BB detonation. Sugestions Please

[Jerry M.]

My 1966 427, 390 hp has detonation issues. Here is the scenario.

1. When cold runs great. 2. When on Turbo blue fuel runs great. 3. At full temp 200 degrees + - I get detonation when accelerating partial throttle. 4. At 3/4 to full throttle no detonation. 5. Timing is set at no vac 4 btdc. 6. Am currently running AC 44's have tried 43's not 45's.

Q's
1. DO I need less vacumn on the vac advance? 2. Differnt power valve? 3. Larger jets? 4. Additives? 5. Different location on carb for vac take off other than current base plate.

I do not want to go to a nice overlaping cam because of judging. It would sound too good to be a 390. Thanks

[Duke W.]

Re: BB detonation. Sugestions Please

According to the specs I have for the '66 L-36 the vacuum advance starts at 6" and is full in at 12". Run some tests with a vacuum gage plumbed into the car so you can see read the vacuum while driving. If it is detonating at greater than 6" of vacuum you can install the '63 327 can that starts at 8" as is all in by 16". This should be okay as long as you engine pulls about 18" at idle, which I think it should.

You should also disconnect the vacuum advance and test to determine if the detonation is still present. If so, you will have to slow the centrifugal advance with stiffer springs. Could be that someone installed lighter springs somewhere along the way, which is good for increasing low end torque, but increases the tendency to detonate at low revs.

The OE centrifugal and vacuum can specs are in the Chassis Service Manual and AMA specs, and you should check both against spec with a dial back light and vacuum pump.

Duke