piston size and compression ratio theory

I apologize if this has been covered before - I did do a search and couldn't find it anywhere in past posts.

I'm rebuilding a 331 hemi and would like to boost static compression ratio from stock 7.5:1 to around 9:1. One obvious choice is to use domed forged pistons. But it seems to me that I could increase compression somewhat by simply boring the cylinders out to take a flat 354 piston. As diggers4life pointed out in one post I found on this topic:

"Compression ratio is the relationship between the volume of the cylinder + combustion chamber at bottom dead center and the volume of the area it compressed into at top dead center."

So it seems to me that if the combustion chamber stays the same size (which it will cuz I'm using 331 heads) but the cylinder volume increases (which it will cuz I'm boring out to use 354 pistons) then the compression ratio should increase. Yes? No?

 

You are correct.

 

Overboring increases compression, but you won't get the 1.5 boost you're looking for just from that. At 7.5:1, you must have a '49-'51 engine; if you could scare up a pair of '53 heads, you'd get a boost from the smaller combustion chambers.

 

I am sure that there are math/engine gurus here that can tell you exactly how much of a C/R increase you will get, but I doubt that it will be much over 8:1. However, you will get an increase in power from both C/R and displacement, so it looks like a win/win situation to me.

 

331 / 8 = 41.4 cubic inches/cyl

7.5:1 compression means the total volume of the cylinder + chamber is 7.5 times as big as the volume of just the chamber. by "chamber" I mean the area between the top of the piston and the head, so you have to take the shape of the piston and the deck height and gasket thickness into consideration, but I am lumping these all together.

so, if the cylinder volume is 41.4 cubic inches, the chamber volume can be calculated:

cylinder + chamber = 7.5 x chamber

which simplifies to: cylinder = 7.5 chamber - chamber

which simplifies to: cylinder = 6.5 chamber

so the chamber volume is 41.4 / 6.5 = 6.36 cubic inches

if you increase the cylinder volume to that of a 354, you get 44.2 cubes, and the compression ratio is

cylinder + chamber / chamber

44.2 + 6.36 / 6.36 = 7.95

so racyredhd got it right, you get 8:1 by increasing bore size to that of a 354. But this assumes the piston dome is the same as the original piston.

 

Cool! Thanks fellas. I could probably get off my ass and try crunching the numbers myself now that I have some encouragement.

 

Oops. Too late. Thanks Squirrel.

 

you're welcome, someone please check my math...

also this assumes the new deck height is the same as the old deck height, so you need to check things like piston compresion height and whatnot.

 

But if you bore it you are going to have to get new pistons anyway.
So why not just buy some domed ones at the get go and forget the bore job?

 

Because unless you are getting stock type replacement pistons they are BUCKS!!! Something like $700 a set but I haven't priced them for a while.

That being said if you are boring up to the 354 pistons you might luck out as stock 354's had compression ratios all the way up to 10:1.

 

Squirrel -

Your numbers are pretty much on the mark.

Smokey - Damn straight!

Van Chop -

There are BOATLOADS of pieces parts available for vintage Hemis, Hot Hemi Heads is the place to look. You can get popup (domed) pistons from them which will increase the CR to whatever you wish.

A vintage hemi is a piece of jewelry, man. Call Hot Hemi Heads and discuss where you want this motor to lay out; they can hook you up, man.

 

OK, so he gets it bored. How much does that cost $250+? A new set of 354 stock cast pistons runs ~$410. So you are already pushing $700 and he still isn't going to get the CR he's looking for. Then what different heads?

 

51-4 engines are 7.5, '55 8.5, Don't know if they upped the compression by changing the chamber or the piston, but cast pistons for 331s are listed as 8.5.

 

I thought this was just a theory question

 

You can up the compression by removing head gasket and skimming the heads too.
Check out any marine applications for higher compression/ performance parts.
Used to be in England that chrysler competition components were often cheaper than standard too.
Ya never know./...
check it out.

 

My mistake--I saw "331" and automatically thought Cadillac!