comp ratio help

[70 340]

I need help. I have a 340. Pistons are forged 9.0 to1 pn(380 99).I wanted to check deck height before rebuild.I installed crank and piston number one TDC.The piston was below the deck .065 thousands.What kind of compression would that give me with standard j heads .024 head gasget?I think the heads were milled Do i need diff pistons?I would like to be at least 9.0 to 1.Thanks for any help.           

[Moparal]

You need some more pistons. They are way to far in the hole. You should be close to zero deck

[70 340]

Which pistons do you recommend? how much comp for the street basic pump gas.The block is 30 over. kind of a buget rebuild.I think the piston pins are floating type they have the snap ring.Does that mater? Were and what to buy. thanks

[Moparal]

Im a big block guy myself. Someone will chime in soon for you. But it sounds like you got 73 74 low compression pistons.

[72bluNblu]

That seems pretty deep in the hole to me for 340 pistons.

But you'll need to know the volume of those J heads before you go any further. If they've been milled to work with those pistons you may not have to do anything, although .065 in the hole isn't exactly ideal as far as setting up your combustion chamber goes. But if the heads have been milled down a bunch you may already be at 9:1 or better depending on the head gasket.

Here's a compression calculator that seems to work pretty well. http://www.kb-silvolite.com/calc.php?action=comp

You'll need to know the volume of chambers on the heads and the piston volume (do the pistons have valve reliefs?)

For example, using the above calculator, if you have 65cc heads, a 6cc piston volume (usually 5-7cc on 340 pistons with valve reliefs), have pistons .065" below the deck, a 4.15" bore on the head gasket and the .024" gasket thickness you posted, you'd end up with a static compression of 8.8:1. But that assumes the volume of the heads and pistons, and small changes make a big deal. J's ran anywhere from 65-72cc's, so the actual volume will make a big difference. If yours are 72's you'll only get 8.2:1 with the same specs as above.

[Chryco Psycho]

typically the 10 -1 ratio pistons were actually above the deck by .010
L2316F is the Speed Pro part #

[70 340]

Yes there are valve reliefs.Will have machine shop check chamber volume first.He also said I will prop need pistons he mentioned keith black.They are not all that expensive.Any experiance with these pistons?What comp ratio should I try for if I do buy new pistons?

[72bluNblu]

The KB 243's are good pistons, I have a set in my 340 build. Much better than cast without having to spend  for forged.

They're really aren't many different compression ratio options for 340's, pretty much the stock 10.2:1 or 8.5:1 unless you go with forged or custom pistons. The KB's should put you in the 10:1 ballpark with stock heads, depending on what the chamber volume ends up being and how thick a head gasket you run.

With KB 243's, a .040" head gasket and just over 64cc chambers I'm looking at ~9.8:1 with my 340.

[Chryco Psycho]

to me the extra cost of a forged piston is less than the cost of the gasket set to tear it down + there will be more damage if a Hyper piston fails , short form I never use hyper it is not worth the risk IMO

[moper]

The pistons you have are cast stock replacements for 72-73 340s from what you describe. For a budget mild build they're fine, but they will work out to be closer to 8:1. The issue is also the heads. They will start around 74ccs from the factory in real life. If there was a vlave job done in the past they usually get bigger. If you want to boost the power, I'd use the KB hypers. They do require a special ring gap for the top ring, but that's it. Unless you really have pinging issues or decide to run a lot of NO2 or boost, you won't break them. The hypers will get you pretty easy up to 9.2:1 or a little more depending. I would not cut the heads to get compression, but you could replace them with the RHS Magnums or Enginequest Magnums and get a big jump in compression from the 63cc chambers they come with. EQ heads are pretty cheap and can be ordered to fit any intake manifold.

[dodge freak 2]

to me the extra cost of a forged piston is less than the cost of the gasket set to tear it down + there will be more damage if a Hyper piston fails , short form I never use hyper it is not worth the risk IMO

I have been hearing that also. Seems they crack easily, one tank of poor fuel and you could be looking for a new short block....no fun trust me.

But then I hear for street engines with no strip use under 450 hp KB are really good but the forged KB are good for up to 650 hp racing engines

 

[moper]

I have been hearing that also. Seems they crack easily, one tank of poor fuel and you could be looking for a new short block....no fun trust me.

But then I hear for street engines with no strip use under 450 hp KB are really good but the forged KB are good for up to 650 hp racing engines

 

All pistons get damaged if there's detonation. Hypers get damaged because the assembler can't take the time to read the directions for ring filing... and really, that's only KBs...lol. They are perfect and survive very, very well when used in the right applications: strokes no longer than 3.75, average piston speeds under 4000fps, and no boost. They dont survive with NOS, boost, or top ring gaps that are typical for plain cast or forged pistons. They are cheap and light. MP uses them in the crate engines and all Magnum engines have had them since their intro.