Timing

[Henry Cloutier]

Hi you all, I'm trying to understand engine timing as related to the 1969 Z28. I realize the factory changed for that year and put the vacuum advance source to ported where there was no advance once the canaster was reconnected following timing adjustment. My history comes from once initial timing was adjusted the vacuum advance was reconnected giving you an instant bump in the timing setting. We messed with the initial adjustment a lot to find the correct amount for the engine configuration as related to performance.
So in 1969 they removed the vacuum source at idle apparently for emission purposes. I've read the articles indicating revamping the vacuum source to the distributor to basically reconfigure the system to prior years. Jump to real time today and I'm running approx. 10 degrees initial and once the vacuum is re-connected there is no real change in timing until the throttle is advanced. I don't have a total timing number but will say the car is impressive with no indications of laying over with higher rpm's. I guess the actual question here what does it matter where the timing comes from as long as the engine gets what it needs?

Thanks as always!

Hank   

[Sauron327]

Navigate this entire site. My cars and others I know have and their cars run much better now. http://www.camaros.org/pdf/timing101.pdf. 302's  suck on the street unless you are winding the sh11t out of it. My 327 300 HP smoked my friend's Z till he hit high RPMs. I drove my friend's Z many times locally and on long trips. Z's are about pride and bragging rights. Or a buyer willing to pay. I recently bought 1971 4 speed Cobra Jet Torino nearly survivor and drove it home. 60K original miles. Pleasure to drive compared to a Z.

[janobyte]

Read JohnsZ tech article on timing, this site.

My 302 has always liked 38 degrees, full advance.

Good fuel.

[Stingr69]

The GM stock ignition curve was not set up for performance.  Reconfiguring the setup is very worthwhile.  You can do it yourself or have it done by others.

This is what I have in my 302 and it works great for me:

302 Ignition Setup

16 degrees initial
20 available in the centrifugal mechanism.
Added up to 36-38 total
Plus 16 more degrees in the vacuum advance can hooked up to manifold vacuum
950 RPM idle speed
Curve starts @ 1100 RPM and goes to 2600 RPM

Tricky part these days is finding the right vacuum advance canister to work with a 302 and full manifold vacuum.  302's do not usually pull enough vacuum (~9" vacuum) at idle to maintain a stable idle with a typical vacuum advance can and full manifold vacuum setup.  A NAPA "VC1810" or "B28" canister is ideal but many manufacturers have discontinued the part number.  A 302 will not pull much vacuum at idle and this is about the only canister that will work with full manifold vacuum at idle.  AC-Delco made a "D1312C" part number and Standard Ignition made a "VC177" so try to look for one of those.  This is stock part on old early 60's (pre-emissions) Corvette SHP engines.

Some might find 16 degrees in the vacuum advance is too much.  With the rest of the curve set up as above, The car might "chug" or occasionally a slight miss a bit at cruise with too much vacuum advance.  If you get that, you could limit the vacuum advance a little to stop it.  Maybe down to 10 degrees.  That said, this is not usually a problem with stock 302's. 



Hope this helps.

[Henry Cloutier]

Thanks for the replies! So truth be told even though I don't know what the all in timing number is since I didn't have a timing tape on the dampner I do know the total is up approx. two+ times the timing tab height. I believe the engine is getting plenty of timing. If I understand the replies (and yes I've read John Z's letter) it comes down to the curve of the timing as related to when the timing comes in?...

Thanks,

Hank

[169INDY]

I use a "DIAL BACK" timing light to walk the mark around the balancer during the timing cycle in lue of a tape applied balancer.

[Henry Cloutier]

I'll check and see if the one I have does that.

Thanks,

Hank

[Stingr69]

Use a cloth measuring tape to go around the damper.  Take the circumference you measured and divide by "10".  Measure that distance to the right of the zero groove on the damper and draw a line with a sharpie.  That line is at 36 degrees.  You can use that sharpie line to set the total timing while the vacuum advance is disconnected.  You need to rev the engine high enough so that the centrifugal stops advancing, then line up the sharpie mark with the zero on the timing tab.  Lock the distributor down and re-check it.  Reconnect the vacuum advance.

[Henry Cloutier]

Snap! I should have thought about this. I have timing tape I temporarily installed to locate 90,180,270 in order to adjust the lifter clearances when I first got the car. I can re-install the tape to locate 36 degrees and then remove the tape. Probably be the weekend before I can mess with it again and I'll report the results back!

Hank

[Henry Cloutier]

Ok, so I had a chance to get under the hood last evening. I discovered I had already marked the dampner at 36 degrees in preparation for this very test and then spaced it completely. At 10 degrees initial total timing was 34 degrees and just under 2500 rpm. I adjusted the initial up to 12 degrees and the total came up to 36 degrees at just under 2500 rpm. With the vacuum advance re-connected the total went up to 40 degrees at about 3200 rpm. I'll test drive in the next few days and report back.

Thank!

Hank   

[lynnbilodeau]

I have put several 480 distributors (stock dist in a 69 Z) on my Sun distributor machine.   My experience is that the total timing does not come  in until close to 4000 rpm (2000 distributor rpm).

Don't for a second think that your mechanical advance has topped out at 2500 rpm unless the distributor has already been modified.   When Stingr69 said "Curve starts @ 1100 RPM and goes to 2600 RPM" I am quite certain that is what he strives for when recurving the distributor.   That means you may have to modify the advance weight and experiment with different springs.  It is a really tedious job to do with the distributor in the engine.  However, if you are willing to put in the time (no pun intended) you CAN achieve the same results as I can with a distributor machine.  It is just much easier on the distributor machine.

Also, my personal experience is to limit total vac advance to 10 to 12 degrees.   Your results may vary.

[NAPA68]

Corvette Central has the B28 vacuum cans. I recently bought one for a 65 Corvette 365hp car. The can checked out just as designed. Many of the aftermarket cans do not operate like the B28........they just fit the distributor.

[Henry Cloutier]

Hi all,
So the test drive was outstanding! The engine pulled hard from idle to 6k rpm. No evidence of knocking at high rpm and still very manageable at lower rpm's. Perhaps it's the 3.73 gearing in the rear making the car more easy to drive down low. I haven't put the timing light back on but I can tell the engine is up on the edge; runs hard but I seriously don't believe she'll take more timing at this point. I checked Corvette Central and if I could get their p/n I'll order one for future installation if needed.

Thanks,

Hank 

[Stingr69]

Sounds good. 

If you hook up any other the vacuum advance canister to manifold vacuum source on a 302 you might notice the idle is a bit unstable.  The B28 canister will not add any wide open throttle power but it will improve your idle - lower the operating temperature while idling and make idle RPM more stable.