Cascade Crew

[redlava]

So far when adjusting my timing on my car I have been just adjusting the initial timing but I want to learn more about the cars total timing

I have been doing some reading and was wondering if I understand this correctly.

When you are setting the timing with a hei distributor you could want to disconnect and plug the vacuum line to the distributor. When setting the total timing you want to warm up the engine and have a helper hold the engine at 3000rpm. At this speed the mechanical advance will completely advance so,

Total timing - mechanical timing advance = initial timing??

The vacuum advance will add even more timing on top of the total timing correct?

How much advance do you normally gain mechanically?

Would the vacuum advance would add on to the total timing?

How many degrees of total timing is a good place to start, 34-36 degrees?

Then I will want to increase it until the engine pings under load, and back off a hair?

That is all I can think of at the moment. Thanks for the help.

[Twilightoptics]

Yes, set timing with the vacuum pod disconnected.


Total timing - mechanical timing advance = initial timing??
Total Timing is max vacuum advance plus max mechanical.



The vacuum advance will add even more timing on top of the total timing correct?
Correct. It will add timing based on vacuum. Manifold vacuum you get all the timing at your highest vacuum (idle, cruise, coasting), and no additional advance when there is little to no vacuum(hard acceleration, wot). Ported vacuum works a little different. You get little to no advance, then as vacuum goes away the advance comes in, then as vacuum approaches none, you get no advance.



How much advance do you normally gain mechanically?
Completely depends on what distributor you have. You can change the weights and springs to change your timing curve and give you more or less timing. You really need to just see what it does with a dialback light.



Would the vacuum advance would add on to the total timing?
Depends on the pod. Typcially as low as 6 and as much as 15 roughly. Some pods are even adjustable. All the motors I've had ran best on manifold vacuum. One had too much advance with the pod so I adjusted it down and it never pinged and ran great.



How many degrees of total timing is a good place to start, 34-36 degrees?
34-36-38 are all worthy Total Mechanical Timings to start with. I hit the dyno and had NO difference between 32-34-36-38º. So I leave mine at 34.



Then I will want to increase it until the engine pings under load, and back off a hair?
Best way is to tune by MPH at the track. Or just set it and leave it. The getting it closer to ping is more for EFI stuff where you can adjust at any given point. For you, any change to the timing changes the whole curve, not just one point.

[redlava]

What is a dialback light? Is that one of the more expensive timing lights with the screen on the back? Any recommendations?

[rjmcgee]

I disagree with Paul on alot of that.

Total timing is base plus mechanical advance. Vacume unhooked from pod, has nothing to do with your total advance. Stock HEI's have alot of mechanical advance, I think mine was like 24* of mechanical advance. That only lets you run 12* base which probably isn't enough for your combo. Not sure the exact specs on your cam, but it sounds simular to my comp 274. The only way to limit the amount of mechanical advance from the factory HEI is to weld the slots up inside under the weights and then file them back until you get the desired amount of advance. Springs only change how fast the advance comes in, not how much comes in. I almost bought a MSD distributor just for the reason of being able to adjust the amount of advance. I got mine down to 10* of mech advance which is perfect on my car. I run 26* base + 10* mechanical advance for a total of 36*. After that is done then hook your vacume can to manifold vacume. I have an adjustable can that I set to about 15*, that gives me about 40* of advance at idle which really helps smooth up the idle. As soon as you go WOT the manifold vacume goes to zero and you mechanical advance comes in and your at 36*. Cruising down the highway at 2500 rpm's you have high manifold vacume which gives you all three, base + mechanical+ vacume. On my car that gives me right at 50* - 52* which is perfect for light load part throttle cruising.

One thing to consider though, stock starter isn't going to cut it when running 26* base timing, gear reduction mini starter is a must.

Figuring out how to set this up probably was the best thing I ever did for my car as far as throttle response, idle quality, and fuel mileage.

[rjmcgee]

[redlava]

I have a Mallory HEI distributor

http://www.jegs.com/p/Mallory/752904/10002/-1

Are you saying that an aftermarket HEI has about 12 degrees mechanical advance? I will need 20 plus degrees of initial timing?

I went and got the specs of the cam if that helps at all

Advertised Duration : 262°/268°
Duration @ .050 IN/EX : 219°/227°
Gross Valve lift IN/EX .468''/.489''
LSA and ICL 112/108
RPM Range: 1400-5800

[rjmcgee]

[redlava]

The reason I was researching this to begin with is that I have the initial timing set at 10 degrees but I noticed that it was starting to run much smoother at 12-14 degrees. I originally thought this was too high. Since I can't afford a new timing light at this time would this work?

If I get a timing tape I can find the mechanical advance. By setting the total timing at 3000rpm and then checking the initial timing at idle. I could then do total - initial to get the mechanical advance?

Also what would the highest initial timing you would run on a stock starter?

[Twilightoptics]

My timing is 28º at idle. Runs the best there.

Depends on who you talk to. Some people call TOTAL timing the TOTAL amount of timing possible. While others deam it strictly mechanical.


http://www.gnetworks.com/v4files/barrygrant/Ign%20timingwithimages.pdf

Read : TOTAL TIMING

"Typically, early small-block Chevy and most small-block
Ford engines with a compression ratio of 9.5:1 will, in most
cases, respond well to 36-degrees of ignition timing (initial
timing plus the mechanical advance), plus a further 10-degrees
from the vacuum advance mechanism, making a total timing of
46-degrees."

[rjmcgee]

[rjmcgee]

[blue89]

Could I get a lecture on vacuum advance? It would seem that you would need more advance at higher rpms and less at idle conditions. So it seems that connecting the vac advance to the manifold would be the wrong way? That would give you max advance at idle and you would loose advance at high rpm?

[Twilightoptics]

Depends on the high RPM you're talking. What kind of load on the engine?

3k cruising needs lots of advance, but your mechanical is way up also.

So you add your mechanical to your vacuum and you get your current total.

Vacuum is just that. Doesn't matter what RPM you're at, it's all about load. Most engines pull more vacuum at cruise than at idle, so your advance would actually be HIGHER at higher rpm cruise, than at idle... if set on the manifold vacuum port.

[chevymad]

You don't want vacuum advance at wot. In fact neither ported or full vacuum have any vacuum at wot. Ported vacuum will have no vacuum at idle.. high vacuum at part throttle, then go away at some higher throttle opening. Manifold vacuum will be high vacuum at idle, then gradually taper off with throttle opening. The factories switched to ported vacuum because retarded timing burns cleaner at idle. However, manifold vacuum will make the idle steadier and smoother. So with a performance motor and cam, hooking to manifold vacuum will give you better idle and run cooler.

Lean cruise mixtures require more time to ignite then the rich wot mixtures. Also during cruise you want your cylinder pressures to peak for the amount of throttle opening, yet not ping. Vacuum advance accomplishes this. You get alot of advance at low load conditions, thus avoiding pinging. For mileage purposes you always want your timing just below the ping threshold.

Best power is not made at that same amount of timing however. Every engine will want a slightly different total timing. It will depend on fuel mixture and head design. Fast burn/vortec heads light the charge very quickly, thus they only need 32-34 total. Old school heads have a slower flame front requiring more like 36. Those are just general ideas, every motor is different. Best power will be accomplished when max cylinder pressure occurs with the crank at 90*. In theory anyway. You have the most pressure along with the most leverage. So you have to find the point where combustion pressure and crank match that.

[d00012]

I like to fancy myself an intelligent man, but when I suspected my car's timing is off, and started to read up on changing the timing, maybe even advance it a little, my brain shut off. I don't get it.

[blue89]

I just went through mine a bit. I did it like this.

I disconnected the vacuum advance diaphram with the engine running and plugged it. Using a dial back timing light with the engine idling I found that it was at 13* advanced. Then I revved it up to about 3-4k RPM and found how much more timing it had. It went up to 29*. This advance was caused by the mechanical advance. So 29-13 is 16* total mech advance.

Then I let it idle again, taking note that it went to 13* again. I then plugged the vacuum advance diaphram back in to the manifold (not vacuum port on the carb) and found how much advance the diaphram added. It went from 13* to 29* again (at idle) so another 16*.

mech =16
vacuum = 16
max advance I wanted is 45*

so 45-16-16 gives my base timing of 13*. So mine was set correctly. Did that help at all?

[Twilightoptics]

You should run more like 34deg total mechanical.

Take the vacuum off. Rev it up. Adjust till you get 34.

Then plug the vacuum line in and go.

Back it off 1 degree at a time if it pings, or try a higher grade fuel.

[aaron_sK]

When I did the Merc I took Paul and Rod's advice and kinda worked it backwards to give the engine and cam what it wanted.

The cam in the FE really likes a lot of initial, like 30* idling at 1K. The vac pod was adding around 10* at 18 inches idle vacuum, so I set my initial at around 20-21* which gave me 30* at idle (off manifold vacuum).

With a stock Ford dizzy, it ads 26*, all in by around 3K. And 46* total timing = no bueno. After messing around with bending the tabs in the dizzy to narrow the advance slot, I dropped it down to around 13* of mechanical.

So now the engine gets 30* at idle, 21-34* going from 1K to 3K at WOT, and around 45* at light cruise.

Don't know if that helps you guys, or even applies to what you're doing, but it worked pretty well for me.