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Hope somebody out there has some hints, as I am facing an issue with my engine. But first things first, as I have to explain how it appeared:

I purchased the vehicle in 2010 from a dealer in Germany, the car came from California, but obviously underwent some partial restoration work by the previous owner.

During 2011 I have done many things on that car and slowly come to a really good condition. However, I replaced the old points distribution to a modern breakerless distribution by means of a Pertronix Ignitor III module with attached Flamethrower III coil. I also replaced all spark plugs and wires to have a consistent system.

At the same time when I did that, I replaced the oil with 10W30 mineral oil, and also filled up the fuel tank with some LiquiMoly carburetor cleaner.

That was it, I started the car and drove a while. Suddenly noticed a significant engine pinging sound when accelerating, which would disappear at higher RPM´s. I checked and tested, and the sound actually only appears when pressing the gas pedal very slightly, like 15-20% or so.

Now I am unsure about the origins of that sound. First I was thinking it is due to the changed ignition and returned it to the original configuration with points. The ping would still appear.
The only way I found up to now which is working to prevent that pinging sound is to adjust the ignition timing to 5-7° behind TDC. Of course this includes a loss of power, increase of fuel consumption and is far from the recommended range.

Finally I am asking you for help, and if anyone ever experienced a similar behaviour of his beautiful 425 engine?

Thanks a lot in advance,
Alex

Sounds like detonation. That happens when the octane is too low. Seeing the compression is over ten you may need to go to a higher octane rated gasoline. I run super unleaded (93 octane) with a couple of gallons of leaded 110 octane added every now and then and have had no problems. The lead also helps protect your valve seats if you don't have a cylinder head rebuild with hardened inserts.

Ahh, detonation... there is nothing quite like the sound of detonation first thing in the morning... NOT!!!

OK, let's cover the bases here. As mentioned in the previous post, make sure you are using the highest octane gas you can get. If it is not high enough then you will need to find some kind of way to increase it further. I am not sure which if any additives will actually do this, but adding 110 AV gas will certainly do so; just vary the proportion until you find what works.

Cleaning the carb may have done something to lean out the mixture which might explain why you suddenly began to hear knock. You might investigate this before doing much else.

Just for thoroughness I will discuss the ignition timing bit in greater detail. There are two classic points at which detonation will occur for the most part; at Wide Open Throttle (WOT) and at part throttle under significant load. WOT Timing is for the most part set using the distributor weights.

Here is an excellent guide on the total process of setting initial timing as well as the mechanical and vacuum advance mechanisms:

http://www.centuryperformance.com/ignition-and-timing-settings-spg-219.html

You are dealing with the latter case which I will address here. First check to see where your vacuum advance signal is coming from; if it is from a manifold point then you are fine but if it is from some point above the throttle blades on the carb then you should move it to the manifold. Next you might seriously consider a variable vacuum advance can which will allow you to back off the timing at partial throttle as you describe.

I run 92 octane gas in my 455 with aluminum heads and a decent amount of advance with no discernable knock (detonation or ping as you prefer). I started with around 10 to 12 degrees of initial advance and never bothered with the mechanical advance as I intend to go fully distributorless this winter so don't want to bother. I did however adjust the vacuum advance to get rid of exactly the same kind of knock you describe; for me it happened when pulling up a hill in my neighborhood that imposed a significant load at partial throttle. Adjusting the vacuum advance in combination with the initial advance allowed me to reach the aforementioned advance which works well enough for now.

The oil you use depends upon the status of the engine; if it is original then I would use a higher multi-vis oil in the 10W-40 range. Some go as high as 20W-50 in the summer. Synthetics are in general better than mineral oil as they last longer and are more resistant to breakdown and contamination. The exception is for break-in of a brand new engine; this should be done with mineral oil which is replaced after 50 miles.

You might also want to make sure you have sufficient ZDDP (a zinc compound which is required for flat-tappet cams to avoid premature wear and is esp. critical during break-in of flat tappet cams). Castrol now has a line of oils for classic cars that claims to have a substitute; given that Castrol is one of the best brands out there I am giving them a chance esp. since I intend to rebuild my motor again within a few years of the first round so I will be able to measure the wear and see if it actually works as advertised. My next cam will be a hydraulic roller cam which will obviate the need for such oils.

The 10W30 oil you used will work but is a tad thin at high temps and will increase the wear level. I suggest moving up to a 10W40 as mentioned above.

Thank you very much for the wealth of information. I will try out as soon as I can, and also check the consistency of that vacuum system. Usually I am driving with 95 or 98 Octance premium petrol and will consider filling with 100 Octane next time.

Not sure how the Octane rating compares to ours, but if it is anything like ours then you have a real problem on your hands since there should be no knock whatsoever. It should be resolvable using the info above, and it may well trace back to the carb, but at those octane levels you should not see any ping from the high compression ratio at normal operational settings.

Good luck, and report back what you find was the cause!