So I got my 383 original 2bbl with .484 purple cam 750 holley had heads done with new springs and headers. so a friend of mine said you just rotate the distributer to the spot were it runs best... my question is I know there's more to it and have been reading about it but wonder is ther no specific # to have your timing at? Because it seems past the timing marks on timing cover like maybe runs 20 ish advanced but what should the total timing be and have read about making the mechanical weights open up sooner like changing springs or you can buy plates to put in distributer to limit the total. My dart has nuts don't get me wrong but I like to learn and do things right. Thanks
Timing questions
- Thread starter jrosey
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Ck with Halifaxhops in the morning.
He will shed light on that Q.
He will shed light on that Q.
The factory only needed timing marks for the technician to set the initial timing at the specified idle rpm.
The advance curve was in the distributor, and generally they expected that to be checked on a distributor machine. You'll see in the service manual those advance specs given in distributor degrees and rpm.
Now you've hot rodded the engine. In general it will need a little more spark lead at idle because the combustion conditions in the chamber are worse. Increased valve overlap help flow at higher rpms but at idle cause more exhaust dilution, reduced compression, and colder conditions.
Although your friend has the right idea, it would need to be done with a load on the engine. In fact best with a load and a CO analyzer. Yea right - like you have one in your desk drawer. If your car has an automatic transmission, you can set the timing and then place the transmission in gear. The goal is for the least drop in rpm (and vacuum) when placed in gear. Thats a good indicator the engine is strong in idle.
A few snips from Chrysler on timing in this post.
total timing reading accurate with no load?
My suggestion is to get some timing tape and measure the timing from as slow as the engine will run, then every 200 or 250 rpm up until it stops advancing or you don't feel safe. Then you can plot the advance curve and see what needs to be done, if anything to the advance.
This chart is helpful when you have nothing else to work from.
Distributor starting point for a curve
You can also use the pre-emmissions era 383 and 440 factory timing as a baseline.
See this thread: Total advance question
Notice the initial was 12.5* BTDC at 550 - 600 rpm. With your cam, its going to need at least a couple degrees more initial, and it may have to idle higher, maybe 700 or 750 rpm.
Be alert to the fact that the distributor may already be advancing at 700 or 750 rpm. IF it is, then when the engine is placed in gear and slows down, timing will decrease. So IF this is happening, it may be neccesary to put more tension on the primary spring inside the distributor.
Direct Connection/Mopar Performance provided a guideline for the higher rpm timing. A copy of that is at the bottom of this article.
How To Limit and Adjust Chrysler Vacuum Advance Cans
That method requires timing tape or a dial back light and of course a tach. In that approach they were most concerned with mid to upper rpm timing and not as concerned with the initial. But I can tell you that using their distributor set as they recommend, initial ended up around 12-15*BTDC.
Finally note. If the distributor you are using was from an early smog era engine (CAP/CAS 1967-74?ish) the advance above idle is very fast and long. That makes it impossible to get the higher initial timing for the pre-CAP era, and what your hot rodded engine will want, without modifications. The best way is to weld up the inside of the slots. However sometimes you can successfully get by using manifold vacuum source for vacuum advance.
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You absolutely must NOT just rotate the distributor to where it idles the best!
Here's why;
At idle, your engine might like 20/25/30 degrees IDK, go ahead and find out.
But as you do that the idle speed will increase. Just keep adding timing and resetting the idle speed. If the engine wants more gas, just open up the mixture screws.
Eventually, you will reach a point where no more timing will produce an increase in rpm, or the mixture screws will run out of gas.
Now, you can find a happy place for your engine, somewhere with a lot of idle-timing.
Now put it into "drive", and try to drive it.
In all likelihood, it will stall. But if it stays running, step on the gas.
In all likelihood, it will stall. But if it stays running, step a lil harder while listening for detonation.
In all likelihood, it will rattle like marbles in a coffee-can.
Here's why;
as for the stalling;
1) you have shut your throttles so far, that the the transfer slots quit flowing, and the circuit which is your primary low-speed fuel delivery system, has dried up, and
2) you have maxed out the mixture screws.
Set this way, it is normal and natural for the engine to stall, because the dried up transfers take a lil time to wake up,when you tip the throttles in, during which time, the AFR goes extremely lean. But as soon as the transfers start working, the AFR goes very rich, because of the maxed out mixture screws.
As for the rattling:
Your distributor has a mechanism inside it that automatically advances the timing with rpm. In a factory D this can often be 25 or 30 degrees.
It usually starts advancing close to 1000 rpm, but could be a lil lower/higher. It is usually finished by 3500 but could be a lil earlier or a lil later.
Your engine wants a specific amount of advance for PowerTiming after that point, and it is usually about 35 degrees. So; the D could be bringing in lets say 25degrees from 1000 to 3500 , which is a slope of ;
25* divided by 3500 less 1000 =.01 per rpm or; 100 x .01= 1 degree per 100rpm.
Now suppose, the way you have set your Idle-Timing as above, that you found that the engine liked 25* at 800rpm. But now, at 2200 rpm, say where about your TC stalls, your timing will be
2200 less 1000 divided by 100 and times 1* per 100=12*, to which you add the 25* at 800=37 degrees. IMO, this is too much PowerTiming for full-throttle use,at 2200. So the engine complains about it by rattling, which usually breaks stuff.
To continue;
by 3000, your PowerTiming will be 45* and by 3500 it maths to 50*. Something will break!
Earlier I said that the max is probably 35 degrees@3500, so at 50* you are at least 15* too many.
This is why you never just twist the D until it runs good.
EDIT
I should add that lifting the throttle at any time while driving, will again shut the transfers off. On deceleration, with a bit of rpm, the manifold vacuum will climb exponentially, as the pistons attempt to create a vacuum. Anything connected to the intake manifold will be subject to that same unnaturally high vacuum. Things like your brake-booster, your PCV, the bottom of your carb, etc.
During the camshaft's overlap cycle the evacuated intake may pull exhaust up into it, which is a reverse flow. Of course the next in line cylinders will draw that in. So now the chambers are full of something, I don't know what to call it, but whatever it is, it doesn't have hardly any fuel in it, and it's probably mostly just air.
Whatever; that can't be good.
Then when you get back on the gas, first the transfers have to wake up, then if the throttles are open far enough, the mains will come on line. From the cockpit, this feels like crap;
well I think it feels like crap. First my face heads for the windshield when I lift. And then when I get back on it, my head gets snapped the other way. I hate that. Ok not hate exactly but more like WTH?!
You might get the impression that you have a real powerhouse of an engine when this happens, but what you really have is a lotta hurky-jerky.
Here's why;
At idle, your engine might like 20/25/30 degrees IDK, go ahead and find out.
But as you do that the idle speed will increase. Just keep adding timing and resetting the idle speed. If the engine wants more gas, just open up the mixture screws.
Eventually, you will reach a point where no more timing will produce an increase in rpm, or the mixture screws will run out of gas.
Now, you can find a happy place for your engine, somewhere with a lot of idle-timing.
Now put it into "drive", and try to drive it.
In all likelihood, it will stall. But if it stays running, step on the gas.
In all likelihood, it will stall. But if it stays running, step a lil harder while listening for detonation.
In all likelihood, it will rattle like marbles in a coffee-can.
Here's why;
as for the stalling;
1) you have shut your throttles so far, that the the transfer slots quit flowing, and the circuit which is your primary low-speed fuel delivery system, has dried up, and
2) you have maxed out the mixture screws.
Set this way, it is normal and natural for the engine to stall, because the dried up transfers take a lil time to wake up,when you tip the throttles in, during which time, the AFR goes extremely lean. But as soon as the transfers start working, the AFR goes very rich, because of the maxed out mixture screws.
As for the rattling:
Your distributor has a mechanism inside it that automatically advances the timing with rpm. In a factory D this can often be 25 or 30 degrees.
It usually starts advancing close to 1000 rpm, but could be a lil lower/higher. It is usually finished by 3500 but could be a lil earlier or a lil later.
Your engine wants a specific amount of advance for PowerTiming after that point, and it is usually about 35 degrees. So; the D could be bringing in lets say 25degrees from 1000 to 3500 , which is a slope of ;
25* divided by 3500 less 1000 =.01 per rpm or; 100 x .01= 1 degree per 100rpm.
Now suppose, the way you have set your Idle-Timing as above, that you found that the engine liked 25* at 800rpm. But now, at 2200 rpm, say where about your TC stalls, your timing will be
2200 less 1000 divided by 100 and times 1* per 100=12*, to which you add the 25* at 800=37 degrees. IMO, this is too much PowerTiming for full-throttle use,at 2200. So the engine complains about it by rattling, which usually breaks stuff.
To continue;
by 3000, your PowerTiming will be 45* and by 3500 it maths to 50*. Something will break!
Earlier I said that the max is probably 35 degrees@3500, so at 50* you are at least 15* too many.
This is why you never just twist the D until it runs good.
EDIT
I should add that lifting the throttle at any time while driving, will again shut the transfers off. On deceleration, with a bit of rpm, the manifold vacuum will climb exponentially, as the pistons attempt to create a vacuum. Anything connected to the intake manifold will be subject to that same unnaturally high vacuum. Things like your brake-booster, your PCV, the bottom of your carb, etc.
During the camshaft's overlap cycle the evacuated intake may pull exhaust up into it, which is a reverse flow. Of course the next in line cylinders will draw that in. So now the chambers are full of something, I don't know what to call it, but whatever it is, it doesn't have hardly any fuel in it, and it's probably mostly just air.
Whatever; that can't be good.
Then when you get back on the gas, first the transfers have to wake up, then if the throttles are open far enough, the mains will come on line. From the cockpit, this feels like crap;
well I think it feels like crap. First my face heads for the windshield when I lift. And then when I get back on it, my head gets snapped the other way. I hate that. Ok not hate exactly but more like WTH?!
You might get the impression that you have a real powerhouse of an engine when this happens, but what you really have is a lotta hurky-jerky.
Last edited:
Sorry,
Some of the above advice is incorrect. Move your dist to get the HIGHEST idle speed. Say it is 30* with THAT cam. Tighten the clamp. Re-adjust idle speed. Put into gear. It will NOT stall. And it will not stall because the engine is making more hp with the added timing & can carry the extra load. This procedure is just a test procedure.
What you have to do now is 'design' your ign timing curve around your new found idle timing; many ways to do it, some described above.
Probably the easiest method is to use an adjustable vac adv unit connected to manifold vacuum [ MVA ]; you can Tee into a bake booster hose or vac fitting.
The beauty of MVA is you can leave your dist in the stock position & add the timing at idle with the VA, which is load sensitive, another important benefit.
More here on MVA, scroll down to post #6. Last 4 lines most important.
www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
Some of the above advice is incorrect. Move your dist to get the HIGHEST idle speed. Say it is 30* with THAT cam. Tighten the clamp. Re-adjust idle speed. Put into gear. It will NOT stall. And it will not stall because the engine is making more hp with the added timing & can carry the extra load. This procedure is just a test procedure.
What you have to do now is 'design' your ign timing curve around your new found idle timing; many ways to do it, some described above.
Probably the easiest method is to use an adjustable vac adv unit connected to manifold vacuum [ MVA ]; you can Tee into a bake booster hose or vac fitting.
The beauty of MVA is you can leave your dist in the stock position & add the timing at idle with the VA, which is load sensitive, another important benefit.
More here on MVA, scroll down to post #6. Last 4 lines most important.
www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
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