Need your thoughts.... I've owned 8 slants over the years, still got 3 driving and a project. But from reading this Forum, I think a lot of you guys (and ladies) are waaaay more into these engines than I have ever been. From your experiences, what are your thoughts for the least invasive and least expensive ways to increase highway mileage in these cars? Mine are all 1975 / 1976 Darts/ Scamp, 225-6 (single-throats) and 904 trans, standard rear ends, 14" tires (both P185-75R14 and P205-75R14) If I stay at 70 mph I can get about 19-22 mpg on trips. The best I've got was at 65 mph 23 mpg steady. These have factory electronic ignition, timed to spec, valves adjusted regularly. I think the fatter tires get worse mpg, but can't prove it, and more psi helps. You folks talk about HEI ignition, and 2-bbl carbs, headers (with headaches?), shaving heads, etc. How 'bout some discussion on this? (Or has it been disgusted to death somewhere else in the Forum, and I missed it? ) Pat

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Make sure the engine is in good mechanical condition. Good compression, proper valve adjustment, etc. No malfunctioning carburetors or chokes allowed. No bad ignition components (plugs, wires, cap, rotor). Clean air filter element. Good alignment.

Distributor recurving which has been discussed at length is first on the modification list. It's cheap, but can be somewhat time consuming to do.

2 1/4" exhaust helps power and economy, but the return on investment is lower than most distributor advance recurves.

Raising the compression ratio also helps power and economy, but to do so requires pretty invasive work unless the cylinder head already needs to come off for corrective work.

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Joshua

Agreed; also, if the engine has some miles on it, check for timing chain slop. This is easy to do: remove the distributor cap and turn the engine manually with the fan and belt in one direction until you see the distributor rotor begin to move. Then turn it the other direction and watch the rotor; if there's any discernible lag between your moving the engine and the rotor turning, the timing chain is slack and will want replacement. It makes sense at that time to do a super-careful job of getting the camshaft timed precisely to optimise your engine's operation for economy. Details are in multiple threads and you can be pointed at one if your engines appear to need this sort of attention.

20-22 mpg at 70 mph in one of these cars with an automatic transmission is quite good. You can probably do better with some effort and expense. Some of these don't add all that much to the cost of a repair needed anyhow; installing a less restrictive exhaust system, for example, or doing a particularly careful tune-up. Others do involve spending money you wouldn't spend if you were just maintaining the car rather than upgrading. If your goal is economy, those extra dollars have to be factored in when deciding what to do.

The state of tune really does have to be tip-top. You can get away with a colder spark plug (an NGK #6, for example, rather than a 5 or 4) with today's cleaner-burning fuels than with the dirtier fuels of the past; this can buy some ping resistance and allow more spark advance for better economy. Rolling resistance is minimised by thoughtful tire and tire-size selection. The rear axle ratio can be changed to match the engine's sweet spot to your selected cruising speed. Maximise the engine operating temperature to the degree you can without making problems (don't go crazy; we're talking correct 195° thermostat vs. colder 180°). Duct cold air (but no rain) to the air cleaner and make sure the thermostatic snorkel works and the stovepipe is intact. For that matter, make sure the manifold heat control valve is working. If any of these cars has an OSAC valve in the vacuum line between carburetor and distributor, usually mounted on the air cleaner housing, bypass it; run the vacuum hose directly from carburetor to distributor. Do you have any unleaded-fuel '75-'76s with catalytic converters?

Better ignition, yep, definitely. I like the HEI upgrade.

Take a look at the engine fan -- if you install a thermostatic clutch-type fan instead of the solid-drive one, you'll use less fuel spinning the fan when it's not needed.

Do the Fuel line mod to waste less fuel clearing a flooded engine on hot restart.

Welcome on the board!

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

Thanks for the feedback. I think the carburetor conditions are ok, two have repair kits in them (NAPA?) with new floats, set to 1975 spec. I think one needs the float level changed slightly, as it has hesitation when coming off idle, but if I tickle the throttle once slightly, ie, squirt more fuel with the accelerator pump, it moves out fine. I've adjusted the %%% out of it, but they all are very sensitive to ambient temperature changes and humidity (RAIN) here. The best running/accelerating one, of course, is the one we just bought 3 years ago from a little old lady and haven't yet messed with the carburetor. Incidently, my carb experience with all the years 1974, 1975, and 1976 is that the 1974 has the absolute worst running fuel system of all. It has the older fat-spark plug heads and they tried a lean-burn carb, and didn't engineer it right. We got rid of that engine for a 1976. Distributer re-curve?? For 70mph power, right? I'll have to research that, as I've never heard how it is done. Exhaust- all these require/have catastrophic converters, and I'm ok with them. Two do have also resonators aft of the mufflers. Those could be eliminated, but it makes the wife's car noticeably quieter. Timing chains- done one, should probably do all. Camshaft timing- That is factory set-up with timing alignment marks installed. I guess I have to read more about that procedure & pro/cons. Tune-ups DO make a noticeable difference- especially spark plugs and distributer caps. I also don't buy Champion, too many failures. Autolight seems ok. Fuel- on trips I run 89 octane or premium, no 87. Thermostats are 195* stock, replaced routinely with the scheduled hoses/water pump replacements. Manif. heat control valve and EGR get regular inspection. I'm not sure about the OSAC valve. Why eliminate it?? HEI-- what will it give me over a freshly tuned factory stock electronic ignition? Thermostatic clutch fans-- I've never seen one stock on 1974-1976 S/6 with or without air conditioning. I assume you mean aftermarket, maybe fiberglas? How many $$ and what is the reliability? Fuel line mods-- Thanks, I read the thread. My fuel lines and filters don't seem to be real close to the exhaust manif, but sit a little off to the side in open space. I hate plastic fuel filters- just a really bad idea, so mine are metal. These cars came stock with a vapor return charcoal cannister from the carb. Vapor boiling has not been a problem up here (Seattle) in the summers. Thanks guys for the good discussion!! You have brought up issues that I never would have thought about. My thought process in asking for feedback is to help get these cars tweeked-out reasonably economically for best MPG performance prior to attacking the next step: building an open-road car for the wife and myself- installing an A500 overdrive into the Dart Swinger 1975. Wife really likes that car, and has told our son it is his only temporarily, 'cuz she wants it! Pat

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

That's not a float adjustment issue.
Shouldn't be the case once they're set up and tuned correctly.
...for which a better spark plug is available (NGK ZFR5N, remember to remove the metal ring washer) that you can't get for the '75-up taper-seat-plug head.
The Holley 1945 was introduced for '74, and those first-year 1945s weren't yet debugged, but it was not a "lean-burn carb". (I have one or two new later revised carbs left on shelf if you wind up deciding you need one..."remanufactured" carbs from the parts store are junk).
Okeh, well, carburetors and catalytic converters don't get along very well under the best of conditions, and pre-'81 cars are much worse than the best of conditions. It's likely the catalysts are partially plugged by now, and there are better-flowing cats available now than there were in the '70s. I would point you at a #9005 from these folks. What state are these cars registered in?
No point in doing so
Don't just line up the dots and guess it's right; it probably isn't. The factory spec was retarded even if it was right, and the tolerances were sloppy, so it often wasn't/isn't to spec. See here.
Used to be, now Chinese trash. Use NGK.
Why? Does the car ping on regular? If not, you're throwing money out the tailpipe and sabotaging your goal.
For better mileage and driveability.
Go read the link.
True, but that's not what you asked.
No fibreglass fans! Which fan to pick depends on your cooling system configuration and whether the car has A/C.
No. The charcoal can handles carb and fuel tank venting. It is not at all the same as a vapour return in the fuel line plumbing.
Big, difficult, costly job that will take a long time to pay for itself and substantially reduce transmission reliability. See here.

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

Please use paragraphs....

Dan probably spent an hour parsing thru that...........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I was trying to be nice and save forum space. Sorry for the run-on. Thanks Dan, and all. Now I'll spend the next few days of any free time following up on your efforts to straighten me out.
I'm actually looking forward to doing the A500 OD trans swap. I've read a bunch of info on the swap, but I haven't seen any mpg reports by anyone who has actually done it on 74-76 A-body cars. I'm proceeding knowing that there may be some triumph and some disappointment. It won't happen this year, but I think I might be able to get the mpg up toward the Honda range, 27-29 mpg. That is the wish, my reality may vary. Pat

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

If fuel economy is the goal I would go with a manual transmission long before I started cutting the floor and transmission tunnel for an A500. The return on investment for installing an A500 will be very low. The car will need to drive a lot of miles before the transmission pays for itself if ever. A lockup 904 is much less expensive, will bolt right in and will help freeway fuel economy. To get the best performance from an A500 will also require an axle ratio change as 2.76 or 2.93 gears plus the A500 overdrive is too little torque multiplication. It will work on perfectly flat roads where acceleration and passing isn't needed, but you will end up dropping into direct drive frequently.

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Joshua

You may actually get better results investigating fuel injection and turbocharging.

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Casually looking for a Clifford hyperpak intake for cheap.

We already decided not to go with a manual trans, hence the OD A500. The lock-up 904 I agree would be better than the 904 in it now. I guess this will be the grand experiment, to see how the OD does without changing rear axle ratios. If bad, that can be done later. It seems that the only time that down-shifting from OD will be an item will be on hill country highways. But maybe not, maybe always, maybe not so much. Grand experiment. The idea is still a road car for mom and dad to travel in.
I'm driving the donor vehicle now. It is a tired 1989 Dakota V-6 with a freshly professionally overhauled A500 OD I found for $600. The trans drives and feels really sweet, and it gives me the opportunity to understand the unit before I jump into the project with both feet.
I applaud the auto industry for going to computer controlled EFI. I think the technology is amazing. Turbo's are neat, but my experience with turbochargers and their control systems in the exhaust systems of light piston aircraft make me want to stay away from high-heat mechanicals. I'm a firm believer in the K.I.S.S. principle of maintenance.
I keep reminding myself that there are better engineered cars out there now, but the Slant-6 A-bodies are so simple and functional, and they can still turn heads. You no longer need to be embarrassed to be seen in one. If I can take this classic economy car and rather inexpensively coax a more "green" MPG out of it on the highway, then I've multiplied the useful life of the vehicle (again). How tree-hugging environmentally friendly is that? If the average new car lasts 8 years before it is wrecked/scrapped, then a 35 year old car has saved the energy and resources of building 3 or 4 replacement cars, by my reckoning. I think that makes all us Slant-6 A-body folks American Heros. Dahm we're good! However, it is un-American to not "buy more stuff," so I guess the philosophy is debatable- elsewhere. Pat

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Josh is right...

If you download the transmission/ratio calculator from Moparts you'll see where he's heading...2.XX gearing and overdrive do not mix unless you plan on a flat land average speed of 75 mph...

Let's take my Feather Duster for example...
I have the A-833 OD box with the .73 OD ratio...stock it came with 2.94's
and something like 195/75R14 tires...so the rpm at 55 mph (back when that was the speed limit...) is about 2100 rpm in 3rd/direct and 1500 rpm in OD...2100 isn't bad, it's a nice spot for the engine...but 1500 is on the ragged edge of being either in the mains or the transition circuit of the Holley 1945...and I'd be more than tempted to downshift to keep the engine from "bucking" around under load...

Fast forward...Say we hang the rpm so it's "smack" in the mains and we recurve the distributor for max advance at something reasonable ("all in" at 1500 is called a "drag race" distributor....), so we keep the 195/75R14's and load the rear with 3.55's...pretty fun in town, and a final "ratio" of 2.59 which is a bit more granny than the A-904 with 2.76's on the highway...So we fast forward to a time where 65 mph is now the norm...with 3.55's and OD you can enjoy 65 mph at about 2100 rpm (all carbs like this as it's in the mains, and the rpm is nearing the "sweet" spot for torque/power which usually = mileage, instead of lugging the poor drivetrain around...)

You want to really have fun...Put the A-500OD in and choose 3.91 or 4.11 gearing... final ratios of 2.85 or 2.99 is about equal to the 2.94 and an A-904...in OD you're now falling into about the 2300 rpm range and the slant will like it there much better...

Something to really consider before you do it and become unhappy with the results...I drove a 1974 duster with the A-230 3 spd and swapped from a 3.21 to a 2.94 then a 2.76 rear ratio... even with the 3.18 1st gear the 2.76 was a clutch burning slug to get off the line at a stop light...not too bad if doing 70 mph up I-5 daily...

-D.Idiot

Like I said- you guys are waaay more into these engines/cars than I have ever been! This is awesome input! You've struggled through all the learning curve to now understand this stuff, and I am very indebted to you all for sharing your hard-earned info with me. I will spend time in the future reviewing your words of wisdom, and hopefully cut down my number of self-induced screw-ups! Pat

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Aerodynamics- airtabs, belly pans, fenderskirts.

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Casually looking for a Clifford hyperpak intake for cheap.

And coast a lot!

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11.02 @ 120.56
http://www.youtube.com/watch?v=EAxRmoDgsdY
Chassis dyno
http://www.youtube.com/watch?v=wd6hFGzLJMc

Go downhill! Attach a sail! Strip the car completely bare of everything but the essentials! Draft behind semis! Attach strong magnets to your front bumper to be pulled by the car in front of you!

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Casually looking for a Clifford hyperpak intake for cheap.