Head gasket thickness and compression ratio changes

Piston to valve clearance one reason. I have actually used thicker than this to get piston to valve clearance rite, along with pocket cutting the piston.

 

At the expense of proper squish. Not good. I wouldn't think that a stock rebuild with stock heads and cam would need to worry about p-to-v clearance unless they remove alot of material from the block and head surfaces.

For a race engine that sees only a narrow high rpm range it's not a problem but for something driven on the street it's detonation waiting to happen.

 

Stock rebuilds aren't counting the HP,, just $$$ cost. With the thicker gasket, if core engine was decked by previous owner, not much of a problem.

 

It can be a problem, my point was that a low rpm stock-type engine is more susceptable to detonation when you have large piston-to-head deck distances (squish), as a .055 gasket would provide.

 

I'm not trying to be smart on this or anything, But i would like to hear your theory on this causing detonation, It could be something I've overlooked. My original post was geared more toward a performance engine (Blown) more than a stock rebuild.

 

Best I come up with for OEM gasket thickness is .047 compressed. Now the .055 was that compressed??

 

We were discussing Scooper's stock rebuild engine. And squish/quench theory is not my theory. The piston has what is called the squish/quench area, the surfaces that come in close proximity to the head deck. Wedge type heads have a corresponding area on the deck surface. True Hemi chambers don't, they are completely open. Flat top pistons can utilize almost the entire head squish/quench surface. Dished pistons will have a smaller area to interact with the head deck surface. The idea is to have the piston and head surface come close enough together at TDC on the compression stroke that mixture that is in these areas are forced into the combustion chamber and/or piston dish, at the center of the cylinder away from the cylinder walls. This homogenizes the mixture for a more complete and quick burn. It has been determined that the best distance between the head deck and piston top is somewhere between .032" and .050". Shape of the piston top (flat/dish) among other things will move this dimension around some. But that distance is determined by where the piston sits at TDC and the thickness of the head gasket.

The other part of the process is called quench and occurrs sometime after TDC. The outer areas of the cylinder are relatively cool because no mixture burn happened there, squish had forced the mixture into the center of the cylinder. As the piston moves away from TDC burned and burning mixture is sucked back into these cooler outer areas and the flame is rapidly put out preventing detonation.

The result is more power because of more complete mixture burn and less chance of detonation due to the cooling effect. They had it all wrong back in the 70s when they tried to prevent detonation strictly by decreasing compression, moving the piston away from the head deck.

 

.055 was after I took it off and put it in the trash. then came back later and measured it just for the fun of it.

 

I dug in the trash and got the part number off the gasket. Doing some searches on the internet, I found this quote...These are described as Economy Head Gaskets.

 

bmc is pretty much right on.
To add to all this, the dish and very little squish of the 70s made the motors run on, detonate with lower octance gas, if the timing was advanced to far, there was carbon build up etc..
Flat tops set to achive .030 to .070 squish will make the charge very active and burn more completely with better combustion control.
The 40p heads if you notice have a greatly different shape combustion chamber with the plug pushed out toward the center line of the bore.
This makes these heads a faster burn, more efficient and require less ignition advance. This gains power by not having to push against the same combustion it's plug initiates thru lesser number of degrees before top center, with the chamber design still optimized to achive max cylinder pressure at about 12 degrees after top center without detonation or other undesirable effects.
Along with higher air/gas flow rates make these heads pretty good for OEM heads.
Lastly, the 40p heads have a smaller chamber volume such that the blocks they were used on often has the pistons set for about .012 below deck so a 9 to 1 CR could be maintained for 87 octane gas in the later EFI motors.
Use these heads on a block with zero deck and you get about a .6 higher compression with .041 gaskets. These figures are subject to a range because you don't have total accuracy on the chamber volumes, deck heights and gaskets used unless everything is measured and calculated.

 

Just to pipe in, MY pistons pushed .20 past the deck, so I had to run a .65 gasket! 16,000 miles on the engine now and I'm afraid to put a set of aftermarket heads on it because of that.

 

Not problem as long as you realize what condition you have.
60 - 20 = 40 a good result.
The compression ring gets close to the deck and improves compression for the same combustion chamber volume as well as not hide much mixture from combustion..
The thick gasket should provide all the clearences but always wise to check so you don't miss something and get into damage.
Only problem is trying to do this on a blower motor where the gaskets can't hold as much boost pressure especially with only 4 head bolts around each cylinder and the exposure the top ring gets to extra combustion heat.

 

Thanks, Good explanition Its always nice to see things thru other eyes sometimes.

 

dyno with old gasket, dyno with new gasket.. with no other changes, and this will tell you EXACTLY what was gained or lost.

hope i helped your confuzzledness

 

Won't work.

The same engine with no changes will not dyno the same every single time you run it. Add to that every dyno has a margin of error as well. From 5 to 15 hp from what I understand.

Now I am talking about chassis dynos...
I don't know enough about engine dynos to make such a statement, but I am fairly confident that the same applies.