[John V.]

Anybody have specs?

What p/n Vac Advance used with the 1112928?

This was a '75/'76 HEI used with Auto Trans and 4 bbl 400.

[abefromen]

1973943

25 degrees

source: 1976 Pontiac MVMA

[John V.]

Thx.

Just to confirm, 25 crank degrees right?

Does it also give the centrifugal advance specs?

[abefromen]

yes, all readings are crank degrees

That dizzy number has the following centrifugal advance:

0* @ 1200 rpm
4* @ 1400
16* @ 4400

[John V.]

Awesome.

I'm a rank amateur on the subject but it looks like Pontiac significantly retarded the ignition advance nos. for this '76 HEI vs. what I'm seeing for the various late '74 HEIs.

I assume this was to meet the ever more stringent emissions rules for '75 & '76.

The reason I wanted to know the specs, I have a 6E22 dated 1112928 HEI that I expect to use for the '73 TA. Owned the TA from '78-'80 then repurchased it in 2016. Intervening years were very unkind to it.

Original engine was a WT code 455 with K65 1112203 unitized distributor. Engine and distributor are now missing. Plan is to recreate what it was in '78 and over the past 4+ years I've been able to acquire many of the pieces it needed to reach this goal. Key to me is the pretty complete '73 455 I acquired last Dec. It included the 1112928 HEI.

I have considered purchasing a correct 1112203 unitized but prices are in the stratosphere. Seems bizarre since the unitized was not offered for the SD and non-SD '73 TAs are comparatively low dollar cars. At least for me, a pricey distributor would be nonsense. It would be cool to run one but from a pure logic standpoint the HEI seems far and away superior and a small fraction of the cost. I planned to go that direction anyway, just a bonus that the 1112928 came "free" with the engine I wound up buying.

I need to get a better handle on the advance requirements for the 4 spd 455.

Here's what I think I should shoot for.

Mech Advance (crank degrees)
Start 0 deg @ 600 rpm
Intermediate 10 deg @ 800 rpm
Max 22 deg @ 2300 rpm

Vacuum Advance (crank degrees)
Start 6"-8" Hg
Max 16 deg @ 13.5" HG

Initial Timing at idle (crank degrees)
12 deg (14 deg max)

Total Timing at 2300 rpm cruising speed
12 deg + 22 deg + 16 deg = 50

Does this seem reasonable?

I've read that all Delco HEI's will provide 20 deg mechanical advance max. But from what I see of the '74 specs, max mechanical advance for the various HEI's shows 18 deg, 22 deg, or 24 deg. I have no idea how each differs, the intermediate and max rpms differ among them but that doesn't seem to be the reason for the different max advance specs.

You didn't label it but I assume the spec you show says the 1112928 provides just 16 deg MAX mech advance at a whopping 4400 rpm.

Is there a stop bushing that limits the max advance and can it be changed out? If so, where can I get an assortment of such bushings?

If my HEI is in good shape (not yet inspected), who is the best source for springs to modify the mechanical advance rate?

The original Vac Can is apparently the Delco VC1837 (ID# AR11). Specs for it show Max 25 deg @ 10.5"-13.5" Hg.

That seems way more advance than desirable for very good street performance.

Is there an original Delco Vac Can for HEI with the specs I need/want?

Can an adjustable Vac Can be User tailored for the desired start and max advance vac signal (" Hg) and advance degree?

Haven't defined the engine specs but it will be street driven and I expect significantly better than stock performance. I'd like to see about 400-440 HP. Will consider aluminum heads. Increased static cr. Probably the stock intake and a Q-Jet. Original log or repro HO exhaust manifolds.

If I can find the parts, I'd like to have the early year EGR hooked up (TA was built in January) with the solenoid timer, etc. I mention this because I've read that EGR would make a difference for distributor advance specs. But wonder how Pontiac's sneaky EGR set-up would impact that advice.

Comments, opinions, and advice appreciated.

[Rocky Rotella]

John,

Pontiac engineers did all kinds of things during the early-to-mid-1970s to offset the negative (performance) effects associated with everything they had thrown at them by GM and the federal government. That includes more stringent emissions regulations (as you said) as well as lower compression ratios, unleaded fuel, EGR, increased fuel economy, catalytic converter, etc. With that said, it's hard to correlate what they were doing with ignition timing

With your engine, assuming it'll be relatively stock, I'd shoot for an initial setting between 10 and 14 degrees, a mechanical/centrifugal setting of 22-24 degrees starting around 1,000 rpm and reaching full advance by 3,200 to 3,400 rpm, and add about 12-15 degrees of vacuum advance.

The HEI you have should make an excellent donor as long as it's in good reusable condition. Here are a couple articles (with videos) I wrote for Car Craft within the past couple of years on HEI mechanical advance curves and vacuum advance.

https://www.hotrod.com/articles/back...i-distributor/

https://www.hotrod.com/articles/quic...acuum-advance/

In the end, using your HEI as a core and swapping around advance parts and installing an adjustable vacuum advance should give you exactly what you need.

[John V.]

Rocky, thanks for the reply and great links. Already read thru the articles once.

Couple immediate questions.

What is the impact of getting full advance at 3200-3400 vs a lot sooner?

I thought my plan was a LOT more than “relatively stock” but I’m an originality guy so maybe I’m over-estimating my expected performance gain vs stock. 😁

If I shoot for more, how would that affect your timing suggestions?

I was anticipating idle at maybe 600-800. Is that logical?

[Rocky Rotella]

Glad to help, John.

Generally speaking, it's tough to get an advance curve that starts at a point above idle speed to come in much sooner than 3,000 rpm or so. There may be some exceptions based on the center plate design, but GM produced hundreds of variations and each is stamped with a three digit number. A start/end spread of 2,200 to 2,400 rpm is pretty common with what we have available today. With that, it's pretty easy to get one to start about 1,000 rpm and have it all in by 3,200 to 3,400 rpm. And from a full throttle performance standpoint, on the street having it come in quicker may not even be noticeable.

In speaking of relatively stock, I'm thinking a QJ, stock-type intake manifold, cast iron heads, 455 to 474-ish displacement, moderate camshaft. Up to or slightly more than 500 hp range. And 600-800 rpm idle should be no issue if you select the right cam.

[abefromen]

Originally Posted by John V.

You didn't label it but I assume the spec you show says the 1112928 provides just 16 deg MAX mech advance at a whopping 4400 rpm.

Yes, that is MAX mech advance for that distributor number.

Suntuned is a member here and is probably the one you should contact for advice.