440 headgasket to lower compression

[kaj750]

Perhaps .060 over it has a hot spot from a thin spot on a cylinder wall. What temp doees the engine run? You could try running evans coolant rather then antifreeze. It is better for transferring heat away from surfaces which can help prevent detonation.
Other then that I would look at a issue with your sniper because its likely in the mid 9's w those heads/pistons and verified by your compression check and shouldn't ping.

Does it stop pinging w better gas?

I can only run 91. Tried a ton of octane booster. No help. I want to grab something crazy like 100 to throw in there, just to see if anything changes.

What do your plugs look like? And what is your base timing and all in advanced timing?

What spark plugs?
To hot of a plug could also cause detonation.

Autolite 85

What happens if you add some octane booster? If you dont put a lot of miles on....maybe that could be your simple solution.
If it doesnt help....then the hot spot or too hot spark plugs theories may be correct. A too lean fuel-air mixture will deonate easily also.
Is the heat crossover in your intake blocked? You want it blocked in hot weather situations. Is the engine inhaling 200 degree underhood air?

octane booster doesn't help. I was driving the car 2-3 times per week. No idea about the crossover. RPM manifold, unknown valley tray (I haven't looked). Highest IATs I've seen is 140. Detonation starts right at 100* IAT. The tune doesn't have anything I can see that would make any weird compensations with 100* IATs.

Something is off here. First, and I may have missed it, but what is the exact gas you're running? What octane rating and how much ethanol is advertised?

Anyway, this shouldn't be pinging... I have more CR and piston psi than you and run on pump 93 all day long in the middle of hot North Carolina summers.

Let's do some math....

Let's assume you have a stock block with a deck height of 10.725 (big assumption), a stock stroke (3.750), and stock rod length (6.768). Those pistons have a compression height of 2.061. Granted, it's just math and not real measured numbers from the block, that means the piston is 0.021 in the hole... good.

I'm sure someone will correct me, but the 906 has a 79cc (also listed at 80 and 88. So not sure honestly...). Let's go with 79cc. Let's assume the builder threw on a very common Fel-Pro head gasket that has a compression thickness of 0.039.

Crunch the numbers and you get 10.18:1 CR, which is right at what you said from the beginning... so the numbers check out. But what doesn't check out is the pinging... that is not a high CR.

Let's go back to timing.. you mentioned using a Sniper for timing, so I'm assuming a Hyperspark for IGN box? If so, did you use the phaser tool for the distributor install and set the sniper to lock out the timing to check it? If you did... then I'm going to assume you're not dealing with a timing issue - which is what this problem sounds like if I'm honest. If the distributor isn't phased and the sniper GCF doesn't have the correct ignition reference angle set for the phase, you could be putting way over 50* of timing. I know you said 10* off the mark for setting, but just confirming you used the phase cap tool, that you locked out the timing control when you sync'ed the ignition to the sniper with a light, and the ignition reference angle is set correctly. (As a side note, sync'ing with less than 15* can cause issues, that's why Holley recommends 15*) And, importantly, that you have the right setting in the GCF for the reference angle (which if memory serves it's 57.5, but you definitely want to check that)... I'm not trying to tell you something you already know, but 90% of sniper/hyperspark issues are improper timing being set/configured.

Others already talked about hot spots... perhaps put a bore scope in the spark plug hole and look for one. If you're pining at 10.1:1 and it's a hot spot, you'll definitely see it.

Beyond what others have said... I've seen a couple super odd things cause pinging noises... like a bad wrist pin hammering the cylinder wall. A way to find that without taking the motor apart is unplug #1, drive around... plug back in and unplug #2, drive around... repeat. See if it's stops pining on a given cylinder.

I'm thinking timing... or something mechanical. OR... you're running the worst gas on the planet, which is pretty likely because of where you live.

Sidebar... Since at cruise it doesn't ping... I'm assuming you're AFR is in a good range... generally between 12.4 and 14.3? Or is it higher than 15+ when it pings?

91oct. I have a bore scope and will add that to my to-do list. Unplugging each cylinder is interesting. I'll try anything. My original thought was timing but I've checked it 10 times to the best of my abilities. AFR at cruise is in the 14s. Seeing 15s would have made it too easy.

I doubt you have 10:1 compression
I used the same pistons (.030 to your .060)
They were like .020 in the hole
With those 906 heads you probably have like 9:1 compression
My engine has 10.7:1 but to get there I had to use Eddy 75cc Aluminum Heads and a .027 Cometic gasket (.047 Quench)

I personally would be looking at timing
whats the total timing?
How much advance and how much total
Do you run a vacuum advance? what happens if you disconnect it?

26* timing at WOT. No vacuum on the Sniper stuff.

Definitely want the heat crossover blocked with fuel injection.

Without taking it apart there is no way of knowing if the heads have been milled, and what thickness head gasket is in there.

If the pistons are only down .020 the it will be near or over 10:1. I've never seen one that was not. What did you have .080 head gaskets to be at 9:1

Have you had a valve cover off to see head casting numbers yet?

I have. The pictures are somewhere, I'm just not sure where I left them, unfortunately.

Seeing the poster has tried just about everything and nothing seems to help I would be curious about one thing. The distributor. What distributor are you running? Has the phasing been checked? Pickup polarity? If in fact it is detonation there should be a way to tune it out.

Sniper EFI, Hyperspark coil and distributor. I used the alignment cap it came with while the engine was a TDC as per the instructions.

not to beat a dead horse, but i posted earlier about timing questions i had. I seems like a timing issue to me and that's easy to get wrong with a sniper controlled hyperspark - if he's actually running a hyperspark

How so?

Probably a combo of issues. Those pistons won't work with open chamber heads and pump gas. Probably oil/water getting into the combustion chamber too. Doesn't take much to push the engine into detonation.

Cylinder 1 did look to have a bit of oil on it.

 

[Geoff 2]

Another thing that causes pre-ign which leads to detonation is carbon build up in the chamber; particles are hot enough to ignite the mixture. once again, a cooler chamber helps.

 

[kaj750]

Everyone will tell you 190-200* running temp is ok. Not in my book. Anything you can do to cool the AIR entering the com chamber, & keep it cool until ign occurs, will help with detonation. I can drive my GTO on a 40C+*[ whatever that is in F* ] day & it will NEVER detonate. 9.9:1 CR, 91 octane [ your 87-88 ], factory iron heads. NGK '4' spark plugs. I spent a looooooooooot of time improving the cooling system & setting up cold air induction. Smaller w/pump pulley, fan blades a tight fit the shroud so that the shroud actually pulls air, alum 2 row rad, Edel Victor water pump. 180* stat. Engine runs just over 180 [ estimated 183 ] under all weather & driving conditions.

I'll bet the det is not as bad on a cold, wet night....

Correct. IATs under 100* are no problem. Runs like a champ.

 

[BLK 68 R/T]

At least 3 of the plugs look suspect to me. Looks like maybe the engine is sucking oil? might be an intake leak on the underside? The electrode part looks okay on most of them but the outer portion where the threads are is black. Does the engine need oil added often to keep it full?

 

[BLK 68 R/T]

I can only run 91. Tried a ton of octane booster. No help. I want to grab something crazy like 100 to throw in there, just to see if anything changes.



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Autolite 85



octane booster doesn't help. I was driving the car 2-3 times per week. No idea about the crossover. RPM manifold, unknown valley tray (I haven't looked). Highest IATs I've seen is 140. Detonation starts right at 100* IAT. The tune doesn't have anything I can see that would make any weird compensations with 100* IATs.



91oct. I have a bore scope and will add that to my to-do list. Unplugging each cylinder is interesting. I'll try anything. My original thought was timing but I've checked it 10 times to the best of my abilities. AFR at cruise is in the 14s. Seeing 15s would have made it too easy.



26* timing at WOT. No vacuum on the Sniper stuff.



I have. The pictures are somewhere, I'm just not sure where I left them, unfortunately.



Sniper EFI, Hyperspark coil and distributor. I used the alignment cap it came with while the engine was a TDC as per the instructions.



How so?



Cylinder 1 did look to have a bit of oil on it.

I would also think the wot throttle timing should be about 10 degrees more? Around 36* and 2800-3000 rpms?

 

[kaj750]

At least 3 of the plugs look suspect to me. Looks like maybe the engine is sucking oil? might be an intake leak on the underside? The electrode part looks okay on most of them but the outer portion where the threads are is black. Does the engine need oil added often to keep it full?

Not sure. I was going to check everything when I swapped the heads. Now that I'm not doing that, I think I'll at least pull the intake mani and see what I can find. At this point, it couldn't hurt.

I would also think the wot throttle timing should be about 10 degrees more? Around 36* and 2800-3000 rpms?

I've been slowly pulling timing in an attempt to stop the detonation. I started at 32*.

It's too the point where I was wondering if I were hearing something else. But since it's only happening during high IATs, it kinda makes sense.

 

[33 IMP]

KAJ750, Do you have access to avgas 100LL? Small local airport?
My 2295 piston 440 (actual 10.5) will run on pump california 91 (557 purple shaft) but I have access to 100LL so I sweeten mine with 25-33% avgas, cause I can.
(The AZ 100 is cheaper than California 91).

 

[autoxcuda]

If I were to try anything, it would be borrowed Holley 750 DP.

The Snipers can be a source for issues.

 

[BLK 68 R/T]

Just my hunch, but I’d be inclined to try a different distributor.

In my personal opinion the cranking compression is not high enough to have potential oil fouling affect it where it would cause detonation.

My assumption is based off of everything in this thread combined with my experiences is there is an issue with the distributor. Something is wrong with the advance.

 

[BLK 68 R/T]

And one more thing. Are you absolutely sure it’s not just an exhaust leak? Once the engine warms up and things expand there’s been more than one person mistake a simple exhaust tick for detonation as it gets louder and increases as the rpm increases.