I have a Crane adjustable vacuum advance can and am trying to get it adjusted for max performance.

The engine is a 77 350 with a mild comp cam, performer manifold, and headers with true dual exh.

The instructions don't give the best description on how to set this thing up.

I am not sure what end result I should be looking for or how to know when I have it there.

Here is what I have
Initial = 13 deg
Initial + max centrifugal = 38 deg which gives 25 deg of centrifugal advance.
total = max centrf. = 51 deg
Vacuum = 14 Hg from manifold

Now with the vacuum adv can hooked up the timing jumps to 60 + deg.

I have the adjustment allen out CCW 9 turns and the lock out plate on the 16 deg notch.

Can anyone explain a little better about how to dial this thing in.

Thanks

 

ttt interested

 

OK let me take a shot at answering. My car had an adjustable vacuum advance can and for me it was a process of trial and error to see how it worked.

Now first of all you are confusing me with the "total=51deg" when the previous line says you have 38 degrees with centrifugal and initial. Now your timing should have 3 components - initial (usually timing at idle), centrifigal (the mechanical advance as you increase RPM's), and vacuum (the advance based on engine load, i.e. - vacuum). For peformance you tune your car to get around 36-38 degrees timing at 2800-3,000 RPM or so, that is centrifigal + initial (don't worry about vacuum advance because that is not a factor at WOT), so disconnect the vacuum advance, run the engine to 2800 or so, put a light to it, and you should be at about 38. Anything much over that and you are going to get some engine damaging detonation. Again, note that at this point vacuum advance is not factored in.

Vacuum advance comes in at light load and cruising situations and you can add another 8-15 degrees to the total, depending on your compression, type of fuel, etc. You adjust this using the cam/lockout plate. In my experience the hex screw at the nipple does very little. It only determines at what point your vacuum advance starts and even then it may change the point from, for instance, 7 psi to 9 psi even though you screw that thing in and out about 30 revolutions. I wouldn't even mess with it unless you have a particularly large cam. I'm also assuming you have the vacuum advance hooked up to the ported (no vacuum at idle) source at the carb.
So anyways the real source for adjusting the vacuum advance is that lock out plate. Mine is not numbered like yours, it only has some notches that, as I understand, change the timing about 2 degrees for every notch. You may need some trial and error to see what advance works best for you, if you get some pinging at cruise you may want to move it up a notch to lock out more advance. The most advance you can get away with without pinging is the best. Like I said, 8-15 or so should work, this will give you a TOTAL when you hook up the vacuum advance back up and bring it up slowly to 2800 of about 45-52 total timing (13 degrees initial+25 centrifigul+ the vacuum advance).
Now this is important to remember - as you adjust the vacuum advance using the cam/lockup plate you have to adjust your initial timing the same rate because the lockout plate limits the travel of the advance only. For instance if you have a 18 degree vacuum advance and you want to change it so it only travels 10 degrees then by adjusting the cam/lockout plate you are adding that 8 degrees to initial timing as a fixed amount. It may sound confusing until you think about it, but just remember to reduce your initial timing as you adjust your vacuum advance. Or, just to make it simple, just remember what your initial timing is and as you change the vacuum advance you have to remember to reset your initial to what it was originally.

So you give to figures on your total - 51 and 60. If 51 is total WITHOUT vacuum advance I would say the first time you run up to speed you are going to detonate your engine to death. If it is WITH vacuum advance I would say you are probably right on the spot. Now where does the 60 come from? That's probably too high and you will get detonation at cruise speed.

Hope this all makes sense.

 

It looks like you've added in initial twice. 13º initial + 25º centrifugal = 38º total. In most applications, a total with vacuum advance fully deployed should be around 50º-52º. If you're running 38º total, set the vacuum advance for 12º-14º. If you're seeing 60º with the vacuum advance set at 16º, your total is actually 44º! How did you set the total and and what rpm does your distributor achieve max mechanical advance?

 

Aside from the math issues, the 25 deg centrifugal with 13 deg initial for a total of 38 sounds about right to start. The vacuum advance can limited to 14 deg is good and you want the can to be fully advanced with manifold vacuum at idle. 13 initial + 25 Centrifugal + 14 Vacuum = 52 degrees which is a good target.

The hex wrench is used to alter the vacuum setting where the vacuum can will pull all the way in. I would shoot for having the can pulled all the way in at about 12" of vacuum if you have 14" of vacuum at idle. After you get this all set up, you will need to road test and go from there. Idle vacuum reading -2" is a good target. (Recheck this when you are done setting up the curve.)

You hook up the vacuum hose and the idle speed will jump considerably. You then adjust the idle screws to get the idle back where you want it. You should have around 27 deg at idle if everything is set right at that point.

Are you "idling on the curve"? If the centrifugal is already advancing at idle, you want to stiffen up the centrifugal advance till it does not begin until about 300 RPM above idle speed.

The road test starts with a WOT check for ping or detonation. If that is not an issue, move on to part throttle testing. If you get ping with part throttle, you may need to bump the vacuum advance can limiter plate a few degrees or so. At this point you should be pretty much set.

Good luck, hope this helps.

-Mark.

 

I spent a month tinkering with my Crane setup,and previous posts are steering you in the right direction.My setup ended up with the blue/silver spring combination,limiter plate set on the very last notch,and 5-6 turns in the vacuum can.(clockwise all the way in,then 5-6 turns out)This combo gives me 35 mechanical(15 initial plus 20 distributer) and 10 more vacuum,which I connected directly to a manifold source.In the 406 there is a quite lumpy cam,and I used the vacuum advance to help smooth out the idle.

 

Sorry about the number confusion, I read a few papers on tuning and some used different terms so I think I confused myself.

The first thing I did was set idle mixture.

I then set Initial to 13 deg with the adv weights taped fully retracted, idle speed at around 400 rpm and vac adv removed and plugged.

I next untaped the weights and started it up, then increased rpm until the timing mark stopped advancing. The dial light indicated this was 51 deg.

51 - 13 = 38 deg provided by the mechanical adv.

Next the vac adv hose was hooked back up and at idle the timing showed the initial 13 deg. When rpm was increased and the timing mark stopped advancing, the light indicated around 60 deg.

I ran the car at different speeds and didn't hear any detonation but to me it feels like the engine isn't producing peak power.

Shouldn't I be able to get a good spin on the tires from stop to pedal to the floor?

 

SpyderD
I have the vac adv hooked to the T on the manifold (same fitting as the brake and transmission).

Should I move it to the carb base port?

 

You can hook up to either manifold (vacuum at idle) or ported (no vacuum at idle) at the carb. You will see many arguments about this, the truth is it really doesn't matter because the vacuum is the same once you get off idle. Try both and see what your engine likes at idle.

 

51 total timing without the vacuum advance is WAY TOO HIGH. You didn't hear pinging because perhaps your engine was not at load. Also, you can't always hear detonation. Go up a hill or make a WOT run and you will damage your engine for sure. The 38 degree timing figure should include initial. Now are you sure you are measuring it right? You don't hear of many distributors that have built in 38 degrees of centrifigul advance.
Good spin on the tires? Well I don't know, it all depends on your setup. Dialing in your timing will help alot but remember if you have a stock late 70's engine with stock gears there is only so much you can do without adding parts.

 

Ok, Here is what I did today.

I did find one problem, I put headers on last week and the plug wires were straight boots. The front and rears needed to be 90 deg boots, well one wire burnt through the fiberglass sleeve and boot so I had one plug shorting to the header.

Initial was set to 13 deg. It had changed from yesterday

Centrifugal when at 2800 rpm was 30 deg. I had to change springs, I started out with blue/blue @1300 rpm, then blue/silver @ 1000 rpm, and finished with silver/silver @ 800 rpm. My target idle is 650 rpm.
The way I did this was to watch the timing mark and slowly increase rpm and note the rpm that the initial 13 deg started to advance, by changing to the silver springs I was able to get 150 rpm above the 650 idle speed. Did I do this right? I could switch back to the blue/silver which would put it at 350 rpm above.

With the silver springs I get 36 deg @ 3400 rpm and 38 deg at 4000 rpm.

I started out adjusting the vac adv can allen so when at 12 Hg using a mighty-vac it was at full travel. Now I have 54 deg at 2800 rpm.

The car performs pretty good now, It will spin the tires from a standing start The best I can tell is there isn't any detonation.

Thanks for all the help

 

ttt