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Discussion Starter #1
Detonation destroyed my Edelbrock RPM 70cc heads. trying to determine if this was due to too much compression or just bad timing. Pistons look ok. I heard a noise like a bolt rattling around under acceleration and lugging and now I know that's what detonation sounds like. Took 5k miles to destroy my Head castings. I always ran ethanol free 91 Octane. BTW I love having a SBC revving at 6200 RPM. I don't know what I had the timing at but had it advanced as far as I could without the motor pinging(hind sight 20/20 should had reduced timing).

I am running
Speed pro Domed pistons .125
Lunati Voodoo Flat Tappet Cam 268-278
Edelbrock 70 CC RPM heads

1. Was this just a timing issue or is my compression too much for a street motor on todays gas.
2. I was getting near 10:1 and I am considering getting a 75cc AFR head vs 70cc Edelbrock heads to lower compression
3. Should I change my pistons and go with a 64cc head.
 

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Ok, so then standard deck of .025" in the hole you'd be just above 10:1 CR.
I think the real issue may have been not knowing what the total advance was at 6,200 rpms.

I guess I wouldn't necessarily change anything in the heads. Curious to see what others advise, though.
 

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Discussion Starter #8
Heads looks like they were deeply pitted around the outside combustion chamber. Specifically around the cylinder opening of the head gasket section of the head. Machine shop said they have never seen such odd pitting and could not be milled out due to depth. Shop suspected detonation caused it. Since I have to buy new heads, I want to determine if I should get 75cc heads and lower compression to help prevent this in the future. I will definitely get car professionally tuned once its all together this time
 

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The same pitting should be present on the tops of the pistons, funny you don't see any damage. I've seen heads welded over and machined back to like new but without seeing the carnage it's hard to make an assessment.

It's also kind of hard to advise on changing compression without knowing where you were before...

Detonation (pinging) can also cause ring and bearing damage so you really should pull the rest of the engine apart and look it over real good.

From there you can spec out proper bore, stroke and chamber for your rebuild to be sure you can run on pump gas without problems.
 

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I've never seen detonation damage a head with out damaging the piston. And I've never seen detonation destroy a head before beating the bearings to death. I'd want to see a picture of a plug and the head before I said or did anything.
 

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0.025" deck and the OE 0.018" embossed steel shim gasket would be a TOTAL of 0.043" Quench.
Using a 0.043" gasket with the stock 0.025" deck causes the quench to be too large. So large that a second flame front can develop under high loads (WOT and kugging) that will cause a knock or ping.
If you had 36* total mechanical timing (with a limited amount of added vacuum advance for cruising economy) at 10-ish to 1 compression with the proper 0.040" quench your 91 octane would have been fine.

I run 11:1 and 93 octane with great results.

Definitely inspect, replace the rod bearings.
 

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No piston damage? No specks of aluminum on plugs? Send heads back to Edelbrock and have them checked for porosity. Doubt that's it but I doubt detonation caused damage to the heads and not the other components. Post a pic for us.
 

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You should have similar damage to the pistons. Doesn't sound right.
 

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0.025" deck and the OE 0.018" embossed steel shim gasket would be a TOTAL of 0.043" Quench.
Using a 0.043" gasket with the stock 0.025" deck causes the quench to be too large. So large that a second flame front can develop under high loads (WOT and kugging) that will cause a knock or ping.
If you had 36* total mechanical timing (with a limited amount of added vacuum advance for cruising economy) at 10-ish to 1 compression with the proper 0.040" quench your 91 octane would have been fine.

I run 11:1 and 93 octane with great results.

Definitely inspect, replace the rod bearings.
This is the best advice you have been given, ASSUMING the damage is from detonation. You need MUCH tighter quench. Has the block been decked? With just a feeler gauge and a straight edge you can find the quench for each piston. Measure all 8, just to be sure your block isn't way out of square.

Forged pistons can escape where cast aluminum is damaged, but that is a very fine line. Should be SOME damage to tops of pistons, even if it isn't as bad as the head.

Pics of both would be helpful.
 
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