Advance Curve - MSD Distributor?

[PHOENIX]

OK I got an earlier birthday present (MSD 8365).

Got it installed and truck is running and holding timing.

Idle seems a little rougher then before; idle speed currently at 780 (manual trans).

I left the (2) heavy springs in when setting the initial timing.

I then changed the setup so I had (1) heavy spring and (1) medium spring.
I drove the truck and it started running rough at around 3000 rpm and started pinging at about 3300 rpm.

I then changed the setup so I had (2) heavy springs again.
I drove the truck and it started running rough at around 3300 rpm and started pinging at about 3600 rpm.

I am guessing that I should remove the blue (21º) advance stop bushing and install the silver (25º) advance stop bushing.

Does this sound right?
See the images below from the msd instructions.





Thanks.

[GMC1tondually4dr]

It might be easier and less wear and tear on your motor if you could take the dist. out and take it to someone that has a distributor machine and knows how to use it. They can set yor dist. up to run very well on the street or wherever. It shouldn't be too expensive. Inquire at the local speed shop.or look in the phone book.

Using your truck for a test bed can work if, as soon as you hear detonation, back off. It could be a long process, thats why I would put it on a distributor machine.

Good luck

[swervin ervin]

Eddie,

every truck, tranny, rear gear, engine, etc. is different. No way to say use this and do this and it works without trial and error. Here's where I would start if it was me.

First, leave the blue bushing in for 21 degress total mechanical advance. This is about the same as a stock GM HEI (20). Start off with the heavy silver and the light silver springs. Hook up the vacuum advance to full vacuum at idle. With the vacuum hose disconnected and plugged, set the initial to around 6 or 8 degrees. Hook the vacuum hose back up and test drive. If this works, try two blue springs and see what happens. In the tests, leave the initial alone. Keep going on the springs up to a light silver and blue. If you get this far and it's OK, raise the initial some. You need a dial back timing light for these tests to really see the curve. The chart is a starting point and is not set in stone.

What you are after is a total between 30-36 degrees of total advance. Right now, I'm running a total of 35, 14 initial and 21 mechanical. If I would run premium gas, I could add another degree or two. I started with the two heavy springs and set the initial to 10 and kept going until I reached 15 for a total of 36. After no pinging, I started messing with springs to end up with a light silver and blue. I found or regular gas, I had a very slight pinging up high, so I backed off the initial 1 degree to 14. This may not work for you. Like I said, every combo is different. You have to use what the truck will allow.

I don't understand what you mean by running rough. No way it should be running rough with the two heavy springs and blue bushing. This is about like a stock curve on a GM HEI. You sure the distributor is installed correctly?

Oh, and congrats on the new distributor.

[PHOENIX]

1.
Idle rough, I wouldnt say rough enough to complain about. Almost like if I installed a very slightly different cam. Just a little rougher then it was before.
i may be able to play with carb and idle speed to change this.

2.
With the (2) heavy springs installed and blue bushing it runs great (better then I can remember) until around 3300 rpm, then if I keep pushing the gas it starts pinging at about 3600 rpm.

Here is why I was thinking about changing the bushing.
If the (2) heavy springs are supposed to have the longest (slowest) curve and the lighter springs have the shortest (fastest) curve why would I want light springs?
I mean with (1) heavy and (1) med. it pings faster (lower rpm) then with the (2) heavy springs.

If you look at chart "A" (2 heavy springs):
Blue bushing - curve stops at 3800 rpm - which is about were I am pinging
Silver bushing - curve stops at 4800 rpm - if pinging started here I wouldnt mind.

If you look at chart "B" (1 heavy - 1 med. springs):
Blue bushing - curve stops at 3600 - which is about were i am pingng

I dont know much of what I am doing. Just trying to follow instructions and what I have read here. Mike Ill try what you said. I am pretty sure the distributor is in right. Like I said from idle to 3300+ its perfect, feels real nice and strong. Then at about 3300 it gets a little rough, then if pushed it pings.
What could be wrong with the distributor install?

I took my time marking the firewall and stabbing the distributor in. It took a few trys and a few oil pump adjustments with a long screw driver, but I would say i went pretty well and is in correctly.

GMC1Ton:
I have thought about taking the truck to a local performance shop to set the curve and carb after I install the exhuast system. I know they could get it better then me. A local shop will dyno, set advance curve, and tune carb for max performance and emissions for about $300.00. Thats just $300 I dont have. A friend did it and his truck was a difference of night and day.

[swervin ervin]

If it's pinging with the two heavy silver springs at 3600, what is your initial set to? No way in the world it should be pinging with these springs. If it is, for sure you have problems somewhere else. You will never be able to get anything to work right until this is figured out. Where do you have the vacuum advance hooked to? Sounds to me like you have too much total advance causing it to ping.

Why not check to see if you are installed right. Pull number one plug. Bring the piston to TDC on the compression stroke. Then line up the timing mark with zero on the pointer. Pull the cap and see where the rotor is pointing. I know you said it was installed right, but check it anyway. Remove all doubt.

Only thing changing bushings does is increase the amount of mechanical advance. I'm guessing you know this though. If you look at the chart for the two heavy silver springs, you see total advance is in with the blue bushing at around 3800 rpm or so. With the silver bushing, it's not all the way in until around 4700. Raising the mechanical advance let's you run less initial. You still want to have between 30 and 36 degrees total. Remember, we are working with the total. This is really all we are concerned about. We want to get all the advance in as soon as we can without any pinging. I don't think changing the bushing will help the pinging but it wouldn't hurt anything to try.

As I said, you need to figure out why it pings with the two heavy springs. Have you checked the curve with a dialback timing light or anything? You can't guess at this stuff. You need to see what it's doing.

[RockHQ]

This article was originally posted at Camaros.net by JohnZ, a former GM engineer. I had a request for a copy of it but it's too long for a PM on this board.

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

[PHOENIX]

I have a timing light with tach built in (i have to switch back and forth).
So how do I determine my total advance timing with this timing/tach gun?
Initial timing is set to about 9 degrees right now.
I played with holding a higher rpm on the engine (about 1500 rpm) and checking the advance at that point, but I do not remember for sure what it was.

I will hook my vacuum advance to manifold vacuum, right now its on ported vacuum, 0" at idle and 15"-23" with any rpm above idle.

Ill also pull the #1 plug and rotate the engine around untill I feel the pressure and the timing mark lines up with "0" on the timing tab. Ill then take the cap off and check to see if the rotor it pointing to #1 plug wire on cap.

Ill be printing this and playing some more after work.

thank again guys!

[PHOENIX]

OK here we go

I set the initial timing to 4-5 degrees and hooked the vacuum advance to manifold vacuum. I then changed my idle speed to compensate for the change from ported vacuum advance to manifold vacuum advance. This is still with the (2) heavy springs installed. Once this was done the idle seemed to be a little smoother. Truck seemed to run fine so I decided to go for a ride. Truck was running great and didnt have any pinging. I got her up to just over 4000rpm. From about 3800 and up it was a little rough though. Also when turning corners and accelerating there were huge flat spots. Please remember the egr is not hooked up right now. As it was also causing a flat spot before, may have been related to my bad timing issues before though.
1. What caused the roughness?
2. What caused the flat spot?
3. Should I hook up my egr right now while doing this stuff, or should I hold off and hook it up after I get the timing and curve set up (it may still cause the flat spot of idle)?

I returned home and decided to check the tdc like Mike said before I proceeded with changing springs and so on. I took the cap off and installed the tdc tool into the #1 spark plug hole. I put a large ratchet on the crank bolt and turned the engine around by hand. When the piston hit the tdc tool I stopped and the rotor was pointing at #1 plug wire on the cap. But when I checked the damper the mark was off, SOB I say as I almost bump my head on the hood latch..........again. Maybe my damper was slipping after all.
Check out this picture and tell me what to do now

[SanitysBane]

new damper or an adjustable pointer

[swervin ervin]

Ok, either your damper is bad or you have the distributor in wrong.

Use the piston stop again. Turn the engine clockwise until it hits, slowly and lightly. Mark the damper at zero on the pointer. Go the other way to the stop. Again, mark the damper. TDC will be exactly center of the two marks. Remove the stop and make sure you are on the compression stroke at TDC. Check the rotor and see where it's pointing now. Should be slightly before the number 1 tower, with before being clockwise before cause the rotor rotates clockwise.

If it's not, you are off with one of the above.

[PHOENIX]

When I try to turn the engine counter clockwise the crank bolt feels like its loosening.
I am using a big ratchet and 5/8" deep socket.
I dont want to mess anything up so what should I do?

When checking earlier the rotor was just barely before (clockwise) of the #1 post on the cap like you describe it should be.

--------------------------------------------------
Can you please explain this to me, I have tried to figure it out and cant understand why
If Top Dead Center (TDC) means: the point at which the piston reaches the top of its travel on the compression stroke.
How come you turn it one way till it stops then backward till it stops and the mid point between the two stopping points it TDC?
I just dont understand this. How come you do this to find TDC and turning it to piston stops on compression stroke isnt the TDC?
Please try to explain as simple as possible

Thanks

[swervin ervin]

You can remove the plugs to ease up on the compression some. You might need to use something to hold the engine from turning and tighten the bolt some too. You're not going to mess anything up by loosening the bolt. Just don't overtighten it and break something. They also make a tool to rotate the engine using the flywheel. They make a engine turning tool you cap over the crank snout but you have to have access to the key way.

OK. TDC is the highest point of piston travel, either on the compression stroke or the exhaust stroke. It will come to TDC twice in each cycle. People just refer to TDC in reference to the compression stroke. You don't need to use the exhaust stroke for anything. You must set the distributor on TDC of the compression stroke. The piston stop keeps the piston from going all the way up. So, you must do the two time method to find the center between the two, which will be exactly TDC, of either exhaust or compression. Compression stroke is when you feel air blow out the plug hole. The reason air comes out is because both valves are closed. On the exhaust stroke, the exhaust valve is open.

[PHOENIX]

Mike,

I dont think I am going to be able to get that bolt tight enough to where it wont come loose when i try to go counter clockwise.
I would have to buy the flywheel tool, so would the below work to get me by today?
Can I turn the engine clockwise to piston stop.
Then mark the damper at "0".
Then take the stop out and rotate clockwise a little more and reinstall the stop.
Then continue to go clockwise till it stops again and mark the damper "0".
Then measure the mid point bewteen these to marks?
This way I can avoid going counter clockwise, if this will work.
----------------------------------------------------------------------
Or will only going clockwise, then counter clockwise work?
I guess I can try to remove the belts and all plugs and see if I can go counter clockwise then.
----------------------------------------------------------------------
The damper is round and i dont understand what mid point would be correct anyway.
Do the marks end up being close together and I measure the mid of the shorter distance or the mid of the longer distance?
----------------------------------------------------------------------

Sorry to be such a PITA.
thanks

[RockHQ]

Sounds like you're making this more difficult than it really needs to be.

First of all if you don't quite understand why you need to turn counter clockwise I'll try to explain. When you install the piston stop (TDC tool) in the spark plug hole, and then turn the crank counter clockwise your 1# piston raises in it's cylinder and is stopped by the TDC tool tip. Now, you're NOT at TDC, but a little "below" it. You mark the spot on the balancer where the timing pointer reads O (or TDC or just an arrow it varies). Here's what you need to do next:


Remove TDC tool, and turn the crank a little so that the piston reaches TDC and starts descending. Then install the tool again, and turn counter clockwise. The piston will now be going back up again, just as if you were turning clockwise like you did earlier. The piston does not know which direction the crank is turning, and will stop equally far away from it's true TDC.

However, the damper will be in a different position, since the damper's true TDC (which we are wanting to exactly measure and mark) has gone past the timing pointer zero by some degrees. Now as you're turning counter clockwise, you'll see that the first mark you made is actually approaching the timing pointer zero again. The mark you made will never reach the pointer's zero, since the piston stop tool will stop the piston from reaching it's highest point. As the piston stops, you need to make a second mark on the damper from the timing pointer zero.

You now have two marks on the damper that both are equally as far away from the piston's REAL TDC, the former being too advanced and the latter being too retarded. However, the real TDC is in the middle of those two marks. Measure the distance of the two marks on the damper radius,
then mark the center and erase the two previous marks made.

Remember, you need to be on the compression stroke of cylinder 1#!

Now get your timing light, hook it up. Disconnect your vacuum advance can, and plug the vacuum hose going to your intake manifold. start the engine. Zero the timing advance dial on the timing light and set your ignition timing to zero at idle. Yes zero.
Now have a friend rev your engine up to around 3200 rpm and hold it there. Probably you will not be able to see the TDC mark you made on the damper on your timing pointer scale, but adjust your timing light's ignition advance dial, untill the damper mark shows at zero (TDC) on the timing pointer. Your friend can now let the engine idle. See the setting on the timing light ignition advance scale. That is how much advance your mechanical timing advance is giving you at 3200 RPM your friend was revving your engine at.

Let's assume you got around 22 degrees of mechanical advance at 3200. Now to get a full 30-36 degrees of ignition advance, you'd need to advance your idle timing between 8-14 degrees. What you're wanting to find out is how much ignition advance you can run without the engine pinging at any given RPM (how "steep" of an advance curve you can run). What you don't really need to be concerned about is initial timing. You need enough initial so the engine will idle correctly and start easily, but aside from that it really makes no difference.

When you're satisfied with these settings hook up the vacuum advance and go for a drive. If the engine pings, it means you're running too much advance under part throttle condition, and must re-adjust your mechanical advance curve, or adjust the vacuum advance (if possible). Ideally, you do not want to compromise full-throttle (zero vacuum advance) timing by altering your initial or total mechanical advance, once set to the desired 30-36 degree range.

Sorry for being so long winded. Hope this helps,

-Axle

[PHOENIX]

That helps a ton. I believe have a clear understanding of what I need to do now.
Ill be playing with this today after work.
Thank you very much, I know that must have taken a little time to write it up.

[swervin ervin]

Going one direction with the engine and using the piston stop will not work. All you are going to do is end up at the same place with the mark. After all, you are going one revolution. Reason you need to go clockwise one time and then go counter clockwise is you are trying to find TDC, which is center of the two marks. Going one direction and then taking the stop out and continue in the same direction is not going to do nothing but end right back where it was.

Think about this some and you will see what I'm saying.

[PHOENIX]

Yeah I understand it now. I just wasnt thinking about it correctly before. I was getting confused, happens pretty easy sometimes .

Anyway after pulling all the plugs and removing the alt. belt I was able to turn the engine counter clockwise without the bolt coming loose.

When we replaced the 305 with the 350 my father took the damper down to a gm dealer and compared it with a 350 damper. I guess they were the same and he just changed the timing mark on the 305 to match the 350. I am not sure what happened I was not there. Have been using that damper and mark ever since. Well tonight I do the tdc of #1 with clockwise and counter clockwise using the tdc tool. I measure between them and find out its different then my "original" mark. I still dont know if the damper slipped, but I am going to assume (for now) that it has not and time it with my new mark. Check out the pictures below for some info on what I found. The marks I made on the damper from piston stop clockwise to piston stop counter clockwise were about 6-3/8" apart, does this sound about right?
I then made the gold mark half way between them, 3-3/16". The black dot on the distributor base indicates the location of the #1 plug wire on the cap.

Also one of my spark plugs had some damage (from detonation?) to the top and had carbon build up (bad timing?). The rest of the plugs looked just fine. I am going to replace all the plugs tomorrow and time it. then play with it and see where it gets me. I have a good feeling about this now, should I based on what I am telling you here and looking at the pictures below?

[GMC Jim]

There has been numerous posts about ignition timing, distributor advance curves. Even some state that vacumm advance has nothing to do with total timing....Per GM specs... total timing is defined as the total of vacuum and centrifugal ... For instance ( Per GM Manual SD-100 "1971-80 Tune-Up Specifications"); the timing for my 1971 402 BB engine is 10-13 centrifugal degrees advance with a total advance ( centrifugal and vacuum ) of 28-35 degrees advance.... These specs are are ar 2000 engine RPM.
The GM shop manuls give the full specs for centrifugal advance curves.. The 402 curve ranges from 0 degrees advance @ 930 RPM to 30 degrees @ 4400 RPM.....
To accurately set these curves requires a shop that has a distributor machine and the knowledge to use it ....It would be next to impossible to set the top end of the curve at 4400 RPM with the distributor installed in the engine!.....Am sure that many will not agree with the methods outlined by GM.... They have manufactured hundreds of different models and types of engines. Many thousands in use the world around in autos, trucks, boats, industrial and other applications and the specs outlined by such large manufacturer should be the guide.... Of course; if the engine has been modified , then that is a different situation!!

[swervin ervin]

Eddie,

I think you got it.

I can't remember how far apart the marks are but it doesn't matter. If you followed the clockwise and counter clockwise method with the stop, center of these marks will be TDC, no doubt about it. The positon of the rotor with the new timing mark is correct. As you can see, you were way advanced with the old mark. No wonder you had pinging. You must have been set at 25 degrees of initial timing or so.

Good job my friend.