Pre ignition problem.... after A518 install

[Paul G]

I have the A518 over drive in and working. As always, one solution leads to another problem, pre ignition or spark knock. Whats happening is; with the overdrive and converter locked the engine is cruising down under 2000 rpm now, 45 mph is @ 1600 rpm, it doesn't get to to 2000 rpm till 60 mph. It was always way over 2000 rpm with the old 727, never a knock issue. I am getting bad knock when I accelerate hard from around 1500 rpm, doesn't quit till I get over 2000 rpm. Then it runs just fine like before. It will cruise at 45-50 mph at 1600 to 1700 rpm fine as long as I stay out of the gas, no knock.

My distributor is set up to run 18° at idle, 36° total, all in around 1500 rpm, not using vacuum advance. I am looking at a few things. Adjusting the throttle switch to take out overdrive and the converter sooner, like around 1/4 pedal instead of 1/2 pedal where I have it now. The other thing, should I find tighter springs for the centrifugal advance? Maybe hold it back till 2000 - 2500 rpm, or will that be detrimental?

I want to try the throttle switch adjustment right away, but the engine is 220° right now. Gonna have to wait till tomorrow. Just got back from a 90 mile round trip cruise to World Market in Scottsdale. Other than the knocking issue, the A518 is definitely the way to go.     

[firefighter3931]

Paul, your motor has a healthy cam right ? If so, it sounds like you are lugging the engine at lower throttle speed which will definately cause it to detonate.

I would try some heavier springs in the distributor to bring the total in at say 2800-3000 and give that a try 



Ron

[flyinlow]

Paul ,

My engine does not like that either. Unlock the converter during the hard acceleration and it will be happier. Either a manual or vacuum switch would work.

Craig

[MSRacing89]

Not sure of your build; but you could try a colder plug and maybe 1º less total timing. 

[flyinlow]

I checked my records. 24* BTDC at idle. 36* BTDC at 2500 RPM. Vacuum  advance  restricted to a maximum of 8* above 10-12  inches of mercury. It will lug if you try to accelerate at slow speeds with the converter locked,unless you feed it the throttle slowly   .  I know you get use to the lower rpms quickly, but even modern cars unlock the converter if you accelerate moderately and they have computerized engine controls that can change the timing in ways our distributors would be hard pressed to copy.

[Paul G]

I discovered that the OD and TC were coming in about the same time. That was dropping RPM drastically at 40 to 45 MPH. The pressure switches I was using to control them were too close together in PSI set point. I am trying different pressure switches for the TC trying to delay lock up a little longer. Have not driven the car with the higher PSI switch on the TC yet. The car is still on jacks doing some other work.

I did  use a little heavier spring in the distributor. I changed out just one spring. That helped. Cant tell if it will hurt performance or not?