[steve25]

In a reprinted interview from the 70s in a 2001 issue of Pontiac enthusiasts mag, Malcolm Mckellar stated that published compression ratio's of 10.75 by the factory where way more like a true 10 to 1!

Only when the HP ratings where switched to net do the compression ratio's reflect true world numbers it would seem.

[TedRamAirII]

I would agree to that. My car ends up (using published numbers) at 10.3 where spec. is 10.75.

[geeteeohguy]

Originally Posted by steve25

Do be looking for any big peak power gains by going up in compression only 1 full point or so!
I will see if I can find my dyno sheets , but in the late 80s on a mild 455 build we base lined the motor with its bowl ported and milled 9 to one comp casting 6H heads and then later that same day after tweaking the motor with over the course of 4 other pulls swapped over to very much milled big valve cast number Heads .

Both sets of heads where bowl ported and flowed very close to each other

The 16 heads kicked the comp up 3 full points, and the end result was far less then we had hoped, we saw like only a 12 to 15 hp gain at peak rev's and about a average 9 ft lb gain top to bottom!

The engines Vacuum increase was very nice though!

The bottom line is that if your compression is in line with what your Cam pick calls for you could be going thru a lot of gyrations for not much gain unless we are talking about a all out race motor!p, or making up for having too much Cam in a street motor.

Do, or Don't.....Years ago I pulled the 12's off of my '67 4oo due to no available gas to run them. I installed a set of huge chambered 66 heads. Dropped the CR from 10.5 or so to 7.25. Fuel mileage and performance plummeted. Went from 16 mpg to 9, and the car was an absolute slug. A few years after that, I yanked the 66's and installed a set of 15's, bringing the CR up to about 9.3. Only 2 points higher than the 7.25. Performance took a dramatic gain: I instantly got 5-6 mpg higher, and the car felt like it had 100 more horsepower. I't been my personal experience over the past several decades that the smaller Pontiacs at least (389--400) like a decent amount of compression.

[STEELCITYFIREBIRD]

So how thick were the factory gaskets in the late 60's??

[Formulajones]

There may be deviations from this, but most of them were a steel shim gasket of about .015" thick.....compressed.

[Formulajones]

Originally Posted by steve25

In a reprinted interview from the 70s in a 2001 issue of Pontiac enthusiasts mag, Malcolm Mckellar stated that published compression ratio's of 10.75 by the factory where way more like a true 10 to 1!

Which is exactly what I've discovered with some virgin engines I've done lately.

[STEELCITYFIREBIRD]

Originally Posted by Formulajones

There may be deviations from this, but most of them were a steel shim gasket of about .015" thick.....compressed.

Wish you could still buy them..assuming you can't?

[Greg Reid]

Again, good info and much appreciated here.
My compression ratio is what it is. This is an original '66 WT 389. I just refreshed it with new rings, bearing, seals, valve job, guides, positive seals and so on..but other than that, the pistons, rods and crank are original. Whatever the compression was in 1966 is what it is today. Because of that, I'm paying attention to this whole 'squish' thing.
I re-measured the gasket and it's actually .046" but still too thick I think.
I'm aiming for the mentioned .040" squish so I figure I need to start at .039" uncompressed, max. Si?

[b-man]

Greg,

I'd be looking to either deck the block to zero and run a Felpro 1016 at .039 crushed thickness or just pony up for a pair of .027" Cometic gaskets if you don't decide to deck the block so you end up at .034" squish.

It's also not too big a deal to take a few ccs out of the chambers before you button it up to drop the compression by say a quarter point.

Bart

[lust4speed]

Most everything in our car club with aluminum heads is 10.3 or a tad lower because of the 91 octane gas here. Had one engine that was a little sensitive to mid-range vacuum advance and sometimes full throttle in the middle of summer. Cam was an off-the-shelf Comp XE284H-10 that is a 240/246 @ .050". Heads were Edelbrock 87cc (89 actual) heads ported out to 305cfm by Butler.

It was an older build, and engine ran into some unrelated problems so it came out and apart. Pistons were .010" out of the hole which made compression exactly 10.6:1. With the .039" thick Felpro gaskets, this made for a nice tight .029" quench.

Detonation was very mild, and some screwing around with limiting vacuum advance and really slowing it coming in allowed the old engine to function properly at 38° total... but the true 10.6 even with that cam was right on the line.

Owner wanted some really docile street manners so a mild Hydraulic roller cam (224/228 on a 112) was designed to grant his wish. We ordered new pistons with a small dish in them to bring the compression down to a true 10.2:1 because of the new smaller cam and previous pinging encounter. Engine is very happy at any ambient temperature now. Side benefit is it idles like a kitten but has a ton more power now in the range he drives in (idle to coming out of passing gear at 4,800 RPM).

[Formulajones]

Originally Posted by STEELCITYFIREBIRD

Wish you could still buy them..assuming you can't?

Yes they are available. Aftermarket companies make them and are sold through Summit etc...

[Formulajones]

Originally Posted by Greg Reid

Again, good info and much appreciated here.
My compression ratio is what it is. This is an original '66 WT 389. I just refreshed it with new rings, bearing, seals, valve job, guides, positive seals and so on..but other than that, the pistons, rods and crank are original. Whatever the compression was in 1966 is what it is today. Because of that, I'm paying attention to this whole 'squish' thing.
I re-measured the gasket and it's actually .046" but still too thick I think.
I'm aiming for the mentioned .040" squish so I figure I need to start at .039" uncompressed, max. Si?

A lot of machine shops, including the one I used recently, doesn't recommend to "zero" deck the engine. Usually .005" in the hole is left to leave room for a future deck clean up if the engine gets refreshed again. Sort of leaving yourself room later down the road. Especially since these old blocks are getting used up and harder to find.

With that thought process in mind, and using a relatively inexpensive (compared to Cometic gasket) .039 compressed Felpro gasket, will get you into an acceptable quench range that works fine for a street car. This is exactly how I just did my Pontiac engine. Pistons are .004" in the hole. Using this gasket my quench is .043" which is just fine. It's 9.68:1 with a factory iron head and a Melling RAIII cam advanced 6 degrees. It's running absolutely perfectly on our cat pee 91 octane and has shown no detonation with 34 degrees total advance even in our current 105 degree temps.

[Formulajones]

other examples I mess with here......

I also run our DZ 302 at 11:1 compression with it's factory iron heads/large dome pistons. It figures to be exactly 11:1 right now, with the piston .005" in the hole. Running the factory DZ cam with 254 @ .050 set 2 degrees advanced. 36 degrees of timing and it too is running beautiful at these high ambient temps with no complaint. But it's a pretty healthy camshaft for a very small engine so I knew from past experience I could get away with it. Actually these types of factory higher HP SBC's with the long duration camshafts that GM ran in them wouldn't run worth a darn without that high compression ratio and become very soggy engines.

Aluminum headed engines are much easier to deal with. One of mine is 10.2:1 like Lust4Speed mentioned in one of his builds. I run a 248 @ .050 cam in it and have had timing as high as 38 degrees and it never complains at all in the AZ heat.

My fathers 571 pushes this further with 10.84:1 using aluminum heads, made best power on the dyno at 34 degrees so that's where it stays. 260-something @ .050 camshaft and it's been running just fine on 91 octane here.

[jonmachota78]

To the OP, if you drop the cr to 9.1:1 then i think you will want to reconsider the cam choice. OF is going to need more CR to have a decent idle and street manners. 2¢

[Greg Reid]

Well, I'm definitely not going to deck the block. I just finished putting the bottom end completely together. Having the chambers looked at is possible as I was considering doing something I overlooked when I had them jobbed a few years ago. I left the press in studs and I'm kinda re-thinking that now.

I really need to cc them and check the piston height closer as it was just a casual check to get the .007" figure. I'm leaning towards trying the 1016. Steelcity's earlier post tells me that I can expect 5 or 6 tenths crush on the gasket. That would drop me down to about .041" or better of squish.

Also, the fuel in GA may be a little more 'potent' than out on the west coast. I have at least one friend here who told me that he's run quite a few original hi compression engines on our 91 octane pump gas with no problems. I don't mind ponying up for premium as I won't be commuting in this car.

Imagine...this started 4 or 5 years ago as a plan to pressure wash this 'field' engine, paint it and drop it in. Came out of a junk car but it ran smooth when we poured gas in and even the choke worked on it.

[STEELCITYFIREBIRD]

Originally Posted by Formulajones

Yes they are available. Aftermarket companies make them and are sold through Summit etc...

If you would, Point me to the brand that is ~0.015", thanks!