever disconnected your vacuum advance for more power was such an idiot that nothing else they did should be viewed as anything other than wrong.

I can't imagine a mild 318 needing larger jets or a drilled out accelerator pump. I would suggest returning the carb back to the out-of-box settings and starting over.

The engine likes 14-35 ignition advance because of the fairly low compression ratio. (X or J heads with only .010 cut off and composition head gaskets will end you up with more like 7.8 to 8.0 compression)

The large ports and intake runners WILL hurt throttle response, but shouldn't have a major effect on fuel economy unless some brain donor set the carb up ridiculously rich in an attempt to compensate. I wonder if that isn't what happened.

A 318 with an XE262 cam and 3.55 gears should get closer to 20 mpg.

How long did you drive it with the kickdown linkage set up wrong? Nothing kills mileage like a screwed up automatic transmission.
 
I agree the compression is probably lower than you think, unless the block was decked, a lot. These engines are almost always further in the hole than spec. Have you done a compression test?

Did they tell you why they thought it needed to be richer? Did they hook it to an analyzer, or did they just guess? You could put the original jets back in and you should be able to compensate for the accelerator pump mod by playing with the stroke. But if you care about mileage I'd get a Carter carb.

I picked up almost 2 mpg over the mopar electronic ignition by installing one of those cheap chinese HEI distributors from eBay.

Your ignition timing sounds right. Why anyone would plug the vacuum advance is a mystery to me.
 
If you put a o2 sensor bung in the exhaust we can meet up with my lm-1 and tune the car. But as for mileage you need to lean the carb out, not fatten it up like toms did for performance. The 650 is not too big. I ran a 750 3310 on my sub 8:1 318 in my duster with 3.91s, weighs 3600 with me in it, and had a summit cam larger than the one you listed and it got around 18.
 
Lots of good points - the first being that if they are stock type pistons you're probably below 8.0 compression, I just went thru this.

Vacuum advance is your friend, put it back on.

600 is more than enough. Put it on the dyno with a wideband and see where your jetting needs to be. I think that with that low compression you'd probably benefit power and economy wise by going a bit lean.

262 isn't that bad of a cam IMO, I just installed a 268. But if you're under 8.0:1 the cam is probably a bit big. You're probably losing power to those larger port and big valve heads, not sure how that impacts MPG though. Trans, stall speed and rear end gear are also big factors.
 
A good thermoquad would help to, best MPG carb hands down.. The other big problem, like has allready been pointed out, is your low compression ratio. This motor would be miles ahead with a good preped set of "302" casting heads, in my opinion.
 
mileage...

you want higher compression 9:1 or better will get better mileage

weight: less weight gives better mileage

trans: manual trans or automatic with lockup converter = good mileage. regular converter on a 727 = very bad lol

i had a 68 dart with a 340 and 4 speed, i used to get 19mpg. sold that car 25 years ago i regret it now haha. it's gonna be a while before i get this duster running

I hope my Dart gets 19 mpg when its done that's better than my v6 4runner! Lol
 
The 318 I have in my car now gets 10-14 mpg now. Like C-130 said, you don't have enough compression to use the larger 340-360 heads. I looked around for a good set of 302's for mine but couldn't find any near me. I have a 360 on the engine stand now waiting for the heads to get done. If it was me I'd get a set of 302's or early (66-67) 273 heads and get them done. That will help some. Re Jet the carb and hook up the vacuum advance. I run a comp cam too, just about the same size as yours. You need to find a way to get the port velocity up, closed chamber heads and a intake with ports to match the heads. Is the 650 a vacuum secondary?
 
Not the best combo for mpgs but, I don't see why you couldn't get at least 14 mpgs out of it. Find someone who actually knows how to tune a carb.

You could also read up and try tuning it yourself. If you keep track of what was in it when you started worst case you could just go back to that setup.
 
Mine:

67 Dart, presently "Craigslist" 318, all stock 70's engine except---

mild solid cam, similar to the 68 340 cam
Edelbrock Performer
was Ed 650 cfm

3.89 (Ford) gear, 225 x 75 x 15 tires, so 28" tire, runs about 3000 at 65 mph.

Dist is Accel / Mopar look-alike, with 20 or so mechanical. 35* total mechanical plus vacuum.

I don't worry much about mpg, and most of my driving is "short hops" around the area. A few days with mostly highway, it gets over 15, bumping 17.

There is something terribly wrong with your combo, and as others have said, most probably very low compression, but also the cam. I don't have any idea how that manifold "is" for that combo.
 
WOW! haha gone for one day and all my problems have been answered lol, thanks guys. Let me see if I can answer all of your questions, I'll just go down in the order you guys replied. I'll upload some pictures of the engine too.

-I don't know how far down my pistons go

-I was told they were 360 heads and I think the machine shop confirmed that, it was a year ago but I think it is safe to assume they are 360 heads.

-I think it is just a stock converter, it is a basic 727 automatic 3 speed.

-it has a 3.55 posi in the back yes I have a tach gauge no vacuum gauge. 60 pmg on the freeway is 3000 rpm's

-it smells a little rich ya but nothing awful, here are the cam specs
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=626&sb=2

-This last fill up I was careful to keep my foot out of it because I wanted to see the best I could get, I nailed it twice the whole time. other than that there was an egg under my foot.

-I def. will keep my vacuum advance hooked up

-I can get a MSD 6al box for $100 but im not sure I need it. cant help mpg's THAT much.

-I drove it the whole time with a messed up kickdown until about halfway through this last tank. it just would go to like 45mph before shifting to 3rd or me letting my foot off, so I could see that helping my mpg's a little but im still wanting like 14mpg haha

-I primarily drive freeway, not like 60mph at 3000 rpm, but like the 50 mph country roads with stop signs and such.

-no I have not done a compression test yet

-the 650 is a mechanical secondary
 

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If you put a o2 sensor bung in the exhaust we can meet up with my lm-1 and tune the car. But as for mileage you need to lean the carb out, not fatten it up like toms did for performance. The 650 is not too big. I ran a 750 3310 on my sub 8:1 318 in my duster with 3.91s, weighs 3600 with me in it, and had a summit cam larger than the one you listed and it got around 18.

Are you in the portland area? I live up north of vancouver in Ridgefield Wa.
I'd love to find someone who knows what they are talking about and help me tune this properly, ideally with a dyno
 
SO in you guys opinion what should I do? I am starting body work and getting ready to paint the car soon. I'd really love to not pull the engine. Can I take off the heads and have them decked a bit more to raise my compression to 9:1 or so? and then have my carb professionally tuned? Would those be the best fixes
 
Are you in the portland area? I live up north of vancouver in Ridgefield Wa.
I'd love to find someone who knows what they are talking about and help me tune this properly, ideally with a dyno

I live a little south of Portland, message me if you want to get together. If nothing else id love to see your car :burnout:
 
That mechanical secondary Holley is the worst thing you could do for that combo. You need a vacuum secondary on that motor. It would be ok on a stick motor but not on that 727!
 
That mechanical secondary Holley is the worst thing you could do for that combo. You need a vacuum secondary on that motor. It would be ok on a stick motor but not on that 727!

double pumper mechanical secondaries are controlled by your right foot.....rear secondaries do not begin to open until the primaries are opened 40 degrees...so you gotta have your foot in it a bit to get secondaires to open...
 
I don't really have the funds right now to buy a new carburetor anyways. It sounds like the Cam and intake are okay it's just my low compression and possibly mixture and jets. Can I fix my compression problem by having my heads ground down or do I need to pull the engine and have the block decked?
 
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