[Kenth]

Dough, most people doesn´t understand that the combustion should be finished and have the cylinder peak pressure at approximate 14 degrees ATDC to obtain maximum effiency from the engine. That is why you see different ignition timing curves for different engines depending on their configuration, combustion chamber shape, compression used, fuel used for example. This is what the engineers have to consider setting up an ignition timing curve that works, and i can guarantee that these engineers had more resources than most of us hobbyists to find the right curve.
Your present timing curve might work without the vacuum advance can and with the throttle nail to the floor at the dragstrip, but for combined street/strip
i would try an ignition timing curve more like an original if i were you, say 10-12 initial, 30-32 total at 4400rpm´s plus 15-20 from the vacuum advance can connected to a manifold source. To this with a racing cam you need to set up the carb properly as i´ve erlier described, i´m sure Cliff R can help you with the carb issue.
I would not be surprised if your car gets even faster after this.
Another thing, i really don´t like the description of the distributor vacuum can as an "ignition timing advance can" as it´s activated most of the time the engine is running. It is in reality an "ignition timing retard can" since retarding the igntion timing is what it really does when you hit the throttle to avoid preignition and ping when the mixture gets rich.
And the Crane can, well i have done some extensive testing on this a few years ago, and the result is:
0 turns it starts at 4,5 in/hg and max at 13.
2 turns it starts at 5 and max at 14.
4 turns starts at 6 and max at 14,5.
6 turns starts at 6,5 and max at 15.
8 turns starts at 7 and max at 15,5.
10 turns starts at 7,5 and max at 18.

I used this with medioker results until i learnt from Cliff R how to set up the idle circuit in my Q-jet right. Since then i´m using my original parts and settings and i would never dream of using the Crane unit again.

Jim, no problem conserning starting issues with the vacuum cannister connected to manifold vacuum.

............Kent

[Doug Christie]

Got the Crane Adjustable Vacuum Advance/Retard (for you Kent LOL) set up. It didn't take all this time to do it, but unless your very lucky, you will have your distributor out several times.

Initial timing is 12*. With vacuum advance hooked up it's 30* and a total of 36* all in by 2000 rpm. I know I have to work on the rate at which the total timing comes in, but that will only be a matter of changing 1 light spring for a medium one.

Anyway, I got a procedure from Geoff B and it really helped speed things up in the learning curve department. See below.

Adjusting the Crane unit [ Simple ].



"Here’s how I do it: You will need a digital tachometer. Analog is not accurate enough.



Disconnect & plug the vac adv.
Warm engine [ & put in Drive if auto ]
With tach hooked up, loosen dist clamp & SLOWLY advance dist. Monitor rpm & lock dist clamp at highest rpm. Changing only 2 to 4 degrees may see a 10 rpm change. Always lock the dist at the HIGHEST rpm. [ This is why a didital tach is needed: to see a 10 rpm change ]

Now, with your timing light, read your initial timing. Don’t be surprised if it is OVER 30 degrees! It’s ok! [ Mine is 36 degrees ]. [ Yep, that’s correct, not a mistake, initial is 36 degrees ].

Decide the static initial you want to run. This is necessary so that the engine is not hard to crank when warm. 10-12 degrees is usually best.
Let’s say you choose 12 degrees & the figure you read from the timing light was 32 degrees. Your Crane vac adv unit, hooked to manifold [ not ported ], needs to supply 20 degrees at idle [ 32 less 12=20 ]

Each notch represents on the Crane plate is 2 dist degrees [ 4 crank ]. I would set it initially at 3 notches back from the highest setting, as a starter.

Screw in allen key & turn CW all the way [ decreases spring tension ].

Refit dist & start the engine & adjust initial timing to 12, with vac adv unplugged still.
Now connect vac adv unit. Check with timing light that you have 32 total. [ Should have what you got in [4] ]

If you do NOT have 32 degrees, you will have to readjust stop plate notch. One notch is 4 crank degrees. If you read 28 on the timing light, you need to back off one notch on the plate & READJUST initial again, because the notched plate also effects Initial setting.

Readjust idle speed & mixture screws for best idle.

Drive the car once you get the 32 degrees set & check for pinging. It may well ping, because the spring is set to LOWEST tension, meaning the vac adv will ‘hang on’ longer under load.
If it pings, turn allen key 2 turns CCW [ increases tension ] & re test.

Continue this until it is ping free.

After you have dialled in the vac adv as above, re check with your timing light that you still have 32 total. If the timing is varying, allen key adjustment went to far & vac adv is varying, which will cause erratic idle etc."

I did set the vacuum cannister up on the kitchen table with a piece of vacuum line with a vacuum guage T'd into it. I suck on the end until I noticed the breaker plate just start to move. Knowing that at idle my car is pulling about 8 in hg vacuum I just adjusted to about 2 in hg lower than that as Kent suggested and put it in the car. I had to do some minor adjustments to it after, but for the most part it's right on.

Hope this helps.

[Ty - 68]

Be careful if you are using the springs from the Crane kit. In my experience they loose their stiffness very quickly i.e. if your total comes in at 3,000 rpm now it may come in at 2,500 in 6 months.

[Doug Christie]

Thanks Ty - 68, Are the B&M any better? I have a set of them also.

[Ty - 68]

Not sure on the B&M. Rocky R. did a bunch of spring and weight testing for a tech article he wrote and his conclusion was that all the aftermarket springs he tested were sub standard (read: junk).

So where do you get good springs? Good question. I have not been able to get an answer to that one. Suntuned used to sell distributor kits (springs, weights, center cam, & vacuum advance) and I have his kit in my HEI. Good components with robust springs. Not sure if he still does this though...

[tininjun67]

After hearing Cliff speak about using ported vacuum, I switched. The truck has been having an idle problem for a few months now. It runs like my race car at idle, which I don't mind, but then it threatens to stall, and then does stall. I live in Texas, and keep the a/c on deep freeze even at the stop lights. It sucks to "2-foot it" all the time, especially if I have co-workers or clients with me. It's embarassing when a car guy's ride won't stay running. Anyway, switched to ported, advanced the timing a little, reset the carb and went for a spin. Feels good, with a full tank (35 gals behind the axle) it will spin the tires from a stop for about 20ft. Not bad for 30" tires and 3.42's. It idles too. It chirps the tires on the 1-2 shift, hasn't done that since I pulled the th400 and put the 4L80E in. I'll keep track and see if the mileage improves. It will probably be worse for a while till I stop lead footin' it.

[Doug Christie]

tininjun67, it sounds to me that if you had your vacuum advance hooked to manifold vacuum before, and it was running rough, you may have a vacuum leak in the vacuum adv. diaphram.

Switching to ported vacuum would eliminate the leak and advancing the timing would do what the vacuum advance should have been doing when hooked to manifold vacuum. Just a thought.

[Kenth]

Dough, i´m glad you´re advancing in the process to make you engine run smoother.

I tried a ported ported source for the vacuum advance in my car today and i could actually make my engine run pretty smooth, much due to my carbs (455SD) relatively generous idle circuit i guess.

My engine: .030" 455, 1971 #96 heads, #744 cam, std intake, 455SD carb, HO manifolds, points ignition 10 initial, 30 @ 4400rpm, 20 in vacuum can @ 15-17 in/hg. TH400 std converter.

With manifold source i had 14.5 in/hg @ 700 rpm in drive, 2.75 turns out on mixture screws.

With ported source i had 12 in/hg @ 650 rpm in drive, 3.5 turns out on mixture screws.

I can live with either settings.

I did try the same a few years ago with an unmodified #7040268 carb, and it would hardly run at idle with a ported source, i guess with it´s leaner idle circuit it just needed more timing at idle, and it did work fine with a manifold source for the vacuum advance, and even better with a modified idle circuit.

The lesson here must be that every part in an engines system, even the smallest parts, has to work in conjuction with eachother.
My observation is that lean mixtures demands higher ignition settings than richer mixtures, both can work well depending on application.

Hope this info helps someone..........please remember, it is not written in stone....

...........Kent

[Doug Christie]

And the stupid thing is, I wanted to pass AirCare by having more timing at idle to take care of high Hydrocarbon and Carbon Monoxide numbers at idle. I went there today, but before I went there I stopped at my friend's shop to check the car out and the HC and CO were both as high as when I failed before.

So he disconnected the vacuum adv/retard, increased the idle, and adjusted the idle mixture screws.HC went from 1200 ppm to 370(pass is 386) and CO went from about 6% to 1.6% (pass is 4.15)

So I went to AirCare and passed with HC 381 ppm and CO 1.67%. The only thing that I really changed from the last time I failed was the intake manifold.

Last year when I first put the engine together I passed with no problem with a stock '74 intake. This year when I failed I had changed from the stock to a Torker 1. Last week I change to a Performer, which is basically a stock design intake and passed.

I spoke to the shop that built the engine originally, and mentioned to them how the AirCare went. He said that he had quite a few customers failing AirCare with single plane intakes, and when they changed to dual plane they passed?

[Kenth]

Since it was 20 years ago i learnt about timing ignition i decided to see if my memory had faded.
So i hooked up my vacuum advance to the ported source next to the fuel inlet on my carb.
Every thing went fine, 12 in/hg at 650 in drive, 15 in/hg in neutral from manifold. I had to open the throttle plates a bit and readjust the mixture screws to 3.5 turns out from lightly seated for smoothest idle.
Then i connected my vacuum gauge to the distributor vacuum line, 0 in/hg at idle, and cruising at 50 mph i had 10-12 in/hg in the vacuum line, this means that the vacuum advance was hardly activated and far from fully activated, this occurs at 15-17 in/hg.
In the manifold there was however 20-21 in/hg when cruising at these speeds.
Not until i was driving at a speed of 65 mph there was enough vacuum signal from the ported source to fully activate the vacuum advance.
This is exactly why i use a full manifold vacuum source, with this i have 14.5" at 700 rpm in drive, 19 in/hg in neutral and 20-22 in/hg at all cruising speeds, mixture screws 2.75 turns out, throttle plates moore closed, this means that i have full vacuum advance whenever the mixture is lean and require this amount of ignition timing.
These are my results and you may use this info as you like and draw your own conclusions, i know i did.

I might as well add this, found it in the Delco Rochester Basic Carburetion Manual:

VACUUM SPARK ADVANCE

Along with the mechanical spark advance, another method is used to advance ignition spark under load. This is commonly called the vacuum operated spark advance unit. The vacuum advance unit supplements the mechanical advance, however, it operates differently as it varies spark timing in relation to engine load. Engine load can best be determined by manifold vacuum so consequently, the vacuum advance unit mounted on the distributor is connected directly to engine manifold vacuum.
Although spark timing is not a direct function of the carburetor, it is used on some applications to control the vacuum operated advance unit on the distributor during the idle and off-idle range. On these units a calibrated port is located in the throttle bore just above the throttle valves. It is connected by a vacuum line directly to the distributor vacuum advance unit.
In the curb idle position the throttle valve is below the spark port so consequently, no vacuum is applied to the advance unit and the spark advance remains in the retarded position. As the throttle valve is opened and engine speed increases the throttle valve moves past the spark port so that manifold vacuum below the throttle valve is applied directly to the vacuum advance unit. Timed spark advance is used on applications where engine design demands retarded spark for smooth idle and performance improvements at low speeds.
On other units the vacuum advance line may be connected directly to the manifold or carburetor bore below the throttle valve. In this case the engine will idle with full spark advance. The full advance spark at idle is used on engine applications where it is not detrimental to engine idle, and improves engine cooling. When setting ignition timing, the vacuum advance line should always be removed, especially on engines using full spark advance at idle as initial ignition timing will be set retarded if this is not done. The end result will be very poor fuel economy. To be sure, always disconnect the vacuum advance line to the manifold or carburetor when setting initial engine spark timing.

Sorry, all dragracers, this is for street cars.........although some of it may be useful for you too.....

...........Kent

[buwalda]

The Delco info from Kenth is interesting reading, plus the effect that manifold versus ported vacuum had on throttle opening/idle mixtire requirements on his engine. It shows that ignition timing requirements for a good idle should be determined BEFORE carb modifications such as enlarging idle feed restictions/drilling holes in throttle blades because these modifications will often be REDUCED or NOT be required at all, once the correct idle timing has been established. It is easy to increase the IFR, but not easy to decrease it.

[kyle_blake]

question about curve

Test driving is the right way to check the timing curve your engine likes correct?

I'm still not sure where the max should come in by, I suppose one just tries a quick rising curve to a 2500 rpm total and then drive it and then change to tighter springs and have it come in later around 4300 then drive?

(does one have to rev the engine in PARK up to 4300 rpm to see if the total is that late.


(4300 is just an arbitrary # I choose )

That part makes me a bit uneasy, I can see why people use the 3000 rpm rule.

Whats your thoughts assuming people are using the 230 duration cams and almost 500 lift.?

[Kenth]

If you´re using a points type distributor, just remove the springs, and set max timing at what you want, f.e. 34 degrees.
Then put the springs back, and check what you have at idle.
If you get 8 degrees or lower, put a stop made from a thinwalled vacuum hose on the pin under the weight plate, and do the procedure again with different thickness of hoses until you get the desired values.
No need to rev the guts out of your engine just to set ignition. HEI is a different story...

...Kent