fresh 440, 10:25 compression with Ebrock performer aluminium heads. Lunati 268/274 cam. 4200 pounds and 3:08 gears.
With the standard Mopar distributor with big and little springs, it pinged at full throttle at 4000 rpm which is 30 degrees(about 5 degrees initial). I put the FBO plate in it, changed springs for full advance at 3400 rpm. With the plate set for 16 degrees crankshaft and 15 initial, now it pings at 3000 rpm. I can now keep backing off to find out where it doesn't ping and that will probably be well below 30. Did we just build this engine with too much compression even for aluminum heads? What is so magical about the usual 34 degrees?

Have you verified your balance timing mark(s) ?

This ^ something doesn't sound right. 30 degrees shouldn't ping under light load.

Are doing this with vacuum advance off or on? If you have vacuum can on, you adding more advance during your test.

Not at WOT.

Kevin

those fbo plates are far from accurate. did you look at the factory advance plate to make sure it is 30 degrees (should have a 15 stamped on it)?

using a dial back to zero timing light the distributor is behaving as I expected, both the unmodified distributor and the modified one. The balancer marks have been checked.
The cam is advanced 2 degrees, does that affect pinging?

The amount of timing needed is dependent on the combustion chamber efficiency. In order to get the most effect out of the combustion event timing needs to occur at a point where the engine can get the most out of it. In the case of our cars that is about 34 degrees BTDC. The more efficient the chamber design the less timing that is needed. An LS1 takes about 27 degrees total as an example.

What spark plug brand and heat range, part number on the plugs?
How old is the fuel in the tank?

Just what i was thinking! Also where is the vac line to the distributor hooked up to? Not familiar with your carb but you want the vac only when you are on the throttle. If you hook it up the other way you will get advance at idle. Pinging is not always from timing, Hot spots(plugs sharp edges) in the combustion chamber can ignite the fuel prematurely causing pinging. With that amount of compression you should run super or something with a higher octane rating.

i think the real culprit is too much cylinder pressure for the gasoline used. that 268 intake lobe probably closes the intake valve at 60-62 degrees ABDC, combined with 10.25:1 compression, way too soon. i bet if you did a compression check that thing is blowing around 200psi or more; if the engine is healthy. i had a 9.3:1 440 blow 190+ with a comp cams 268 intake lobe and it wasn't a happy combo. closing the intake valve that early may work ok on 8:1 but i believe the sweet spot is around 70ABDC. keep in mind that for every .010" of carbon build up the static compression ratio will raise about 1/2 point with a 440 and .010" isn't a lot. of course other checks like verifying TDC on the dampner should be done too.

Agree that may be too much compression for that cam.

Also - is there any useful quench? what is your deck height? Head gasket thickness? Which Eddy heads - big chambers or small chambers?
Is that 10.25 comp ratio an actual calculation or just what the piston maker calls their piston? It could be even more than you think.
What octane gas?
What carb and what have you done to it? it could be going lean and causing the ping.
4200# and 3.08 gears is contributing to your problem.

Yup, that's what I'm thinking as well. That is a fairly small cam which will build pretty good cylinder pressure given that high of a static CR and the cam IVC event.

Check out Wallace Racing DCR Calculator for a better feel how your current 10.25 and the cam IVC pan out. Typically it is said that up to 9.5 DCR is about the max for a 94 octane pump gas.

For a comparison, here are the results for my SB 360 build which used a 238/244 @0.050" hudraulic flat tappet cam in a blueprinted 10.5:1 static CR motor:


The actual cranking compression pressure I was seeing on that engine was 185-195 psi, so I consider the above DCR numbers to be pretty close. That combination ran on 91 octane pump gas, very fast ignition advance (mechanical), 21 initial that topped out at 35 deg. Factory iron heads, chambers polished, using NGK BKR6E-11 spark plug, V-Grove (#2756).

What is the fuel being used?
Octane, Anti-Knock Index, etc

A 1995 Magnum V8 with SMPFI will ping on at least one cylinder
with scantool measured 28 degrees of total advance
at full throttle and 4000 rpm
at 90 degrees F at 480 feet above sea level elevation
with ‘Regular” Sunoco gasoline (87 AKI on the gas station pump)
but actually about 85 AKI when tested.

With 93 AKI Sunoco in the same conditions
it will not ping.

The high swirl,
heart shaped
Magnum V8 combustion chamber with quench clearance of about 0.087 and 62% quench area
is more Octane tolerant than the older wedge 440 chamber.

Knocking on a 440 V8 30 degrees of total advance at 4000 rpm wide open throttle 92 AKI gasoline
- maybe 180 degrees of coolant temperature?
would depend on intake air temperature
.... near certain to knock at 90 degrees F/ 70 degrees F Dew Point
.... unlikely to knock at 32 degrees F/ 30 degrees F Dew Point

The Performance Trends Engine Analyzer 3 computer program
can make an educated guess at this,
taking into account
Dynamic Compression Ratio,
Volumetric Efficiency,
and exhaust pipe back pressure.

Thanks for all the input. I will do a compression check and see what I get. Use that link and do the calculator. The heads are the 84cc ones. The best gasoline here is 91 octane with 10% ethanol. I don't know if the engine builder verified the 10.25 but it is likely. Although the carb is not completely dialed in, per the A/F meter it isn't going lean at full throttle. The engine builder has the same engine in a Charger with a wilder cam and much steeper gears and torque converter. I don't think I would want to go to a much wilder cam. I would rather try a thicker head gasket or shave some off the tops of the pistons. I drive the car on long trips and around town. The power before the ping sets in is nice, but I want to feel what it is really capable of.

i use an Icon 836 piston with those heads (9.8:1 comp) with noticeably more cam than you have and it's very pump gas friendly in a 3.23 geared b-body. i've got the cylinder pressure down a little low but i do use a fairly aggressive timing curve with a vacuum advance. drives great and very reliable.

I made a mistake on the cam. It is a Lunati 262/268 so milder than what I first posted. Worse situation for 10.25:1 with 91 octane, I suppose I forgot to mention that I am at sea level also.

And your advancing the cam 2 degrees works against you too…..the problem is you have an early intake valve closing event….the cylinder is sealing early and you are building compression…big time. I run about 200psi with 10.2 and stealth heads, Lunati cam (the one with .493/.513 lift) and I get away with it at about 34 degreees mechanical (also have vacuum) but I do have quench.

learn how to do the math on a lobe profile and try to find something that closes the intake valve 70-72 degrees ABDC. to keep duration down you'll need something with a 112-114LSA. the summit 6401 might work. it's actually 280/290 seat timing on a 114LSA; closes the intake valve at 72 degrees ABDC. may not be a 100% cure but much better than what you have. it's difficult to find anything that doesn't have a 108-110LSA anymore. with a heavy car/tall gear valve and ignition timing will be the key.

as i think more about this a stock magnum cam would be better than the lunati. they have a 115LSA and close the intake valve in the mid 70's range ABDC. they are not 268/284 as the factory claims. they have a long closing ramp that the factory doesn't claim all of. true seat timing is close to 276/292.

If its lean AFR it can make it ping & rattle fix that 1st