Spark knock at WOT

[REHolder]

Just replaced head gasket and upgraded to electronic ignition on my 77 Spider. Engine starts easily and drives well unless I put my foot thru the floor, at this point I get pinging and spark knock. Upon research, I discovered that this engine has 10:1 Pistons, High Performance Stage 1 Cams and a rebuilt performance head with larger Valves and porting by Vicks Autosport in Texas. Carb is a single Weber 32/36 DGV. Anyone have recommendations? There's got to be many out there with Fiat engines built like this one. I think I must be running too lean. If you have faced this issue, I would appreciate your help.

Thanks

[18Fiatsandcounting]

I have no direct experience, but in general there are several things that you can try, going from #1 (easiest) to #6 (hardest):

1. Retard the ignition timing somewhat. Most Fiats like a total advance of around 35 to 37 degrees at 3500 rpm and above, but that's with stock compression. You might try reducing to 30 degrees or so.

2. Run higher octane gas (if you aren't already), or use a can of octane booster in each tank. Some brands of gas have different anti-knock behavior even though the octane rating is the same, so you could try another brand of gas.

3. Install larger main jets in the carburetor, in particular the secondary main jet. This will richen the mixture.

4. Put in a cooler thermostat so that the engine runs at 175 or 180 degrees instead of the usual 190.

5. Add some cooling ducts to direct cooler (outside) air to the carb intake. This likely won't do much, though.

6. Change pistons for a lower compression, or put in different cams. You might also be able to tune the camshaft timing with adjustable cam pulleys, but that gets fairly involved in knowing how to set the cam timing to reduce knock.

A 10:1 compression isn't terribly high, and I would think that (1) to (4) above would help.

-Bryan

[REHolder]

Thanks for your suggestions Bryan. I have been running 93 octane from different brands. Do you know of an octane additive to try? Retarding the ignition makes for difficult starting. My next step was to increase the secondary main jet size to a 165. I had not thought about running a cooler thermostat but I had thought about running cooler plugs.

[davidbruce]

Just replaced head gasket and upgraded to electronic ignition on my 77 Spider.

You don't indicate if this condition was present before this work was done. Was the knock present before the head gasket and ignition change? If not I would be looking at the work performed to see what changed.

[Nut124]

I would measure cranking compression first. What RPM is the knock happening?

10:1 if true, should run fine. Your cam timing may be off causing excessive dynamic compression. This could cause knock at mid rpm range at WOT.

[18Fiatsandcounting]

Your cam timing may be off causing excessive dynamic compression.

Nut has a good thought here, and I've seen some aftermarket cams that had the hole for the pulley dowel pin drilled in the wrong position so that the cam timing was off by 10 degrees or so. You have to remove the cam and while a crude measurement can be done at home with a dial gauge and a cambox, a precise measurement would require a shop that specializes in cam grinding or measurement.

But, I'd start with the other things first. One question: What kind of electronic ignition? Is the timing advancing as it should? Roughly 10 degrees BTDC at idle, advancing smoothly to about 35 degrees at 3500 rpm or so?

-Bryan