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post #1 of 25 (permalink) Old 10-08-2013, 06:58 PM Thread Starter
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66 389 timing

Wondering if someone else has gone thru this already and knows what to get, do, etc..

What I have-

66 gto , 28k original miles. Yes, seriously it does. 210-220 psi per cylinder in compression test. 2 speed auto trans.

Problem - pinging under load on 93 octane gas with octane booster in it and lead additive.

I have retarded the timing from the factory 6 deg btdc to approx 3 deg btdc and installed 160 degree thermostat to keep a bit cooler to reduce any temp induced detonation. Pinging has reduced a good bit, I will prob drop timing down to 0 deg and see how that goes.

What I want as an end product is to be able to run the car on 93 octane without sacrificing much performance or causing any problems. Has anyone installed or tested a modified distributor advance with heavier springs and lighter weights?

Nearest place with the super blue high test racing fuel is about 20 miles away.

Anyone know any of the tricks?
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post #2 of 25 (permalink) Old 10-08-2013, 08:06 PM
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You could try a thicker set of head gaskets to reduce the CR,

Cometic head gaskets

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post #3 of 25 (permalink) Old 10-08-2013, 08:09 PM
 
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You can try to band-aid it with timing adjustments, but you're probably destroying that engine...quickly. You usually can't hear detonation.

I'd talk to the pros. Check with Central Virginia Machine, Butler Performance, Cliff Ruggles or any of the reputable Pontiac builders.

1968 Pontiac GTO
1983 Pontiac Bonneville (G) wagon
2008 Pontiac G8 base
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post #4 of 25 (permalink) Old 10-08-2013, 08:31 PM Thread Starter
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I can hear a subtle rattle for lack of a better description at mid range rpm under load. It has decreased with the timing change, and only seems to occur when engine is at operating temp. I have removed the exhaust manifold heat valve to take that possibility out.
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post #5 of 25 (permalink) Old 10-08-2013, 09:33 PM
 
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, thicker head gasket will get your compression down, but sacrifice the quench in the fuel mixture. Beats beating your engine to a pulp though.

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post #6 of 25 (permalink) Old 10-08-2013, 10:17 PM
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Originally Posted by Kjp View Post
What I want as an end product is to be able to run the car on 93 octane without sacrificing much performance or causing any problems.
The only way you're going to get that "end product" is to address the root problem, which is too much compression for 93 octane. Pulling out timing is killing performance, so is running the cooler thermostat. Fat head gaskets would also hurt because they ruin the quench.

You've really got only 4 viable options:
1) Run fuel with enough octane (race gas or convert whole fuel system to E85)
2) Drop compression by installing dished pistons (D-shaped dishes to save the quench area)
3) Drop compression by switching to different iron heads with larger chambers
4) Swap to aftermarket aluminum heads (you can get away with more compression with aluminum, but you'd still want to regard 10.3 to 10.5 : 1 as an upper limit).

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post #7 of 25 (permalink) Old 10-09-2013, 09:45 AM Thread Starter
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Thats kind of where I'm headed. I'd hate to ruin the originality of the vehicle, but with modern gasoline octane ratings we are all in the same boat.

My thought as a winter project would be install some 9.5-1 or 10-1 pistons and a mild cam to keep the output where it was with the 10.75-1 pistons. That way, original heads, etc.. and overall external look is there, but its a vehicle that can be driven anywhere.

Or, without getting too deep into the internals I may do the aftermarket heads.
Any suggestions on best combo to go with? Edelbrock? Which version runs better on 90+ octane , the 87cc or 72cc heads?
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post #8 of 25 (permalink) Old 10-09-2013, 11:47 AM
 
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66 timeing

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Originally Posted by Kjp View Post
Thats kind of where I'm headed. I'd hate to ruin the originality of the vehicle, but with modern gasoline octane ratings we are all in the same boat.

My thought as a winter project would be install some 9.5-1 or 10-1 pistons and a mild cam to keep the output where it was with the 10.75-1 pistons. That way, original heads, etc.. and overall external look is there, but its a vehicle that can be driven anywhere.

Or, without getting too deep into the internals I may do the aftermarket heads.
Any suggestions on best combo to go with? Edelbrock? Which version runs better on 90+ octane , the 87cc or 72cc heads?
The 87cc bigger the cc the lower the compression. You can find 96cc like the 7K3 head, 114cc 7M5 head #66 are also a 114cc and the 96 is a 96cc and are fairly easy to find .I replaced my points with a Mallory unilte conversion around 100.00 buck's fits in stock cap with window. Works great. One of my 1968 goats still has orig 400 with 6400.00 original miles and she and most Pontiac have lifter noise it is common on Pontiac's for your valve train to make noise.
You can also check to make sure your lifters are pumping up. You can do this by pushing down on lifter end of the push rod you should be able to get it to move up and down, a solid lifter will not allow that. They are Hyd lifters some times getting crap in them wont allow them to bleed down or if there old they will not pump up spring gets weak. And when replacing the lifters make sure there Pontiac and Not Chevys there is a difference between them....Hope this helps happy motoring
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post #9 of 25 (permalink) Old 10-09-2013, 12:10 PM
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The way to plan it out is to decide on what you want for a target compression ratio, then see what it takes to get there.

First let me say that this whole topic tends to devolve into a religious war any time it comes up. There are people who can and do push these limits successfully, but doing so requires everything to be perfect and the engine constantly kept in perfect tune, cooling system optmized, planets aligned, animal sacrifices made to the appropriate spirits, etc.
What I'm going to give you are guidelines that I have found and also believe to be reliable, but I won't get into an arugment about them.

If you're running iron heads on 93 octane, then your target compression ratio should be in the vicinity of 9.3:1 to 9.5:1 (with closer to 9.3 being preferred).

With aluminum heads, you can (and should) run more. Somewhere in the neighborhood of 10.2:1 to 10.5.1. Aluminum is a better conductor of heat and tends to pull heat out of the chamber, which lessens the tendency towards detonation and also has a negative impact on combustion efficiency (you lose more energy to waste heat instead of using it to make power).

Zero-decking the block is a good thing to do for several reasons: it "hides" sharp cylinder edges that tend to form hot spots that lead to detonation, and it also promotes cylinder turbulence (due to the quench pad area) which keeps things nice and mixed, promoting an even burn. Zero-decking the block also raises compression ratio, so you have to know "going into it" whether you're going to have that done. (In order to get an accurate compression ratio you have to know what your deck clearance is NOW anyway - which means you've got to pull the heads and maasure it.) Pontiacs "usually" have the pistons "about" 0.020 "down the hole" when the piston is at TDC.

You also need to have accurate measurements of combustion chamber volume. Don't assume your heads are always going to have volumes that match factory specs. The volumes vary even on untouched heads, and the heads may have been milled. Even 2 or 3 cc's makes a difference.

You also need to know the bore size on your engine - what it actually is.

So with all that (did you get the point about needing to measure everything on YOUR engine?) If we make some basic assumptions which are:
389 Pontiac, bored +0.030
Standard stroke 3.750
Head gasket compressed thickness .045
Head gasket bore size 4.160
Deck clearance .020
Chamber size 69 cc's (usual for 389 D port heads)
Flat top pistons, 6 cc's in the valve pockets

That puts your static compression ratio at 10.06:1 --- too high for iron heads on 93 octane.

If we take the same parameters and ONLY change the heads, install 72cc aluminum heads, then you'd be at 9.76:1 --- safe, but actually too low for aluminum.

Keep the aluminum heads, zero deck the block: 10.187:1 --- dang near perfect PLUS you get the added benefit of improved quench from the zero-decking operation.

Lots of ways to skin this cat depedning on your other goals and constraints. For instance, keep the heads you have, don't zero deck the block, swap pistons for a set that have 14 cc's of D-shaped (D-shape VERY important) dish, and you're at 9.31:1 with iron heads --- again, dang near perfect for iron heads and 93 octane - AND - the engine still looks 100% stock (but probably won't make as much power as it would with aluminum heads)

One final thought: "Walking the razor's edge" on compression ratio in a street engine, in my opinion, just isn't worth it. On my 461 (which runs high 11-second et's in a 4000 lb car), the power difference between 10:1 and 10.5:1 would only make about 8 HP. Not worth the risk to me. Now if it was a race car and I was searching for every last smidgen of power, sure (but it'd be on race gas too), but in a street engine? Nah.

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post #10 of 25 (permalink) Old 10-09-2013, 12:28 PM
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If I had a '66 GTO with 28k original miles on it, here's what I'd do: If I drove the car less than 2500 miles every year, I'd buy race gas. (It's what I do with my '65 GTO). Not octane booster (unless it's TEL 130). Real race gas. Your '66n needs at least 100 octane with 200+ psi compression. Period. If I had your car (28k original miles, remember?) and wanted to drive it all the time, I'd simply bolt on a set of aluminum Edelbrock heads and run it. You'll be all set with 72cc E heads (10:1 compression) and 91-93 octane fuel. You won't have to tear apart the rest of your original, tight, and mechanically sound engine, either. The thought of tearing apart a 28k survivor 389 and refitting pistons, etc, to me is barbaric. But I'm a real square when it comes to this type of stuff. So: race gas or aluminum heads. Band-aids like thick head gaskets, timing curves, egr, etc, will NOT work.
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