Distributor recurve

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That 79 dist was maybe this one was used in 78 and 80

1977 spec.jpeg


78 specs.jpeg
 
Different heat range plugs per application. Light duty basic car application, heavy duty got the 318 bbd 1-3/16 venturi verus the car standard 1-1/16 venturi

spark plug-2.jpeg
 
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Plotted out the one for a '78 2 bbl.
Specs looks like potentially decent starting point for cammed engine that needs more intial.
upload_2021-7-20_14-0-28.png
 
rich006 said:
The reason I'm fudging with the distributor is I already have "a lot of detonation and lousy fuel economy" (15 mpg in town). My goal with the distributor is to reduce the mechanical advance to eliminate the ping.

I have stock 2" exhaust all the way back. I think the Super Six cars came with a 2.25" pipe. I asked the muffler shop to give me a 2.25" pipe with a 2" muffler, but the guy convinced me it wouldn't make a difference. I think he was wrong and I am considering telling him to do it anyway. This is what I am looking at:
and maybe this:
and maybe even this:
I don't want excess noise, but I do want the engine to run well.
Click to expand...
You do not want to reduce the pipe size at the muffler to 2 inch, that would restrict flow - bad dawg!
Run the whole system in 2 1/2 inch and be done with it.
 
Here's what I measured with the 1979 springs and governor plate in the 1974 distributor (vacuum line to distributor was disconnected and plugged). Initial timing was 10.5 degrees BTC. I only get 7.7 degrees of mechanical advance, which is all in by 1750 RPM. This doesn't seem right.
Timing BTC vs. RPM-3.png

And here's the vacuum advance chart for the 11.0R vacuum pod. There's no advance until vacuum is about 8 inches, and it maxes out at about 19 degrees advance (above initial) by 14 inches.
Timing BTC vs. Inches applied.png
 
I went for a drive and there's no detonation, even at WOT at 80 mph. So that's nice. But I still need to adjust the fuel/air mixture.
 
That power curve is terrible.
No wonder it doesn't detonate.
but it won't make decent power in your engine.
And even with the VA, it has no where near any potential to make decent fuel economy.

Take a look at the chart in post #30; that's looking pretty good for your combo. I had a similar combo to yours that I recently sold; 9.5Scr, small solid-lifter cam, SS intake, but small exhaust like yours. Mine was set up for fuel economy with bottom end power to pull a 2.45 rear gear. My curve ended up similar to post #30.
 
rich006 said:
Here's what I measured with the 1979 springs and governor plate in the 1974 distributor (vacuum line to distributor was disconnected and plugged). Initial timing was 10.5 degrees BTC. I only get 7.7 degrees of mechanical advance, which is all in by 1750 RPM. This doesn't seem right.
Click to expand...
I agree.
rich006 said:
And here's the vacuum advance chart for the 11.0R vacuum pod. There's no advance until vacuum is about 8 inches, and it maxes out at about 19 degrees advance (above initial) by 14 inches.
Click to expand...
That seems to be working ok and pretty close to specs in the books.

So how to figure out what's going on with the mechanical advance.
??

Looking at the measurements and we see the advance begins above 1000 rpm.
Assuming the weights are moving freely, this indicates one or both springs is applying too much tension on the wieghts.
The long looped spring should apply no tension when the distributor is at rest.
So look at both springs, make sure they move freely and only the primary spring is holding the weight against the inside of the slot.

upload_2021-7-20_23-47-45.png


Then we see the advance slow greatly at 1250.
Its likely that's the long looped spring engaging. But best thing will be to take a look at the relationship of the springs and the weights.
Look at the inital postion, and then move the weight out until the second spring engages.

How to adjust the spring tension or degrees before the secondary spring engages
Distributor curve help needed

Thes two posts show what to look for
Distributor curve help needed
Distributor curve help needed
 
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Bugman said:
View attachment 1715768491

View attachment 1715768492
Click to expand...
The tag number on the donor distributor is 3874876 and it was on a 2bbbl auto car. If I'm reading the chart right, it is only supposed to provide a maximum of about 7 degrees of mechanical advance at the crank (~4.7 at 2200 rpm compared to ~1.2 at 600 rpm, so 3.5 degrees difference at the distributor, times 2). I was seeing 7.7, consistent with the chart. Combined with the 12 degree initial setting, the total advance would be about 19 on the Super Six car from the factory. Does that seem right? I know my engine likes up to 20 degrees at idle, so I would have no problem bumping the total to about 28 with the distributor as is.

If I want to allow more centrifugal advance, I guess I will need a longer long-loop spring.
 
When using engine as distributor test machine, it is helpfull to reset timing to 0 degrees and use lowest possible idle speed. Turn on heater fan and head lights,this uses Alternator to place load on engine,helps steady idle speed.
 
Your measure ments are close enough.Not much advance past 20 degrees,yes thats about right. Short answer use distributer as is except change vaccum advance to the 8.5 one 17 degrees instead of 22. With fast burn rich mixture and vaccum advance Peak combustion pressure occurs much too early, rough runing and ping could result.That 5 degree difference would allow base timing to be increased 5 degrees and timing at low power high vaccum cruise remain the same
 
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I did try setting initial to 22 and I definitely had ping there.

Bugman said:
When using engine as distributor test machine, it is helpfull to reset timing to 0 degrees and use lowest possible idle speed. Turn on heater fan and head lights,this uses Alternator to place load on engine,helps steady idle speed.
Click to expand...

Good info, thanks.

Bugman said:
Your measure ments are close enough.Not much advance past 20 degrees,yes thats about right. Short answer use distributer as is except change vaccum advance to the 8.5 one 17 degrees instead of 22.
Click to expand...

The only reason I used the 11 vacuum advance can is that my 8.5 can leaks vacuum, about an inch per second. I guess it might work OK but it would be hard to test with a handheld vacuum pump, and I wouldn't have confidence running it. It seems all the aftermarket cans are 8.5. I guess I'll get one of those.
 
If you get cruise leaned out the 11 can might be perfect.Use to be 3 cans 7 trucks, 8.5 and 11,cars
 
For vaccum advance testing .One simple in the drive way test. fully warmed up. Normal lean cruise carb, engine under load from alternator. Rpm around 2000 -2200 with vaccum gauge hooked up advance and retard timing to find the exact point where max vaccum occurs. Check timing ,put that number in your note book.Next with hand held digital tach note rpm.Engine should be runing smooth at this point. Slowly advance timing rpm may rise slighty, advance timing till rpm starts to drop, check timing make note.Some where between these two points is where vaccum advance should be. If more timing is added rpm will drop and engine will start runing rough. Air/fuel ratio will have some effect
 
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