Originally Posted by sbnova
HI my sbc 350/400 hp has a pinging problem. I currently have a mallory unilite distributor with vacum advance. I tried setting my total timing at 33,34,35 degrees and it did not help. I went back to base timing of 6,8, and 10 degrees and no change. The fuel has been changed to fresh 93 octane tried octane boost no good, I am thinking distributor I am currently trying the vacum advance adjustment by turning the allan key in the vacum advance canister counter clockwise to reduce the advance curve so far ng. does anyone have any suggestions? can the distributor be defective? should I try another maybe msd or pertronix? thanks for any help..
There were three different timing tab/damper TDC line combinations used on the Gen 1 SBC (not counting the plastic timing cover engines like the L31 Vortec, etc.). Mixing/matching the tabs and dampers can cause a false reading when you go to time it. More on the differences HERE
Some info on setting up a timing curve is HERE
Other possible causes for preignition is a hot spot in one or more cylinders. This can be a sharp edge left over from having the heads decked, or a bit of casting flash.
Be sure the spark plug heat range is not too hot. Obviously, vacuum leaks can dilute and lean the air/fuel mixture and that can add to being detonation prone. So be sure there are no leaks. You can remove all the hoses except for the vacuum advance and plug the ports to eliminate vacuum leaks coming from the vacuum accessories. Be sure there are no leaking intake or carb gaskets and that the PCV system is working as it should.
Check the color of the spark plugs to be sure the air/fuel ratio is not overly lean. Be sure the float level is right, fuel pressure adaquate (about 5 psi) and the filters are flowing properly (if in doubt, replace).
Another area to look at is the dynamic compression ratio (DCR). If the cam is very mild and the static compression ratio (SCR) is too high, the DCR will also be too high for pump gas in some cases.
is one of the CR calculators that are online. Plug your numbers in and see what you get.
To see what the dynamic compression ratio is you need to know the rod length (stock is 5.7") and the intake closing point of the camshaft. This lets you estimate whether a combination of parts will be able to use pump gas w/o detonation, for example.
A couple calculators for dynamic compression ratio:
Another dynamic compression ratio calculator
is a short page on the compatibility of different cam durations to the compression ratio.
One last thing- do yourself a favor and read up on "quench"
. It's an important part of the engine build and can make the difference in having an octane hungry pinging engine or a smooth running engine w/o detonation.