427/390 timing question

[Gary Wilkerson]

There's been alot of discussion about inital timing vs. overall timing in a small block. I think th general accepted amount is 36 deg's., a little more if it'll take it.
At that point the inital timing may or may not be at the recommended inital setting.
If I have that understanding correct, does the same hold true for a big block?

Gary

[Duke W.]

Re: 427/390 timing question

SB distributors have 24-30 degrees centrifugal with the lower amount typical of the early 327s and the 30 degree number beginning with the '65 L-79 and carrying through on all 327s through '67.

As a rule you want 36-38 deg. total WOT timing on SBs, which is the sum of initial plus full centrifugal, and getting the centrifugal in as quicky as possible is advantageous for low end torque, but the rate of timing advance is generally limited by detonation with today's available fuel octanes, so the OE centrifugal curve is sometimes as aggressive as you can get.

Total idle timing should be in the range of 20-32, with the higher overlap cams (less idle vacuum) needing an amount toward the top end. Total idle timing is the sum of initial plus full vacuum, plus maybe a few degrees of centrifugal for engines with centrifugal curves that start below idle speed - like 64-65 L-76 and L-84.

The same basic rules apply to BBs, but I think peak power is achieved in the range near 40 degrees of WOT timing. Total idle timing should be in the same range as SBs.

The ignition map for your particular engine is in your CSM and AMA specs. As a starting point you can increase initial timing until total WOT timing is about 40 degrees as long as it doesn't detonate. Then you can experiment with quickening the centrifugal curve until you reach the detonation limit.

Duke

[Duke W.]

Re: 427/390 timing question

SB distributors have 24-30 degrees centrifugal with the lower amount typical of the early 327s and the 30 degree number beginning with the '65 L-79 and carrying through on all 327s through '67.

As a rule you want 36-38 deg. total WOT timing on SBs, which is the sum of initial plus full centrifugal, and getting the centrifugal in as quicky as possible is advantageous for low end torque, but the rate of timing advance is generally limited by detonation with today's available fuel octanes, so the OE centrifugal curve is sometimes as aggressive as you can get.

Total idle timing should be in the range of 20-32, with the higher overlap cams (less idle vacuum) needing an amount toward the top end. Total idle timing is the sum of initial plus full vacuum, plus maybe a few degrees of centrifugal for engines with centrifugal curves that start below idle speed - like 64-65 L-76 and L-84.

The same basic rules apply to BBs, but I think peak power is achieved in the range near 40 degrees of WOT timing. Total idle timing should be in the same range as SBs.

The ignition map for your particular engine is in your CSM and AMA specs. As a starting point you can increase initial timing until total WOT timing is about 40 degrees as long as it doesn't detonate. Then you can experiment with quickening the centrifugal curve until you reach the detonation limit.

Duke

[Gary Wilkerson]

Thanks Duke! I'll proceed as you suggest *NM*

[Gary Wilkerson]

Thanks Duke! I'll proceed as you suggest *NM*