After installing a new timing chain and doing all of the usual tuneup things, I am planning to try advancing my timing to get a little better highway fuel economy.

My 73 Dart calls for 0° advance at 750 RPM in neutral. The EGR is inoperative now and the catalytic converter is long gone (if it ever had one).

As I am only doing highway speeds of 60 mph, I am interested to know what others are using for their initial timing and what highway gas mileage they are getting with their cars.

Frank

You're going to need to check your governor...if it's a 15 you're SOL since that already ramps up to 30 mech adv. so you'll get some ping at all levels of initial advance.... if you have brazed the slots down or have another governor like a "9"... then you can go for more initial out of it. With the 9 (18 degrees mech advance), you should be able to run up to 12 degrees of initial....after that you should check to see what your vacc advance it set up for, up to 20 more degrees would be good for highway if it will take it when the vacc. adv is active)....

Depending on gas quality I use 8 to 14 degrees of initial advance in my car and get 18-22 on the highway, but I'm also not running a 'normal' setup either....


-D.Idiot

Stock , worn out Super 6 ,new chain, 3100lb, 3.23 , 5 speed, .70 OD, highway miles, 8BTDC , 24mpg cdn

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Yeah....Im the one who destroyed this rare, vintage automobile.....

8 degrees initial, 30 degrees total and another 15 degrees of vacuum advance, 3600 lb station wagon gets 18 to 21 mph.
(mostly the 18 because I drive the car hard almost all of the time)
DD

Latest tune: 15 initial, 29 total + 17 vacuum. 80 Aspen, 2.94 axle, 904 with lockup. 2150 RPM @ 60 mph. At that speed, car returns 26 mpg.

Mitch

Set mine at 8*, 9L governer, light yellow spring and heavy spring. 8.5 vacuum advance can adjusted to come on later than usual. Get around 20 on the highway with 225-70R15 tires, 2.94 rear with a stock "high stall" factory converter in a 904. Turn 2500 at 60.

I pulled my distributor over the weekend and found the following numbers on the tag:
22 3
3755042

Inside the distributor, the vacuum pod is marked 8.5R. I didn't want to disassemble it to check the centrifugal markings.

I looked up the distributor information in my Motor manual and found the the following information for that distributor tag in 1973:

Advance Starts: 0.5-4.5 @ 550
Intermediate Advance: 9-11 @ 900
Full Advance: 14 @ 2000
Inches of vacuum to start plunger: 7
Max Advance Dist. Deg. @ vacuum: 10 @ 11

The above specs are at the distributor. Double the numbers to get crankshaft specs.

Anyway, I found that my gas mileage is about 24.5 mpg (imperial) or 20.4 mpg (US). The odd time I was out on the highway during the summer a couple of years ago (with my worn timing chain and about 2° of advance), I was able to get 28-29 mpg (23.3-24.1 mpg US). I'm sure winter driving doesn't help.

I set my timing to 5° BTDC @ 550 RPM. To check my centrifugal and vacuum advance, I marked my damper for 30-32° and 45-55°. I checked my highway speed and found that I am running at 2450 RPM at 60 mph. So, my advance is:

centrifugal only: 25° @ 2450 RPM
centrifugal + vacuum: 42° @ 2450 RPM
centrifugal only: 30-32° @ 4000 RPM (bounces around)
centrifugal + vacuum: ~50° @ 4000 RPM (bounces around)

With my timing advanced to 5°, my gas mileage has increased. My first reading is 25 mpg (20.8 mpg US) but probably would have been higher if it wasn't for the snow storm we had on Monday. It'll take a few fill-ups to get a good idea of my new fuel economy.

I have two questions:
1) I put the car up on blocks and compared the driveshaft revs to wheel revs. I estimate that I have a 2.75:1 axle ratio but I can't remember what ratios were available for my car. Is this right or am I off a bit?
2) With my new advance setting, I now notice that I am getting light knock while climbing hills -- probably due to a severe carbon-buildup. I think I need to add the vacuum hose octopus (actually quadropus?) back on my engine to enable EGR. Do I have to look for a 1973 model or is there a later version that will work as well?

Frank

Hey Frank, have you tried putting some more tension on the vacuum advance pod? If you put a 3/32" allan wrench into the nipple, you can adjust the tension on the spring, requiring more vacuum to actuate it. It will still give the same total advance, just later. I believe you turn the screw counter-clockwise for more tension.

very similar to my latest curve. 3600+ lbs car plus load, 3.07:1 rear axle ratio, 904, 2850 rpm @ 85 mph she gives me 17 mpg. 2400 rpm @ 70 she gives 19 mpg. I alway plan to cruise @ 60mph but I dunno, she always finds her way out around 70-80.

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Mitch has my favorite specs so far.

I run my daily driver 68 Dart at 12-15 initial, 28 full mechanical, with 21 deg vacuum, IIRC. Has A500 lockup trans and 3.23 gears, gets 21-24 hwy at 65-70 MPH.

My 64 Dart with 5spd, 3.91s, and TBI, I run 12-15 initial, 26-28 full mechanical, 22 deg vacuum. 23-27 hwy at 70-75 MPH. EFI is more dialed in than carb on 68 and car is lighter.

A common mistake is to run too much final mechanical advance. The 225 does not like anything over about 30, and 28 is more like it, unless your compression is really low.

Lou

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Home of Slant6-powered fun machines since 1988

I don't doubt your recommendation not to run more than 28-30° of mechanical advance. However, I think this is more important for maximizing acceleration which I have been trying to avoid. On the highway, I think total advance (mechancial + vacuum) is more important and it sounds like you have around 50° compared with my 42°.

Since my timing is bouncing around at higher RPMs, I suspect that my distributor shaft bearings are getting worn. Does Mopar Performance still have the performance distributor available for the slant six? If so, does anyone know its specs?

Frank

Yep you are correct about the acceleration/cruise parameters. The bouncing around could also be a worn/stretched timing chain.

Yes, my totals are in the high 40s. I would shoot for at least 45 total (vac + mech) on the hwy. Mine are set up for best accel (26-29 total mech), and good economy (+16-22 vac) - no reason why you can't have both. I haven't gone beyond about 50, but worth exploring up there too. The cylinder pressure/filling is quite low at part throttle cruise, so igniting the mix earlier creates a longer more complete burn.

Lou

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Home of Slant6-powered fun machines since 1988

I hope my timing chain isn't worn as it only has about 4000 miles on since I changed it last month.

I want to try advancing my timing a bit further but, with the light knock I'm getting at 5°, I'll need to hold off for a little while. With luck, some of the carbon build-up might burn off but since I'm using about 1 quart of oil per 540 miles, I think it won't be easy. I'll probably have to try some EGR to keep the knock down.

Frank

Try to run some water or something to clean out carbon?
I think Lou´s idea is good, get the most performance with the mechanical part, and economy with vacuum.
Guess I want the best performance up hill, so I get up hill before night, then my vacuun can take over again, for the mileage part.

I tried to read up on cleaning carbon out of engines. I found a procedure that seems to make sense: http://www.motorminute.com/Service/CarbonRemoval.pdf.

I also found a part number for the Mopar Performance electronic distributor: P3690788. I'll try to find specs on it so I can decide whether it'll have a payback in better gas mileage. This is my winter beater and I'm really hoping to get another winter out of it without spending too much more.

Frank