[81chevycamino]

So when I initially installed my engine I dropped the distributor in, set the initial timing to 15-16 and fired it up. It has run fine ever since. Never pings or stumbles and runs strong. I had thought the max advance was 34 degrees but messing around today I have no idea what I am doing.

I have the vacuum advance hooked to ported vacuum and while revving the engine slowly it seem to just keep advancing. I need to clean the damper marks but I feel like it just keeps on advancing the faster the engine goes until it’s at like 50-60 degrees. I have very little understanding of the recurve procedure or how you make it “all in” at 34 degrees. I have read so much I think I’m even more confused now. Do you check “all in” with or without vacuum advance hooked up? Can someone in simple words help explain what I should do please?

 

[36 sedan]

Mechanical timing curves are checked with vacuum advance disconnected.
What you are seeing is the mechanical and vacuum advance doing their jobs as you slowly accelerate, disconnect the vacuum and you'll see the mechanical advance curve by itself.

 

[BogiesAnnex1]

You can’t set this without the engine under operational load. Without the load on the crankshaft it takes very little throttle to net out a lot of RPMs. In this circumstance, there is a lot of vacuum resulting from the not much opened throttle, that brings up the amount of vacuum advance which would not happen under load as that takes a lot more throttle opening for the RPM so manifold vacuum is much less thus so would the vacuum advance be much less.

So in operation of moving the vehicle there would not normally be a condition of high RPM generating a lot of centrifugal advance combined with a high vacuum that resulted in a high amount of vacuum advance.

Bogie

 

[81chevycamino]

Mechanical timing curves are checked with vacuum advance disconnected.
What you are seeing is the mechanical and vacuum advance doing their jobs as you slowly accelerate, disconnect the vacuum and you'll see the mechanical advance curve by itself.

Ahhh...so with the vacuum disconnected what should my mechanical advance be limited/adjusted to? I see more people list 34 degrees @ xx rpm. How is this decided?

 

[55 Tony]

Ahhh...so with the vacuum disconnected what should my mechanical advance be limited/adjusted to? I see more people list 34 degrees @ xx rpm. How is this decided?

A lot depends on your engine build.

 

[ExcellenceAuto]

You can install a timing tape on your balancer, and use a conventional timing light.
From the front, under the hood you can rev it and watch the mark by yourself. Have somebody in the car looking at the tach, take note of the RPMs when the timing stops to rise. Set it at 34 at its highest point, see how it drives from there. You have another 2° to go up after that if it doesn't ping. Others will be able to add to this and explain it more clearly I'm sure!

 

[2old2fast]

You can’t set this without the engine under operational load. Without the load on the crankshaft it takes very little throttle to net out a lot of RPMs. In this circumstance, there is a lot of vacuum resulting from the not much opened throttle, that brings up the amount of vacuum advance which would not happen under load as that takes a lot more throttle opening for the RPM so manifold vacuum is much less thus so would the vacuum advance be much less.

So in operation of moving the vehicle there would not normally be a condition of high RPM generating a lot of centrifugal advance combined with a high vacuum that resulted in a high amount of vacuum advance.

Bogie

Sorry but that was clear as mud !

 

[2old2fast]

If it ain't broke , don't fix it . 81CC , timing has been written about ad nauseum , start reading .....

 

[chasracer]

You said it was running fine. So as mentioned above, disconnect the vacuum advance, plug the vacuum connection and check the timing as you bring up the RPM. Yes you need to mark the dampener or have a tape on it or make your own tape. When the timing stops going up, that is your total mechanical advance. It could be 32, 36, 37, 34 - doesn't matter. If the engine is happy, you're happy. Now, if you want to move it to a particular number and see if it is better you can do that and if it doesn't then you can return to the current setting. Don't get caught up in "everyone says it should be this number" that's BS and remember that it's just a possible starting point to find out what your engine combination wants.

 

[RWENUTS]

Deciding your motor’s timing takes trial and error.
There is no set numbers to go by.
On a new motor I’d just set my initial at 10 degrees. Get it hot and go for a drive to my favourite location. Slightly uphill, in high gear I’d put a load on it. Squeeze the pedal down about 1/2 way. I’d advance the timing a little at a time until it would ping. Pinging sounds like rattling a rock in a tin can. Then I’d park, check the timing, write it down and retard that setting 2 degrees. Test once more to be sure it didn’t ping. Make sure the dizzy was locked down and back to the garage. Shut it off, wait 5 minutes and restart. If it cranked hard then once running retard timing a couple more degrees.
My best initial was when it didn’t ping and cranked over easy.
Don’t forget! Vac advance is plugged for these tests.
I usually aimed for 32 total all in by 3000 rpm.
If my best initial was 18 then I knew I needed 14 mechanical from the dizzy. I’d mod the dizzy to get that number. Test some more and if all was well then I’d add a little vac advance. Usually about 10 degrees more. Made a limiter to control the amount.
I’d leave all my work alone. Learned early on that not much to be gained by further fine tuning.
Mind you I never had a concerne running at higher rpm down the highway Or really anywhere. I liked to run my motors where they were happy. Usually 28-3200 rpm. And I lived at higher (3500 feet) elevation.
So you just gotta experiment.

 

[81chevycamino]

Thank you everyone for the advice. It has been running great and never pings even in OF with the converter locked up and giving it some gas on inclines.
I now understand figuring out the mechanical, but should I be doing any tuning on the vacuum advance or just let it ride?

 

[RWENUTS]

The vac advance is the last thing to fine tune.
Have you seen this?

Where should I set the timing on my performance engine?

Bad-Ass Racing Engines specializes in building some of the baddest street and race engines around!

www.badasscars.com

And some info on hei mods.

Hot rodding the HEI distributor

www.crankshaftcoalition.com

 

[2old2fast]

I have to chuckle sometimes when I read , you need this many ° , or that many ° , settings are , at best , ballpark . Consider that " newer" computer controlled engines run far better , cleaner, smoother & make more power over all because the onboard computer constantly adjusts the timing & fuel trim to optimize engine performance i. e. There is no single optimal setting , to be optimal under all conditions , means the settings must change !

 

[BogiesAnnex1]

Depends mostly on compression ratio and combustion chamber design. Lower ratios need more advance though under SMOG regulations they got less because there was a specific target of lower NOx production which goes up with combustion temperature so steps were taken to reduce that by both reducing compression and backing off timing. This does not apply to such an extent on modern EFI engines with multi-stage converters, so it is a time period from the early 1970’s to mid 1990’s built engines where you will encounter this. So the amount of optimum ignition advance will vary with this and chamber shape.

The other large factor is combustion chamber shape. Modern chamber shape that uses the Ricardo heart shape wedge chamber for in line valves are much better and faster burners of the mixture than older chamber shapes which as with reduced compression ratio were intentionally designed to lazy burners to get the flame temperature down for less NOx production. For a hot rod engine using old 1972ish through 1995ish heads the engine will hunger for more ignition advance. For modern chambers with compression ratios around 9 to 1 dyno testing shows a range between 32 to 36 total degrees as delivering best power. This is from a standard SAE test that loads the engine to maintain a crankshaft acceleration rate of 300 RPM per second. The cam is also going to affect the amount of timing advance, this is trickier in that at lower RPMs it will want more advance than a mild cam then switches to less at higher RPMs. All of this is a dependency on mixture density trapped in the cylinder. It is important to understand that compression ratio and compression pressure are not the same thing. Compression ratio, at least the static, is simply a measure of physical volumes. Compression pressure is a measure of how much mixture gets into that volume. Since true gasses are compressible which really means their density thus pressure is flexible
based on another bunch of criteria. The simple end result of mixture density is the result of throttle position, induction reversion of the flow, and the time available to take a breath.

So all of this and more leads to a range that works. For your specific if you can’t afford a ton of dyno time then you have to hit the street and experiment.

Bogie

 

[Bonneville462]

Best to disconnect that vacuum advance and leave it disconnected. Then keep adding initial until it has trouble starting and back off two degrees.. Next check for full throttle ping with good gasoline. If it pings take some of the mechanical off by limiting the advance..

 

[81chevycamino]

Best to disconnect that vacuum advance and leave it disconnected. Then keep adding initial until it has trouble starting and back off two degrees.. Next check for full throttle ping with good gasoline. If it pings take some of the mechanical off by limiting the advance..

So I gather all adjustments are made without vacuum advance. Anything that happens when vacuum advance is connected is okay?

 

[RWENUTS]

Yes!
Leave the vac advance plugged off until you’ve found the timing your motor likes. Then plug it in and adjust to fine tune.

 

[75gmck25]

The primary purpose of vacuum advance is to increase timing for better gas mileage at cruise RPM. As suggested, disconnect the vacuum line and tune the initial and mechanical advance. Gas mileage may suck without vacuum advance, but you won’t hurt the engine.

Then reconnect the vacuum line and while cruising at about 50-60 mph, listen for pinging during light throttle acceleration. If there is significant and frequent pinging you need to get a vacuum can with less advance. Many stock HEI cans add as much as 20-22 degrees, but there are others that are 10-15 degrees, or adjustable cans that go even lower.

FYI - some vacuum advance specs list crankshaft degrees (e. g., 20), while others list distributor degrees ( half of crankshaft, or 10 degrees).

Bruce

 

[timothale]

When I was an engineer at Ford I was buying a 460 van or truck, sometimes both every year. during the years when ford retarded the cams for NOX, The first thing I did was a double roller chain and gear set , setting the cam 0, reset the timing. The Ford vacuum advance cans were adjustable by pulling off the hose and inserting a 1/8 allen wrench to adjust spring tension. The advance curve plotted by running on a dyno and slowly increasing rpm and finding the best timing was not always best for an engine when on a fast acceleration. Air flow dynamics during aceleration is not the same as at a fixed rpm adjusting the timing.

 

[81chevycamino]

Thank you all.
So with vacuum advance plugged I was at 20 degrees of initial. I dialed it back to about 14 degrees and all in it was at 30. If I understand that means I have 16 degrees of mechanical. Does this sound right? No issues with starting, but can’t road test it just yet.