Timing question

[rich weyand]

Did you mean to say "oil pump driveshaft?"   

Uh, yup.  My brain was right, my fingers mis-spelled oil. 

[OldsFan]

Rich, bd - the explanation of the oil pump driveshaft, etc. was great.  The gears of the rotor and cam will mesh before the oil pump driveshaft does...  Makes sense now.  A bit embarrassing to say I didn't know that, but hey, you learn something new every day, I guess.   

Either way, I'd much rather ask than break expensive engine parts. 

Because it was so cold out today, I didn't bother with re-indexing the distributor.  I need to get my truck into the warm garage to work on this further.  Regardless, after your explanation, I wanted to see how it was setup.  I found this link below with some pretty decent pics and a bit of description.
http://www.chevyhiperformance.com/techarticles/0609ch_installing_a_chevy_distributor/  So I am hoping to get my truck into the garage tomorrow and re-index that distributor. 

In the meantime, I have a related question.  The truck runs almost exactly the same as it did before.  It idles well, runs well, accelerates without hesitation, etc. and yet, the max advance is set to 32 degrees right now when it was around 40 before.  How can that be?  I really expected it to be less responsive and "slower", but it (thankfully) seems just about the same as before...  Do you think the knock sensor was "constantly" sensing knocking and retarding the timing, giving the impression that it was only reaching around 32 degrees?

Thanks again!

[rich weyand]

Hey, none of us was born knowing this stuff.  What's really stupid is not asking when you don't know, and dynamiting something expensive!

I think it could have been the knock sensor protecting the engine, but I don't have one of those so I don't know.  Someone else on here will know for sure.

My base timing is 16*, with 36* of total timing, with no knock sensor, and it seems very happy with that.

[OldsFan]

Hey guys - just FYI, I got a nasty cold probably from one of my family members (kids and teachers bring all that stuff home, )...  Anyhow, I got the truck in the nice, warm garage and as soon as I am feeling better, I am going to re-index my distributor, so don't give up on this thread just yet.

I did a couple of things, despite being sick.  I measured and realized that the HEI will fit past the cruise module.  Just to make sure, I loosened it up and confirmed that it swings past.  It is a pretty tight tolerance, but it does get past.

I also checked the timing with a new Craftsman timing gun (http://www.sears.com/craftsman-professional-digital-timing-light/p-00914066000P)...  I set the dial-back to 14 degrees, hoping to see the zero line up on the timing tab and tape.  It did, but it was really jumpy (anywhere from 3 before to 3 after).  When I set the gun back to zero, 14 lines up and pretty much just right.  I double checked with my old gun (no dials, just a light) and things line up solidly.  I am going to assume it is the electronics in the light and not the engine - but it would be really hard to actually rely on that feature...

[OldsFan]

I got the chance to pull and re-index my distributor.  When I pulled it, I decided "well why not just clean the whole thing up and put in a new vac advance can and...."  The vac can wouldn't hold any vacuum, so I replaced it with a known good one that I had from an Oldsmobile.

Well, after putting the distributor back in, firing up the engine and setting the timing, I was pretty pleased.  12 degrees at idle, 32 at 3K.  Seems to be happy with cleaned up setup.  The air cleaner housing drops back onto the carb, no problem.

That's when I decided to hook the vac advance can up.  So using full manifold vacuum, the advance goes from 12 to 40 degrees!  That's 28 degrees of advance which seems like a lot.  More importantly, I get a loud (enough to hear over the engine) "clatter" from the distributor.  It isn't really grinding but very fast clicking.  It varies with engine speed and is only present at high vacuum (> 18 degrees or so).

I think that the teeth of the advance plate on the HEI shaft are hitting the magnetic teeth on the pickup.  You can kind of see it here: http://image.stockcarracing.com/f/images/12565867+w799+h499+cr1+ar0/scrp_0412_02_z%2Bstock_car_distributor_installation%2B.jpg 
and here:
http://www.chevythunder.com/ECM%20HEI.jpg
 

Have you guys ever had a noise from an HEI distributor before?
Does 28 degrees of vacuum advance difference sound like too much to you?

Thanks!

[rich weyand]

GM HEI in stock trim should add about 15 degrees of vacuum advance at idle.  Sounds to me like something's broke or been modified.

[rich weyand]

BTW, the article in this thread has a great discussion of all these issues, as well as ported or manifold vacuum.
http://www.corvetteactioncenter.com/forums/c3-technical-performance/60830-ported-vs-manifold-source-vacuum-advance.html

[OldsFan]

Hey Rich...  That's a great link - covers a lot of different stuff as well as the eternal question of whether to use manifold or ported vac for the advance.

I realized you were right, something had to be broken or messed up.  Take a look at the picture.  The top canister is what I just pulled back out of the distributor and the one one the bottom is the broken original one.   You can see the two part numbers (AS 5 and 691 20).  Take a look at the actual advance "windows" that are cut, though.  The AS 5 has a much larger stroke at full vacuum than the original.  I think that's why it was giving such a ridiculous amount of advance.

Also, the orange arrow points out two tiny spot welds.  I think there is probably a block off or plate or something that was originally welded on there.  I am pretty sure the AS 5 one was off a friend's SBO from a few years ago.

Bottom line - I gotta get a new vacuum advance can.  The only reason I grabbed the one I had was that no local stores even stock a vacuum advance can anymore.  I can't get one before Monday.  Sure,  I could probably MacGyver something up to stop the advance on the AS 5, but c'mon - a working new part is better than a hacked up old one, right?

Well, I'll let you know how it goes.  I appreciate your responses!

[OldsFan]

I finally got a vacuum advance can that seems to work right (Echlin 1868) from Napa. I setup the timing again at 10 degrees and 30 degrees. All that went well.

I do have a couple questions, though and if you, Rich, or someone else knows - I'd really like to understand the answers:

1) The vacuum hose routing ("UHJ" is the label type) says I need 4 degrees BTDC, 550 RPM in Drive or 650 with the AC idle solenoid active.
Setting the initial timing I had the vacuum advance disconnected. To dial in the 550 RPM, though - do I connect the vac advance or not?  Sounds like a stupid question, but I want to make sure I've got this setup right.     If I set it to 550 without vac advance, it lands around 750 or so when vac advance is connected.

2) The vac line which was connected to the vac advance is kind of odd.  I noticed that it runs through the "distributor delay valve" and when cold, appears to continuously hold 20 inches Hg, even under load.  At some point after it warms up, it then switches to full manifold vacuum and rises and falls without the delay valve. That seems wrong - but - according to the vac diagram, it looks like the right hose.  Any idea why this is the case?  I included the vac diagram below if it helps... 

Any info appreciated...  Thanks!

[rich weyand]

They did a lot of crazy stuff with ignition timing beginning on January 1, 1968 for emissions purposes.  This was by law.  My dad specified his 1968 Ford Country Squire wen he ordered it to be built prior to December 31, 1967, so his 390-4V wouldn't have any of that crap on it; that's why I remember the date.

They retarded the timing spec to 4* BTDC, and ran it on ported vacuum, which at idle (where pollutants are worst) ran so retarded with no vacuum advance that it exhausted the charge still burning into the manifold.  They then used an AIR (Air Injection something or other) which ran on one of the fan belts to pump air into the manifold to use the still burning charge to burn up all the obnoxious stuff (mostly NOx) that got created when the charge was ignited under pressure.  This made manifolds/heads/engines run hot when left idling, and was not friendly to exhaust valves.  Some models used a selector valve that, when the engine started overheating, would switch from ported vacuum to manifold vacuum to give it a chance to cool down.  The vacuum delay gizmo was to hold the spark advanced when the engine was cold because they were running the engine so retarded that you would open the throttle on a cold engine and it would stumble until the centrifugal advance came in.

And none of this will work right if you still don't have the AIR pump and all the other gizmos on the engine.

So, the solution to all this is to go back to the pre-1968 setup, which was optimized for performance and driveability, though not emissions:

1) rip all that vacuum stuff off (vacuum delay valve and vacuum selector, if you have them), and connect the carburetor manifold vacuum port (driver's side on your Edelbrock) directly to the vacuum advance cylinder on the distributor.

2) set the base timing to 14-16* BTDC with the vacuum advance disconnected and the carburetor port closed off.  SBCs want 34-36* of total timing when all the centrifugal advance is in, and stock centrifugal advance on all Chevy HEIs of this period was 20*.  I run mine with 36* of total timing with no problem; some people go even higher, up to about 40*.  You can keep setting it higher until you get knocking, then back it off two degrees, but that will make you sensitive to the gas you buy -- if you get some marginal gas at a cheapo station, you may find you need to back it off more.  36* BTDC seems good no matter what kind of pump gas I'm running in it.  You disconnect the vacuum advance to basically make it easier to set the timing right, because the vacuum advance hunts a little bit, so disconnecting it just gives you a stable image under the timing light.  Otherwise you would set it to about 30* BTDC with the vacuum advance pulled in, which stock is 15*.  Simpler to just disconnect it (and block the carb port) and get a rock solid image to set the timing with.

[OldsFan]

Hey Rich.  I know you are right - I should just tear off all of the emissions stuff - but I am trying to keep my vintage emissions system in place for the guys out there who realize that early 80's emissions controls with A.I.R. systems are just the ticket for the ultimate performance in racing applications. 

Just kidding.  Truth is, I am thinking I may not pass the emissions test here in Delaware if I am missing the vacuum controls, etc.  What I am considering doing is just hooking up the vac advance to manifold vac and capping the existing line.  There are so many other vac-based things, though, I was not sure if it'd screw up some other stuff (like the tip-in valve whatever it is, etc.). 

I gave it a shot yesterday, though.  I have it set to factory settings (4 degrees BTDC, etc.) just to test and it actually doesn't run poorly.  I am going to increase the base timing though, because it tops out at 25 degrees right now and I think it is leaving some gas mileage and a bit of power on the table that way.

Anyhow - Thanks for your help on this stuff!  Much appreciated!