rookie timing question

[Don W.]

I'm trying to set my timing on my 69 427/390 and achieve 36 to 38 degrees at 2500 to 2800 and/or WOT per Dukes' and Lars' performance timing methodology. My initial idle timing at about 650 rpm with the vacuum advance disconnected and plugged.

I bought an Innova digital timing light with rpm display and advance settings.

Question: if my idle timing is 14 degrees with the vacuum advance disconnected, and put the advance setting to 35 degrees and rev it up 3800 rpm; do I try and move the timing line back to 14 degrees or to the 0 degree mark?

Thank you,

Don

[Don W.]

Re: rookie timing question

I found the answer. Set the time light to the spark advance trying to achieve, say 30 degrees, rev and hold the engine at 3,000 rpms with the vacuum off and plugged and adjust the time mark to the zero line by rotating the distributor.

[Duke W.]

Re: rookie timing question

The OE centrifugal for Don's '69 L-36 HT400 is start @ 900, 26 @ 3800, and the initial timing spec is 4, so total OE WOT advance is only 30, which is well short of the 36-40 recommended by the Chevrolet Power Manuals of the era.

The reason for the less than optimized OE spark advance map is emissions. Keeping the timing retarded below optimum for most operating conditions increases EGT, which aids oxidation of HC and CO with the injected air, but it costs performance and fuel economy.

Unfortunately Don's distributor has been modified. Centrifugal has been limited to 20 and it's all in at 2800. The limited centrifugal needs 16 initial to get even 36, which yields too much advance at low to medium revs causing detonation on CA 91 PON premium.

The solution will be to modify the centrifugal to achieve 30 centrifugal. Then initial can be set at 8, then choose from available springs to get full centrifugal in ASAP without detonation. That and fixing the TH400 electrical kick down circuit as discussed in another thread should get Don's '69 L-36 dialed in as best as possible for our less than stellar octane rating CA "premium" gasoline.

BTW, I expected to find ported vacuum advance, but it's full time though I could find no obvious signs of modification. So if it was converted from ported to full time it must have been done by internally modifying the carb vacuum advance circuit. It has a 12" B26 VAC and idle behavior is 600 @ 14" in Drive, so the B26 is perfect.

Duke

[Don W.]

Re: rookie timing question

Here are my results from yesterday afternoon's timing events:
15 degrees at idle, 650 rpm, 35 degrees @ 3000 rpm, test drive = detonating
11 degrees @ idle, 600 rpm, 31 degrees @ 3000 rpm, test drive = ran fine

31-11=20; therefore I only have 20 degrees total centrifugal advance in my distributor.

Don