rough Idea on compression ratio

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reno340

reno340

I Am The Big Dog
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I have a 1972 Duster 340 that is getting an engine rebuild. I was looking at some silvolite pistons for it. I copied this from the description
Brand: Silvolite Pistons
Part Number: 1267
Condition: New


Silv-O-Lite Pistons for 1968-71 Chrysler 340 5.6L V8 Standard Size, matched set



Flat Top Piston, Four valve reliefs



Bore Size: 4.040" / 103.124mm

Compression Height: 1.824"

Pin Diameter: 0.984

Piston Ring Thickness: 5/64" x 5/64" x 3/16"

Weight: typically 868 to 871 grams w/ pin, but varies by production run.

I bought a set of heads from Aerohead racing that they state are between 67-70cc open chamber heads (casting number 3769974).

Can anyone tell me what I can expect the compression ratio to be? at least a rough estimate would suffice.
Thanks
 
Assuming 70 cc chambers and 10 cc's for the 4 valve reliefs:
- With Mr Gasket 1121G head gasket: 9.2
- With Felpro 1008 head gasket: 8.95
- With Felpro PT5883 head gasket: 8.7

These are a tad shorter than stock pistons; those had a compression height of 1.840" and IIRC 2 valve reliefs. Weight is very close to stock, maybe about 5-6 grams heavier.
 
reno340 said:
So these are basically 1972 340 stock replacement pistons?
Click to expand...

Yes. Why the description says "68-71" is a mystery, as the 72-73 pistons had the 4 valve reliefs. The 68-71 had 2.
 
My goal here is to try to find a good set of pistons for my 340 that will put me into the 9 to 9.5:1 compression ratio. I am on a very tight budget here. (A bunch of unexpected life expenses came up all at once while I am trying to build this) My block is excellent but I need pistons and I can use those pistons that I looked up as they are very affordable ($200.00) I don't want to pay a lot more for just for a little more CR. I will be doing this in a month or so and am gathering my parts. Any help here is appreciated.
 
Looks like you are there with those pistons and Mr Gasket 1121G head gaskets....Those are pretty close to the stock shim head gasket thickness. BTW, the catalog says that the pin is offset in those pistons, so they need to be installed in a certain direction.... notch to front would be what I would expect.

Are you boring the cylinders? Your original description posted is for stock 4.040" pistons size. If the bores are worn or tapered or out-of-round, your engine may not do well without overboring.

The listed Sealed Power ring pack for the 340 is the E286K. The ring heights are the same for the E251K's for the 360 but there may be something different like in the ring twist for the 340 vs 360. The E286K crosses over the a Hastings 2M694, which is a moly faced top ring set that is a little less expensive. I can't say anything about the relative quality of the 2 brands.
 
reno340 said:
So these are basically 1972 340 stock replacement pistons?
Click to expand...

No..the 72-73 piston has a compression height of 1.74....the 68-71 piston has a compression height of 1.84......so your 1.82 compression height is a little less ,...but without knowing the deck height on the block we are all guessing...
 
The lower the compression ratio comes in at, the more important will be the cam selection.

For a streeter I like to work backwards from the application, choosing a cam from where the power needs to be. Then knowing what Dcr to run,, with what fuel it wants to run, I can back-caculate the Scr. Then I can shuffle the guts around until the Scr comes close, and we are done.well sorta. The heads still have to do the work.
 
70aarcuda said:
No..the 72-73 piston has a compression height of 1.74....the 68-71 piston has a compression height of 1.84......so your 1.82 compression height is a little less ,...but without knowing the deck height on the block we are all guessing...
Click to expand...
Yes, all my numbers assume that the block has not ever been decked. If the old pistons are stock, then then it is a fair assumption that the deck is stock.
 
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