After about a thousand miles of highway driving (around 3000 rpm) fuel consumption looks to be at 18.5 mpg. In town is lower, but I have not calculated it. This big overlapping cam does not lend its self to low rpm economy, and Holley 390 tends to run rich on transition circuit below 2600 rpm (50 mph) unless pulling up a hill. This car has a 8 ¾â€￾ Suregrip with 3.55 gear.

I guess I have reached a panicle tuning this carburetor, I don’t see it getting any better unless a cam change is made, and or carburetor change is made.

I’m contemplating pulling the engine & transmission this winter. I suspect some sort of ring problem in number 4&6, and possibly a burnt valve. This will give me a chance to renew dry gaskets & seals, freshening engine up with a minimum of expense, and increase compression if block has to go to machine shop.

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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

Try running the stock 72-up mechanical cam but with more compression.

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Casually looking for a Clifford hyperpak intake for cheap.

What cam are you using?

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70 Plymouth Valiant 4dr

Going for a slightly milder cam with your current compression would help (say an Erson 270/270 or an OCG 791 with a 111 LSA to make best use of the compression and limit overlap for mileage). Also the 3.55's aren't going to help mileage 18-19 was the limit I could get with the mint duster using a similar engine combo above, the offy shorty intake, 390 cfm holley, and 3.55's behind an A-230 manual 3 spd...this same engine could get 24 mpg highway with the A-833OD and the 3.55's.

So FYI, getting the highway rpms down to 2300 rpm and gaining a little more pressure and less bleed down at overlap while retaining the volumetric efficiency of the high CR engine should put it back on track.

2 cents

-D.Idiot

VBW:
[quote]What cam are you using?[/quote]

Unknown manufacture, no cam card, but I did graph events using a degree wheel:
[img]http://i294.photobucket.com/albums/mm87/wjajr/Dart%20Engine/100_1373.jpg[/img]

Lift at valve:
Exh = 0.460â€￾ center line = 113*
Int = 0.444â€￾ center line = 99*
105.5*
Duration @ 0.050â€￾: exh 236*, int 235* Overlap 22*
Duration @ 0.006â€￾: exh 324*, int 315* overlap 101*

This cam starts to perform above 3500 rpm.

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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

There is your fuel economy problem right there. Even with 3.55 rear gears your motor will not turn over 3k on the highway. That cam's power band is way too high for a fuel economy minded street motor.

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Casually looking for a Clifford hyperpak intake for cheap.

I have had success with good MPG and big cams. I would try the carburetor first, and the gears are hurting you.

I have not been able to get good MPG with Holley 4bbls, but I have not tried a known good 390.

FWIW, 18.5 MPG with 3.55 gears, all your motor work, and an automatic is not bad at all. I would say you'll be very hard pressed to get over 20 MPG with an auto and 3.55s, unless you go MP-EFI and all the latest gizmos. A lockup trans or manual trans will get you big gains.

If you are worried about saving money, you will spend more to make it 10% better than you will likely ever save by driving the car 1000-4000 mi a year. If you want better numbers for MPG, that's different, and I treat MPG like this usually, as a performance metric.

Lou

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Home of Slant6-powered fun machines since 1988

Lou,

At this point I’m happy to have this thing running lean enough so I don’t gas myself pulling into the garage or sitting at a light with pig rich A/F ratio. Out on the road at 60 mph running rpm is 3000, and A/F ratio gage shows mixture moving lean to rich and back with slightest accelerator movement, and I can see on gage when running on transition circuit, main jets, and power valve. It runs richest when on transition circuit which is most light throttle opening driving under 50 mph.

Exhaust pipes are no longer pig rich black, but a lighter gray. I am going to run this set-up for now, and spend the new carburetor dough for new rear seat upholstery, and front bucket foam.

By the way I have second strongest yellow secondary spring installed, as this engine was starving for fuel under WOT at 3000 rpm with heavier springs. Secondaries now open around 2600 rpm under WOT in third just above automatic kick down rpm, but at small throttle opening secondaries stay closed below 3500 rpm. Engine is real responsive to wide throttle openings over 3500 rpm, she pulls easily and quickly from 60 to 80 during passing, practically when manually down shifted.

I can live with 18.5 mpg, it is better than 16 mpg I got with my first 273 powered three on the tree Dart, and would blow the doors off of it. The Lebaron with its Hemi 2.6 only gets around 22 mpg, and is a whizzing slug.

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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

Sounds like a plan.

Lou

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Home of Slant6-powered fun machines since 1988

I got sizable mileage gains with more aggressive timing. I agree with Lou, the Mopar auto, while durable, is not the most efficient. However my old auto Duster averaged 22 mpg and would get 26-27 on the highway. This beat the Toyota I had at the time.

Sam

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Base timing is 14 to 15*, recurve disallows any advance before 1400 rpm. Car idles in park between 1100 to 1200 rpm. Also I have a higher stall converter, 2600 behind a v8, and stalls somewhere around 2300 rpm behind this engine.

I think I need to kick up dynamic compression to 8-8.2 or so, which would require static to be 10.5:1 or a little more. I haven’t run the numbers lately and don’t recall the exact recipe.

With more compression, some of the low rpm overlap inefficiencies will be lessened which I suspect should increase fuel economy a wee bit.

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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

What timing are you running total? What vacuum do you pull at cruise, say at 45 on the level? Where would the timing land cruising at 45 on the level? On my car it is close to 50. Mileage on the trip to Lou's jumped from 21.5 to 26.7 when I advanced the timing from 35 at this point in the map to 50. That would be 2200rpm and 45-50 KPA. You could feel the new life in the engine when I did this.

To be fair I also put more air in the tires for the trip back, so part of the improvement might be attributed to this.
Sam

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Yes, vac advance is essential for good mileage, and 45 deg total at part/light throttle cruise is a good target.

8-8.2 dynamic comp can give you plenty good mileage. Most stock 225s would have been well below this.

Lou

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Home of Slant6-powered fun machines since 1988

Sam:
[quote]What vacuum do you pull at cruise, say at 45 on the level?[/quote]

15 to 16 in Hg - rpm would be around 2250 with total advance in high 40’s. 50-52* total advance would be at 2800 rpm. Confession: I made several additional notches on damper pulley at ~10 degree increments to estimate advance above 10 degrees. Slightest opening of throttle drops vacuum to 10â€￾ or less in 2200 rpm range out on the open road.

Once rpm exceeds constant flat ground 4000 rpm vacuum jumps into 18-19â€￾ range. This seems to be when cam savaging becomes optimal, and at WOT a power surge can be felt in seat of pants.

Like others have said this is a racing cam designed for high rpm operation. That rpm is just above all legal posted speed limits in this neck of the woods.

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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