65 (&'64 ?) T.I. versus Points timing spec diff's

[Wayne M.]

After reading Jack Corso's thread bellow on his C3 400hp TI, I had to see what's called for in my '65 documents. I found the following discrepancies between the Owner's Manual (first edition), the '65 AIM (K66 instructions sheet), and the '65 Shop Manual Supplement.

Advances are quoted in nominal degrees (no +/-) with vac advance line disconnected and engine running at specified RPM:

350hp L79: Owners M; 6 deg BTDC @ 750 rpm (points)
AIM (K66); 10 deg @ 650 (mag pulse)
Shop M; 8 deg @ 650 - 750 (points)

365hp L76: Owners M; 10 deg BTDC @ 750 (points)
AIM (K66); 12 deg @650 (mag pulse)
Shop M: 12 @ 750 - 850 (points)

375hp L84: Owners M; 10 deg BTDC @ 850 (points)
AIM (K66); 12 deg @ 650 (mag pulse)
Shop M; 12 deg @ 850 min.

In all cases, the TI system calls for 2 degrees more than the Owner's M and the same as for the Shop Manual, except for L79 which says K66 requires 4 degrees more than the Owners M and 2 deg more than the Shop M. And in all cases the K66 degrees are specified at a lower rpm than the points systems.

Do some of these discrepancies have to do with compensating for the transistorized ignition characteristic of centrifugal advance decreasing a half degree (distrib degrees, or 1.0 degree crankshaft) for every increase of 500 rpm beyond 3000 rpm ?

I wonder if the '64 K66 sheets in the AIM show a similar pattern ? I do have the K66 AIM sheets for 1966, and the '66 Shop Manual, and everything seems consistent between points and mag pulse, by then.

[Loren L.]

Midkiff's weekends

We need to find Midkiff something to do on the weekends to keep him out of the garage. Your head begins to hurt before you finish reading his questions.........

[Duke W.]

Re: 65 (&'64 ?) T.I. versus Points timing spec diff's

An electronic characteristic of the TI is to retard a couple of degrees at high revs relative to points. This has been discussed before, and is likely the reason for the slightly greater TI initial timing specs.

Initial timing is just a tuning parameter. Total WOT timing (initial plus full centrifugal) for points should be in the range of 34-40 degrees, so add 2 for TI, The total centrifugal for the '65 L-79 is 30 deg. @ 5100. Run it as high as you can in this range without detonation. Otherwise back it off until the detonation ceases.

Corvette News Vol.8 No. 2 has a table of tuneup specs. Initial timing for L-79 is listed as 10 nominal, 8-12 range. The data does not distinguish between TI and points.

Three degrees retard from the optimum timing for max power will reduce power about one percent, so the engine doesn't take much of a hit if you have to back off initial a few degrees to prevent detonation.

Duke