have a little rattle when going up hills. if running about 70 mph and start up a hill i hear it steady if i don't back out a little. this is in my 73' 302. i put mid grade gas in and no change. low comp. built engine. 600 holly and one step up cam. 30 over flat tops. timming is 10 btdc. have pertronix elect. ing. could the plug heat range or gap be a factor? thanks for any help ...frank...:bananaman :bananaman L.T.N.G.
Your engine is originally made with 8.0:1 compression. If you have flat top pistons then you just boosted your compression which would require a higher octane gas. You may need to go with 92 octane and or retard the timing 1 degree at a time to get to the point where it doesn't knock. A hotter plug may also contribute to spark knock but theres really no need to use it. Use the standard stock plug. Retarding timing lowers power output so you may want to try octane boost if a full tank of 92 octane doesn't help.
Depending on the rear gear ratio, 70 mph would suggest the centrifigal weights in the distributor have gone into to much advance for the engine at the rpm involved. It is a relitivley simple matter to reduce the total timing advance by tightening up one of the return springs a small amount to reduce the advance under your new set of engine conditions and higher compression. Fooling with other areas to get out of the problem will not be very productive when it is to much advance that is the problem.
I forgot to say that flat top pistons is only half the story. In a 73 motor the piston top is down in the bore some .014 " so the question becomes, is the flat top only filling in the dish area or is it also coming to the top of the block deck? There is a difference in compression between the two. Flat top to block deck is still only less than 9.5 Cr even at .030 oversize unless the heads have been milled .020 or more. Already been there.
try unplugging your vac advance( too much unloaded total timing) IF it does not have a vac advance you need to curvr your dist...usually 36 degrees TOTAL timing works well on windsor engines
i have a 2.79 rear gear with 24 in. tires. this is with a c4 tranny. i have a 3.00 rear end to put in if that will help thanks...frank...:bananaman L.T.N.G.
I think K.Merring is on to somthing. If those pistons are down in the bore 0.014" then part of your problem could be the quench area. If I remember correctly your tops of your pistons should be .035" from your heads for good flame travel. Much more and pinging is a problem. Hopefully it is as simple as the distributor. if not you might be able to help by changing the head gasket to one that is thinner or thicker depending on what you need. "Strong turbulence in the combustion chamber will help control detonation by swirling the flame front throughout the chamber, starting little fires all over, burning up end gas in lots of places, rather than leaving pockets to explode. Turbulence can be induced by shaping the port, by shrouding the intake valve, and by putting a squish band (also called a "quench band") in the chamber. That's a ledge or section of the chamber that the piston comes really, really close to (40 thousandths or less. 30 thou is good; less than 20 is too close) making the gas mix in that zone squirt out into the chamber. Ideally, it's aimed to squirt at the spark plug.
I like the idea of unplugging the vacuum advance and trying it that way. Make sure you plug the line though. If that fixes it, Accel makes an adjustable vacuum advance that allows you to limit the timing advance from vacuum. If you have a stock distributor, they come with very heavy springs that are usually good for just about anything. Now, i think everyone is assuming that this is pinging...is the noise definitely coming from the engine, or could this be something else, caused by the engine having to work...such as a catalytic converter heat shield (had lots of those rattle). Does it make the noise at night, only during the day, all the time?? When it's floored? 10 degrees of timing shouldn't be too much base timing, i wouldn't take any out.
cacollo, you have a good point. i was playing with it yesterday and it was doing some strange stuff. i also remember it doing it when the six was in it. it very well could just be an rpm rattle. i noticed that going down the interstate that just giving it a little gas it makes the sound but if i gas it kind of steady it doesn't. wouldn't spark knock come from gasing it heavy?...frank...:bananaman L.T.N.G.
