Hi, I have the timing set at about 12 intial, with 36 all in by 2,500, this is without vac advanced hooked up, I am using a MSD Billet dist,
What is the difference between bringing it all in by 2,500 or 3,000 ? should I change it so it comes in by 3,000?

Help me with my timing curve I am confused. Thanks

 

2500 is kinda soon to bring it all in. Id shoot for around 3200 - 3600 RPM to have it all in. Even though that engine has around 8.2:1 compression, the large chambers and lack of swirl make it have a tendency to detonate, and 36 degrees at lower RPMs with the vac advance on top of that will cause it to do just that.

The reason why 3600 RPM or so is usually around the area where you want to bring it "all in" and then keep it there is complicated, but that's usually the RPM that seems to work well for setting the point of peak mechanical advance.

 

And with the MSD Billet that is controlled by the spring rates I don't here any pinging with the motor the way it is now.

If it was not to detonate would you get more power by bringing it in sooner?

What about controlling how much vacuum advance the dist adds, I have a adjustment for this just not sure If I need to do it or not.

 

Anyone?

 

Timing is something that you need to have the right amount of. Too much causes the cylinder pressure to rise too fast, and you loose power to compressing a burned mixture. Too late, and the peak pressure arrives after the piston has gone a good bit past TDC and power is wasted. Really you need the correct timing. Even if it doesn't detonate it may still have too much timing. Lack of power and overheating are good indicators of this.

What I said above are some quick rules of thumb. For your engine, Id imagine something like 34-36 degrees at 3200-3600 RPM and maybe another 6-10 degrees of vacuum advance with a max timing of 38-40 degrees at P/T would work ok.

You will need to experiment a bit to see what works best. Something like a g-tech helps a lot here. The true timing may be quite different than the normal rules of thumb. For example my 350 with more compression and fast burn style heads makes peak power at WOT with around 28 degrees of advance. The way I tuned it was to datalog (my car has an OBD-II PCM) the time it took to go from 0-100 MPH and varied the WOT timing until I found what worked best and got the lowest time.

 

Ok so i'll try to set base at about 12, and then have 36 all in by 3,000. When I hook the vac advance in the timing tends to shoot up quite far, at what rpm should I check timing with vac advance hooked up? How much should it read about with vac advance hooked up? Thanks for your help.

 

Are you running a carb? You may find this thread useful
.
https://www.thirdgen.org/forums/carb...ture-full.html

You can easily add another 15 degrees (in some cases more) of vacuum advance at idle. You have to be sure that at your idle vacuum, the vacuum advance can is all in at that vacuum level. Otherwise you idle may hunt up and down as the vacuum fluctuates.
Here are a couple of other tech articles about ignition timing too.

http://www.hotrodders.com/forum/dist...1-a-59033.html

http://classicinlines.com/spark.asp

 

Yes i am running a 650 DP Carb, I have the timing hooked to ported vac at the moment. I am wondering how total I should let the vac add on top of the 34-36 total at 3,000.

 

Here's the ignition and carb tune I did just yesterday.
You can see my engine specs in my signature.
I just made the change from ported vacuum to full manifold vacuum too, which I should add, has made a night and day difference in some preliminary road testing.
Base timing set at 14°. Idle vacuum (with no vacuum advance connected) was 9-10" at 800 rpm. I also have a MSD distributor and have the two blue springs installed. This means I'm "all in" by about 2800 rpm. I have the total advance bushed to 18° maximum advance. This limits my total advance, without any vacuum to 32°. I'm keeping this value deliberately low until I get to the track and do some testing. I have 10.25:1 compression with iron heads so I'm staying conservative on the mechanical timing at the moment (I could go to 34° with these heads).
Now for the vacuum advance. I have a fully adjustable vacuum can. It allows adjustment for both vacuum set point as well as total amount. Since my idle vacuum was at worst 9", I adjusted the can to have all the advance supplied before that value. The total available was 22° so I kept it at that. This means at idle, I have 14° base timing plus all the 22° of vacuum advance for an idle total of 36°. Idle rpm went way up because of this as did my idle vacuum. It's now at 12". I have several adjustments with the Barry Grant carb that allow me to reduce idle speed. These include the idle stop screw and the secondary idle stop screw (which all Holleys have) as well as the "Idle-Eaze" feature, which is unique to BG carbs. I brought my idle speed back down to 750-800 rpm.
To answer your question, you can have all of the vacuum advance added on top of your total mechanical plus base timing with no problems. What you'll find is that the only time ALL of the advance will be available will be when you're cruising at 3000 rpm at part throttle. The vacuum will have to be high enough to pull the vacuum advance in and the rpms high enough to bring all of the mechanical advance in. At part throttle, the engine will tolerate, or even prefer, plenty of spark lead. Once you mash the gas pedal, your vacuum will drop and the advance will return to whatever amount you have dialed in with respect to rpm.
If you haven't already, invest in a vacuum gauge. Plumb it into the full manifold vacuum port and observe the vacuum values at idle and as you cruise. Obviously get enough vacuum line to allow the gauge to brought into the car as you test.
Hope that helps.

 

Thanks, I plan to mess with it this week, I am waiting on a timing light from E Bay to show up, I picked up the old Sears unit that I have read good things about. I Plan to use about 12 Intial, and 36 total all in by 3,000 then try the ported vs full vac.

 

I haven't re-read the posts. Do you have an adjustable vacuum unit?

 

It can be adjusted with a part inside of the Dist.

 

Ok, I checked the timing yesterday, I have the two Blue Springs and I think the Blue bushing in the Dist.

My Two Tachs I use for my Small engines would not read right so I had my Son sit in the car and handle the rpm's for me.

What I got after the car was warm was about 12 with no Vac and at idle around 700 RPM. I then checked it at 2,500 and I had just under 30, Then at 3,000 it was all in around 34. I checked 3,500 and 4,000 and it stayed the same.

I then checked it with vac adv hooked to ported vac and I got a max of about 52.

With Manifold Vac I was up around 20 at idle and still a max of about 52.
How does these Numbers Sound? After I verifie that the pointer is correct ( I think it is but just want to check) I'll check it again, I may bump the int to 14 which should put my total at 36 correct?

 

Your timing curve sounds very similar to mine with the two blue springs although my bushing (don't recall the color) limits the total mechanical to 18°. I'm keeping my total (excluding vacuum) deliberatetly low until I get to the track for some testing)
Your total combined advance of 52° is at what's argueably an accepted standard for Gen 1 SBC. I say that knowing that all engines are not created equal.
Bumping your initial to 14° will put your total available at 54°. Keeping in mind the previously mentioned standard, I'm running 54° and feel nothing unusal at cruise. 14 initial, 18 mechanical and 22 vacuum is my max if I'm cruising at 3000 rpm. That's about 60 mph in drive (not OD). Seems okay.
I mentioned this before but it bears repeating: do you know the specs of your vacuum unit? Amount of vacuum needed to initiate and what's needed to be all in? Total available? I've set my advance to be all in about 2" below my idle vacuum. I'll also mention again that my idle vacuum went up considerably with the switch to a full manifold source. It was 9" and has risen to 12". I've made changes accordingly.
Sounds like you're well on track.

 

I didn't check the Vac again after I set the carb, and the dist was hooked to ported vac at that time. I had 17 for Vac.

My Vac advance is the stock MSD Billet unit, I am unsure of the specs, I can limit the amount with a plate inside the dist.

Right now I am allowing full vac advance.

 

From what I'm reading on the MSD website, they don't make an adjustable vacuum unit for their distributors. Just a fixed unit (of which I can't find any specs other than it gives 16" of advance) although there's a mechanism which allows it be locked out. This may explain why there's no allen headed screw inside your canister.
This isn't to say you don't have an aftermarket part like I do.
The disc with the serrated edge in the picture is what sets the amount of advance. The allen screw inside controls the set point.

 

at WOT there is no vacuum so there would be no vacuum advance added to the total mechanical timing.

total timing is just that... if the car likes 34 degrees at WOT this all the total total (mechanical timing) it will ever see at or need WOT.


vacuum advance adds timing at idle & part throttle (where you have none, low, & moderate loads)

so if you see 40 degrees at idle as you open the throttle and accelerate the vacuum advance diminishes (because vacuum drops until you reach "0" at WOT) until all you have it total mechanical timing.

make sense?

 

On the Billet model they have a lock out option but they also have a 4 way adjustable vac advance, I don't think it controls how sensitive it is to vac just the amount that it allows. Its in the instructions with the Dist.

 

FRMULA88 yes I do understand that part of, Now I am stuck on if I should use ported or Manifold Vac.

 

I'd suggest that unless you get a fully adjustable unit, one that allows for adjustment of the vacuum level at which all the advance is in, or by luck, the unit you have is fully deployed below your idle vacuum level in gear, then full manifold advance may give you trouble trying to establish a stable idle under all conditions. This is the primary reason I found that all lot of people abandon the idea and go back to ported.
Personally, the change in the drivability of my car more than offsets the cost of the unit and the time it takes to tune it.
Good luck.

 

It's the total timing available at cruise that's the concern when running a lot of vacuum advance and the typical 34-36 degrees of mechanical along with it. This would be seen at crusie rpms that have all of the mechanical deployed as well as all of the vacuum advance. 52 degrees is not uncommon and often desirable.

 

I think the way my DIst is now I have all of the vacuum advance that is available. I can reduce it four different ways if I add the adjuster plate in there. Next Wed Me and a buddy are going to confirm TDC. then I'll play with the timing a bit more.

 

Confirmed TDC and the Point is spot on, I'll Advance the timing a touch and try it out.