Engine Knock

[David H.]

Not to kick a dead horse, but in my research for engine rebuilding I came across this article on the Corvette Restorer CD, 1961-62 Edition that has cleared a lot of things up for me, so thought I would share my find.
This makes it very clear and simple how to deal with the octane issue and the spark knock issue.
Hope it helps others with this delemma.

[Duke W.]

Re: Engine Knock

How about a synopsis of the article?

Duke

[John H.]

Re: Engine Knock

I came across this article..

What article? Got a reference for the title or the issue it was in?

[David H.]

Re: Engine Knock

Well, that's interesting. I attached the two page article to my post and even tested to be sure they could be read ok. Now they dont appear to be with the post. I will look it up and post the issue it was in.
At the top of the article it says 11104.txt.
It is in Volume 11, number 1, summer 1984.
Article is called "Engine Knock Remedies

You should read the entire article but to summarize:
Dont use additives or high octane fuels to try to correct for the knocking.
It states that oil in the combustion chamber will aggravate the knocking. Make sure the cooling system is in good condition including the water jackets, radiator and fan clutch.
Another overlooked item is the plastic advance limiter sleeve in the distributor. Make sure its there on most pre '75 Corvettes.
The problem is not the compression ratio but more the compression pressure. Compression ratio is only a comparison of two volumes. Compression pressure is the amount of air fuel actually compressed in the combustion chamber at the instant before ignition. They state that the hotter and leaner the mix the more it wants to knock. Most 327 high compression engines were delivered with two (2) steel shim head gaskets under each head. At overhaul time or valve grind time, a favorite go fast trick was to use only one shim head gasket because fuel quality was not a problem. GM produced a thin .016 inch gasket that was used a lot on rebuilds then. (pn 3916336) If you have these on your engine now, they are definately going to cause a knock. It states that they have found that a thicker teflon composition gasket such as the Fel-Pro Perma-Torque works well. Or GM # 14011041 for small blocks with 4" or 4 1/8" bore works well but not on smaller than 4" bore. Apparently this lowers compression enough so that any small block will run fine on regular leaded or premium unleaded pump gas.
They comment that if you retard your timing, water jacket temps go up aggrevating the knock problem you are trying to fix.
Other tips include: dont use a thermostat hotter than 180 and never leave the thermostat out. Use the coldest spark plug you can find like AC-44 in small blocks and AC-43N in big blocks. Check the heat riser valve for sticking closed or partially closed.
Even tho this article was in 1984 it seems to still be appropriate for today.

[Duke W.]

Re: Engine Knock

Who was the author?

The advice is still mostly okay in a qualitative sense, except for the spark plug recommendations, and as long as you have a full time vacuum advance you have to retard timing at least 4 deg. below the minimum recommended setting before it will exacerbate knock. And if you extend the logic, it's one reason why I recommend that ported vacuum advance be changed to full time as long as a properly selected VAC is installed, which may be different than OE.

Twenty-five years of research - data gathering and analysis - on my part and others have pretty much quantified the CR that you can run on various OE engine configurations with today's fuels, and it's not that hard to take the measurements required to know the true CR and run it through a calculator. (see article in the current issue of The Corvette Restorer)

....lots of information about all this is in the archives.

Duke