73, 340 compression question

[crash340]

So, 340's built mid 72 and beyond (the blue motors), are low compression motors, small valves etc, do the obtain the low compression by using open chamber headed?, if I swap heads to earlier 340 big valve heads will I automatically get better compression slightly ( or by going to say Indy heads). I know I may not get the 10.5 compression of a 340 due to pjston difference but a little gain would be nice.

[cudabob496]

you need to know things like stroke, depth of pistons below deck, and head combustion chamber
volume, to calculate your Compression Ratio.

http://www.csgnetwork.com/compcalc.html

[crash340]

Thanks Cudabob, yep understand that's how to calculate c/r, just want to know if that's how the later 340's got there lower c/r by use of open chambers in the heads, broadly speaking.

[cudabob496]

Thanks Cudabob, yep understand that's how to calculate c/r, just want to know if that's how the later 340's got there lower c/r by use of open chambers in the heads, broadly speaking.

that's usually how they did it, but not 100%

[72cudamaan]

Older 340's had a dome piston to raise the compression

[Chryco Psycho]

Basically no , the 340/360 heads used approx. the same size combustion chamber in all years , it was the piston that changed the compression , the early 340 piston was actually slightly above the deck

[cudabob496]

Basically no , the 340/360 heads used approx. the same size combustion chamber in all years , it was the piston that changed the compression , the early 340 piston was actually slightly above the deck

OK, learned something new. Maybe I was thinking big blocks? Or did big blocks
change piston design too?

[crash340]

Thanks all, was just thinking I could maybe buy my aluminum heads for the 408 build and throw them on my late 72 blue 340 in the interim and end up with slightly raised c/r.

[Chryco Psycho]

you would actually lose , the Alum head will lower compression heat by 1 point so unless the heads increase the compression by more then 1 point you are losing that way too .
Big blocks had 2 series of heads , small closed chamber 78 cc & larger open chamber heads 88cc
 but they also used different pistons , std 440 used approx. 9:1 pistons , Magnums had taller pistons with valve reliefs putting them closer to 10:1 , Later years after 72 use even shorter pistons with effectively 7.8:1 compression

[crash340]

Ahhh the font of knowledge, thanks CP.

[cudabob496]

you can compensate somewhat, for cylinder pressure, when switching to alum, by running a hotter thermostat.
I felt a power increase when going from 170 to 200 degrees coolant temp.

[dpcd67]

I am with you; I have a 73 340 I just built. I kept the 360 heads that came on it, but the lower compression is due to the fact that the pistons are below the deck by at least .1 inch. (And as mentioned, old 340s were zero or above the deck. So, I got some pistons that are zero deck clearance and the compression is supposed to be about 9.5 to 1.  If you are interested, I will look up which pistons I got; I think they were sealed power forged. That was last summer and I forgot.

[crash340]

Thanks dpcd, I appreciate the offer, no need, it was just a random thought passing through my ever speeding brain to do a simple head swap for now to gain me some cr and larger valves, the plan is to pull the engine and store it as its a numbers matching item, i'm going to pick up a 360 block, stoke it, head it and more than likely inject it. until then though, I have a six pack I was going to just throw on it for fun for a while and figured if I have the intake off and the exhaust off, a few bolts and a set of heads would be a breeze, changing out pistons etc at this stage is not on the white board.

Greg

[mrbill426]

On that note, if one were to install earlier ("taller") 340 pistons in this later block, how much valve lift/duration would be safe without worrying about piston to valve clearance?  Assuming a hydraulic cam were used, and very little more than a one-time resurface were done to the heads.  What were they, 10:1 pistons?

 

[cudabob496]

On that note, if one were to install earlier ("taller") 340 pistons in this later block, how much valve lift/duration would be safe without worrying about piston to valve clearance?  Assuming a hydraulic cam were used, and very little more than a one-time resurface were done to the heads.  What were they, 10:1 pistons?

 

Generally I think you need at least .1 inches clearance piston to valve clearance. That's what I was told when changing cams on my 440. But taking some measurements initially is the right thing to do.