Slant Six Forum
https://slantsix.org/forum/

setting base ignition timing
https://slantsix.org/forum/viewtopic.php?t=29922		 Page 1 of 1
Author:  newport77 [ Tue Jul 08, 2008 12:35 pm ]
Post subject:  setting base ignition timing
I'm trying to set the ignition timing on my '73 slant 6.
Factory spec is TDC but I have always had it a little advanced- i.e. 4 degrees BTDC. I assume the TDC spec was for emissions reasons.

In an effort to see if I could increase my fuel mileage as much as possible, I decided to advance it further.

I want to know if it is OK to simply keep advancing base timing until the car pings on acceleration, then back it off a bit? Is there anything bad about having it too far advanced?

I advanced it to 8 degrees and it didn't ping, then to 11 degrees and it still doesn't ping.

I checked the vacuum and centrifugal advances for proper operation, by the way.

I'm just surprised I can set it at 11 degrees BTDC and it doesn't ping.

I have 2 other slant 6s and one pings if I set it any more than 5 degrees BTDC, the other I can get it to 8 degrees BTDC and then it will ping. Of course those cars have different carburetors so I'm sure the jet size is different, which affects the richness of the mixture, which will also affect whether it will ping or not.

I'm just wondering if it's OK to just keep advancing it- is that a sound method of doing this? I didn't think I could advance the timing this much and it would be OK.
Author:  Aggressive Ted [ Tue Jul 08, 2008 12:54 pm ]
Post subject: 
I run my 74 Dart at 16 degrees initial with no ping, however I have my vacuum advance backed off 4 turns so it doesn't kick in at idle and throw the initial base timing off. Before I modified my 1920 carb, it used to go lean right off idle at 20 degrees with a #57 jet.

Overall I have found that 16 degrees initial and 16 degrees mechanical and 22 of vacuum advance by 2500 rpm gives me the best performance and mileage with an automatic.
Author:  newport77 [ Tue Jul 08, 2008 2:18 pm ]
Post subject: 
Thanks Ted.

The jet in your carb is definitely leaner than mine. I'm pretty sure I have a #61 jet.

You must have switched to a 1920 carburetor. In '74 they came with the 1945, didn't they?

What kind of gas mileage are you getting, around town and highway?
Author:  Aggressive Ted [ Tue Jul 08, 2008 2:48 pm ]
Post subject:  Mileage
newport77,

Yes, it came with a 1945 and a #61-2 jet in it. I have gone through two of them now and the best it got was 20 mpg on trips, and 18 mpg most of the time. It was a pretty gutless setup that pinged allot!

Doc suggested that I make the switch to the 1920 because they are easier to tune and get much better mileage. My first experience with one was about a '64 version 1920 which would average 21 mpg and 22 on trips with a #57 jet.
It was an old, used up, left for dead carb sitting under a pine tree and full of needles and almost all white with corrosion. So the experience with that one made we want to try something a little newer.

Last year I found a new "1968-72" OEM Economaster version of the 1920. It had a #55 jet in it which really didn't do well on the 11% grades we have around here. The "Argentine SlantSixer", Juan coached me on making some modifications (drilled out the idle air bleed, etc..) to the carb to where it will light up the tires (posi) and still get good mileage, 23.5 mpg average with the #61-2 from my old 1945. I drive about 130 miles a day round trip to work and back and drive 65 to 70 when I can. The back roads are 35 to 40 mph with lots of stop and go during rush hour on the freeway. Mileage is not bad for the way the engine feels. Often times I think it's a 318 under the hood, not a SL6. :D
Over all I am very pleased with the results and glad that I followed Doc's suggestion. :D :D :D
Author:  SlantSixDan [ Tue Jul 08, 2008 3:02 pm ]
Post subject: 
Large amounts of initial advance are not the best way to go. You can use this technique temporarily to determine if your engine might benefit from more advance, but you'll have better results overall if you will recurve the distributor to give you the additional advance at driving speeds, with a base/initial setting of somewhere between 5 and 10 degrees, ideally 7-8°. Too much initial advance eats into your fuel economy, driveability, and ease of starting, and can create problems with getting the carburetor dialed-in correctly.
Author:  Aggressive Ted [ Wed Jul 09, 2008 8:01 am ]
Post subject: 
newport77,

It is very hard to find any new MOPAR springs to recurve a distributor. You have to get ones from old V8 distributors. If you haven't done a recurve before, it is alot of trial and error even if you have a distributor machine. It took me three times before I got the recurve nailed. Doc helped allot.....
My engine at 16 degrees initial starts with about a half a turn of the starter. One pump on the gas pedal and hit the key and it is running before you can release the key. It doesn't matter if it's cold or it's hot, it starts immediately!

I keep a log book for mileage and every change I make to the engine since my commutes are fairly long, 130 mile round trip 5 days a week.
Fuel economy went up 1.5 mpg from running at 8 degrees to running at 16 degrees. So I leave it a 16 for the easier starting and better mileage. Our local carb shop owner talked me into trying 16 degrees so I gave it try and documented the results over the last year. My engine does not ping at all like it used to with stock tuning.
Author:  newport77 [ Wed Jul 09, 2008 10:34 am ]
Post subject: 
Thanks for the tips guys.

What I have found is that I actually have more advance than spec from the centrifugal advance (this is perhaps not surprising of the springs in the distributor are worn). i.e. I looked in the service manual and I get more advance at 2000 rpm than they say I should. It's not a lot, but it's more.

I also get a little more advance than spec from the vacum advance. I.e. They say I should have a certain amount of advance at 11 in Hg, but I have more than they say. I have to look at my records again to see the eact amount that I wrote down when I did this.

I'll have to test whether 11 in base timing helps fuel mileage by test driving it on the highway. Based on what Dan said, I don't think advancing the base timing further will help much. It may actually make my fuel mileage worse.

Fuel mileage and trying to save $ was my purpose in checking all of this.

I have never recurved a distributor before, so I'm not sure how to do it.

One question- I know the vacuum advance adds additional degrees to the base timing, and that the centrifugal advance also adds degrees to the base timing. When you're driving on the highway, are the effects of the vacuum advance and the centrifugal advance also additive? i.e. on the highway you're at 2500 rpm at least and probably 15 in Hg vacuum.

I mean, hypothetically let's say I have 8 degrees base timing, 12 degrees additional advance from the vacuum advance, and 15 additional degrees from the centrifugal advance. Is my total timing 35 degrees?

Or is my total timing just whichever one is more- i.e. in this case the centrifugal advance adds more advance than the vacuum advance, so would the effect of the vacuum advance be nil, meaning I'd just have 20 degrees of advance.
Author:  newport77 [ Wed Jul 09, 2008 10:42 am ]
Post subject: 
I forgot to say that I'm getting about 20.5 - 21 mpg on the highway going 65 mph in the '73. I have a #61 jet in the Holley 1920 so I have thought of putting in a smaller jet, but decided to look at ignition timing first.

I know I can get better gas mileage than this because I used to have the Holley 1945 on my 75 Dart and that got 24 or 25 mpg on the highway. However, it was a dog when it came to off the line acceleration, and when trying maintaining highway speeds up hills and things. I switched to a Super Six carburetor and it still gets 22 mpg highway. This is my basis for comparison.

I know the 73 should be able to do at least as well as the 75 in terms of gas mileage. They both have automatic transmissions and the same rear axle ratio (2.76:1).
Author:  Aggressive Ted [ Wed Jul 09, 2008 3:11 pm ]
Post subject: 
newport77.

The total advance is additive (initital + mechanical + vacuum).

You can get there a number of ways. For example, at 2500 rpm I am at 52 degrees, (16 initial + 16 mechanical + 20 vacuum).

That is how I able to get 23.5 to 24.5 mpg mixed driving with a led foot.

I wouldn't go too low on the jet size because you will get into the power valve too much which will kill mileage.
Your old 1945 should have come with a #58 jet. So you know how that feels and the mileage you got. You might drop down gradually from the #61 and find the best compromise.

As long as your not pinging, try and get a little more advance cranked in. I am using a VC-208 vacuum canister to get the extra advance at cruise. The easier on the pedal you are the better mileage you will get with that set up. I try to keep the vacuum at 15 to 20" at cruise. If your dipping down to 10" at cruise, then the main jet is too small.

Send me your email and I will send you the recurve instruction manual.
Author:  newport77 [ Wed Jul 09, 2008 7:45 pm ]
Post subject: 
Thanks Ted. Thanks Dan.
-newport77 (Scott)
Author:  DusterIdiot [ Wed Jul 09, 2008 7:48 pm ]
Post subject:  Nicht!
Quote:
Your old 1945 should have come with a #58 jet.
A 1974 Holley 1945 only comes with a #58 jet stock and the lever/linkage setting are adjusted from a different point than it's 1975+ bretheren...

For 1975 with the addition of a catalytic convertor, the #61 or #612 jet became standard for the car version 1945's...

FYI,

-D.Idiot
Author:  newport77 [ Thu Jul 10, 2008 5:59 am ]
Post subject: 
Good to know.
The jet in my 75 1945 didn't have a number on it.
I didn't know what size it was.
All I can tell you is that it was SSLLLOOOOOOWWWW on acceleration.
Author:  MitchB [ Thu Jul 10, 2008 6:36 am ]
Post subject: 
Quote:
Good to know.
The jet in my 75 1945 didn't have a number on it.
I didn't know what size it was.
All I can tell you is that it was SSLLLOOOOOOWWWW on acceleration.
Probably because the power circuit was setup lean. I enlarged the power valve restriction by a good amount - something like 20%, and this makes a big difference.

Mitch
Author:  Wizard [ Thu Jul 10, 2008 6:47 am ]
Post subject: 
Same here is that power circuit can come in any time when vacuum drop enough? Well, mine has too weak spring and need more washers added.

Correct assumpation?

Cheers, Wizard
Page 1 of 1 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/