C3: L46 spark plug heat range w/HEI

[Dan Pepper]

I've searched the archives, but haven't found a thread specific to this situation. I'd like a recommendation for spark plugs. The car is a '69 L46, stock except for a Delco HEI distributor. I'm not sure how the HEI unit will affect the heat range of plugs to use. I assume the distributor is correct for this application as it was installed/tuned by Dave Hurlinger of James Garner's AIR team fame, so I figure he knew what he was doing when he dropped it in over 10 years/5k miles ago.

The car has had a history of pinging as attested to by the previous owner - who used copious amounts of fake lead additives and "octane" booster - which I don't use - to ease the pinging. I've adjusted carb mixture, idle speed and timing (4 DBTDC) per the CSM, but don't have a dial back timing light to check total advance. Vacuum is steady at idle. It is also beginning to diesel on shut down. The car runs better on 92 unleaded cut 75/25 with 114 racing fuel, but there shouldn't be any reason it won't run on pump gas, right?

I removed the plugs today to find them dark, dry and very, very crusty - not sure if it's caused by too much heat (detonation) or all the additives the guy was pouring into the tank to try to get the pinging to subside - or a combination of both. The plugs I pulled out are Champion RJ12YC. On the assumption that the advance, vacuum and fuel mixture settings are right, I'm thinking of going to the factory suggested R43 plug (cooler?), but have read in the archives that higher compression engines do better with hotter plugs. What do the experts suggest?

[Duke W.]

Re: C3: L46 spark plug heat range w/HEI

AC heat range "5" is best for normal street and highway operation in SBs, however heat range "4" will work okay for TIs because the TI produces double the per spark ignition energy as the single point, so it will fire fouled plugs that the single point will not.

Your engine probably has ported vacuum advance, so idle timing is base timing only, which is severely retarded relative to the 25-30 degrees total idle timing that is best for engine efficiency. The retarded timing heats up the exhaust gas to promote oxidation in the exhaust, but it also heats up the combustion chamber boundary surface temps, which will promotes detonation and run-on after shutdown.

You can eliminate or mitigate this problem by converting to full time vacuum advance by routing the vacuum can signal line to a convenient source of full time manifold vacuum, such as the choke vacuum break line.

You will also need to change your vacuum can to one that is suitable for full time vacuum advance based on the engine's manifold vacuum characteristics, however, in order to make a recommendation I need to know the OE vacuum and centrifugal advance specs for you engine. They are in the AMA specs and CSM.

If you report them here, I can make some more specific recommendations.

Duke

[Dan Pepper]

Duke - Here are the vac and adv. specs for an L46

From the CSM:

Centrifugal:
C-4732
0 @ 1000
10 @ 1700
26 @ 5000

Vacuum:
C-3955
0 @ 7"
15 @ 15.5"

Note that the manual contradicts itself here by stating initial timing at 8 degrees BTDC under the CSM distributor setting chart, whereas in the tune-up spec chart it states 4 degrees BTDC.

Will the HEI factor into selecting a new vac can? Also, I chose Champion RJ12 plugs as that's all the parts house had in the AC45 equiv. heat range. What do you recommend for gap? THanks.

[Duke W.]

Re: Duke - Here are the vac and adv. specs for an

It's finally dawned on me that you do not have the OE ignition system, so all bets are off. Your engine is modified, and you have to determine the current ignition map before you can do anything.

I suggest you buy or borrow a dial back light and a Mighty Vac, as this is the most effective way to determine the map. With these two pieces of equipment I can document a timing map in less than two minutes, not including instrument set-up and tear down time, which is most of the effort.

1. Determine if the engine has ported (OE) or full time vacuum advance.

2. Tie up the centrifugal advance with a strong rubber band and use the Mighty Vac to pump down the vacuum can in 2" increments and the timing light to check timing at each vacuum increment and the vacuum can start and max points - deg@vacuum. Dial the timing mark back to the initial setting at each vacuum increment, so the timing you read is only that added by the vacuum advance.

3. Determine the centrifugal curve. Go to 6000 RPM to be sure you get the max advance. Determine where it starts and where it maxes out. Depending on the distributor, it may not start until as low as 500, so start the test from a very low speed, and determine the speed at which the advance increases. This is the start point.

I never assume than anybody gets an ignition map right for a given engine set up and fuel octane quality because in my entire life I have probably only run into two other guys who understand what an ignition map is and the timing requirements over the entire operating range from idle - through cruise - and WOT across the rev range, which is effectively an infinite number of operating conditions. Timing is added to the initial setting by a combination of vacuum and engine revs - two independent variables, and since the throttle is an analog control system, it has effectively and infinite number of settings.

However, this can be simplified by boiling it down to three basic cases - idle, typical cruise, and WOT across the rev range, which is simulated by disconnecting the vacuum can. Then you can characterize the WOT timing curve from the start to end point.

Proceed no farther until you baseline what you have.

Duke