I need a little insight, maybe one of you has had this problem before and can save me some time. I have a 390 bored+.040, 10:1 compression, I'm running a edlebrock intake and carb, headers, K&N air filter, and a Crane cam HMV-272, and a stock dist. with Mallory unilite. The problem I have is I get spark knock, I have the initial timing set at 12 deg.btdc................any guide lines ? I know I will have to curve the dist, and probably block the total mechanical...........just wondering if anyone has had a similar engine build, and run into the same problem.


Charley Emmons
1967 F100 SB
390 C6 auto

 

Hey Charley, welcome to FTE! Just have a few questions. When does the knock occur, during light throttle, at cruse or under full throttle? Have you tried backing down on the initial timing? How tall of gear are you running? Have you looked at the color of the plugs? What type and heat range are you running? Does it have a vacuum advance? What do you mean by blocking the total mechanical? Has it always knocked or just started? Sorry for all the questions, just trying to get a little more info so we can help you out.

Barry

 

[font size="1" color="#FF0000"]LAST EDITED ON 02-Mar-01 AT 02:55 AM (EST)[/font][p]The base timming seems good, that's what I run, but mine was custom curved for my setup. My cranking compression is 195~200 and I run cheap gas and haven't noticed any knocking. I blocked off the exhaust crossover to help cool the intake and run a spacer under the carb. The temp below the carb is about 127 deg and the engine seems to run about 170 at standing idle. Also, try to keep the engine temp about 160~170, I use an Edelbrock water pump and A/C pullies that cause the pump to spin faster. Keeping the engine cool is very important. I use the Milodon 160 deg high flow thermostat and ground the water ports in the heads to match the intake for better flow. Run about 70% water and some Water Wetter or Problend's 40 below and see if that helps.
As a side note, a cold air intake can help a lot, even more so with headers making under hood heat. The stock air cleaner can be adapted to pull cold air from the pass side of the rad.

 

Hey, no problem with all the questions, I will now try to answer them. I get the spark knock under heavy load (accelerating hard)at about 2500rpm, this engine has always done this since I built it two years ago. I do have the vacuum advance on the dist, but vacuum doesnt come into play under heavey acceleration. The plugs look normal--light tan color. I am running 3.25 gears in the rear. I have tried to back down the initial timing, but it doesn't run or start as well. What "blocking the mechanical advance" does is limit the amount of travel for the weights on the mechanical advance. My cranking compression is about 165psi, and I run premium(ouch!)and I have a 185 deg. thermostat in it. Lets see I think that answered all the questions..........oh ya this site is great

Charley Emmons
1967 F100 SB
390 C6 auto

 

Thanks for bearing with all the questions. Like to try to find out as much as possible before I start shooting from the hip. Karl has some real good advice, these big blocks don’t like to run hot! Just curious if the knocking went away when you backed down on the initial, not that you should run it there. Sounds like you have too much timing coming in or you’re lean on full throttle. First thing I would do is change that thermostat for a high-flow 160-degree unit.

Now about the timing, I’d have a distributor shop set it up for your engine combination. Am guessing you would want around 24 degrees total in the distributor (crank degrees). With an initial of 12 degrees, that would give you 36 degrees total. As far as when it all comes in, it depends on the gas you run. With 92 octane, I’d shoot for around 3500RPM. This is just a ballpark, give the shop all the information on your engine and application.

Have you done any tweaking with the carburetor? Edelbrock offers a ton of info for tuning on their web site http://www.edelbrock.com/automotive/eps_sect2.html. Look up the Calibration Reference Chart for your model carb. and then follow the steps to richen up the mixture for the power mode. The chart will show the range from three stages lean to three stages rich. Go up just one stage if the knock is light, two stages if it’s more. Do all this after the distributor has been curved, thermostat changed, and it still knocks.

I hope this helps and gets rid of your knocking. Thanks for the accolades, this is a great site, great people!

Barry