Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - I have stock Gen IV 454 & 781 Heads, Compression ????
View Single Post
  #14 (permalink)  
Old 11-30-2011, 01:53 PM
techinspector1's Avatar
techinspector1 techinspector1 is offline
Senior Curmudgeon
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,096
Wiki Edits: 326

Thanks: 781
Thanked 1,063 Times in 882 Posts
Originally Posted by wild_man84
MY guess would be to read the same literature as the piston eyebrows and find the negative side of the dome piston and just subtract that? then, Also having to check clearance from deck height to be sure valves are in the clear and the quench is where it needs to be.. Of coarse I have no clue how that process would need to go. But, Using what Ive just learned and what your about to teach me on DOME, I know Im about to learn!!!
This is what makes it worth the effort. A young man willing to learn.
Yep wild_man, it's just that simple. We'll start with this piston....
Subtract the dome volume from the combustion chamber volume before you begin your calculations. ex: 118-25.5=92.5, so that's our new combustion chamber volume to calculate from. We have whacked off the piston dome and stuffed it up into the chamber, reducing the volume of the combustion chamber. Now, visualize the piston being perfectly flat on top with a slight valve relief divot which is worth 3 cc's.....see here.....
We'll call the notch "piston crown volume" and figure it in when we calculate. Or, we can borrow 3 cc's of material from the combustion chamber and stick it into the valve relief of the piston. We'll magically fill the valve relief with 3 cc's of material, leaving a perfectly smooth piston crown and an 95.5 cc combustion chamber volume (92.5-3=95.5). Doing your own calcs allows you to be whimsical if you want to.

What to do with the pop-up on the intake stroke though? It's sitting there on the piston crown and taking up space (volume) as the piston descends in the bore with the intake valve open. That 25.5 cc's of piston dome could have been 25.5 cc's of fuel/air mixture if a flat-top piston had been used, so I think we have to consider it and deduct 25.5 cc's from the cylinder volume.

So, BBC bored whatever, let's use 4.310" as a bore size with a 4.000" crank.
(.7854 X 4.310 X 4.310 X 4 X 16.387 = 956.32 cc's. Now, we deduct the 25.5 cc's from the 956.32 cc's and find an actual cylinder volume of 930.82 cc's.

Next, combustion chamber volume. You have become proficient at pouring voids and doing calculations to arrive at an exact answer. You're makin' a little money on the side, pourin' for the local speed shop. You pour these heads at 121.0 cc's across both heads. Someone has spent some time on them, equalizing the volumes chamber to chamber. If they aren't all the same, then you have different static compression ratios cylinder to cylinder.
We've stuffed 'em with the domes off the pistons and now we'll take back 3 cc's in just a minute, leaving a chamber volume of 95.5 cc's.

Now, we know that the block deck height is ~9.800" stock, but you measured this block at 9.785" with your 12" dial caliper...someone has taken a light cut on it sometime in the past. You hate electronic calipers because everytime you pick them up for use, the battery is dead. Anyway, we need that figure to know how much space there will be for the reciprocating assembly (crank, rods, pistons). Half the crank throw is 2.000", rod length is 6.135", piston compression height is 1.645"
so the length of the "stack" is these 3 added together and arriving at 9.780". This will leave a piston deck height volume of (.7854 x 4.31 x 4.31 x .005 x 16.387 = 1.19 cc's).

The value of the smooth piston crown will be zero.

A commonly used head gasket would have a bore of 4.370" and a thickness of 0.039", yielding 9.58 cc's.

1. cylinder volume 930.82 cc's
2. chamber volume 95.5 cc's
3. piston deck height volume 1.19
4. piston crown volume 0
5. head gasket volume 9.58

Your turn....

Last edited by techinspector1; 11-30-2011 at 02:02 PM.
Reply With Quote