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Discussion Starter · #1 ·
I rebuilt a 302 a little bit ago, .030" over block. I used flat top hyperutectic pistons, E7 heads, and a .047 compressed thickness head gasket. The block had a 0 deck height as measured the ******* way (straightedge across the bore). Assuming 62 cc combustion chambers, this combo should be in the area of 9.5:1 static compression.

But I am having trouble with detonation under heavy load. I am currently only running 4 degrees of initial advance, and though I haven't measured recently, if I recall correctly that should be 32 degrees total. This is on the 87 octane left in the tank.

I can try higher octane fuels, but wonder what the effect of changing to thinner head gaskets would be. What I have read tonight is that it would improve the quench affect, and promote better mixture of the fuel/air, thus improving resistance to detonation. A switch to a .039 thick gasket would also yield a .2 increase in static compression.

What do you all think? Would it be worth the cost to change to better/thinner head gaskets?
 

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The gasket change is not going to make a huge difference--not worth the effort IMHO.

Have you removed and plugged the hose to the distributor's vacuum advance? Also, was TDC on the balancer checked when the lower end was assembled? Was the camshaft degree'd in?

I suggest looking at camshaft events instead and maybe going with a somewhat more aggressive camshaft with more overlap.

Or, simply try run the high octane fuel that you have been avoiding since it is the simplest fix.
 

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With 9.5 you should have no issues at all with detonation. Check the timing again and make sure you are using the correct timing point and balancer combo. Run without the vacuum advance as Dennis suggested if using it. Other things would be bad gas or the A/F ratio is off.
 

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Discussion Starter · #4 ·
Yeah, I didn't expect any problems with 9.5:1 and detonation. I actually think that the heads have been milled considerably, and maybe compression is a bit higher, but that is just a theory.

The gas is a couple months old. Not the freshest, but wouldn't expect a prob there either.

I don't have a means of degreeing the cam. I'll look into it. I'd love to buy this and a head cc kit, but that will not be in my budget too soon.
 

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Yeah, I didn't expect any problems with 9.5:1 and detonation. I actually think that the heads have been milled considerably, and maybe compression is a bit higher, but that is just a theory.

The gas is a couple months old. Not the freshest, but wouldn't expect a prob there either.

I don't have a means of degreeing the cam. I'll look into it. I'd love to buy this and a head cc kit, but that will not be in my budget too soon.
to find top dead center .... do a search here on piston stops . cheap tool and easy to make one ..

a few threads you'll find will tell how to use it .
 

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While .035 to.040 quench is optimal in a street car, Under .050 quench should be OK in this case. Today's gas goes bad fast. Try fresh fuel anyway. Also verify you have the correct spark plugs as the wrong heat range can cause det. BTDT. :mad: Do as suggested to verify your TDC, and remember at high loads and RPMs your total timing and curve are the focus - not initial. Also, at higher loads the vacuum advance system is non-functional and does not play a part in this issue. Do you have a timing light? Is it a 'dumb' version (flashes on spark) or a dial-back type?

David
 

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Discussion Starter · #7 ·
I drove it in to town today and got fresh gas, out of 2 gas stations 89 was the highest I found, so got that and a octane booster just for kicks, and set to 10 initial, and I think that should be 38 total. Got some preignition under heavy load, but noticeably better. I babied it home and will play with the ignition curve. I have a dial back light.

I'm running autolite 25 plugs, gapped at .044. I thought about opening that up a little too...I have crane cams points replacer kit, and have run up to .055 with no problems in the past.

I'll try to verify TDC. This is a fairly new balancer, and shows no sign of wear. I had it on my last engine without problem.
 

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Discussion Starter · #8 ·
A couple of interesting things I found tonight.

I double checked total timing - I was correct, 10 initial yields 38 total. After revving the motor, I was able to hear a decent tick that I hadn't heard from the cab before. After it idled down, the tick became milder, then went away.

Then I pulled the spark plugs. 1 looks too clean, 2,3,4 look too rich, 5,6,7,8 look perfect.

My theory is that a lifter is bleeding down too quickly, and by a valve not opening fully its causing a lean mix in cylinder number 1, causing detonation.

What do you guys think?
 

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A couple of interesting things I found tonight.

I double checked total timing - I was correct, 10 initial yields 38 total. After revving the motor, I was able to hear a decent tick that I hadn't heard from the cab before. After it idled down, the tick became milder, then went away.

Then I pulled the spark plugs. 1 looks too clean, 2,3,4 look too rich, 5,6,7,8 look perfect.

My theory is that a lifter is bleeding down too quickly, and by a valve not opening fully its causing a lean mix in cylinder number 1, causing detonation.

What do you guys think?
might have a intake gasket sucking air on left bank (5-8)
 

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Discussion Starter · #10 · (Edited)
Thanks for your input, DanH. I'll test for that...

I'm a little worried about #1. The tick I hear sounds to be coming from #1 area (listened with stethoscope), and compression being lower than the other cylinders. Cylinders range from 155-135, most at 145. But #1 was 120.

I've already taken the valve cover off and inspected the rocker arms and pushrods.

I should note that the engine only has 50 miles or so on the rebuild, and I'm aware that things are still wearing in.

I intend to make a piston stop this weekend and test that theory out.
 

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A bad valve guide can cause tick also if rockers are adjusted right. This can cause the valve to not seat correctly causing lower compression.
 
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