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Discussion Starter #1
Hey everyone. Back again with another probably simple fix.

It would appear that I am getting very light preignition.. the weird thing is i can't hear anything except for when i'm at part throttle cruise.

Every time I check my plugs, if I look deep down inside on the porcelain and inside walls of the threaded portion I can see a very few small shiny specs.

I'm at 10.2:1 CR
.040" quench
93 octane pump gas
14 initial 34 total timing all in by 3200
No vacuum advance
Autolite 3924 plugs gapped to .040"
236 at .050 cam

Am I running too hot of a plug? I really don't think it's timing... for this is not a very radical curve. I run nothing other than premium gas. Usually from Shell or Chevron stations.

Or maybe I'm just paranoid and think I'm having preignition?

I tend to overlook simple problems and it helps me to type this all out and hear ya'lls opinions.

Also if anyone has read my last posts in regards to carburetor jetting, I have it backed down to 74/78 now. Still probably rich, but I'd like to figure out this "preignition" issue and my timing curve before I jump back into that situation.

Anything and everything can help. Thanks.
 

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I suspect you are hearing detonation rather than preignition. The initial advance set at 14 degrees is a little too early for 10.2:1 compression ratio on 93 octane pump gas. Use a colder heat range spark plugs and move the initial advance back to 12 degrees and see if the preignition goes away. One step colder heat range spark plugs may not make any difference but they will not hurt.
 

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I'm guessing you're using iron heads. That's the reason we keep bangin' on 9.5:1 maximum static compression ratio with iron heads on pump gas.
You can't hear pre-ignition, but you can hear DETONATION. In my experience, shiny specks on the spark plug insulation are indicative of the piston disassembling itself. Keep running the motor that way and you will find yourself with some zero compression cylinders due to holes in the piston crowns. Rather than re-engineer the whole mess, I'd be thinkin about adding water/alcohol injection. Edelbrock doesn't make their units any longer, but several years ago, I fitted one using water and isopropyl alcohol to a 390 Ford and eliminated the detonation.
Do It Yourself Water Alcohol Methanol Injection
Water/Methanol Injection Kit for Forced Induction Gasoline Engines | AEM
Snow Performance : Snow Performance Water Methanol Injection Systems. The Best Most Accurate Gas and Diesel Water Methanol Injection Strategies for Fuel Injected Carbureted Turbocharged Supercharged
https://www.youtube.com/watch?v=HkPFZWd8wj4
One other thought, if the detonation is only happening at part throttle cruise, maybe the vacuum cannister is putting too much advance into the system. Change cannisters to a Crane adjustable unit, using the limiter plate that comes with it to limit vacuum advance.
http://www.cranecams.com/333.pdf

.
 

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Discussion Starter #7
Actually I'm running aluminum heads. I've been told it's due to the cheap 10% ethanol gas we get around here. I'm planning on dropping down the plug heat range as mousefink said.. that's all I can think of other than doing the methanol Inext ion as tech suggested.
 

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How do yall feel about running an octane booster? Or is that stuff garbage?
Resoundingly YES. That stuff is a scam with a capital 's'. When you read the label and it indicates it raises the octane level one or two 'points' be advised that 'points' are .1 of an octane.

You would be much better off buying 5 gallons of racing gas at 104, 110, 116 octane and mixing with pump gas to achieve a desired octane. The only issue is that racing gas is alot of jing and mostly leaded which won't do you catalytic converter any good if you have one.

I see you're a relatively young guy. I've seen all of my son's friends and him show up with that stuff in the trunk as some form of magic in a can. It's such a marketing ploy - 'if they sell it, then it must work' and then mix that with a testimonial from one of the Sunday morning garage shows that have painted, spotless floors without a chip out of them and it makes the impressionable willing to buy that crap. When I was young an old guy told me 'believe half of what you see and none of what you hear and figure it out from there.' I determined that was sound advice and if I apply that when something sounds too good to be true, then I usually figure out that it is.

Sorry about ranting, but it irks me when the impressionable or uneducated are taken advantage of because they are impressionable or uneducated.

Best of luck to you - Jim

p.s. - the old guy was my grandfather that was your age during the Great Depression
 

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Also I'm not running any vacuum advance currently
You shold definately connect the vacuum advance if you have a distributor that is equipped with one, especically for stop and go driving. Distributors without a vacuum advance is for engines that see full throttle launches, such as at the drag strip. In that case, a vacuum advance is not needed.

Detonation and pre-ignition is basicly the same with similar results. The cause is what makes them different. Detonatipn is a premature explosion in the combustion chamber caused by excessively high compression, too much timing advance, low octane fuel or a combination of any of the three. Pre-ignition is a spontaneous explosion if the fuel/air mixture by an overheated engine, spark plug heat range too high, or a hot spot in the combustion chamber which occurs before ignition.
 

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Discussion Starter #12
How do yall feel about running an octane booster? Or is that stuff garbage?
Resoundingly YES. That stuff is a scam with a capital 's'. When you read the label and it indicates it raises the octane level one or two 'points' be advised that 'points' are .1 of an octane.

You would be much better off buying 5 gallons of racing gas at 104, 110, 116 octane and mixing with pump gas to achieve a desired octane. The only issue is that racing gas is alot of jing and mostly leaded which won't do you catalytic converter any good if you have one.

I see you're a relatively young guy. I've seen all of my son's friends and him show up with that stuff in the trunk as some form of magic in a can. It's such a marketing ploy - 'if they sell it, then it must work' and then mix that with a testimonial from one of the Sunday morning garage shows that have painted, spotless floors without a chip out of them and it makes the impressionable willing to buy that crap. When I was young an old guy told me 'believe half of what you see and none of what you hear and figure it out from there.' I determined that was sound advice and if I apply that when something sounds too good to be true, then I usually figure out that it is.

Sorry about ranting, but it irks me when the impressionable or uneducated are taken advantage of because they are impressionable or uneducated.

Best of luck to you - Jim

p.s. - the old guy was my grandfather that was your age during the Great Depression

That's exactly the answer I was looking for. That's why I ask yall about this before I go and waste my money on it. Thanks jim. I was thinking about buying some race gas from the local speed shop and doing a 1:5 ratio mix (I have a 12 gallon tank, figured 2 gallons ought a bump it up enough). But if I were to buy 5 gallons a month and just keep it in a 5 gallon can in my garage, would that be okay? What's the shelf life on that stuff? I know I've been told this ethanol gas doesn't keep very long at all due to the ethanol absorbing moisture in the air (I don't have a sealed tank)
 

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Discussion Starter #13
Well I threw 2 gallows of 110 leaded in with my existing 6 or so gallons of 93 and boy let me tell you... this car runs like a bat out of hell now!!

Now my concern is...

Is gas that junk nowadays that a 10.2:1 motor is having detonation issues in 93 octane?

Or

Did I screw up on the build somewhere?

Should I compensate with a cam with a larger valve overlap?
 

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A compression ratio of 10.2:1 is on the borderline for 97 octane pump gas with the initial timing advance set at 12 degrees or where the engine starts and runs best. You can reduce the initial timing back to 10 degrees and that may help prevent detonation on pump gas but your engine will lose power and may overheat. Pump gas is bad now and is going to get worse for high compression engines.

Installing a longer valve overlap camshaft in order to use low octane pump gas will make your car run like a porch dog. Don't do it.
 

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Discussion Starter #15
A compression ratio of 10.2:1 is on the borderline for 97 octane pump gas with the initial timing advance set at 12 degrees or where the engine starts and runs best. You can reduce the initial timing back to 10 degrees and that may help prevent detonation on pump gas but your engine will lose power and may overheat. Pump gas is bad now and is going to get worse for high compression engines.

Installing a longer valve overlap camshaft in order to use low octane pump gas will make your car run like a porch dog. Don't do it.

Well. That sucks haha. I thought i was being modest at 10.2:1 when i built the engine... i guess I was wrong. So whats the actual street gas limit then? 9.5:1 for iron heads and 10:1 for aluminum? I was always told 10:1 iron and 11:1 aluminum.. but I guess that was back when gas wasn't up to 20% ethanol. I guess I'll stick to mixing pump gas with a few gallons of 110. If I do a 20/80 mix of 110 and 93 I should get 97.5 octane fuel. It's insane how much better thr engine runs now. It idles cleaner, it's more responsive, it even sounds and smells better.
 
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