Ignition Timing Map for L76 Stock Distributor

[Dennis K.]

Hi Guys,

Having timing problems with my 65 L76 (this is the 327 with the 30-30 solid lifter cam). Engine is 100% stock including distributor (except I have removed points inplace of mallory unit). I am having detonation issues on POT after setting initial as per spec (10 degress BTDC @ 750rpm w/o vac advance). I suspect now that my mechanical advance is not being limited/restricted properly so I continue to get too much advance as the revs climb. Does anyone have a map for the distrobutor for this L76 engine? My next step is to pull the distributor and put it on a distibutor machine and "dial it in." I just don't know where/when to get it "all in" and what it should look like across RPM range... figure I would play with springs and restrictor to advance flyweights. Thoughts?

Thanks for your help.

Dennis

[Duke W.]

Re: Ignition Timing Map for L76 Stock Distributor

You can get those specs and a lot of other data by downloading the '65 Corvette information from the GM Heritage site. It's also in the 1965 Corvette Shop Manual supplement.

Your engine has the most aggresive spark advance map of any OE Corvette engine - same as the FI engine with the 30-30 cam, but as built it should not detonate on premium fuel at only ten initial because the DCR is very low due to the late closing inlet valve.

It's possible that it has a different cam, but check the centrifugal curve against specs and you can slow it down with stiffer springs if necessary. Also make sure to have a 236-26 or B28 VAC and that it functions properly.

BTW, the centrifugal curve starts at 700, so you have to set the initial at less revs, at which it will barely run. A better way is to use a dial back timing light and set the total WOT advance at 34-38 degrees beyond where the centrifugal maxes out.

A common problem with the old Delco single points is the rubber advance limit bushing disintegrates and allows over advance. Like most vintage Corvettes, the dist. is probably in bad need of a disassembly, cleaning, thorough inspection, and "blueprinting".

Duke

[Dennis K.]

Re: Ignition Timing Map for L76 Stock Distributor

Thx Duke. Good info on heritage site! Interesting to see that the info on the heritage site differs from the owners manual regarding timing settings. As far as timing curve - this one is all in by 2350 RPM! Interesting.

Will have this on distributor machine over the weekend. Suspect the bushings are bad...

Cheers,
Dennis

[Wayne M.]

Re: Ignition Timing Map for L76 Stock Distributor

Thx Duke. Good info on heritage site! Interesting to see that the info on the heritage site differs from the owners manual regarding timing settings. As far as timing curve - this one is all in by 2350 RPM! Interesting.

Will have this on distributor machine over the weekend. Suspect the bushings are bad...

Cheers,
Dennis

Dennis -- I see from the '65 Corvette Shop Manual, that the 1111069 distr, shows no centrifugal advance @ 800 RPM, all in at 2350 RPM with 24 degrees; zero vac advance with the 1116236 can at 4" Hg; and 16.5 engine degrees @ 8.2" Hg.

The Delco Test Specifications manual DR-324S-2 is only slightly different, probably incorporating tolerances:
460 dist rpm (920 engine) for start of centrif. advance (0-2 deg distr). All-in centrif. advance at 1175 distr. rpm (2350 engine) with degrees 11-13 distr. (22-26 crank).

On the vacuum side, Delco says (for the 236 can): 3-5" Hg start advance; 5.7-8.2" for max adv.; gives 8.2 distr deg (16.4 crank).

[Dennis K.]

Re: Ignition Timing Map for L76 Stock Distributor

Thx Guys.

Pulled the distributor and ran it on the dist machine. I fully expected to find that the "restrictors" which limit the mechanical advance were deteriorated or failed... but that was not the case. I got 15 degrees of distributor advance (and nothing more) at approx. 2400rpm and higher.... this translates to 30 degrees on the crank, right?

So... according to the 65 Shop Manual I got from the heritage site... I should have no more than 24 degrees at 2350 rpm. This could cause my detnotation under POT and WOT. I will work to dial the mech advance back to 12 degrees on distributor (24 on the crank)

However... after looking at the 65 Shop Manual (see page 43) - I am a bit confused about what GM has published. The distirbutor table on that page indicates that:
1. Centrifugal Advance begins at 800 rpm
2. Max Degrees = 24 degrees at 2350 rpm
3. Vac Advance Begins at 4 In Hg with Max Advance at 16.5 degrees at 8.2 In Hg
4. Initial timing is 12degrees BTC @ 700rpm

The Centrifugal Advance Graph on that same page indicates the following (which seems to contradict the table - am I reading this right)?
1. Centrifugal Advance begins at 800 rpm - OK
2. Max Degrees = 24 degrees at 4,000 rpm - ???
3. Centrifugal Advance should be at approx 15 degrees at 1600 rpm - OK
4. Vac Advance - neither of the two curves seem to align with the table on the same page... and certainly not the vac can for my application!

I am a bit confused here after trying to make sense of the graph (as I was hoping to use this graph to select the correct springs) - not sure where GM is indicating crank degrees, distributor degrees, all-in advance vs. mechanical.

Thoughts?

Regards,
Dennis

[Duke W.]

Re: Ignition Timing Map for L76 Stock Distributor

Degrees advance on the dist. testing machine are in dist. degrees, and crank degrees are double, so 15 dist. equals 30 at the crankshaft. The same applies to dist. RPM; 2350 at the crank is 1175 distributor. You were not clear whether you were quoting RPM at the dist. or crank.

On a distributor maching the 069 should measure 12 @ 1175 max.

I believe some very early 365/375HP engine may have received '63 distributors that had 24 max at 4600 (cranshaft). Assuming you have an 069 distributor, (Does it still have the band? What is the number?) the vacuum and centrifugal specs are as listed, so ignore the graphs. On a dist machine the VAC should provied about 8 dist. degrees at about 8".

Most shop manuals have errors. They were published once and never corrected. The AMA specs are usually more reliable because it was an engineering document that was revised as necessary during the model year run.

The total centrifugal advance is determined by the slot length and limit bushing and how much the weights can retract, which can depend on the weight geometery. Some weights allow retraction to the "football" limit, but some won't go that far because the weights themselves contact before they touch the football.

What I'm suggesting is you may have the wrong weights. The OE weights might have a three digit number stamped on them. If so, post it and other 365 HP owners may be able to confirm if they are OE.

You should do whatever is necessary to limit the centrifugal to (crankshaft) 24 deg. at 2350, and run the initial in the range of 10-18. I'd try to run at least 14, and if it still detonates, slow down the centrifugal.

It would help if you posted all test and spec data in the same format as the specs listing in the manual so clear comparisons can be made.

Duke

[Joe C.]

Re: Ignition Timing Map for L76 Stock Distributor


All numbers here are in crankshaft degrees.

Total initial plus centrifugal should be no more than about 38 degrees. You may start with 36 degrees, and you'll like it better, and never mind what the specs say, is if you set your static timing to 16 degrees. Then limit your centrifugal to 20 degrees by welding up the limit slot. If you do this there is no need for the bushing. If you'd feel better using one, then get a bronze one. Don't use the hard rubber or plastic ones that come in most kits.

Finally, be sure that your vacuum advance is adding no more than another 17 crank degrees during steady state cruising, and idling.

If you use these specs, then there should still be a very large amount of "wiggle room" before your L76 detonates. If the engine still detonates on pump premium (92-93 PON), then check in this order:

1. Mixture too lean.
2. Valve lash set too loose.
3. Spark plug heat range too hot.
4. Hot spots due to excess carbon in chambers/piston crowns.
5. Cam phased too far advanced (the typical inlet centerline with the 346 cam is 110 degrees, meaning that it should be installed 4 degrees advanced).
5. Static compression ratio too high.

[Dennis K.]

Re: Ignition Timing Map for L76 Stock Distributor

Thanks Guys.

My distributor machine measures crank degrees, so I am sure of all of my work... just got tired of trying to make sense from conflicting data from Shop Manual. That being said, my vac advance is working perfectly and my problem is in the mechanical advance. Will get on it this weekend.

Cheers,
Dennis