My cam is the Lunati 402A3 grind. I have Brodix Race Rite heads with 115cc chamber, and I have a very small dome, I think it is only 23cc. Anyway, my 468ci BBC should have only 10.2:1CR, with a DCR of I believe around 8. I've got what I'm guessing is a pinging problem. My dad has called three people insisting that my motor will need to run 100 octane. When I had done my calculations and asked around here I/we designed it to run 93. I refuse to believe this...one guy told me that you can only run 93 octane with 9.5:1. I tried to explain to him some 'dynamic compression' or whatnot, about the intake valve closing really depending on compression...he said 'that's cylinder pressure though, not compression'. ..... I am going to run up to get a few gallons of 100 octane though, because there was a few gallons of 87 in my tank when I filled with 93, so maybe it is not enough. Anyway, I just wanted to see what you think..thanks.

Have you looked into your timing curve / advance / total timing? I think that a true 10/1 with AL heads you would be in fine shape with 92 octanre fuel. My 049 iron heads at 9.5/1 loves 90 octane fuel and am considering giving it a belly full of the "regular" fuel at 87 octane. Prime consideration for a strip / steet ride is that it's go to run on plain jane pump gas. Focus on your timing and AF and I bet it goes away!!!

We're really working hard on it. I added 3 gal of 101 octane for equal measure jusssst incas something about there being a few gal of 87 in the tank messed with the 93 I put in. Anyway, we've tuned the carb a bit (it's a 4781 Holley) but that doesn't really seem to be doing it. We can move the dist in any direction and not really get it to go away. It possibly sounds like pinging, BUT I would think pinging would be very sort of erratic, not in rhythm. It sounds like there is only one cylinder with a problem, in my opinion. It's like 'bink...bink...bink' rather than 'binkbinkbinkbink' if that makes sense. Anyway, just kind of thinking aloud..thanks.

Detonation sounds like a spray bomb rattle. Usually heard during moderate to full throttle under load.

Have you checked the lash on your rockers? Do you happen to have alum valve covers?

I ask because on my SBC if my rockers are even a little loose they will make some ugly noise under the tall alum covers and if they're really loose it runs horrible (of course) and sounds like something is really really wrong.

See it really does sound like my roller rockers are hitting my chrome valve covers. I have two gaskets under there, but feeling with the hand and listening with a screw driver it doesn't sound like it really is hitting. I had a bunch of clearance with the two gaskets, too. The thing that gets me is that besides this noise..it really sounds strong! And it feels good with a rev. My dad didn't want me to take valve covers off yet so we'll have to see next weekend. I called it quits for tonight due to other stuff and gotta be at school this week.

One thing I also wanted to do was to, while the engine was running, pull a wire off the dist, listen, replace, move to the next one, etc..and see if the problem went away on a cylinder or if it really was a 'whole' engine problem. Would that maybe be a good thing to do? What would it tell me? I would think it would at least tell me if it was a pinging problem or a problem with a specific cylinder.

I'm also not having any sort of cranking issues, isn't that a sign that timing is off sometimes?

Anyway, thanks for any ideas..

You really should pull off the valve covers next, its just a few bolts and the gaskets can probably be re-used.

You may have had enough clearence with everything cold and tight, but once things heat up they grow, and I think you have alum heads so your block is growing at a different rate than your heads, as are the chrome valve covers. Usually isn't a problem, but you never know.

I asked this in the other post: Have you been letting it idle before finishing the cam break-in?

Try a slower advance curve on the ignition(heavier weights).A cranking comp. of around 200 lbs or so is getting closer to boarderline with pump gas....If you had to you could even pull the cam back 3 or 4 degrees

By any chance you have an exhaust manifold/header leak??

Just thinking out loud is all?????

pdq67

Without knowing the deck height and the head gasket you used, I ran the numbers on Pat Kellys compression calculator and came up with 10.3:1. This figure is ASSuming a "0" deck height and a .041 thickness x 4.54 dia gasket bore. (felpro 1017-1), and as you stated, a 23cc dome. Have you ever used a mechanics stethoscope? It's amazing what you can hear going on inside your engine and how fast you can pinpoint where a noise is coming from. I have heard a long skinny screwdriver can work in a pinch. Put the handle on/by your ear and move the end across the valve cover and listen to whats going on in there.

Think a header leak could sound like a rocker arm hitting a valve cover?

A header leak can sound like, loose rocker arm/lash.

Think a header leak could sound like a rocker arm hitting a valve cover?

Yes.

Does it make the noise while idling or just under load? If it does it while idling then I doubt it's detonation.

Steve

definetly could be an exhaust leak!! worth checking!

The tranny is in neutral when the car makes the noise, RPMs roughly 1500...so no load really.

It IS highly possible that I didn't tighten down a few bolts on the header...and I had always thought it was coming from from the side where bolts could be a bit loose. I will check though, really can't remember if I or my dad tightened down the bolts.

I guess really I can rule out a timing problem in the distributor because just tuning it by hand rotating the base doesn't cause a change really.

I hope that is it, but never know, I will still most likely pull a cover to check. Seems that there was some spitting from the carb (could see something spitting) but that was eliminated visually, but now there is noise, so who knows...anyway, thanks everyone! Will check soon..

When checking the timing disconnect the vacume advance,it may be pulling to much timing in.If it still rattles then your problem is somewhere else.Bad fuel,engine to lean,weak ignition.Engines detonate under load,if this engine is detonating under no load with all the timing pulled out start looking at one of the other things listed.