Chevy Nova Forum banner

1 - 20 of 35 Posts

·
Registered
Joined
·
3,895 Posts
Read your plugs.:yes:
:poke:



Really, it's typically best to run as much advance as possible. Your engine will basically tell you how much you will ultimately have to run. Every engine is different. Different cams, compressions, intakes and carbs (just to name a few of the more obvious) will all effect what type of timing the engine will like.

Just experiment.

I rarely run any vacuum advance (because most of my builds barely make vacuum) so I set them with as much initial timing as possible and use the stop bushings to adjust the total timing and the different spring combo's to get the advance curve I need.
For example, the last tune I did liked 18 initial and 40 total....all in by 3000. It was a pump gas BBC 468 (right at 10:1, solid cam, Team G int) in a truck with a 3500 converter. Knocked out 4 degrees and added a little octane (110 Purple) for a 175 shot.
 

·
Registered
Joined
·
2,146 Posts
It really depends on what your engine likes. Are we talking mild street performance? hot street/strip or all out race? I usually start off with 32-34* for mild street performance engines that run pump gas to 100 octane. I try to keep the initial timing under 16* initial so the starter doesn't "grunt" when starting. For hot street/strip engines I run them at 36* total and 18-22* initial. Again there are variances. For an all out race engine 36-40* total. If I can I lock out the timing on an all out race engine. Usually I have no problems with starting. Most engines I've done have the start retard function in their MSD. Just my .02
 

·
Premium Member
Joined
·
15,966 Posts
there is a lot of variables in Ignition timing.

As some say X° this is not always the case.

It is a work in progress. In days long ago, these tuning practices were a secret. A speed shop with a Sun Dist machine could make a car do wonders.

Now that we have dist we can change. All we need to do is understand what we are doing.

It can and to some be a very demanding obstacle.

The main rule to follow, for ultimate performance, drivable and idle quality there is no set number.

There are base line setting to be safe. These bases lines can in some cases be dead on, in others a total disaster.



All these setting will be without Vac advance hooked up.
Initial timing! At this point this is all we are dealing with!
The initial timing at idle speed around 12°-16°, you pic this setting based on the idle quality, in gear and in neutral.
(as in my engine with a wild cam, I require 26)
Very important that the engine is warm, and carb air adjustments have been made. Move the timing around till this parameter is found.
Now that we are satisfied we have a good initial timing.
We are now looking for the total timing for full throttle performance.

We write down X° at X idle speed is the initial setting.


So we check the total! Bring the rpm up till the full advance has been achieved.
make note of RPM at full advance and the advance amount Z°.
the base line is for most all engine s 36°

But lets say your getting 40° or as low as 25°

We then know the dist has A° advance built in.
You take the total and subtract the initial and we get the mechanical advance amount

- Z°
____
=A°


Now lets say we have a total advance of 40°
And the timing is high from detonation or poor engine performance.
But we have a initial of 16° and we have 24° advance in dist!
So we want to change the timing total to 36°

we want to chnage the mechanical advance to only 20°
16° (X)
+20° (Z)
_____
Total 36° (A)
We must find a curve kit to accomplish this for the model dist we have. Some dist come with adjustable mechanical advance.

Now, lets say you don't have total full mechanical advance till 4000rpm, and your cam has a power starting range of 2600 rpm. This is what the weight change or springs do, to change when the mechanical advance comes in total. In most cases we end up modifing the weights to do what we want.

Once you have all these variables set, then you hook up your vacuum advance, reset the idle speed. If the vacuum advance causes detonation, you may need a adjustable one to reduce the amount of timing applied by this unit. Most are in the 45°-52° range at full vacuum.

hope this helps
Al
 

·
Premium Member
Joined
·
7,732 Posts
Set it to 36-38 total and experiment from there. You shouldn't exceed 40. Don't worry about initial until after you find your max total. I run 38 degrees total with my 406. You need to learn how to measure total advance.
 

·
Super Moderator
Joined
·
9,776 Posts
Discussion Starter #8
Yeah, I gave it a quick shot the other day, but I couldn't get a solid reading when the RPMs went up. I'm using a Snap-On MT1261a digital dialback light...



...and the MSD paperwork starts out with "Note: do not use digital or dial back timing lights", so...might be OOL for the moment.
 

·
Premium Member
Joined
·
8,405 Posts
Yeah, not all timing lights work with a MSD. I think if you shouldn't advancing and retarding the initial until you can read the total.
If you are at 12 and your total is 38 and you advance the initial to 16 then your total is 42 which is too much.
 

·
Premium Member
Joined
·
771 Posts
I have my timing set at 12* initial currently (at idle, vacuum advance disconnected)

Where should it be, and how do I figure out what the best setting is?
If you mainly use it at the track, the initial timing is almost irrelevant. Since you'd be launching where the full advance would be coming in, it doesn't matter. On the street, it's a little different. The car will be very jumpy and run hotter. If you're concerned about emissions, then colder timing is the answer. As long as you burn decent fuel, the correct inital timing is whatever makes you happy. The distributer has mechanical advance anyway, just keep the max advance under control.

Initial timing is overrated
 

·
Super Moderator
Joined
·
9,776 Posts
Discussion Starter #11
I think if you shouldn't advancing and retarding the initial until you can read the total.
Good point...I'll see if I can borrow a light and get that nailed down first.

The distributer has mechanical advance anyway, just keep the max advance under control.
This is a street/strip car, so it has to be at least somewhat well behaved driving around town.

More news when I figure out what the max is.

Thanks everyone!
 

·
Registered
Joined
·
3,895 Posts
Go to Sears and get one of their Craftsman dial back lights. I've got three and they've worked flawlessly on just about every type of ignition system out there. MSD 6, 7 (all versions of both), Digital 6 and 7 (all versions of both), Jacobs and Mallory Hyfire are just a few that come to mind.
 

·
Premium Member
Joined
·
771 Posts
Go to Sears and get one of their Craftsman dial back lights. I've got three and they've worked flawlessly on just about every type of ignition system out there. MSD 6, 7 (all versions of both), Digital 6 and 7 (all versions of both), Jacobs and Mallory Hyfire are just a few that come to mind.
I have one from Sears and works just fine. I had a Mac light, but dropped it and cracked the whole end of it. :eek: The Sears metal deal has been treating me fine for years. The only thing I miss is the brightness of my old one. MSD's play games with all kinds of electronics. I had problems on a dyno once.
 

·
Premium Member
Joined
·
15,966 Posts

·
Registered
Joined
·
3,895 Posts
And naturally...I had a generic Sears light that I bought back in...77 maybe...and I sold it at a yard sale last summer. Figures.
That's sorta how I ended up with three.:yes: Friends with nice tools find themselves in a hard way and know I'll give them a little more than the local pawn shops.:D The only difference is sales are final.:devil: Most times I'll let them borrow whatever they need down the road because they are very respectful and know the rules....Bring it back clean and in the same (if not better) shape then when you got it or forget ever borrowing anything else.:rolleyes: I've had to repo a few tools in the past and cut off a few "Friends" but all-in-all it's been fine.
 

·
Super Moderator
Joined
·
9,776 Posts
Discussion Starter #18
I did some tuning today, and it's running better.

First step was hooking up the vacuum gauge and setting the idle mix screws. I found they were way lean (which explains why it had been threatening to stall at idle in gear) so I fixed that. I ended up with a 1000 rpm idle (in park) with 13 in/hg vacuum, which was a lot higher vacuum than I was expecting. (BTW Al, I used the big vacuum gauge I bought from you, worked great...)

I made some timing changes next. Initial is now set at around 15*, total is around 38* all in at 3000. (This is with my Snap-On light, I still need to verify this with another light...)

I hooked up a mityvac to the advance can (at idle) and pumped up 20* of advance with that. That seems a bit on the high side, but the RPMs were going up, so I think the mechanical was kicking in some too. I'm going to see if I can dig up the part number on it and look up the specs, I *think* I want more like 10-12* from the vacuum. I may try limiting the can travel a bit if it's really 20.

On the other hand...the butt-dyno is reporting very favorable results, and I'm not hearing any pinging on Shell 93. I took it for a spin around the block after the changes, and it's a lot more driveable than it was earlier, very smooth, snappy, and packs a nice punch. :yes::yes: Not that it was that bad before, but...it's even better now. I also dropped the idle down to 900 or so, which works out to a nice rumbly level. Me likey...

Overall I'm pleased :devil:
 

·
Premium Member
Joined
·
771 Posts
I hooked up a mityvac to the advance can (at idle) and pumped up 20* of advance with that. That seems a bit on the high side, but the RPMs were going up, so I think the mechanical was kicking in some too. I'm going to see if I can dig up the part number on it and look up the specs, I *think* I want more like 10-12* from the vacuum. I may try limiting the can travel a bit if it's really 20.
I'm not so sure that the vacuum advance and the mechanical advance add on top of each other. I think it's overlapping, whichever comes on first or both. You can't get 10* from vacuum and add 15* from the mech for 25*. Street car? I guess if I had to rely on pump gas, I'd hook up the vacuum and not worry about the total timing. That is if you're not looking for a street race! It's a whole lot easier to start the engine without all the initial timing, I'll tell you that.
 

·
Super Moderator
Joined
·
9,776 Posts
Discussion Starter #20
I'm not so sure that the vacuum advance and the mechanical advance add on top of each other. I think it's overlapping, whichever comes on first or both. You can't get 10* from vacuum and add 15* from the mech for 25*. Street car? I guess if I had to rely on pump gas, I'd hook up the vacuum and not worry about the total timing. That is if you're not looking for a street race! It's a whole lot easier to start the engine without all the initial timing, I'll tell you that.
The vacuum advance can rotates the pickup relative to the distributor body. The mechanical advance rotates the stator on the shaft relative to the shaft itself. Two independent mechanisms, so they do add up.

What I meant was that at 900 RPM idle, there's no mechanical advance involved yet. As I put some vacuum on the advance can with the mityvac, the timing changed (because of the vacuum can) which made the RPMs go up. A side effect of the RPMs going up is that the mechanical advance started to kick in, which effectively ruined my measuring what the max vacuum advance is.
 
1 - 20 of 35 Posts
Top