Played with the timing this weekend,and decided to try full manifold vacuum instead of ported vacuum.After setting the timing and fooling with the adjustable vacuum can for best performance,the engine runs cooler and smoother! No more smelly fumes at idle and car has better mileage.At first I had to much timing at cruise and it would surge at light throttle.I set my initial to 8 btdc vacuum disconnected.Vacuum can set to 14 deg max advance.At idle with vacuum connected timing at 22 btdc.Engine idles great and starts up quick.Is this to much timing at idle? Aloha's

 

A Norwegian friend of mine once told me to not worry about your timing at idle 'cause nobody ever drives at idle.

 

It's not too much at idle but the important question is with the mechanical initial at 8 what is your total mechanical at 3,000 RPM? Should be 36.

 

I disagree with that philosophy. My engine spends a decent amount of time idling after a track session, or right before an autocross lap, and even at a stoplight on occasion. I prefer it to be thermally efficient so it idles as smoothly as possible, and cools down as quickly as possible.

It's also nice when the timing is accurate at 2000 RPM, and not just at 3000 RPM WOT.

Just my particular philosophy.

 

I agree with setting your total mechanical advance to 36 and then add vacuum to make sure you don't exceed 52 at 3000

 

I read about 30 deg mechanical advance all in by 2800 rpm.14 deg vacuum advance=44 total.If I increase the initial anymore I will get surging at cruise rpm.Cam is advanced 4 deg,could this be why I cannot increase the timing anymore? I set the vacuum can to come all in at
13hg as my idle vacuum is 15hg, 2 inches below idle vacuum recommended per HEI timing paper.Does this sound correct? Since switching over to manifold vacuum car runs much better and mileage has increased along with no smelly fumes at idle.I could adjust the vacuum can for less advance if you guys think that would be better than what it is set up for now.....Suggestions?

 

If you're not experiencing any kick-back when the starter is engaged, then don't worry about too much timing at idle.

 

what octane gas are you using? It should be 36 at 2800-3000 rpm. Is the vacuum can adjustable so you can reduce the 14 to maybe 10?

 

The main thing to be concerned with when setting timing is to have 34-38 degrees of advance, all in by 2,500-3,000 rpm. If your mechanical advance is working properly, it should be giving you somewhere around 20 degrees of advance, so lets say you have the mech adv springs adjusted so that you have 36 deg of advance at 2800 rpm (I am using the numbers from my car, yours may be different, but the math will still work out the same way). If your mech adv is giving 20 degrees, that means your initial timing at idle, with no vacuum advance will be 16 degrees. Get it? 16+20=36. Now, plug the vacuum advance can in, and, if attached to manifold vacuum, adjust it so you have 32 degrees of advance at idle. 16+16=32. That will give you 52 degrees of total advance (16+20+16=52)on the highway. Back off of the initial timing if you experience pinging or knocking under load, and back off the vacuum advance if you experience surging on the highway. The only thing you'll have to monkey with will be adjusting when the vacuum advance comes in. And remember, you will need to make carb adjustments after setting up timing like this.

My -1980 'vette has a relatively stock engine (Edelbrock Performer, Holley 600, block hugger headers and 2 1/2" dual exhaust with Maagnflow mufflers, no cats) and this is how my timing is set up. 36 degrees, all in at 2800 rpm. At idle, 16 degrees with vacuum disconnected, 32 with vacuum attached to manifold vacuum. It runs great on 87 octane gas.


Scott

 

Much thanks! Our ethanol free gas is just 87 octane.Will try to dial in more timing.
On a side note,after going to full manifold vacuum my fuel percolation after shutdown has ceased! How's that .....

 

I got 15 at idle and 36 at 2800, thats 51 total on full vacuum and it idles and runs smooth and cooler than any other way.

 

Good Numbers!

 

See how the numbers work for Tim H's ride? His distributor is giving him 21 degrees of mechanical advance, so 15 at idle with no vac adv. 15+21=36. If his vacuum advance can is giving him another 15 degrees, then he's idling at 30 degrees, and cruising down the highway at 51. At idle with vac adv: 15+15=30. On the highway with vacuum advance: 15+21+15=51.


Scott

 

Now I don't know if im doing something right or not!!!!!

 

Depends on compression ratio. Old school timing is full throttle at low rpm in top gear if it pings slightly its on the money

 

Will this work for a L-48 Compression engine with 3.08 gears? Cause I could never get mine to ping..

 

Same here,I know when it had too much timing it would surge at cruise and pop thru carb at times. Alohas

 

Most of all you really need a dial back timing light to read the numbers right.
But if you don't, un plug the dist from any vacuum source and set you initial timing to the top the the timing tab which is 12, now plug the dist into full all the time vacuum and adjust the idle down to what you like usually around 700 in gear and you will notice the engine runs alot better. As soon as you can get the right timing light is when you set it all perfect.
See I set my advance with the timing light at 36@ 2800 and the idle timing just happens to fall on 15.

 

surging at cruise might be a lean mixture or too much vacume advance.

 

Dat's a roger! Going to play with the timing tomorrow.
.Happy Thanksgiving to all!