Pistons, Heads, and Compression

[mongo]

After a long delay, it looks like my 95 3.4 block is just about out of the shop. I haven't started on my 02 Grand Am heads yet. Time to order some pistons. I had planned on buying a set for a 3400 from a Transport van as they have the correct dishes to clear the valves on the aluminum heads. If I want to modify my compression ratio I would like to know if there were different stock dish sizes (depth/volume) or am I stuck shaving my heads?

[mongo]

Sorry if the question above seems repetitive since Compression has been a hot topic around here lately but my question is more of economics and part availability. For example:

If there are different dishes available in stock pistons... and I need new pistons anyway... there is no additional cost to my build.

If I get done with my heads and they did not need shaving but I do it anyway to increase the compression, I have some cost (less than $100) involved.

If my heads cannot be shaved enough to reach my desired CR and pistons are not available in stock form, the cost of custom pistons would be excessive for a stock/mild performance build using almost exclusively off the shelf parts....

[95Grand-Am]

What compressoin are you trying to get? How do you plan on using the engine, racing or street?

[LeftVentricle]

Using iron head head gaskets with aluminum heads can get you a bit of compression because they're a bit thinner than the aluminum ones. For example, a gen 2 3.1 has a stock compression of 8.8:1. With the iron head gaskets it's bumped up to a little over 9.3.

If you're just looking for crazy compression, iron head pistons with the aluminum heads will net you something ludicrous like 13:1.

[geoffinbc]

The heads should only be shaved for cleanup. They cannot be shaved excessively in order to increase compression. You will weaken the deck, which is especially bad if you are using the engine for high performance.

You can use a stock head gasket with iron head pistons but you will end up with 13.67:1 compression.

You CANNOT use an iron head gasket because the pistons stick out of the bore .020". However a lot of after market pistons are made short in order to allow cleanup of the cylinder block so they may sit flush with the top and allow you to use an iron head gasket. Your shop could also fly cut the pistons to zero deck and they you could use and iron head gasket and I would wager your compression will be in the 10's. But then you need a full balance job.

[LeftVentricle]

You CANNOT use an iron head gasket because the pistons stick out of the bore .020".

Does that only apply to the 3.4 SFI, or to aluminum head gen 2s as well?

[mongo]

I am using a 3.4 block with Gen 3 3400 heads. I am hoping to end up with 9.5:1 to 10:1 compression. The engine will be a daily driver. If I am not mistaken, using the Transport pistons and 3400 gaskets will give me a CR around 8.5:1. Using standard 3.4 pistons sticks me around 13:1 or so which is not acceptable for my purposes. It seems there is a CR gap between 9:1 and 11:1 for my parts combination that is economically feasible. By the time I pay for fly cutting pistons, I could have bought custom pistons. By the time I pay for custom pistons, I could have bought a crate motor. The day I am willing to spring for a crate motor is the day I start looking for a new hobby car with a V-8. I couldn't live with myself bragging about a car someone else did the work on!

So, in a nutshell, I am willing to pony up SOME cash to increase my CR but there is a limit to how far I will go. If nothing else, I will live with a CR around 8.5 and put my money elsewhere in the car... probably the interior...

[geoffinbc]

Well a stock 3400 is 9.64:1 so what wrong with that. That seems to fall right into your range. Mill the heads .005 or .010 and you will be over 9.7:1

[mongo]

That is right where I want the compression to be... Perhaps I was mistaken in my calculations putting it at 8.5:1...

Thanks for setting me straight!

[geoffinbc]

Maybe you entered the wrong Piston to Deck clearance. Remember the pistons stick out .020 which on our compression calculator neds to be entered as -.020. Entering .020 without the minus would tell the calculator that the piston is down in the block .020"

[mongo]

Knowing that the pistons stick up .020 on the iron block 3.4 and that I am using Gen 3 heads... which head gaskets are best for my application? Given the piston to head clearance, the only thing preventing piston slap is the distance created by the crushed head gaskets. I am building this engine for reliability. Sometimes reliability and performance go hand in hand... sometimes you get a choice of one or the other. Also, are there any brands of gaskets to avoid?

[geoffinbc]

You just need to use a 3400 gasket which is .060" thick. Buy GM or Felpro thats the only thnig I ever use and I think that goes for most people here too.