Head Gasket Compression Question

[Anonymous]

Taken from another thread...Thanks MarkP

Well, the 'stock' gasket IS the steel gasket. All of the aftermarket gaskets are composite type and are thicker than the steel ones, which causes lowered compression unless the head is milled by the respective amount. For instance, the stock steel gaskets are about .020" thick installed, while the popular FelPro version is .055" thick installed. In an otherwise stock engine, using the FelPro gasket lowers the 1980 3.3L compression from the original 8.5:1 to 7.6:1 - dismal at best (I did this 2 years ago, not knowing it at the time, and being in too big of a rush to check). Ford made a replacement gasket of the composite type which is .027" thick as a fix for leaking head gaskets in warranty situations. I have a couple of these left for sale, if you want one. They seal better than the steel ones, which can be tricky to seal and assemble unless you're good at it.

So I have a 81 Big Bell stock rebuild with a OZ head. Nothing was taken off the block. I understand that steel gaskets were not used in OZ. I have a FelPro gasket. Anyone care to take a stab at what my compression is provided the head is stock, and what it would be if I used the steel or .027 gasket. I wonder if this is why I have been able to run so much advance.

 

[xctasy]

Addo or Mustangaroo posted the cc's of a 250 2V a while back. My pick is 59 to 61 cc, so plug 60 cc into the equation, and remember, Aussie blocks had the piston riding up less than 25 thou from the top of the block. They used steel gaskets for a while, only changing to composite with the cross-flow in 1976.

The compression computer from the http://falconperformance.sundog.net/compcalculator.asp site is what to use.

 

[addo]

Sure about the steel gaskets, XE? I popped a virgin 221 with a composite gasket. Twice, actually. It was thinner than a lot of replacements though - perhaps like what MarkP stated.

58cc was probably intended on the 250-2V head, but I can't recall offhand.

 

[Anonymous]

plug 60 cc into the equation

plugging that in and .55 on the head gasket gets 7.8:1, not good. I left out that the block was bored .30, I'm not sure how to plug that in. I also have the dished pistons. The composite would get me to 8.3:1.

So do I go to a machine shop? I don't want to go back to the same guy. I know some work has been done on the head because hardened seats have been installed. Can this be done in the car? What sort of shop should I look for and what do I tell/ask them? How much should I expect to pay? I am getting one of the compsites and already have a steel. I have installed the adjustable rocker arm assembly.

If I have a shop do the work can they weld in the port divider with the engine in the car?

 

[xctasy]

The dish is often about 6 cc or so, the top of the piston is about 30 thou below on most new pistons, about a 1.5" deck.

The bore goes up to 3.71, and the gasket should be designed to take an over 60 thou overbore, or about 3.75"

Plugin' it in, and use 58 cc's.

Number Of Cylinders 6 Bore Diameter [3.71]
Stroke Length [3.126]
Combustion Chamber Volume [58]
Head Gasket Compressed Thickness [55]
Head Gasket Bore Diameter [3.75]
Piston To Deck Clearance [30]
Select Piston Type Dished
Dish/Valve Relief/Dome Volume [ 6 cc]
Volumetric Efficiency: 85%
RPM: 5500

Results:
Displacement, [cubic inches] 203
Displacement, [liters] 3.3
Static compression ratio 8:1
Cubic Feet per Minute required @ 5500 rpm, [cfm] 274
Estimated Horsepower @ 5500 rpm
Assumes altitude of sealevel, barometric pressure of 14.696 and 60° air supply to carburator 140

So that's it, an 8:1 compression ratio at best.

I'd go with a steel gasket, and get 8.6:1, and save the price of a re-mill of the head.

But it'd run nicer with a little more squeeze.

 

[Mustangaroo]

My Aussie 2v head has a 47cc it's been milled about as far as it will go, my son Jason's head is 58cc and doesn't appear to ever been milled. Jason engine's block is decked to .000 and is using standard felpro gasket and using Ford Tempo HSC flat top pistons and his c/r is about 9:5.1 my engine has stock dish pistons 4-5cc dish and .025 block, steel headgasket .018 and 47cc head running about 10:5.1 no pinging even using 89* octane gas. Hope this info is helpful?

 

[Anonymous]

Okefenokee,

The best way to be sure of your CR is to CC your head. Since you have to pull it to change head gasket anyway, you’ll have the opportunity to CC your head and have your port divider welded in at the same time. Once you know your CCs you can calculate how much you need to mill the head and/or deck the block based on XE's data above. IIRC 2 CCs = .010
Edit: For example if your at 56 CCs and you mill off .020 and get 52 CCs with the .027 head gasket all things being equal your CR will go from 8.8:1 to 9.3:1 and you will still be able to run on regular gas.

CobraSix has the process pretty well documented on his web site:
http://kastang.tripod.com/ccing.html