I run an au head and gasket on my ef 4.0 and want to find out if they raise the comp at all compared to a stock gasket?
I have a pinging problem and i think it may have started when i did the head swap.
This is about the last thing i will try to find out before i give up on this problem, because i think my wifes ef doesnt want to get fixed sometimes.
I feel like a dog chasing my tail

they are about 25thou thinner then a regular ACL/ford ect. gasket.

so yes it will raise the compression.

going to the AU gasket will raise the compression by 0.5




what head did you put on it? and how much was it milled?

more likely the head was machined flat again and thats whats done it.

25thou from the head has less impact on compression then 25thou less gasket thicknes.

but was 25thou taken off?

ive heard of much more than that taken at a time

you'd have to take off about 40thou from the head to equal 25thou less gasket.


thats a damn lot of metal, if it needed that much the head was warped and shouldn't have been used.

so comp on an EF 6cyl will go from 9.3 to 9.8 with just an AU gasket... is that correct?

close enough to it.
every 25thou of gasket is worth about 4.25cc of combustion space

most people that change the gasket to an AU and have the head skimmed would be pushing 9.9 to 10:1. depending on actual bore size.

Damn i must have forgotten this post.
I run a stock au head that hasnt been shaved but im not sure exactly what au engine this head came off, part number starts with a wr.
After reading these posts i reckon ill be going back to the stock ef gasket and hopefully stock compression ratio.
If the au head has a smaller combustion chamber size than the ef that wont help my problem either.
Anyways thanks alot for all the replies, muchly appreciated

Good post but needs to be more clear as im aware that when u have a standard un opened engine and you are replacing the headgasket what comp ratio will u have once the head is back on.

things that have to be considered

How much thou has been machined off the head plus AU head gasket equal blah blah comp ratio??

there is NO magic formula that says X gasket plus Y taken from head = Z compression.

like i posted befor, every 25thou of gasket is worth about 4.25cc of combustion space.
the chamber needs to be cc'd to measure it.
The rest is up to your brain and your calculator.