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ggaagg57
06-01-02, 09:30 AM
74 block milled ,heads too. if you set timing at 8 decrees it pings if you set it at 4 or less it want run good . running 93 oct . and 104 aditive still pings,
will running a colder plug or a hotter one help? if so which one thanks

Tom Bryant
06-06-02, 12:18 PM
First off we need to understand pinging. It occures when there is more than one source of ignition in the combustion chamber. One would be the spark plug and another would be a hot spot somewhere else in the chamber. When the two seperate flame fronts collide you hear a pinging sound.

Knocking is caused by the combustion mixture firing when the piston is still traveling upwards; much earlier than normal spark advance. In this case the ignited mixture actually tries to force the piston back down the cylinder causing a hard knocking sound. Kind of like beating on the piston with a hammer and can have the same results if not corrected. Knocking is caused by pre-ignition due to a hot spot in the combustion chamber or on the piston top. Also can be caused by severely advanced ignition timing. This is a much more severe condition than pinging but either one can, in time, burn holes in pistons or break ring lands.

Try disconnecting your vacuum advance and plug the vacuum line with a punch or screw. If the pinging goes away move your timing back up toward where it should be. If you find that you can run nearly normal timing with the vacuum advance disconnected you have too soft of an advance unit. Install an adjustable vacuum advance unit, available from Summit, Jegs or anyone that sells high performance ignition parts. It is adjustable with an Allen wrench through the vacuum tube . You will want to tighten the spring til the pinging stops. This causes it to need more vacuum to advance the distributor.

If you can't get the pinging to stop by disconnecting the advance unit you may have a piece of carbon on the piston top that is glowing hot and causing pre-ignition or a second source of ignition like a glow plug in a deisel. GM sells a Top Engine Cleaner that you pour into the carb while reving the engine for about half the can. Then you drown the engine, stalling it, with the rest. after a period of time passes you restart the engine and rev it to blow out all of the desolved carbon and deposits. The catylitic converter must first be disconnected on 1975 and newer cars.

If it still pings you may have to pull the heads. You said that both the block and the heads have been cut. You may need to use a thicker composit head gasket to lower the compression ratio by up to 1 full point. Fel-Pro will list the compressed volumn of their gaskets so if you know your deck height and chamber volumn you can calculate the new comprssion ratio as well as the old one. I'm not sure if that formula is in the Tools area of the Corvette Action Center or not but the guys at Summit's tech line should be able to help. Lacking the numbers to do the math just go with the thickest one available.

Other anti-pinging things you can do is to make sure there are no sharp edges in the combustion chamber or on the piston tops. De-burring the chambers of any casting flash or machining burrs helps but polishing is even better. The valve reliefs on the piston tops usually have a sharp edge from the machining process. This edge will get hot and cause pre-ignition. Soften this edge with some fine emery cloth. Anything you can do to eliminate any edge that can get hot will help a lot. Also the smoother surfaces helps to promote smooth flame propogation across the surface of the piston for a more even burn.

Other items worth mentioning are aluminum heads. They will tolerate higher compression without pinging. Thermal barrier coatings on the piston tops, combustion chambers and valve faces do an amazing job of smoothing out the flame travel and keeping the heat in the chamber where it can do it's work. Heat is horsepower. They also keep the chambers from getting hot spots that contribute to pinging.

From the simple to the expensive there are many things that can be done to reduce or eliminate pinging. Hope this gives you some direction.

Tom

ggaagg57
06-06-02, 03:26 PM
thanks for the reply , it had a dual point dist . in it i replace it with the stock one, this seem to have fixed the car . once again thank for the reply . george

Tom Bryant
06-08-02, 05:42 PM
Did the dual point have lighter advance springs in it? They were probably too light.

Tom

ggaagg57
06-08-02, 07:43 PM
i guess it was advancing to fast and getting full advance at a low rpm. but it runs fine now with a skip every now and then, im sure it needs wires. but there is no ping.or knocking. thank again for the replys. ggaagg57