[dhcarguy]

My engine was in the car when I bought it almost 20 yrs. ago. I have put 21,000+ miles on it. (total on engine 40,000+ miles). I've always had to add race fuel to the 94 octane and back down timing a couple of degrees to stop it from detonating.
With 93 octane, I need to add a little more race fuel. Since I retired, I drive the car a lot more. It's getting expensive.(1 gal of 116 oct. to <7 gals of 93 oct). I have asked a few local (smart) car guys about installing thicker head gaskets, like cometic .066 crushed.( felpro 8518PT are in it now). I am getting mixed answers, and "quench" was mentioned also. NOT sure what quench.
From my documentation book, I have a receipt from "G&R Performance" in Babylon, NY., 502 Rutgers Rd. Dated Oct. 1995. This is what's listed:
1- boiled block and installed cam bearings.
2- Milled the block. (I was told it was "0" decked)
3-bore and hone .030 over
4- polish and radius crank
5- clevite main bearings
6- balance assy.
7- miloden rod bolts
8-seal power rings
9- Lunati cam and lifters (230@50 dur. 460 lift).
10- boil and peen #77 heads. NOT ported, bronze guides, stock SS valves.
11- miloden screw in studs
12- heavy duty seal power springs
13- HS 1.5 roller rockers
14- I put on 65' tripower intake and correct carbs
15- stock exhaust manifolds and 2.5" Pypes "X" pipe system
Do you think the Cometic gaskets will help or make things worse???
thanks, Dave

Cam card

[n20ta2]

Im not sure on the gasket thickness curing the detonation issue but I think Cometic gaskets require a certain deck finish to seal properly. Donyou have them on there now?

[n20ta2]

50 RA For the mls gaskets I believe.

[68WarDog]

#64 or 6x4 heads.

[Schurkey]

Originally Posted by dhcarguy

1- boiled block and installed cam bearings.
2- Milled the block. (I was told it was "0" decked)
3-bore and hone .030 over
4- polish and radius crank
5- clevite main bearings
6- balance assy.
7- miloden rod bolts
8-seal power rings
9- Lunati cam and lifters (230@50 dur. 460 lift).
10- boil and peen #77 heads. NOT ported, bronze guides, stock SS valves.
11- miloden screw in studs
12- heavy duty seal power springs
13- HS 1.5 roller rockers
14- I put on 65' tripower intake and correct carbs
15- stock exhaust manifolds and 2.5" Pypes "X" pipe system
Do you think the Cometic gaskets will help or make things worse???

1. WHAT PISTONS? What displacement?

2. Who has tuned the carbs, and who has tuned the distributor advance curves--vacuum and centrifugal? What is the initial advance? What is the total timing? Vacuum advance on manifold or ported vacuum?

3. Was the cam "degreed" or merely installed dot-to-dot?

4. What is the cranking compression with a fully-charged battery? What is your elevation?

Thicker head gaskets are likely to make things worse unless you go unreasonably thick so as to gain "open" combustion chambers, with a loss of power. I would absolutely not rip the engine apart until I had played with the carbs and distributor, and made sure the cam was in time.

[Tom Vaught]

How far down in the hole are your pistons?

A .060 Gasket only leaves you a few thousands before you wander into high probability
of combustion issues and detonation. A .040 gasket would be better.

Tom V.

[JLMounce]

What cid is this engine? Is it still a 389 under the #77 heads?

Your short bio indicates you are in PA. I believe there is fairly wide adoption of e85 at the pump in many areas of PA. This could be a decent solution for you that doesn't require taking the heads off.

I dealt with this exact same issue in my bird. Purchased the car with the engine already in it and it pinged even on 100 octane fuel @ 7.52/gallon. I ended up going through the entire top end and replaced the heads with KRE D ports and went to a larger roller cam. That solved the issue and gave me a good bit more power to boot.

However it was an expensive proposition in itself. E85 would provide the knock resistance necessary for your combination. You'd need to set up your fuel system to handle the added fuel requirements and ability to handle E85. If you're still running a carb, that would need modified to provide proper fuel metering as well. It would however be more budget friendly then swapping heads.

I will say this, if you're going to go to the point of taking the heads off, I wouldn't bother with thicker head gaskets. There's real possibility, depending on how far in the hole your pistons are that loss of quench distance may exacerbate your problem. If the heads come off, measure your piston to deck height, determine proper head gasket thickness to achieve around .040 quench distance, then choose a head to go on the engine that will achieve around 9.3:1 compression if you're staying with a factory iron head, or around 10:1 compression if you're going to a modern combustion chamber aluminum head.

If you're going to that trouble, make sure you solve the problem for good. One thing to consider is that going to a later style head may require a number of changes that could include changing the intake manifold or even changing pistons depending on the type installed and how much net valve lift you have. With your older closed chamber heads, I would lay odd you have the 8 valve relief pistons in that engine, which may be contributing to the problem. They would however allow you to move to a later head without worrying much above piston to valve clearance.

[Formulajones]

How big is the engine? What's the compression ratio.

At a quick glance I'm not so certain the camshaft is doing you any favors as far as cylinder pressures are concerned.

Thick head gaskets aren't really going to help the issue, actually that may likely make matters worse.

Cam install position is a big player. Lots of other small things factor in, carb, distributor, heat, etc...

Need more info

[25stevem]

You have a lot more fuel and air entering the cylinders with that cam, but the exh side of all the street production 326/389 and 421 heads just plain sucks!

The motor can not pump the burnt gasses out efficiently and if your making the mistake of retarding your distributor timing, then that helps the pinging situation out at low speed, but makes things worse during heavy throttle since the motor was relying on cylinder pressure too much to pump out the exhaust thru those stock restrictive ports.

I have seen this countless times over the decades with these heads !


I also guarantee your carbs are too lean with that cam now in the motor.

Your jetting on the center carb needs to go up to .068” to .070” and the end carbs need to go up to .070”.

And this is for starters!

What do your plugs look like ?

Take some side photos of the ground electrode on 4 of the plugs and post them up.

What does the motors hot cranking compression check in at?

[dhcarguy]

My engine is a 1966 389cu in. that has been bored and honed .030 over.
Stock 1965 #77 heads. Harland Sharp 1.5 roller rockers, heavy duty springs.
Felpro #8518 gaskets in the car currently.
They are Flat top pistons. Was told they are zero(flush with deck. Not sure
Carbs are tuned with cam in mind. Car does run real good
Was told by previous owner compression is near 11:1. I have no way to know for sure.

All this is above my paygrade. I am NOT a mechanic. I can take anything apart and put back together, but I do not know what makes the internals of an engine perform perfectly.

Thanks guys for your time

[Jay S]

The .060” Cometic head gasket would not change the static compression enough to be worth the time, it might even run worse.

I think it would take a minimum of a .090” thick Cometic to have any success. Probably about .1” would work the best, then use cometic’s thick intake gaskets to line the intake back up. I have an engine that runs .14” thick Cometics, and another that has .075”.

[shaker455]

Has your distributor been built for your combo?
Was your carb dialed in with a wideband?
How hot does you car run?.. ever get over 190 degrees?
Are your exhaust crossovers active?

[MidnightAuto]

77 heads on stock motor is 10.75 to 1 I believe.

[25stevem]

With some quick math I just did and assuming
1) your deck height is zero.
2) your head gasket volume is 9.4 CCs.
3) your piston notch volume ( 4 notches) is 6.7Ccs
4) your chamber volume is 70 CCs
And we know your at .030” over, then yes your compression is then between 10.6 and 11 to one with the veritables involved in the above numbers.

[Jay S]

389 has smaller valve reliefs than the later engines. Less than 6.7 cc. Maybe 4? It probably is almost 11:1

[dhcarguy]

Originally Posted by Jay S

389 has smaller valve reliefs than the later engines. Less than 6.7 cc. Maybe 4? It probably is almost 11:1

They are Flat Top pistons

[dhcarguy]

Originally Posted by MidnightAuto

77 heads on stock motor is 10.75 to 1 I believe.

Engine is Not stock

[dhcarguy]

[QUOTE=shaker455;6384233]Has your distributor been built for your combo?
Was your carb dialed in with a wideband?
How hot does you car run?.. ever get over 190 degrees?
Are your exhaust crossovers active?[/QUOTE

1- stock dizzy with Pertronix ignition.
2- correct tripower carbs., bigger than stock jets and opened venturi tubes slightly. 1/4" phenolic spacers on all three. Adjusted with vacuum gage.
3-Only runs above 190 when on the highway driving over 60mph. will get to 205degrees. Rear gear is 3:90 with NO overdrive. ,4sp Muncie.
4- crossovers are not blocked.

Car will stay under 190 degrees all day long around town ,even in traffic in 90degree weather.

[Half-Inch Stud]

Well, the thick Cometic is a solution for High-Compr 400 heads on a 455. 0.080" Cometics hang in waiting if/when a Spare 455 gets going again. The theoretical Ping/fouling is mooted by the long-stroke slug speed for plenty of squish effect.

Otherwise, my running engine went with Ross Blower (dished) pistons for the RA96 IIs on the 470.

[25stevem]

The pistons have to have at least 2 valve notches if there aftermarket.

Please when you have time and before making any changes do a cranking compression test and report back .