[1969GPSJ]

So until I get aluminum heads for my 428 (16 casting, 72 cc, big valve, screw in stud heads), I have been considering using thicker Cometic head gaskets with my stock cast iron heads. Specifically I am looking at the 4.16 bore 0.080 or 0.098 gaskets. With the .080 compressed thickness gaskets, my static compression ratio would be around 10.06:1, with the 0.098, it would be around 9.70. Just trying to get the engine into the pump gas friendly zone to allow me to enjoy the car while I am getting another engine ready.


1) How does using a thicker head gasket affect engine operation outside of the obvious effect on the compression ratio?
2) What do you think the positive effects of the thicker gaskets would be?
3) What do you think the negative effects would be?
4) Which would be a better choice for my iron head - mostly stock fuel injected 1969 4 speed GP?



When I get the 72 cc aluminum D ports and a typical 0.041 gasket, compression would be 10.96:1 - should be pump gas friendly for aluminum heads.

[Goatracer1]

Are you racing the car? If not I don't see any problems using a thicker head gasket on a strictly street driven car especially for a short term solution.

[Gach]

I would never go with that tick of a head gasket. .080….What you need to determine is what is your deck height is. Are they -.010 in the hole or more. Or is it zero deck. In order to find out what your true compression ratio is. What cc the heads are also.

I edit it because Way too much information for what he’s asking.

[Johnny406]

Your #16 heads are likely going to be closer to 80cfm+ than 72cfm. If your block wasn't decked/resurfaced, you are likely to have your pistons .008-.015 in the hole as well. Measure both of those to verify and come up with actual compression instead of advertised. I'm betting you will be fine with what you have.

[1969GPSJ]

Nope, the 16 casting heads I have cc'd have been consistently right around 72 ccs (low end 68 ccs, high end 74 ccs, but the low end may have had been resurfaced). Pontiac was pretty good with the RAIII and HO "d" port chamber machining consistency. Post 1970 heads may have had larger chambers, but the '68 through '1970 open chamber high compression D port heads used on the performance 428s and 400s did not.

[1969GPSJ]

Originally Posted by 1969GPSJ

Nope, the 16 casting heads I have cc'd have been consistently right around 72 ccs (low end 68 ccs, high end 74 ccs, but the low end may have had been resurfaced). Pontiac was pretty good with the RAIII and HO "d" port chamber machining consistency. Post 1970 heads may have had larger chambers, but the '68 through '1970 open chamber high compression D port heads used on the performance 428s and 400s did not.

This specific 428 engine has not previously been apart, so the finish of the head and block would be as they were when they left the factory. I may end up using the Copper gaskets from Butler or another source.

Again, this is a somewhat temporary answer to getting the car back on the road. I have not dealt with Cometic's MLS gaskets in the past - I just have not needed anything that specialized, and the Fel-Pros worked just fine. I am just wanting a little more gasket to reduce compression from the factory 10.5:1 to 10 or 9.7.

[steve25]

Ok, I’ll let it slide this time, lol!

[younggto]

Do your deck surfaces and heads have the appropriate surface finish for Cometics?

[hurryinhoosier62]

Originally Posted by younggto

Do your deck surfaces and heads have the appropriate surface finish for Cometics?

Best question I have seen asked! ALL MLS head gaskets require a specific surface finish on the heads and decks. If yours don’t meet the specs don’t bother installing MLS head gaskets.

[chiefbigb]

You can lay the eye brows back in the combustion chamber and get 3cc there. Most of the untouched 72cc heads I have checked are 74 to 75cc

[Gach]

Originally Posted by 1969GPSJ

Nope, the 16 casting heads I have cc'd have been consistently right around 72 ccs (low end 68 ccs, high end 74 ccs, but the low end may have had been resurfaced). Pontiac was pretty good with the RAIII and HO "d" port chamber machining consistency. Post 1970 heads may have had larger chambers, but the '68 through '1970 open chamber high compression D port heads used on the performance 428s and 400s did not.

Are you running pump gas now ? Or you mixing it. I had 69 Grand Prix 428, and never had a issue with runing 93 octane pump gas.

[1969GPSJ]

Originally Posted by Gach

Are you running pump gas now ? Or you mixing it. I had 69 Grand Prix 428, and never had a issue with runing 93 octane pump gas.

Car has not been on the road for quite a while, but it was very, very octane sensitive - even with the best (non race) pump gas available, so much so that I had to dial back the timing so much (from 9-10 degrees advanced to 1-2 degrees retarded) that it ran poorly, and was very sluggish. With higher octane fuel, I could run up to 12 degrees advanced, and the car's throttle response was night and day different. Maybe with a cam change it will be less sensitive to the octane. It has the original 067 in it now.

[Gach]

If it’s that sensitive my guess is you got something else going on. Really I ran mine with no issue on 93 octane, for years. Here’s a possibility and I’ve seen this a more then a few times. Some times your running what you think is total timing 32-36 degrees, but if there’s issues with the distributor, for example you put a time light on it and say at 1500 rpms. Time in while it says 32-36 degrees But ! You bring it up to say 3000-3500 and it keeps advancing, I’ve seen the advance up to 50 degrees. But you don’t know if that’s happening say your first instinct is kicking it back 10 degrees then it feels sluggish. Just something did you want to check out and make sure that it’s not advancing. Honestly I never had any issues with my 69 Grand Prix 428. Running on 93 octane. Just a thought something you might want to check out.

[Gach]

What do you mean by the original oh 067 ? Do you know what number the heads are on the motor. Another question what octane pump gas fuel was you running

[1969GPSJ]

Originally Posted by Gach

If it’s that sensitive my guess is you got something else going on. Really I ran mine with no issue on 93 octane, for years. Here’s a possibility and I’ve seen this a more then a few times. Some times your running what you think is total timing 32-36 degrees, but if there’s issues with the distributor, for example you put a time light on it and say at 1500 rpms. Time in while it says 32-36 degrees But ! You bring it up to say 3000-3500 and it keeps advancing, I’ve seen the advance up to 50 degrees. But you don’t know if that’s happening say your first instinct is kicking it back 10 degrees then it feels sluggish. Just something did you want to check out and make sure that it’s not advancing. Honestly I never had any issues with my 69 Grand Prix 428. Running on 93 octane. Just a thought something you might want to check out.

Hi Gach, in my first post of the thread I mentioned that the heads are 16 casting big valve, screw in stud 72cc (nominally) heads. I have not had the heads off of this engine. The engine, being a '68 428 originally in an automatic car came from the factory with either the 9779066 or 9779067 camshaft and the 1111959 distributor. When I got it, it had a 1970 RA III stickshift quadrajet - which was rather odd, but a windfall.

In the past, I put a timing light on it (I have a dial back light, so I can check the timing throughout the range) and it was in spec. During an inspection it was obvious that there weren't any missing bushings or pins in the distributor. Springs were stock, weights were stock. Vacuum can was fine and functioning. Could not find any problems with the advance mechanism at the time. Checked the balancer with a piston stop, and the balancer had not slipped (at that time). I don't now have faith that the elastomer is serviceable, and I expect to change the balancer as part of getting the engine back to running and reliable service. Again, it ran well with a blend of race gas and 93 octane, but sounded like ball bearings in a tin can with just the 93 at 9-10 degrees of timing. To drive it on 93, I had to retard the timing by 12 degrees from stock. Not fun.

[Dragncar]

If you need something to get you by until a full rebuild, maybe water injection. Its bolt on, don;t even have to pull the heads.
Modern ones are 500ish $

Designed for boost but they have a controller so they should be able to work without it. Call tech. This one is for a carb or throttle body.
https://www.summitracing.com/parts/sno-20010

Or old school. 90$ on E Bay. I ran one back in the 80s for a bit.
https://www.ebay.com/p/1728425361

Might get you by and better than a quench lilling .080 head gasket.

[1969GPSJ]

Originally Posted by Dragncar

If you need something to get you by until a full rebuild, maybe water injection. Its bolt on, don;t even have to pull the heads.
Modern ones are 500ish $

Designed for boost but they have a controller so they should be able to work without it. Call tech. This one is for a carb or throttle body.
https://www.summitracing.com/parts/sno-20010

Or old school. 90$ on E Bay. I ran one back in the 80s for a bit.
https://www.ebay.com/p/1728425361

Might get you by and better than a quench lilling .080 head gasket.

I used water injection on another car a number of years ago. Edelbrock actually had a system at that time. Worked really well with 50% distilled water and 50% methanol. I just seemed to run out at the most inconvenient times. If you used tap water the spray nozzel would end up getting hard water deposits and clog.

[Dragncar]

Water injection today is not the same as the old Edelbrock Vari-Injections. Technology always moves forward.
I doubt the folks running these modern water-meth injection are saying "don't waste your money".
There is some interesting new systems out there, most made for boosted engines. You have to look for old school NA carb water-meth systems.
But it does work, dyno proven .
https://prometh.com/collections/carb...ts/mist-a-carb

https://www.summitracing.com/search/...ection-systems

Here is a MT article on a boosted 302 in a 65 Mustang.

https://www.motortrend.com/how-to/ho...onation-blues/

428 Pontiacs, that 390 HP 428 had the highest horsepower of any Pontiac production engine.
With D port heads and cast pistons. It should have been in GTOs and Firebirds. Chevy had 427s but we could not have 428s ?
And just think is we would have been allowed to fight fair with high CR 428s and 455s with forged rods/pistons and RAIV heads and the rest of the goodies ?

[Gach]

Originally Posted by 1969GPSJ

Moonshine (with a little water) or Vodka also worked.

They do make 120 Proof Vodka..LOL just carry gal of distilled water and a fifth of vodka. Its funny with todays marketing they can sell to allot of still wet behind the ears. It did work pretty good on my buddy’s Chevelle. Long enough to make to next package store.LOL. Funny guy saw the bottle in his trunk he said. No No I’m not a alcoholic.

[Cliff R]

NEVER for any reason use a thick gasket to lower compression on one of these engines for pump gas. Here I will not for any reason build one of these engines with over .040" quench. These crappy combustion chambers don't like that deal at all and you'll just knock all the balls right out of it and make it want to run hotter, overheat, and ping harder all else being equal.

I've tuned scores of 400', 428's and 455's pre-1971 with higher compression to run fine on this new fuel. Not exactly sure why yours is so octane sensitive? I've ran into a few where I had to shorten up the timing curve and reduce the amount added by the VA, but have always been about to get a good end result IF they were using factory camshafts.

I have ran into a good many that you were NOT going to make happy with any sort of tuning. Most of those were 455's and a few big block Chevy engines.

Having had several 428's in the Ventura over the years I will say that the dished piston versions are finicky due to the added quench area. One of my 428's was only 8.8 to 1 compression and would ping if you tried to run very much total timing on it. It also heated up very quickly and the temps would "creep" up in slow traffic and at stoplights making it rattle even worse till you got moving and cooled it back down some. This was WAY back before I knew about tight quench and selecting the right camshaft. It ended up having a bad crankshaft and had to be removed but I was glad because it was not happy anyhow.

I did some research and talked to the folks at HO Racing and installed the HC-01A camshaft in the next engine. Since I cracked the #96 heads with the overheating previous engine I used 6X-4's instead. That engines was an absolute HOME-RUN and powered the car for nearly 20 years until I built my first 455 for it.

In any case as it pertains to what you are doing here I would NOT lower the compression. Change the cam instead and pick one with a wide LSA and about 15-20 degrees more seat timing than the 067 cam. Moving up to a real 068 would really help, and even on up to the Summit 2802 would fit the bill nicely and still have good idle quality and street manners while lowering dynamic compression at the same time.

I'll add here that static compression is only one "player" in this game, and whoever came up with the proverbial "brick wall" of 9.5 to 1 for pump gas should be banned from posting on the Internet for at least 100 years. That is one of the dumbist statements I've ever heard when it comes to this hobby......FWIW......