# 66 389 timing



## Kjp (Oct 8, 2013)

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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## 05GTO (Oct 6, 2004)

You could try a thicker set of head gaskets to reduce the CR,

Cometic head gaskets


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## jmt455 (Mar 26, 2012)

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.


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## Kjp (Oct 8, 2013)

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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## Instg8ter (Sep 28, 2010)

:agree, 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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## BearGFR (Aug 25, 2008)

Kjp said:


> 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).

Bear


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## Kjp (Oct 8, 2013)

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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## GoatGuru (Oct 1, 2013)

*66 timeing*



Kjp said:


> 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.
> 
> ...


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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## BearGFR (Aug 25, 2008)

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.

Bear


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## geeteeohguy (Feb 2, 2008)

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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## Kjp (Oct 8, 2013)

Thanks All!

Heads on the car are the 093, 68cc factory ones.

I found a source for the VL 110 race gas about 15 miles away, bought a few 5 gallon drums of it. Mixed in one drum with the 93 octane, and no pinging. The stuff is expensive, about $55 per drum. 

I do expect to drive the car a good bit, drive across the country if I wanted to, so the Aluminum E 72cc heads are probably the best option. Or if I can locate a set of cast iron heads with the 87cc to run on available 93 octane to keep the compression in the mid 9's, and keep the factory look.

If I understand correct, If I would mill the original heads out to 72cc, compression would drop down to an almost acceptable 9.76:1. So milling out my cast irons to approx 74cc would drop this down below 9.5:1.

I was working online with this comp calculator - Compression Calculator by ZealAutowerks
It was giving me a reading that looked like 87cc may be the best option to keep ratio in the mid 9's? Not sure on the accuracy with it, since when I put in the stock numbers, I was getting an 11.51:1 ratio.... a full point higher than what the factory said!


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## BearGFR (Aug 25, 2008)

You really do have to measure everything and take it all into account. Even though the factory specs say 10.75:1, in actuality it's nowhere close to that. 10.75 was the "blue print" spec - meaning that's what you'd have if you machined everything in the engine to the tightest possible published factory numbers. Production-line engines never came close to those measurements.

I have an Excel spreadsheet I wrote to do all the calculations and also to make it easy to tweak numbers here and there to see what effect the changes have on compression.

I know I've posted it on here several times in the past, if you use the search tool on here you can probably find it.

Bear


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## geeteeohguy (Feb 2, 2008)

Another thing: the valve angles on the '66 and earlier 389's are slightly different than the '67 and up engines. With a big camshaft, late model heads will hit the pistons. But not with smaller oem type cams, like the 068. So, you may be able to get a set of 87cc iron heads and run them. I installed a pair of #15 heads on my '67 GTO, and the CR is right at about 9.3:1 (they were 87cc before the clean-up and valve job...more like 84-85 now). What I learned was that the cost of upgrading the 87cc iron heads was almost the price of superior, lighter aluminum heads. I would not do it again. You have several head choices, most common being the #15 (1970 455 heads) and the sought after and pricey #64 heads (1970 GTO 455 only). There are other iron heads that are 'close'. Also, a set of 092 heads is about 69-70cc, so you could get a set cheap, have them modified to 78-80cc and keep your original 093 heads to maintain the car's value over time.


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## GoatGuru (Oct 1, 2013)

geeteeohguy said:


> 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.


Stay away from aluminum heads keep with the cast unless you want to run CHEVY VALVES and SPRINGS like Edelbrock does. In my opinion Edelbrock heads are junk and are not worth the price. I have bought a set and took them off in the first week. I don't know why this other guy keeps trying to get you to buy aluminum heads. You will run into head gasket issues due to the dissimilar metals (AKA aluminum expands/contracts at adifferent rate than your cast block) no reason to chance doing it again. If you do go that route get Kre-D .http://www.tinindianperformance.com/KRE%20aluminum%20d%20port%20pontiac%20head.$1,950.00 a pair 
Edelbrock htm.EDL-61575 for hydraulic roller cam for $2,319 or EDL-61579 for hydraulic flat-tappet cam for $2,199 per pair

Read more: Edelbrock Performer Pontiac Aluminum D-Port Cylinder Heads - Project Pure Poncho - High Performance Pontiac Magazine. Tell me, you do the research and here is the info. I put a set of Kre-D on one of my stock 68 gto's and it runs like a charm with a 9.1:1cr unlike the Edelbrocks. And there Pontiac valves & springs. P.S find your self a set of 6X-8 heads they are a large chamber head that will give you 7.6:1cr Or a set of 6X-4s, but the 6X-4 are a 350 head with a small chamber and they are a 8.2:1cr and after 1977 they have hardened seats already. 2:11 intake 1:66 exhaust with screw in studs. All stock 6X heads can be reconditioned, milled, and performance enhanced. The other thing is AV Gas (aviation fuel 150 octane) get it and blend it with regular gas to give you 130 octane and it will be cut enough you want disintegrate the rubber components in your fuel system. Best way right there saves all the problems and arguments. Your motor is running at least 10.7:1cr. The thicker head gasket wont drop the CR enough to make it able to run on pump gas. You need to be at 9:1 max or less to run on pump gas without pinging or knocking. Changing the timing curve will not work either and can potentially harm your motor. To lean or to rich and you will melt your pistons or valves or both. P.S. My opinion from personal experience change the lifters and cam and go with a Mallory Unilite distributor module fits in the stock cap. No timing flutter and a set of 6X heads and your golden.


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## rickm (Feb 8, 2012)

drastic altering of that engine to me would be a crime.


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## BearGFR (Aug 25, 2008)

Um... A valve, a spring, or any other part for that matter doesn't know what "make" it is. What matters are the materials, dimensions, specifications, and the craftsmanship of the part. My round port Edelbrock heads are fitted with 2.185 intakes and 1.77 exhausts and they've been mildly ported by one of the best in the business. Compression at present is right at 10.0:1, which is a few ticks low for aluminum. The car weighs 4000+ pounds with me in it, runs high 11's at the track, and completed the entire Hot Rod Power Tour last summer with nary a peep - all on 93 octane.

When reading any magazine, the first thing one should do is pay attention to all the ads. You'll find there's a direct correlation between the ad content and the vendors / parts that are "recommended" in the magazine's articles.

The recommendation for the aluminum head swap is very simple. It's the one solution that fixes the compression problem (and provides hardened valve seats for unleaded fuel) so that he can actually enjoy the car and drive it anywhere on 93 octane without losing any performance and also not requiring any other changes or machine work to the engine. It also preserves the ability to restore it back to 100% original at any time, all with the least amount of fuss. It's probably also the least expensive way for him to get to where he's said he wants to go. All he'll need are the ready-to-run heads, a new set of head bolts to match them (lengths often vary from stock Pontiac D-port), and gaskets.

I's also steer him towards the KRE's or the newer D-port Edelbrocks just because they're also D-port heads and won't require changing anything on the exhaust system.

Bear


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## Kjp (Oct 8, 2013)

Some good options and very good advice here! 

I would prefer to keep the 'original look' as much as possible, but also want to be able to drive and enjoy the vehicle too. 

72cc Edelbrock or KRE alum D port heads seem like a very good bolt on option.

Can the 093 original heads be modified or milled out to a larger CC size to take care of the compression issue? This way, I have the original look, numbers, etc.., but also the benefit of using the original heads.

Let's face it, gasoline is not what it used to be, and I don't think it is going to be getting any better with more additives, ethanol, etc. blended into it.


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## GoatGuru (Oct 1, 2013)

Kjp said:


> Some good options and very good advice here!
> 
> I would prefer to keep the 'original look' as much as possible, but also want to be able to drive and enjoy the vehicle too.
> 
> ...


Butler Performance - Specializing in Pontiac Engines Heads and Performance Parts Yes you should be able to if you go to the right person who deals with Pontiac. Here you go its not cheap but what is any more.. Travis


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## BearGFR (Aug 25, 2008)

Kjp said:


> Some good options and very good advice here!
> 
> I would prefer to keep the 'original look' as much as possible, but also want to be able to drive and enjoy the vehicle too.
> 
> ...


 I've attached a copy of my compression ratio tool to this post. It's a Microsoft Word document with an embedded Excel spreadsheet (read the directions at the top of the document).

I've saved this copy with the _usual_ 389 measurements. Please notice I said _USUAL_. Pontiac heads have been known to vary in chamber size a few cc's either direction, even untouched from the factory. As you'll see when you play around with the tool, even a few cc's makes a significant difference. Regardless of what anyone else might try to tell you, you're just not going to know what the compression ratio on your engine is until you tear it down and get all the measurements.

For example, if I _ASSUME_ your engine has all the usual Pontiac specs (which are standard 389 bore, standard 389 stroke, piston deck clearance of 0.020", head gaskets with 4.16 bore and 0.042 compressed thickness, 6 cc's of volume in the piston top valve pockets, 68 cc's in the head chambers) then your engine works out to 10.096:1 compression (nowhere near 10.7) but that's still too much for 93 octane. As an example of how things affect CR, do nothing but bore the block 0.030 over, and all of a sudden your CR is now 10.230:1. Let's keep going and say a previous owner did a valve job on the heads, and they needed to be milled some (say, oh 0.025) to get them flat, and that reduced chamber volume from 68 cc's to 63 cc's. Now you're at 10.789:1. Take it one more step and say he also had the block zero-decked while it was apart. Now you're at 11.3:1 (!) all with very minor machine operations that could have been done over the years.

Now see why measuring everything is so important?

To answer your question: "Can the 093 original heads be modified or milled out to a larger CC size to take care of the compression issue?", let's go back to the tool and use our basic assumptions (only we'll keep the 0.030 overbore just in case). Playing only with chamber volume, you'd need 78 cc's to get it down to 9.278:1 - that's punching out each chamber by an additional 10 cc's.
The MOST you can mill _off_ an untouched 65-67 head is about 0.065, and that _reduces_ the chamber size by 10 cc's. Beyond 0.065, the head decks are too thin and the head is trash. So to get an additional 10 cc's into the chambers you'd have to cut close to that amount (0.065) off the entire chamber surface. Besides being very difficult _and expensive _to do (cutting the contoured surface of the chamber face as opposed to the flat surface of the deck) is going to leave them very thin, right where you need strength the most. After all, the chamber top is all you have to withstand the pressure and heat of the combustion process. This area sees a whole lot more stress than the deck surface does. I dunno, and I'll happily defer to anyone who's had direct first-hand experience doing this (as opposed to just reading magazines) but it doesn't seem like a good idea to me. 

(Your mileage may vary, void where prohibited by law, slippery when wet) 

Bear


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## geeteeohguy (Feb 2, 2008)

Aluminum and iron are, indeed, dissimilar metals. That said, not a big deal. Most cars these days, and built within the past 25 years, have iron blocks and aluminum heads, like the two Toyota trucks I drive daily that have 270,000 and 202,000 miles on them that have never been apart. There are a lot of different options out there, and diligent research will show the most prudent choice for our original poster. Lots of folks running E-heads like them a lot and are enjoying good service out of them.


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## Kjp (Oct 8, 2013)

Well, about ready to pull the trigger on this project. Have been running the car on race gas all of last season. Aluminum heads with the 87cc chambers seem to be the safest choice. 72cc will work, but with the 389's valve angle, this may be cutting it a bit close. I spoke with Butler is about 99% sure this will work, and Edelbrock is positive as well. As long as I don't put any cam in with a greater than .450-ish lift. Most I'd do is a set of the Ram air cast headers and maybe an intake/carb swap to take advantage of the increased flow thru the heads.

Has anyone had any experience or know of anyone running Edelbrocks or KREs on a 389 or similar vintage Pontiac with the valve angle limitations?

This is the most non-intrusive way to run the car and be able to take it cross country if I want to.


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## PontiacJim (Dec 29, 2012)

First, todays gas has ethanol and ethanol makes the engine run leaner. First thing I would do is fatten up the carb jets several sizes to get the engine to run richer.

Have you considered a water injection set-up that activates under wide open throttle? Seems this is overlooked as a means to cool the incoming mixture and reduce detonation. Do a little research on it and there are a number of systems available.

Your 093 heads are closed chamber. It is possible to open them up to a semi-open chamber that may give you the extra cc's you need. H-O Specialties had a _Pontiac Engine Design and Blueprinting_ booklet. Pg 68, Pre-1968 closed diagram with area to remove. http://pontiacformula.free.fr/schema/Pontiac_Heads.pdf Ames also sells this booklet.

I would also make sure my distributor was working correctly with its mechanical and vacuum advance. The springs, weights, and vacuum advance are all possible adjustments that affect your timing curve which might need to be tailored to your engine's needs. 

It is also possible to select a different cam grind that would lower your dynamic compression while keeping your 10.75 static compression. You can discuss this with a Pontiac engine builder or cam supplier. Here is a discussion that gives you some idea of what this is all about. http://pontiacformula.free.fr/schema/Pontiac_Heads.pdf

I think I would exhaust all possibilities that might be incorporated in your original engine using the iron heads before going aluminum. *Bear* has pretty much covered the aluminum head approach, so its your call.:thumbsup:


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## geeteeohguy (Feb 2, 2008)

If you put 87cc aluminum heads on a 389, it'll be low powered and inefficient. You do not want to run 9:1 with aluminum heads. You want a minimum of 10:1 CR to make any power due to aluminum's thermal loss. Takes a full CR point to make up for this loss that iron heads are so efficient at. 72cc heads are the ones you want, or even a bit smaller. Valve angle should not be a problem if the cam is reasonable. You will need to check it. Again, running 87cc heads on a 389/400 engine=turd.


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## PontiacJim (Dec 29, 2012)

Also found this tidbit: *Gasoline quality*: With musclecar-era leaded gas, the lead is attracted to the hotter (core-nose) part of the plug, causing glazing. The spark runs down the core nose instead of jumping the gap. Going to a slightly colder plug helps prevent lead-glazing. However, with today’s cleaner-burning oxygenated unleaded gas, an equivalent engine needs to run plugs about 1-2 heat ranges hotter than originally specified (many plug manufacturers have revised their catalogs accordingly).


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## Kjp (Oct 8, 2013)

Thanks guys, I purchased the 72cc Edelbrocks and am in the process of disassembly/ reassembly. The most I'd go on a cam upgrade, if needed, would be the 068 grind from Melling, along with new lifters, pushrods, and 1.5 rockers. Of course, I'm in there so I guess why not now... That should be very safe with the valve angle from all the references I've seen and been forwarded. Of course, I am planning on testing and rotating the engine by hand when I get the first head on.

Believe me, I've tried different timing, fuel mixtures, etc... Nothing keeps this car happy with the stock heads except a good healthy mix of VP 110 and some 93 octane. And can't exactly carry a trunk full of the race gas to the GTOAA and POCI nationals next summer, need to fill up with 93 or best available along the way.


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